The Stabilization of Transition States by Cyclodextrins and other Catalysts

The Stabilization of Transition States by Cyclodextrins and other Catalysts OSWALD S. TEE Department of Chemistry and Biochemistry, Concordia University, Montreal, Canada

1 Introduction 1 2 Cyclodextrins 3 Effects on reactivity 7 3 Transition state stabilization 9 The Kurz approach 9 Cyclodextrin mediated reactions 11 4 Non-covalent catalysis 13 Intramolecular reactions 13 Decarboxylation 15 Bromination-debromination 17 5 Covalent catalysis 22 Ester cleavage 22 Amide cleavage 45 Deprotonation 46 6 Other catalysts 46 Acids and bases 47 Metal ions 52 Amylose 55 Micelles 55 Catalytic antibodies 56 Enzymes 60 7 Future prospects 62 Acknowledgements 63 References 63 Appendix 69

1 Introduction

Enzymes fascinate (and exasperate) chemists because they can catalyse reactions at ambient temperatures and at modest pH, often with high substrate selectivity, regioselectivity, and enantioselectivity . Moreover, they do all this at rates that are 106-10” times faster than the uncatalysed reaction. The origins of these impressive feats almost certainly lie in supramolecular behaviour (Lehn, 1985, 1988) since enzymes invariably form 1 ADVANCES IN PHYSICAL ORGANIC CHEMISTRY

Copvrrghr 0 IYY4 Academic Press I.imrwd

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enzyme .substrate complexes from which the catalysed reactions ensue. Many static and dynamic studies of enzyme behaviour have provided ample evidence of such complexes and great progress has been made in elucidating many of the mechanisms by which enzymes transform their substrates into products (Walsh, 1979; Fersht, 1985; Page and Williams, 1987; Liebman and Greenberg, 1988; Dugas, 1989). At the same time, there have been significant advances in understanding the factors underlying the catalytic abilities of enzymes (Jencks, 1969, 1975; Bender, 1971; Lienhard, 1973; Gandour and Schowen, 1978; Page, 1984; Fersht, 1985), although at times it has seemed as though there were too many theories of enzymatic catalysis, based on the multiplicity of ideas about the efficiency of intramolecular processes (Page, 1984, 1987; Menger, 1985; Page and Jencks, 1987)! The underlying principle of enzyme catalysis was expounded many years ago by Haldane (1930) and Pauling (1946). According to them, catalysis results from stabilization by the enzyme of the reaction transition state, relative to that of the initial state. This view was developed by Kurz (1963) into a quantitative approach to transition state binding, and hence of transition state stabilization, albeit in the context of catalysis by acids and bases (Kurz, 1963, 1972). His approach was taken up and used by enzymologists (Wolfenden, 1972; Lienhard, 1973; Jencks, 1975; Schowen, 1978; Fersht, 1985; Kraut, 1988), so much so that it is now implicit in many modern studies of enzyme action (see, for example: Fersht et al., 1986, 1987; Leatherbarrow and Fersht, 1987). Of particular note, Kurz’s innovation helped to develop the use of “transition state analogues” (Jencks, 1969) as efficient enzyme inhibitors, either for the purposes of mechanistic studies or for possible pharmaceutical use (Wolfenden, 1972; Wolfenden and Frick, 1987; Wolfenden and Kati, 1991). In turn, the availability of transition state analogues as haptens has been critical to the recent development of “catalytic antibodies” (Schultz, 1988, 1989a,b). The fascination of chemists with enzymes has led, in recent years, to many attempts to model or mimic their action (e.g. Bender, 1971, 1987; Breslow, 1982, 1986a,b; Page, 1984; Tagaki and Ogino, 1985; Kirby, 1987; Stoddart, 1987; Schultz, 1988, 1989a,b; Dugas, 1989; Chin, 1991). The object of such studies has been to understand enzyme action and, in a broader sense, catalysis better, and possibly to learn how to synthesize catalysts (“artificial enzymes”) for specific purposes (Breslow, 1982; Schultz, 1988). Many such studies have employed model systems based on the binding and catalytic properties of cyclodextrins (CDs) or their derivatives (Bender and Komiyama, 1978; Breslow, 1980, 1982, 1986a,b; Tabushi, 1982; Komiyama and Bender, 1984; Bender, 1987; D’Souza and Bender, 1987; Tee, 1989). At the same time, CDs have commanded another, more practical and populous audience due to their many potential applications in the food, pharmaceutical, and cosmetic industries (Szejtli, 1982; Pagington, 1987). These differing interests in the chemistry of CDs have led to an explosion in the literature

TRANSITION STATE STABILIZATION

3

concerning these molecules in recent years, especially now that they are produced commercially and are available relatively cheaply. The present review deals with a particular aspect of the chemistry of cyclodextrins: the effects that they can have on organic reactions by virtue of their abilities to bind to many organic and inorganic species (Bender and Komiyama, 1978; Saenger, 1980; Szejtli, 1982). It is a considerable expansion of an earlier work (Tee, 1989) which first showed how the Kurz approach to transition state stabilization can be employed profitably in discussing reactions mediated by cyclodextrins. Most of the large amount of data that are analysed is collected in tables in the Appendix so as to avoid breaking up the discussion in the main text too frequently. While the main emphasis of this review is on catalysis, since this is of greater interest, the Kurz method can also be applied to retardation. In fact, for some of the systems discussed later, the smooth transition from retardation, through inactivity, to full catalysis can be quantified and analysed in relation to the structure of the species concerned. At the end of the review there are some examples involving catalysis by acids and bases, metal ions, micelles, amylose, catalytic antibodies, and enzymes to give the reader a feeling for how Kurz’s approach may be usefully applied to other catalysts. Very few of these examples, or those involving cyclodextrins, were discussed in the original literature in the same terms. It is hoped that the present treatment will stimulate further use and exploration of the Kurz approach to analysing transition state stabilization. 2 Cyclodextrins

These water-soluble molecules are cyclic oligomers of a-D-glucose formed by the action of certain bacterial amylases on starches (Bender and Komiyama, 1978; Saenger, 1980; Szejtli, 1982). a-Cyclodextrin (cyclohexaamylose) has six glucose units joined a(1,4) in a torus [l], whereas p-cyclodextrin (cycloheptaamylose) and y-cyclodextrin (cyclooctaamylose) have seven and eight units, respectively. The form of cyclodextrins (CDs) is variously described as being “conical”, “toroidal”, “bucket shaped”, or “doughnut shaped” [2]. Regardless of the adjective used and the finer details of their structure, the most important feature of CDs is the cavity, because this enables them to form inclusion complexes in solution and in the solid state. By virtue of their cavities, CDs possess the requisite amount of preorganization and the convergent surfaces (Cram, 1983, 1988) necessary for them to function as hosts for small molecular guests of an appropriate size, shape, and polarity. The depths of CD cavities are all the same (approximately 7.5 A), being determined by the width of a glucose molecule, but the sizes of their cavities differ in diameter (a-CD about 5.0, p-CD about 7.0 and y-CD about 9.0A) (Bender and

4

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CH;! \ OH

Komiyama, 1978; Szejtli, 1982), giving rise to a gradation in binding affinity. The geometrical features of CDs, plus their relative rigidity, obviously impose constraints on their ability to form guest-host (inclusion) complexes with organic and inorganic species (Bender and Komiyama, 1978; Saenger, 1980; Szejtli, 1982; Atwood et af., 1984). Nevertheless, CDs have been labelled “promiscuous” for their propensity to act as hosts to a wide variety of small- to medium-sized guests (Stoddart and Zarzycki, 1988). It is the ability of CDs to form complexes that enables them to influence chemical reactions through supramolecular effects (Sirlin, 1984; Lehn, 1985, 1988). In what follows, some of the basic aspects of C D binding, relevant to the reactions discussed later, are presented. More detailed discussions of CD inclusion complexes can be found in the references already cited. Broadly speaking, the cavity sizes of a-,p-, and y-CD are appropriate for binding simple derivatives of benzene, naphthalene, and anthracene, respectively (Sanemasa and Akamine, 1987; Fujiki et al., 1988; Sanemasa et af., 1989). Many studies of the inclusion of aromatics, particularly of dyes and other molecules with strong chromophores, have been reported, and these have been useful in delineating the main features of C D binding (Bender and Komiyama, 1978; Saenger, 1980; Szejtli, 1982; Atwood et al., 1984; Stoddart and Zarzycki, 1988). In contrast, the affinity of small to medium aliphatic molecules for CDs have been less well studied, most

TRANSITION STATE STABILIZATION

5

probably for practical reasons. Nevertheless, there have been studies with various surfactants (On0 et al., 1979; Satake et al., 1985, 1986; Diaz et al., 1988; Palepu and Reinsborough, 1988; Palepu et al., 1989), alkanes (Sanemasa et al., 1990), and a particularly interesting study of the binding of many alcohols to both a- and p-CD (Matsui and Mochida, 1979; see also, Matsui et al., 1985; Fujiwara et al., 1987). For the most part, CDs form simple 1 : 1 host-guest complexes with suitable guests. But it is important to note that 2 : 1 binding can be significant with longer aliphatics (Palepu and Reinsborough, 1988; Palepu et al., 1989; Sanemasa et al., 1990), aromatics (Sanemasa and Akamine, 1987; Fujiki et al., 1988), azo dyes (Bender and Komiyama, 1978; Szejtli, 1982), and aryl-alkyl guests (Tee and Du, 1988, 1992), and this can influence reactivity. Also, there is now evidence of 1: 1 : 1 binding of C D two guests (Hamai, 1989a,b) which has been implicated in some reactions (Ramamurthy, 1986; Tee and Bozzi, 1990). The ability of a C D to form inclusion complexes in aqueous solution results from its cavity, the interior of which is less polar than water and hydrophobic. The apparent polarity of the C D cavity seems to depend on the probe used. Some studies have suggested a similarity to dioxane (Bender and Komiyama, 1978; Hamai, 1982), while others favour ethanol (Cox et al., 1984; Heredia et al., 1985). No doubt the particular observations are affected by the presence or absence of specific interactions, such as hydrogen bonding, between the guest and the CD host, as well as by the depth of penetration of the guest/probe. Decarboxylation studies, to be discussed more fully later, suggest an environment like 50% aqueous 2-propanol (Straub and Bender, 1972a,b). Various other factors have been cited (Bender and Komiyama, 1978; Szejtli, 1982) as contributing to the binding ability of CDs. However, the principal factors seem to be the hydrophobicity of the guest and the appropriateness of its size and shape in relation to that of the C D cavity (Tabushi, 1982). These factors are evident in the binding of alcohols to CDs (Matsui et al., 1985) and of other guests with alkyl groups (Tee, 1989; Tee et al.. 1990b). For illustrative purposes, and because of its relevance to a later section, the binding of alcohols will be discussed in some detail. For linear, primary alcohols (n-alkanols) the strength of complexation with CDs, expressed by pKs = -logKs, where Ks is the dissociation constant of the complex, correlates strongly with their coefficients for partition ( P , ) between diethyl ether and water (Matsui and Mochida, 1979; Matsui et al., 1985), with slopes close to 1 ( l a and lb). It has also been

+

a-CD: P-CD:

+ 1.25; pKs = 0.94 log P, + 0.58; pKs = 0.91 log P,

r = 0.994

(la)

r = 0.994

(lb)

noted (Tee, 1989; Tee et al., 1990b) that for these alcohols, and other linear

0 . S. TEE

6

3.0

normal A

6 2.0

au

u"

branched cyclic

1.0

V

2-alkanols

+

tertiary

0.0 1 .o

2.0

3.0

pK, (a-CD)

Fig. 1 Correlation between the binding of aliphatic alcohols to p-CD and to a-CD: (-) the least-squares line for n-alkanols; (----) pKs (p-CD) = pK, (a-CD); above this line a given alcohol binds strongly to p-CD than to a-CD. Data from Matsui and Mochida (1979) and Matsui et al. (1985).

aliphatics, pKs values vary linearly with N , the number of carbon atoms in the chain. These observations are reasonable since, as remarked above, the binding of guests to CDs is largely governed by their size and hydrophobicity (Tabushi, 1982). Obviously, the sizes of extended n-alkyl chains increase linearly with N , but so also do various measures of hydrophobicity, such as the logarithms of partition coefficients, critical micelle concentrations, solubilities (Hansch, 1971; Leo et al., 1971; Hansch and Leo, 1979; Tanford, 1980; Menger and Venkataram, 1986). Equations (la) and (lb) represent two nearly parallel lines with a vertical difference of about 0.7, indicating that a given linear alcohol binds about five times more tightly to a-CD than to p-CD. This makes sense in terms of the sizes of the a- and p-CD cavities (about 5 and about 7 & respectively) in relation to the cross-section of methylene chains (about 4.5 A) (Sanemasa et al., 1990). With bulkier types of alcohols (secondary, tertiary, cyclic, and branched) there is a tendency towards stronger binding in the larger cavity of p-CD. This feature is clearly seen in Fig. 1 which plots values of pKs for p-CD against those for a-CD. For the linear n-alkanols there is straight-line correlation ( r = 0.9991) with a slope of 1.10. Other, bulkier alcohols deviate above this line, showing the tendency to a stronger affinity with p-CD. Points for the bulkiest alcohols (branched, tertiary, cyclic >C,) lie above the dashed line corresponding to pKs (p-CD) = pKs (a-CD), since such alcohols are bound more strongly by p-CD (Fig. 1). One other feature of CDs is relevant to later discussion: the acidity of their secondary hydroxyl groups, with pK, values about 12.2 (VanEtten et

TRANSITION STATE

STABILIZATION

7

al., 1967b; Gelb et al., 1980, 1982). The conjugate anions may function as nucleophiles or general bases and react with substrates included in the CD cavity (Bender and Komiyama, 1978; Komiyama and Inoue, 1980c; Daffe and Fastrez, 1983; Cheng et al., 1985; Tee, 1989; Tee et al., 1993a). By virtue of their complexing ability, CDs may influence the course of chemical reactions in respect of rates and/or product selectivity. In consequence, there is a large body of data in the literature on the effect of CDs on many types of reactions (Fendler and Fendler, 1975; Bender and Komiyama, 1978; Szejtli, 1982; Tabushi, 1982; Sirlin, 1984; Ramamurthy, 1986; Ramamurthy and Eaton, 1988). The present review concentrates on reactions for which sufficient kinetic data are available to allow quantification of the effects of CDs on transition state stability, in an attempt to understand how cyclodextrins influence reactivity in either a positive or negative sense. EFFECTS ON REACTIVITY

The kinetics of reactions which are influenced in a simple way by CDs may be viewed in the following manner (Bender and Komiyama, 1978; Szejtli, 1982; Tee and Takasaki, 1985). For a substrate S that undergoes an “uncatalysed” reaction (2) in a given medium and a “catalysed” reaction through a 1 : l substrate/CD complex (3), the expected variation of the observed rate constant with [CD] is given by (4). k

s-P

S+CD=S-CD-

hc

P+CD

K.

Equation (4) corresponds to saturation-type (Michaelis-Menten) kinetics and rate constants obtained over a suitable range of [CD], sufficient to reflect the hyperbolic curvature, can be analysed to provide the limiting rate constant, k,, and the dissociation constant, K s (VanEtten et al., 1967a; Bender and Komiyama, 1978; Szejtli, 1982; Sirlin, 1984; Tee and Takasaki, 1985). The rate constant ku is normally determined directly (at zero [CD]), and sometimes Ks can be corroborated by other means (Connors, 1987). Traditionally, data corresponding to (4) are analysed by using a Lineweaver-Burk approach, but an Eadie-Hofstee treatment is preferable for statistical reasons (Dowd and Riggs, 1965; VanEtten et al., 1967a; Bender and Komiyama, 1978). With the present, widespread availability of

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cheap microcomputers and appropriate software, it is now feasible to analyse data more directly in terms of (4), using non-linear least-squares fitting techniques (Bevington, 1969; Leatherbarrow, 1990; Duggleby, 1991). In our own work, we have settled on this last approach, usually keeping k , fixed at the measured value, and treating k, and Ks as the constants to be fitted (Tee and Takasaki, 1985). Using such non-linear fitting gives a more consistent approach to data analysis, particularly when one has to use expressions more complex than (4), because of additional processes such as non-productive 2: 1 binding or reactions with a second CD molecule (Tee and Du, 1988, 1992). Generally speaking, discussions of the effects of CDs on reaction rates are given in terms of k,lk,, K s , and, sometimes, k,lKs. Most often, the ratio k,lk, is emphasized since this quantity measures the maximal rate acceleration (or retardation) due to binding to the CD. Obviously, Ks measures the strength of binding of S to CD, but it conveys no information whatsoever about the mediation of the reaction by the CD or the mode of binding in the transition state which may be very different from that of the substrate (Tee, 1989; Tee et al., 1990b). Sometimes use is made of the apparent second order rate constant for the reaction of the substrate with the CD ( 5 ) , where S+CD-P

ki

k2 = k,/Ks ( 3 ) , since this rate constant measures the selectivity of the CD for different substrates. This usage is analogous to the use of kcat/KMfor measuring the “specificity” of enzymes (Fersht, 1985). In cases of catalysis where saturation kinetics are not observed, because binding of the substrate to the CD is weak and K s is relatively large, k2 may be obtainable from the linear increase of kobsdwith [CD]. Provided due attention is paid to the potential deprotonation of the substrate, and of the cyclodextrins (VanEtten et af., 1967a,b; Gelb et af., 1980, 1982; Tee and Takasaki, 1985), the value of Ks should not be pH dependent. However, for many reactions, such as the widely studied ester cleavage, k , , k,, and k2 are all dependent on the pH of the medium. This makes direct comparisons between the observed constants for different CD-mediated reactions either difficult or problematical. However, in general, the ratios k,lk, and k21k, are independent of pH and so are more useful for comparative purposes. As remarked already, k,lk, measures the maximal acceleration at levels of the CD sufficient to saturate complexation of the substrate. By looking carefully at the variations of this ratio with structure one may obtain insights into the mode of transition state binding (VanEtten et al., 1967a,b; Bender and Komiyama, 1978). More useful is the ratio k21k, ( = k c / K s k , ) because it takes into account the effect of substrate binding and it scales the reactivity of S towards the CD to its intrinsic reactivity in the absence of CD.

TRANSITION STATE STABILIZATION

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Enzymologists have used the analogous ratio k,,,lKM k, in full realization of its significance and usefulness (Wolfenden and Kati, 1991). However, k,lk, has been used only occasionally by chemists (Sirlin, 1984; Tee and Takasaki, 1985) without realizing that the ratio, or rather its reciprocal (k,lkl = K.rs), has another, much greater significance. The utility of K,, is the main focus of this review; its significance will be made apparent in the next section. 3 Transition state stabilization

Following on from the early ideas of Haldane (1930) and Pauling (1946), it has become widely accepted that the principal factor in enzymic catalysis is stabilization of the reaction transition state by binding to the enzyme (Jencks, 1969, 1975; Lienhard, 1973; Schowen, 1978; Page, 1984; Fersht, 1985). Likewise, lowering of the free energy of the transition state must be crucial in catalysis by other agents. Therefore, any method that can provide quantitative information about the strength of such stabilization has great potential for use in the study of catalysis, whether it be enzymic or non-enzymic. Application of the method to different substrates and catalysts might furnish insight into the nature of the catalysis involved and, in particular, into the manner in which catalysts bind to transition states and thereby stabilize them. Thirty years ago, Kurz (1963) devised a very simple method, based on transition state theory, whereby the energy of stabilization of transition states by catalysts may be estimated. He used the method to probe the transition states of acid- and base-catalysed reactions, and developed the idea of transition state pK, values (Kurz, 1972). The approach was taken up by enzymologists (Wolfenden, 1972; Lienhard, 1973; Jencks, 1975; Schowen, 1978; Kraut, 1988), and it proved to be very influential in the formulation of the ideas about enzyme catalysis referred to in the previous paragraph and in the Introduction. It is, therefore, surprising that the Kurz method has been ignored by most physical organic (and inorganic) chemists studying the mechanisms of catalysed reactions. Very recently, however, essentially the same method has been applied to organic reactions catalysed by metal ions (Dunn and Buncel, 1989; Pregel et al., 1990; Ercolani and Mandolini, 1990; Cacciapaglia et af., 1989, 1992), and the present author has shown how the Kurz approach can be used in discussions of reactions mediated by cyclodextrins (Tee, 1989; Tee et al., 1990b; Tee and Du, 1992). THE KURZ APPROACH

Consider two reactions, one of which is “uncatalysed” (6a) and the other of which (6b) is influenced by some “catalyst”, cat. According to simple

0 . S . TEE

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transition state theory (Glasstone et al., 1941; Laidler, 1987), the rate constant for the uncatalysed reaction is given by (7a), and that for the catalysed reaction by (7b), where v = kBT/h, and the transition state of the catalysed process (6b) is considered for mathematical and thermodynamic purposes to be that of reaction (6a) bound to the catalyst (TSecat). It is assumed that the average frequency of passage over the barrier (v) is the same for (7a) and (7b), and that the transmission coefficients are equal for the two processes. Kraut (1988) considers the possible consequences when these assumptions are relaxed. A+B+. . .

A +B+. . .

k'

=

+ cat

k

k'

products products

k = v[TS]/[A][B] . . .

(7a)

v[TS.cat]/[A][B] . . . [cat]

(7b)

With the assumptions, just given, division of (7a) and (7b) leads to a simple definition (8) of an apparent constant for the dissociation of TS.cat into TS and catalyst. Obviously, KTS is a quasi-equilibrium constant, since

KTS = [TS][cat]/[TS cat] = k/k'

(8)

actual, reversible dissociation of TSscat into TS and catalyst is unlikely, if not impossible. Nevertheless, KTs (or more accurately AG$s = -RTln KTS) provides a useful measure of the relative energies of the transition states for the normal and the catalysed reactions, under standard conditions, regardless of their actual structures. It is important to note that the derivation of KTS, given above, involves no ussumptions about the mechanisms of either the catalysed or uncatalysed reactions. Therefore, it is possible to use values of K,rs (and pKPrs = -log KTs) and their variations with substrate or catalyst structure (or some other reaction parameter) as probes of transition state structure (Kurz, 1972; Tee, 1989). Clearly, complications may arise when the mechanisms of the catalysed and uncatalysed reactions are quite different, but under such circumstances one can reasonably hope that trends in KPrs and other kinetic parameters may be such as to point to the discrepancy and that they may even suggest a resolution. It is not the purpose of the present review to give a critical appraisal of the Kurz approach; that can be found in the review by Kraut (1988). Rather, it is to show how this simple method can be used in the study of reactions influenced by cyclodextrins. Some examples involving catalysed

TRANSITION STATE STABILIZATION

11

reactions of other types which may be of interest to a wider audience of physical organic chemists are also presented. CYCLODEXTRIN MEDIATED REACTIONS

Application of the Kurz approach to CD-mediated reactions, whether they be accelerated or retarded, is straightforward (Tee, 1989), provided appropriate kinetic data are available. From the rate constants k , for the normal, “uncatalysed” reaction (2) and for the mediated (“catalysed”) reaction ( k 2 = k , / K s ) as in (3), application of simple transition state theory, in the manner shown above, leads to (9), where now KTs is the apparent dissociation constant of the transition state of the CD-mediated reaction (symbolized here as T S - C D ) into the transition state of the normal reaction (TS) and the CD. This constant and its logarithm, which is proportional to a free energy difference, is a valuable probe of the kinetic effects of CDs on reactions.

As outlined in Section 2, discussions of catalysis (or inhibition) by CDs are generally in terms of k,lk,, K s , and, to a lesser extent, k2 = k , / K s . This last quantity has the same usefulness (and significance) as does kCat/KMfor enzymes (Fersht, 1985) in that it is a measure of the substrate selectivity of the CD (VanEtten et al., 1967b; Tee and Takasaki, 1985). With proteolytic enzymes such as a-chymotrypsin, there is no major problem with the use of k,,,lKM since the peptide bonds formed between different amino acids have fairly similar intrinsic reactivities ( k , ) (Berezin et al., 1971; Dorovska et al., 1972; Fersht, 1985), but comparisons between substrates having quite different reactivities require some kind of scaling, and this can be achieved by looking at k21k,. As remarked already, such ratios have occasionally been used (Sirlin, 1984; Tee and Takasaki, 1985), but it was not recognized at the time that k21k, is simply the reciprocal of K r s , as seen in (9). While purists of thermodynamics may cavil that KTS is not a true equilibrium constant, it does correspond to an energy of great interest and importance: the free energy difference between the transition states of the uncatalysed and catalysed reactions [(2) and (3), respectively] under standard conditions. Alternatively, one may prefer to consider this difference as the free energy of transfer of the transition state from aqueous solution to a 1 M solution of the catalyst, as has been done recently (Dunn and Buncel, 1989; Pregel et al., 1990). Whatever the case, the significance of Kvrs can most easily be appreciated by consideration of the Gibbs energy diagram in Fig. 2. As indicated there, the relative free energies of various species involved in reactions (2) and (3) are directly accessible from the

0 . S . TEE

12

TS+cat A

G

S.cat

products

Fig. 2 Relative Gibbs energies for the species involved in a reaction which is uncatalysed (S -+ TS + P) and mediated by a catalyst (S cat --* TS .cat + P). For a specified [cat] the free energy differences can be directly calculated from the measurable constants k,, k, and K s , and the derived values k2 and KTs, as indicated. pKTs = -logKTs is a measure of the stabilization of the transition state by the catalyst.

+

measurable quantities k , , k,, and K s (or k , and k 2 ) . As long as these constants are all measured under the same conditions, the apparent “equilibrium constant” KTS (through its logarithm) gives a direct measure of the binding energy of the transition state to the catalyst for those conditions, regardless of the mechanism (Schowen, 1978). The diagram in Fig. 2 also serves to emphasize that stabilization of the transition state by the catalyst is primarily responsible for any rate increase. To a considerable extent the binding of S is irrelevant, except that strong substrate binding necessarily detracts from catalysis. In fact, according to (9), the change in rate is determined by the strength of binding of TS, relative to that of S (k,lk, = K s / K T s ) (Lienhard, 1973). This emphasis has been termed the “fundamentalist view” by Schowen (1978). A much more agnostic view of the importance of transition state stabilization has recently been presented by Menger (1992). Obviously, strong binding of the substrate to the catalyst may distort the structure of S towards that of TS, thereby making reaction easier. However, such distortion simply reflects the complementarity of the catalyst and the transition state (Fersht, 1985). From a purely thermodynamic point of view, the formation of a strong S.catalyst complex lowers free energy by an additional amount that must be overcome in the process of activation of the k, process (3) (Fig. 2). Living organisms and their enzymes have evolved so

TRANSITION STATE STABILIZATION

13

as largely to avoid this problem by having working levels of [S] close to K , ; thus the free energy difference between S enzyme and S.enzyme is quite small and the cost in free energy is minimal (Jencks, 1969; Lienhard, 1973; Fersht, 1985). As pointed out above, values of KTS are obtainable from rate data without making any assumptions about the reaction mechanism. Therefore, one may use Kr.7 and its variation with structure as a criterion of mechanism, in the same way that physical organic chemists use variations in other kinetic parameters ( B r ~ n s t e dplots, Hammett plots, etc.). For present purposes, the value of KTS can be useful for differentiating between the modes of binding in the S . C D complex and the TS .CD transition state, between different modes of transition state binding, and hence between different types of catalysis (Tee, 1989). According to Bender and Komiyama (1978), CDs may show two basic forms of catalysis: “non-covalent” and “covalent”. In the former case the C D binds to the substrate(s) and provides an environment for the reaction that is different from the bulk solvent, whereas in the latter case there are also distinct covalent interactions between the substrate(s) and some functional group(s) on the C D in the rate-limiting step of the reaction. Therefore, it seems reasonable to expect that values of KTS for these two types of catalysis may show different sensitivities to structural change, since the partial bonding involved in covalent catalysis will normally lead to stronger interactions with the CD and possibly to more stringent geometric requirements than non-covalent catalysis.

+

4

Non-covalent catalysis

In this form of catalysis, inclusion of the substrate in the C D cavity provides an environment for the reaction that is different from that of the bulk, normally aqueous, medium. In the traditional view, the catalytic effect arises from the less polar nature of the cavity (a microdielectric effect) and/or from the conformational restraints imposed on the substrate by the geometry of inclusion (Bender and Komiyama, 1978). However, catalysis may also arise as a result of differential solvation effects at the interface of the CD cavity with the exterior aqueous environment (Tee and Bennett, 1988a,b; Tee, 1989). INTRAMOLECULAR REACTIONS

A simple example of non-covalent catalysis is the intramolecular acyl transfer [3] to [4] which is catalysed by a-CD but retarded by p-CD (Griffiths and Bender, 1973). As seen by the constants in Table 1, the

0 . S . TEE

14

Table 1 Non-covalent catalysis of intramolecular acyl transfer [3]+ (41.”

7.3 0.19

LY

P

6.6 5.2

48 0.96

“Based on data from Griffiths and Bender (1973).

difference in behaviour of the two CDs lies in the substrate binding ( K s ) , and not in the transition state binding ( K T s ) . The binding of the transition state to each CD is very similar, but the stronger binding of the reactant to p-CD in the initial state leads to rate retardation (k,lk, < 1). Presumably, the substrate [3] (or as [3’]) sits deeper and more tightly in the larger cavity of p-CD so that access to the transition state geometry is made more difficult. It is noteworthy that the “transition state analogue” [5] binds to a-CD (inhibition constant, K I = 12 f 2 mM) with almost the same strength as the actual reaction transition state which presumably resembles the tetrahedral intermediate [6]. In another example of intramolecular participation, the attack of the carboxylate ion group of mono-p-carboxyphenyl esters of substituted glutaric acids, the rate of anhydride formation is sharply reduced by p-CD (VanderJagt et al., 1970). Apparently, the substrates bind to p-CD in a conformation that is unsuitable for reaction. At the same time, the large rate reductions must also mean that the transition state of the reaction cannot be bound by p-CD in such a way as to be significantly stabilized.

02Nd0H - 02NwH OCOtBu

OCOtBu

__f

131

r l

OzN

0

t Bu OH

TRANSITION STATE STABILIZATION

15

Several other intramolecular reactions showed only slight rate accelerations or retardations (VanderJagt et af., 1970). Of potential synthetic use, it has been found that both intramolecular and intermolecular Diels-Alder reactions can be catalysed by p-CD (Sternbach and Rossana, 1982; Breslow and Guo, 1988). DECARBOXYLATION

The rate of decarboxylation of activated carboxylate anions [e.g. (lO)], shows strong solvent dependence. It is not surprising, therefore, that these reactions have been used to probe the microsolvent effects of micelles and CDs (Fendler and Fendler, 1975). In particular, it was anticipated that complexation with a CD might result in catalysis by providing an environment for the reaction that is less polar than water. X-Ph(CN)CHCO;

+ X-Ph(CN)CH-

+ COZ

( 10)

In keeping with this expectation, Straub and Bender (1972a) found that the decarboxylation of phenylcyanoacetate anions (10) shows catalysis in the presence of p-CD, albeit modest [Appendix, Table A4.11. The rate accelerations show little variation (12-23, at 60.4”C) even though the reactivity of the anions spans two orders of magnitude and Ks varies with the position and size of the substituent. Consequently, the values of pKTs vary in parallel with pKs (slope = 1.08k0.13; r = 0.957) which strongly suggests that the binding of the transition state in the CD cavity is very similar to that of the substrate, S. The magnitude of the rate accelerations caused by p-CD is comparable to that brought about by a change from water to 55% (w/w) aqueous 2-propano1, but significantly less than those in wholly organic media: 100% 2-propanol (2600); dioxane (2800). Also, the activation parameters for reaction in the mixed solvent and for the S - C D complex in water are very similar (Straub and Bender, 1972a). Presumably, these findings mean that the aryl ring of S is situated largely in the C D cavity, with the anionic moiety directed towards the exterior, so that the reaction centre is situated in a “mixed” environment near the interface between the bulk aqueous medium and the less polar C D cavity. Data for the 4-chlorophenyl derivative were obtained at three temperatures (Table A4.1). At the lower temperatures, the rate acceleration is greater because the transition state binding is strengthened more than the substrate binding. The data may be analysed to elicit the enthalpic and entropic contributions to the free energy of transition state stabilization, obtainable from the variation of AC&( =AH+, - TAS+s) with temperature (Table 2). If desired, the data may be further dissected since, from ( 9 ) ,

0 . S. TEE

16

Table 2 Thermodynamic parameters for the P-cyclodextrin-catalysed decarboxylation of the 4-chlorophenylcyanoacetate anion."

Temp./"C

AAC' 2.32 2.22 2.09 AAH* = 5.22 AAS = 9.40

35.4 45.4 60.4

AG: 2.79 2.77 2.68 AH(' - 4.21 AY! 2 4.57

AG;~

5.11 4.99 4.77 A P ' S = 9.42 A& = 13.9

"From the data of Straub and Bender (1972a) (see Table Al ). Free energies and enthalpies are in kcal mo1-l: entropies are in cal K-' mol-'.

-RTlnKTS = -RTln(k,/k,) -RTlnKs, and so AC;, is given by (ll), where AAG' = (AC; - AC:) is the difference in activation free energies of the two kinetic steps. The relationship (11) is evident in the diagram in Fig. 2. Likewise, for the enthalpy and entropy, the separate contributions are AH!;s = AAH$ AH: and AS& = AASs + AS: (Table 2).

+

As seen in Table 2, AH;.s = 9.42 kcal mol-' and AS;, = 13.9 e.u., and so the free energy of transition state stabilization (approximately 5 kcal mol-') results from a favourable enthalpy change, partly offset by an unfavourable entropy change. A similar situation pertains to binding of the substrate also (Table 2). Thus, the similarity between transition state binding and substrate binding, pointed out above from the correlation of pKTs with pKs, is evident in thermodynamic parameters as well. The decarboxylation of benzoylacetic acids in acidic solution proceeds with intramolecular proton transfer [7] + (81. This feature of the reaction appears to limit charge separation in the transition state since the rates in water are very insensitive to the electronic nature of the substituents ( p = +0.03), unlike the reaction of their anions ( p = +1.42) (Straub and Bender, 1972b). The reaction of the acids shows catalysis by p-CD, with limiting accelerations of 2-8 (Table A4.1). Values of Ks and of KTs do not vary greatly with the aryl substituent, probably because the hydrophilic keto and carboxyl groups of [7] do not allow the benzoyl function to penetrate deeply into the C D cavity in either the initial state or the transition state. The modest catalysis presumably arises because binding to the p-CD heIps to bring the reactive groups together and to stabilize the cyclic transition state. It is highly unlikely that catalysis results from a microsolvent effect since the decarboxylation reaction [7] + [8] is not particularly sensitive to the solvent (Straub and Bender, 1972b).

17

TRANSITION STATE STABILIZATION

[71

ketone

BROMINATION-DEBROMIN ATION

Ionic reactions of neutral substrates can show large solvent dependence, due to the differential solvent stabilization of the ionic intermediates and their associated dipolar transition states (Reichardt, 1988). This is the case for the electrophilic addition of bromine to alkenes (Ruasse, 1990, 1992; Ruasse et al., 1991) and the bromination of phenol (Tee and Bennett, 1988a), both of which have Grunwald-Winstein rn values approximately equal to I so that the reactions are very much slower in media less polar than water. Such processes, therefore, would be expected to be retarded or even inhibited by CDs for two reasons: (a) the formation of complexes with the C D lowers the free concentrations of the reactants; and (b) slower reaction within the microenvironment of the less polar C D cavity (if it were sterically possible). Contrary to the above expectations, the bromination of anisole (Tee and Bennett, 1984) and of phenols (Tee and Bennett, 1988a) in the presence of a-CD is not strongly retarded, so that some form of catalysis must occur. In some cases, actual rate increases are observed in spite of the several complexations that reduce the free reactant concentrations. Analysis of the effects of substituents on the kinetics leads to the conclusion that the catalysis by a-CD most probably results from reaction of CD-bound bromine with free substrate (12a) and that the a-CD.Br2 complex is 3-31 times more reactive than free Br2 towards phenols and phenoxide ions (cf. Tee et al., 1989). For the kinetically equivalent reaction of the substrate. C D complex with free bromine (12b), the rate constants (k:) for phenols do not correlate sensibly with the nature and position of the substituents, and for three of the phenoxide ions they have unrealistically high values, greater than 10" M - ' s - ' . S + C D + Br2 eS

+ CD.Br2

KH

S

+ CD + Br2

S . CD + Brz

k?

products

+ CD

(12a)

products

+ CD

(12b)

kg

For reactions, such as phenol bromination, in which two substrates are required to produce the rate-limiting transition state the value of KTs may

0 . S. TEE

18

be calculated most easily from (14), the ratio of the second-order rate constant for the normal reaction (13a) and the third-order rate constant for the CD-catalysed reaction (13b) [see Section 3, (S)], where TS and T S - C D are the transition states corresponding to k2" and k3c,respectively. Note that k3C = k q / K B or k!lKs, from (12a) and (12b).

This approach has been applied (Tee, 1989) to kinetic data for the bromination of phenols and phenoxide ions catalysed by a-CD. For 15 different substrates (nine phenols and six phenoxides) K,rs values vary only between 0.07 and 0.8mM, with most being between 0.1 and 0 . 5 m M , indicating very similar transition state stabilization for substrates with a range of reactivity of 40 million (Table A4.2). Moreover, the values of K,rs show no clear correlation with K s . This lack of dependence of KTS on the structure of the substrate is strong evidence that the transition state for the catalysed process is one in which the phenol moiety is basically outside the CD cavity while the bromine is inside ([9] -+ [ 101). The same conclusion was

X =OH

[91 arrived at in the original paper (Tee and Bennett, 1988a), but using slightly different arguments. In particular, it was noted that the Hammett p €or the catalysed and uncatalysed reactions (kf and k2") are virtually equal, suggesting that the organic substrate remains in a largely aqueous environment. Also, as noted above, rate constants ( k ; ) for the alternative mechanism (12b) vary less sensibly and some are physically unreasonable. The debrominations of a series of 4-alkyl-4-bromo-2,5-cyclohexadienones (ipso-dienones [ll])were also studied and found to undergo strong catalysis by a-CD (Tee and Bennett, 1988b). These reactions were chosen for scrutiny since they should serve as good models for the reverse of the

TRANS IT1ON STATE STAB ILlZATlO N

19

brominations just discussed. Values of K,, for the debrominations fall in the M (Table A4.3) and are insensitive to narrow range of 6 x lo-' to 12 X the structure of the dienone. If, in the transition state for debromination, the @so-dienone were bound inside the cavity of a-CD, particularly through its alkyl group, one would expect a greater dependence of KTs on the size and shape of the alkyl group(s). Thus, for debromination, as for bromination, the catalysis data suggest a transition state in which the organic moiety is largely outside the C D cavity, and the two bromine atoms involved in the reaction are essentially inside ( [ l l ] -+ [12]). It is gratifying (and reassuring)

that the two separate studies of a-CD catalysed bromination and debromination arrived at the same description of the transition state that the two reactions have in common. The origin of the CD catalysis of bromination and debromination probably relates to solvation; yet it cannot be a simple microsolvent effect since brominations are much slower in media less polar than water, as remarked above. Most probably the catalysis arises from a differential effect of the aqueous exterior, where the organic moiety resides, and the less polar CD cavity containing the bromines. For bromination, solvent reorganization around the developing bromide ion is less necessary (than in the normal aqueous reaction) since it is being formed in the CD cavity (Tee and Bennett, 1988a). For debromination, nucleophilic attack can occur by a largely desolvated bromide ion which thus behaves as a stronger nucleophile (Tee and Bennett, 1988b). Debromination of the @so-dienone [13] (+ [14]), formed during the

0 . S. TEE

20

course of the bromination of 5-methylsalicylic acid, is subject to intramolecular general acid catalysis by the carboxyl group (Tee and Iyengar, 1985; Tee et al., 1986). The effect of (u-CD on this reaction was studied (Takasaki and Tee, 1989) to see how the two very different types of catalysis interact with one another, since enzymes normally use several catalytic effects to achieve large rate accelerations (Jencks, 1975; Gandour and Schowen, 1978; Fersht, 1985; Page and Williams, 1987). Conceivably, the two forms of catalysis might interact with one another in three different ways: destructively, one interfering with the other (worst case); independently, each contributing its individual acceleration (acceptable); or constructively, each amplifying the effect of the other (best possible result). In the event, it was found that the two forms of catalysis act together on a single transition state to give an impressive rate enhancement of 12 million. However, each form of catalysis operates more or less independently of the other (Takasaki and Tee, 1989), an effect termed “cocatalysis”. Analysis of the kinetic data showed that the component of catalysis due to the a-CD (3400 times) is within the range of values found for other ipso-dienones (2400-4600), even though the anion of [13] is 3500 times more reactive than the analogous dienone lacking the 2-carboxylate group (Scheme 1). Moreover, the KTs of 0 . 0 8 8 m M for [13] is in the middle of the range of the values for other ips0 dienones (Table A4.3), indicating the same degree of transition state stabilization by a-CD. Therefore, the findings for the CD-catalysed debromination of [ 131 are also consistent with the transition 0

0

0

CD.Br-

Scheme 1

12000000!

TRANSITION STATE STABILIZATION

21

state having the dienone moiety outside of the C D cavity (as for [11]-+ [12]). Furthermore, the fact that the two forms of catalysis do not interfere with each other may be taken as evidence that they take place in two spatially distinct regions: internal general acid catalysis in an aqueous environment outside the CD cavity; nucleophilic bromide ion attack inside the C D cavity. The effects of a-CD on the bromination of other substrates have been studied recently (Javed, 1990; Tee et al., 1990a; Tee and Javed, 1993), the object being to see if the catalytic effects observed earlier with phenols (Tee and Bennett, 1988a) are peculiar to these substrates or more general. Broadly speaking, various aromatic and heteroaromatic substrates (Table A4.4) showed behaviour (k$lk2, = 1.7 to 10; KTS = 0.2 to 1.2 mM) very similar to that of phenols, and so the catalytic effect appears to be fairly general. The oxidation of formic acid by bromine also shows catalysis by a-CD (Han et af., 1989; Tee et al., 1990a). The first finding was that the four p-halogenophenols (X = F, CI, Br, or 1) have remarkably similar transition state stabilization ( KTs = 0.40, 0.43, 0.46, and 0.29mM), even though these substrates have a wide range of ability to bind to a-CD ( K s = 120, 3.6, 1.4, and 0.47mM) (Table A4.4). This finding is inconsistent with inclusion of the phenol by the C D during the catalysed bromination and so affords yet further support for the view of the transition state implied in [9] -+[lo]. Three salicylate (2-hydroxybenzoate) anions, which have unusual reactivity towards bromine that has been attributed to intramolecular proton transfer assisting electrophilic attack (Tee and Iyengar, 1985, 1990), exhibit modest catalysis (k;\lk2, = 3 to 10) and have KTS values similar to phenols. Pyridones and their N-methyl derivatives, three heteroaromatic acid anions, and four phenoxy derivatives show comparable catalysis (k$lk2, = 1.7 to 9.5) and KrrSvalues (Table A4.4). To provide an example of a reaction that is very different to electrophilic aromatic substitution, the oxidation of formic acid by bromine was also studied. This reaction, which involves electrophilic attack on the formate anion (15) (Cox and McTigue, 1964; Smith, 1972; Herbine et al., 1980; Brusa and Colussi, 1980), is catalysed by a-CD (ktlk,, = 11) (Tee et al., 1990a), and the degree of transition state stabilization (KTS = 0 . 1 8 m ~ )is similar to that for phenols (Table A4.2) and most of the other substrates (Table A4.4). Br2 + HCO;

+

2Br-

+ H+ + C 0 2

(15)

Combining the results for 34 different substrates (Tables A4.2 and A4.4), there is a good correlation of logk,, with logkz,, covering 10 orders of magnitude, with unit slope (1.01; r = 0.993) (Fig. 3). Because k3= = k $ / K B (12a), logkt also correlates with logk,, in the same way. Apparently, then,

0. S TEE

22

0

2

4

6

8

10

Fig. 3 Correlation of the third-order rate constants for a-CD catalysis of bromine attack with the second-order rate constants for the uncatalysed reaction. Data from Tables A4.2 and A4.4 (Tee and Javed, 1993).

the nature of the catalysis of bromine attack (discussed above) is much the same for all of these 34 substrates, with only very minor variations in the extent of catalysis for the different structural types. In the same vein, the amount of transition state stabilization provided by a-CD is virtually constant for substrates with a 10” range of reactivity, further supporting the reaction scheme, expressed in (12a) and illustrated by [9]+ [lo], in which the substrate remains essentially outside the CD cavity. 5 Covalent catalysis

The term “covalent catalysis” was chosen by Bender and Komiyama (1978) to classify reactions in which there are covalent interactions between a functional group on the CD and the substrate during the rate-limiting step of the reaction. The reaction in this category which has been most studied is the cleavage of aryl esters (Bender and Komiyama, 1978; Matsui et al., 1985; Tee, 1989). ESTER CLEAVAGE

In most cases the esterolysis takes place by nucleophilic attack of an ionized hydroxyl of the C D (VanEtten et al., 1967a), leading to acyl transfer (VanEtten et al., 1967b). Under the reaction conditions the acylated CD

TRANSITION STATE STABILIZATION

23

which is produced is normally fairly resistant to hydrolysis so that overall the ester hydrolysis is not formally catalysed. Because of the partial covalent interaction between the ester substrate and the CD in the transition state for acyl transfer quite low values of KTS can be found (Tee, 1989). Moreover, they show a strong dependence on the position and size of substituents, rather than on their electronic character (Komiyama and Bender, 1978; Matsui ef al., 1985). These features emerge from the data in the classic paper by Bender and coworkers (VanEtten et al., 1967a), much of which is presented in Table A5.1. Broadly speaking, they found that meta-substituted phenyl acetates are superior to their para isomers as substrates for cleavage by both a- and p-CD, a finding supported by much subsequent work (e.g. Matsui et al., 1985; Tee and Takasaki, 1985; Tee et al., 1990b). This difference in behaviour is strongly correlated to differences in transition state binding, as shown below. The transition state for the cleavage of phenyl acetate by a-CD has KTS = 0.81 m M (Table A5.1). Acetates with para substituents have larger values (weaker transition state binding) whereas for meta groups the values are generally lower (stronger transition state binding). Thus, the values of K-rs are consistent with the view that mefa substituents, regardless of their electronic nature, position the phenyl group of the ester in the CD cavity in a geometry which facilitates the attack of an ionized hydroxyl group and the formation of the transition state for acyl transfer (Scheme 2A). In contrast, para substituents position the ester in the CD cavity in such a way that nucleophilic attack is more difficult and they also tend to interfere with transition state binding (Scheme 2B). Support for the above view comes from NMR studies of the binding of phenyl and nitrophenyl acetates to a-CD (Komiyama and Hirai, 1980). These indicate that the nitro groups are located in the CD cavity and that the acetoxyl groups of the esters are held outside, more or less close to the secondary hydroxyls of the CD. It was calculated that the distance between the ester carbonyl carbon and the secondary hydroxyls decreases as p-nitro > phenyl> m-nitrophenyl, consistent with the observed order of rate acceleration (Komiyama and Bender, 1984). The cleavage of phenyl acetates by p-CD shows the same general features as that by a-CD (Table A5.1), although there are quantitative differences that must arise from the larger cavity size of p-CD. Generally, the mefa-substituted esters are not cleaved as well as by a-CD and the pura-substituted esters are cleaved better. Thus, the distinction between the kinetic parameters for two series of esters is less dramatic for p-CD, presumably because of the looser fit of substituted phenyl groups in p-CD. This trend is continued with the two entries for y C D (which has a still larger cavity) where the differences between the meta and para isomers of t-butylphenyl acetate are quite small (Tables A5.1). Nevertheless, the

0 . S . TEE

24

I

Me

Me

B

x-0-

oxo I

Scheme 2

depictions in Scheme 2 are still appropriate as the difference between metuand para-substituted isomers is generally substantial. This difference is clearly shown by the p K ~ svalues plotted in Fig. 4, which are calculated from the extensive data for ester cleavage by p-CD (Tables A5.2 and A5.3) accumulated by Fujita and coworkers (Matsui et al., 1985; Fujita, 1988). For rn-alkyl and halogen substituents there is a good

TRANSITION STATE STABILIZATION

25

6.0 -

ro

5.0 -

V

y'

V

Q

4.0

-

3.0

-

2.0

0

2.5

3.0

3.5

4.0

Fig. 4 Correlation of constants for transition state stabilization (pK,,) and substrate binding (pK,) for the cleavage of meta- and para-substituted phenyl acetates by /3-CD. The substituents are alkyl groups and the four halogens. The two deviant points are for longish p-alkyl groups (n-butyl and n-pentyl). Data from Tables A5.2 and A5.3.

linear correlation ( r = 0.992) between the free energy of transition state binding (expressed by pKTs) and that of substrate binding (pK,), with a slope of 1.63 k 0.07, strongly supporting the view that for metu substituents the S . C D complex and the T S - C D complex have similar geometries (Scheme 2A). In contrast, the correlation is poorer ( r = 0.788) for puru substituents and the slope is closer to zero (0.38 k 0.11); only in the case of two long, flexible alkyl groups (n-butyl and n-pentyl) is transition state binding improved significantly (Fig. 4), perhaps because they can accommodate better to the cavity. Fujita and coworkers (Matsui ef ul., 1985; Fujita, 1988) have also collected a large body of data for the basic cleavage of metu-substituted esters by a-CD. The observed accelerations (k,lk,) vary from 41 (X = H ) to 360 (X = CHO), with most being in the range 100-250 (Table A5.4). The strongest transition state stabilization is for the m-iodo substituent (KTS = 2 . 8 p ~ ) ,but since this also gives the strongest substrate binding (Ks = 0.48 mM), the acceleration of 170 is not exceptional. The plot of pK,,,s versus pKs (Fig. 5) shows a fair correlation ( r = 0.928) between transition state binding and substrate binding, with near unit slope (1.09), even though it includes substituents of various structural and electronic types. This correlation is also consistent with the mechanism outlined in Scheme 2A. The correlations presented in Figs 4 and 5 are in stark contrast to the disorder shown in a plot of logk,lk,, against the Hammett u constants for

0 . S . TEE

26

6.0

.

R

O

H

V

OR

A

Hal

0

CN

0

COR

A

NO,

5.0 u)

y’

4.0

3.0 1.o

0

2.0

3.0

P K S

Fig. 5 Correlation of constants for transition state stabilization (pKTs) and substrate binding (pK,) for the cleavage of meta-substituted phenyl acetates by a-cyclodextrin.

Data from Table A5.4.

meta and para substituents (VanEtten el al., 1967a), which the late Professor Myron Bender often claimed was “the world’s worst Hammett plot” (e.g. Bender, 1987). His point in doing so was to emphasize that it is the position of a substituent, rather than its electronic nature, that largely determines its effect on the acceleration of CD-induced phenyl acetate cleavage (Komiyama and Bender, 1978). This view is supported by the linear correlations of logk, with various parameters found by using multiple regression analysis (Matsui et a f . , 1985). The correlation equations show that the electronic contribution of a substituent is virtually the same as that of the normal reaction ( k , ) so that it cancels out in the acceleration ratio ( k c / k u ) .The correlations also reveal an unfavourable steric term for para substituents, whereas bulky meta substituents improve the esterolytic ability, again consistent with the portrayals in Scheme 2. Unlike the phenyl acetates in Tables A5.1 to A5.4, basic cleavage of ethyl benzoates, ethyl cinnamates, and Ph(CH2),,COOEt ( n = 1, 2, 3) is slower with p-CD, except in the case of some benzoates which exhibit quite modest rate enhancements; with a-CD the cinnamate esters mainly show inhibition (Tanaka et a f . , 1976). All of these substrates show saturation kinetics, with K s in the millimolar range, and so their KTs values are all high (Table A5.5). On the other hand, esterolysis of phenyl benzoates shows more enhancement (k,lk, -- 10) with a-CD (VanEtten et al., 1967b). Thus, as has been shown by Menger and Ladika (1987) for ferrocenylacrylate esters, a good leaving group (normally phenoxy) seems to be a requirement for large rate accelerations.

TRANSITION STATE STABILIZATION

27

The “best” substrate found by Bender’s group was rn-t-butylphenyl acetate, undergoing cleavage by p-CD (VanEtten et al., 1967a). For this ester, k,lk, = 250 and Ks = 0.13 m M , so that KTs = 0.52 p ~ considerably , lower than for most other phenyl acetates studied (Tables A5.1 to A5.4). Thus, the binding of the t-butyl group in the p-CD cavity stabilizes the transition state much better than other rneta substituents. However, the acceleration is no larger than that for other groups because the substrate binding is equally improved by an rn-t-butyl group. In 20.5% aqueous CH3CN the rate acceleration is raised to 940 because substrate binding is weakened ( K s = 2.3 mM) (VanEtten et al., 1967b) somewhat more than the transition state binding ( K T S = 3.3 p ~ )In. the same medium, replacing all the primary hydroxyls of p-CD with mesyloxy groups (CH3SO20-) further enhances cleavage due to even weaker substrate binding and stronger transition state binding (k,lk, = 1550; K s = 3.1 mM; KTs = 2 . 0 p ~ ) im, plying that the t-butyl group of the ester penetrates deeply enough into p-CD cavity to interact significantly with the substituents on the primary side, perhaps because they can fold inwards closing off the bottom of the CD cavity. In strong contrast, methylating the secondary hydroxyls completely destroys the rate acceleration because the nucleophilic sites on the wide end of the CD cavity are all blocked (VanEtten et ul., 1967a,b). The pioneering studies of Bender’s group were followed by many attempts to increase the efficiency of esterolysis by cyclodextrins and several approaches have been tried, most notably in Breslow’s laboratory. One may “optimize” the structure of the substrate (Trainor and Breslow, 1981; Breslow et al., 1983), modify the cyclodextrin (Emert and Breslow, 1975; Breslow et al., 1980; Fujita et al., 1980), or alter the solvent (Siegel and Breslow, 1975). The last of these is the easiest to achieve but detailed studies are made tedious by the necessity to redetermine all of the relevant equilibrium and rate constants, and the acidity dependence of the catalysed and uncatalysed processes, in the new medium. The study by Siegel and Breslow (1975) is one of few involving solvent variation and having sufficient kinetic data to allow calculation of K r s for different media. First, they showed that various organic species bind to p-CD in DMSO solution, though not as well as in water. A medium change from 0% to 50% (v/v) aqueous DMSO to 100% DMSO weakens the binding of rn-t-butylphenyl acetate substantially: K s = 0.1 to 2.0 to 18 mM. For basic cleavage of the same ester, with and without p-CD, the change from 0% to 60% (v/v) aqueous DMSO increases k , by 25, k, by 48, and the acceleration ( k , / k , ) rises from 270 to 510 (Table 3 ) . As the authors emphasize, the reaction at kinetic saturation ( k , ) is 13000 times faster in 60% aqueous DMSO than the background reaction ( k , ) in water containing the same buffer. To get at the origins of this acceleration i t is necessary to dig deeper and to look at the effect of solvent change on transition state stabilization.

0 . S. TEE

28

Table 3 Basic cleavage of m-t-butylphenyl acetate by P-cyclodextrin in water and in 60% (vlv) aqueous DMSO.' % DMSO 0 60

k,104is-1 0.3 7.5

k,ls-

I

k,lk,

KslmM

KTSIpM

0.008 0.38

270 5 10

0.10

0.37 9.8

k2h-I

s-'

80 76

"From the data of Siege1 and Breslow (1975). In a buffer corresponding to an aqueous pH of 9.5. 'Assumed value, given that Ks = 2 m M in 50% aq DMSO. Any other value in the millimole range would not alter the arguments in the text. Note that the assumed value is incorporated into both K , , and k Z .

Assuming K s = 5 mM in 60% (v/v) aqueous DMSO (since it is 2 mM in ~ ~water: 50% aqueous DMSO), K,rs = 9 . 8 p ~ ,as compared to 0 . 3 7 in transition state binding is 26 times weaker in the mixed solvent. More surprising, however, k2 (= k , / K s ) is the same in both media (Table 3). Thus, the much faster cleavage of the ester by p-CD in 60% aqueous DMSO originates from two factors: (i) the enhanced nucleophilicity and basicity of anions in the mixed medium (Reichardt, 1988); and (ii) substantially weaker substrate binding (1/5O) in 60% aqueous DMSO while transition state binding is weakened less (1126). Of the two, the first factor is much more important. The virtual equality of k2 in the two media arises because the 48-fold increase in k , is matched by the 50-fold increase in K s (Table 3). Obviously, the esterolytic ability of a CD can be improved by replacing one of its primary or secondary hydroxyl groups by a stronger nucleophilic group such as thiol, amino, or imidazolyl (Fendler and Fendler, 1975; Bender and Komiyama, 1978; Fikes et al., 1992). However, such replacements bring about gross changes in reactivity which obscure the effect of CD binding on the reaction. It is more informative in this respect to make more subtle changes to the CD to modify its ability to bind substrates and transition states.

4Px'

C02-p-N02Ph

-X-CO2-p-NO2Ph

[lsa] X = C=C [15b] X = none [ 1 5 ~ ] X = CH2 [16a]

X = C=C

[16b]

X = trans-CH=CH

TRANSITION STATE STABILIZATION

29

With such considerations in mind, presumably, Breslow and coworkers (Emert and Breslow, 1975; Breslow et ai., 1980) prepared modified p-CD with seven pendant N-methyl (or ethyl) formamido groups, in place of the primary hydroxyl groups. These groups may form a flexible floor to the p-CD cavity which might adjust itself to suit the binding of different substrates and transition states. A capped p-CD derivative with a diphenyloxy moiety was also prepared and studied. Accelerations of up to I million were observed, corresponding to low KTS values, down to 7.5 x lo-’ M (Table A5.6). However, these impressive values are intrinsic to the esters, which had been carefully designed using CPK (space filling) models for optimal transition state binding. In actuality, the flexible capping has only small effects on the efficiency of ester cleavage by p-CD. For various esters the values of kJk, were raised by 7- to 20-fold, due partly to slightly weaker substrate binding and/or marginal improvements in transition state binding (Table A5.6). In a related study, Fujita et al. (1980) modified p-CD by replacing one of the primary hydroxyl groups by S-methyl or S-t-butyl; they also prepared a derivative capped on the primary side with a diphenylmethyl unit. The efficacy of these derivatives in cleaving rn- and p-nitrophenyl acetates was measured (Table A5.6). Similar to Breslow’s work, it was found that the presence of the S-methyl group has little effect on either K s or K,rs, suggesting that it does not intrude far into the CD cavity. The larger S-t-butyl group presumably provides more of an intrusive floor since it lowers K s and KTS to a lesser extent, resulting in lower accelerations. With the diphenylmethyl capped p-CD, binding of the m- and p-nitrophenyl acetate substrates is strengthened considerably ( K s = 6.1 + 0.11 and 4.80+ 0.012 mM, respectively), and so is transition state binding to a lesser extent (KTs = 0.085- 0.017 m ~ and ; 620- 3.2 FM), so that the accelerations are reduced (k,lk, = 72- 6.5 and 7.7- 3.9). Covalent modification represents only one way to alter the binding properties of a CD. Obviously, changing the solvent system is another way, but this will normally affect reactivity at the same time (VanEtten et al., 1967a,b; Siege1 and Breslow, 1975), as already discussed in relation to the data in Table 3. But, as presented later, there is an even more subtle way to modify the binding capacity of the CD cavity, by the addition of an inert spacer molecule or “spectator”. Besides the expected inhibition observed in most cases, there are instances where the addition of a potential inhibitor brings about rate increases due to improved transition state binding (Tee and Hoeven, 1989; Tee and Bozzi, 1990; Tee et al., 1993b). Several of the entries in Table A5.6 also represent many of the efforts by Breslow’s group to “improve” substrates for cleavage by p-CD. The adamantylpropiolate ester [ 15a] exhibits a healthy acceleration of 2150, which is raised to 14000 by flexible capping and to 15000 by judicious placement of a t-butyl group (Breslow et ul., 1980); KTS values for these

0 . S. TEE

30

situations are about lo-’ M. In contrast, cleavage of the adamantanecarboxylate ester [15b] is retarded 28-fold (k,lk, = 0.036; K T S = 42mM) by p-CD (Komiyama and Inoue, 1980a), and that of the homologous adamantylacetate [15c] is raised only threefold (Komiyama and Inoue, 1980b). Clearly, the size, shape, and rigidity of the side chain on the adamantane skeleton (which is the primary binding site of the esters [15]) greatly affects access of the secondary alkoxide nucleophile to the ester carbonyl and hence the stabilization of the cleavage transition state. Similar considerations apply to esters binding in the CD cavity through a ferrocene group (Fc): the Fc-propiolate [ 16a] is accelerated by 1.4 x 10’ and the Fc-acrylate [ 16b] by 7.5 x 10’; for these esters KTs drops to 3.6X lo-’ and 9.3 x 1 0 - ” ~ , respectively. Capping affects these values only slightly (Table A5.6). As noted above, a good phenoxy leaving group on ferrocenylacrylate esters such as [16b] appears to be mandatory for large accelerations (Menger and Ladika, 1987). Further developments of ferrocene based esters led to even faster acyl transfers to p-CD (Trainor and Breslow, 1981; Breslow et al., 1983), the most spectacular rate accelerations, up to 6 million, being with the derivatives [ 171 and (181 in which an acrylate moiety is conformationally

Fe

restricted by a ring (Table 4). Since K s values are in the normal millimolar range, the accelerations are solely due to much improved transition state binding: in one case KTS is reduced to 9.7 x 1 0 - ’ ” ~ . As impressive as these developments have been, chemists still have a way to go to catch up with “Mother Nature”. For enzymes KTs may be as low as 10-20 M , since K M is generally in the range lo-’ to 10- M and k J k , values are up to 1014 or more (Lienhard, 1973; Kraut, 1988) (see Enzymes, Section 6). Further lowering of KTs for “artificial enzymes” below 1 0 - l ” ~will no doubt require more covalent interactions in the transition state, with better catalytic groups. Nevertheless, the transition state stabilization evident in Table 4 is comparable to that which has been achieved so far with catalytic antibodies (Section 6). The esters in Table 4 also provide two excellent examples of enantioselectivity. This behaviour was revealed when Breslow and coworkers

TRANSITION STATE STABILIZATION

31

Table 4 The "best" esters for acylation of P-cyclodextrin." ~ 7 1

k,lku

KslmM K.~SIM

Selectivity

3.2 x loh 3.8 1.2 x lo-' 20

1181

Enantiomer 1.6 x 10' 14.6 2.9 x lo-' : 1

5.9 x loh 5.7 0.7 x lo-"' 62

Enantiomer 0.5 x lo4 4.7 4.9 x l o r x : 1

"Based on the data of Trainor and Breslow (1981) and Breslow er al. (1983). Reactions in 60% aqueous DMSO, at 30°C.

Table 5

Enantioselectivity in the cleavage of sarin by a-cyclodextrin."

Enantiomer

kJkU

KSImM

KSImM

(S)-( +)-Sarin

4.4 160 36

6.0 40 6.7

1.4 0.26 5.4

(I?)-( -)-Sarin

Selectivity

"Based on the data of Van Hooidonk and Breebart-Hansen (1970)

noted biphasic kinetics due to the different reactivities of the two enantiomeric forms of the esters. The selectivities of 20 and 62 are substantial, and the values of K s and K,, show that they are almost solely due to differences in the stabilization of the two diastereomeric transition states, rather than to differential binding of the enantiomeric substrates. The enantioselectivity just discussed arises because CDs are inherently chiral due the asymmetry of their D-glucose units. Many attempts have been made to exploit this attribute for chemical purposes and some success has been achieved in synthesis (Bender and Komiyama, 1978), and in the physical separation of enantiomers (Szejtli, 1982; Armstrong et al., 1986), the latter now being in general use in chromatographic resolution. More limited success has been obtained in studies of kinetic resolution, comparing the reactivity of one enantiomer to the other (Bender and Komiyama, 1978; Szejtli, 1982). For the cleavage of various aryl esters by CDs and by modified p-CD derivatives, Fornasier et al. (1983, 1987a,b) found selectivities up to 19. Similarly, the cleavage of oxazolones by CDs shows values up to 12 (Daffe and Fastrez, 1983). With two N-carbomethoxyphenylalanine esters very low selectivities of only 1.2-2.3 have been observed (Ihara et al., 1986). More significant is the enantioselectivity shown by the cholinesterase inhibitor, Sarin [19] (Van Hooidonk and Breebart-Hansen, 1970). This nerve agent (Benschop and De Jong, 1988) is cleaved by a-CD, with a 36-fold preference for the more potent R ( - ) enantiomer (Table 5 ) . The

0 . S. TEE

32

enantioselectivity arises almost equally from two sources, weaker substrate binding of the R-Sarin and stronger stabilization of its cleavage transition state. Therefore, the greater selectivity in this case seems to arise partly because the ester is highly asymmetric and partly because chiral phosphorus is the reaction centre.

Me\ F-P=O / i-Pro

[19]

For the most part, the enantioselectivities that have been observed so far are modest to non-existent. In all probability the problem is that, while CDs are inherently chiral (because D-glucose is chiral), they are relatively symmetrical. Thus, a-CD has approximate C6 symmetry and p-CD has approximate C, symmetry. In the absence of strong guest-host interactions both have virtual cylindrical symmetry. Conceivably, for substantial enantioselectivity there must be strong interactions between a highly asymmetric substrate and a group on the CD in a highly asymmetric local environment so that the diastereomeric transition states have distinctly different energies. Such strong interactions are only likely when covalent bonds are being formed in the transition state, that is, during covalent catalysis. This is presumably the situation in the cases cited above which do show significant enantioselectivity . In any event, it is very unlikely that high enantioselectivity can arise solely from differential substrate binding. Since transition state binding is necessarily much stronger than substrate binding for significant catalysis (9), it is most probable that large enantioselectivities will originate primarily from differential transition state binding. As outlined in Section 2, simple aliphatic compounds with short t o medium length alkyl chains bind to CDs, and the strength of the binding increases in proportion to the chain length (C, to C,) and size (Matsui et al., 1985; Tee, 1989; Tee et al., 1990b). Thus, with aryl esters having medium length alkanoate chains it is possible that inclusion of the acyl chain of the ester may become dominant in the initial state, and possibly in the transition state for esterolysis also. Several studies support this expectation. Bender’s group (VanEtten et al., 1967a) studied the basic cleavage by a-CD of three 4-carboxyphenyl esters with different acyl chain lengths (Table A5.7). Reaction of the acetate is accelerated, whereas that of the isobutyrate and t-butylacetate esters are retarded. The data in Table A5.7 show that the change in behaviour with the two larger acyl groups arises because there is a greater increase in the strength of binding of the substrates ( K , = 150- 1.1 mM) than of the transition states (K,,,, = 28+ 5.8 mM), even though the latter improves by a factor of 5 . More recently, Bonora et al. (1985) studied the cleavage of a series of p-nitrophenyl alkanoates (C2, C4, C6, C8 and C,,) by a-CD and by p-CD in

TRANSITION STATE STABILIZATION

33

basic solution (Table A5.8). For both CDs the values of Ks diminish significantly with the length of the alkanoate chain, implying that there has been a switch from binding of the aryl group to binding of the alkyl chain. Studies of the variation of the circular dichroism of the ester chromophore induced by CD binding support this conclusion. Moreover, the variation of the kinetic parameters with chain length suggests that transition state binding also involves alkyl binding.

In another recent study, a comparison was made between the basic cleavage of m- and p-nitrophenyl alkanoates (C2, Cj, C4, Cs and C,) by aand p-CD (Tee et al., 1990b). The strategy was to make use of the normal difference in behaviour observed with meta and para substituents, discussed earlier (e.g. Fig. 4), to probe the importance of alkyl inclusion. If ester cleavage proceeded through a transition state with aryl inclusion [20] appropriate kinetic parameters should be sensitive to the position of the nitro group, and not particularly to the length of the acyl chain. However, if reaction involved a transition state with the acyl group bound in the C D cavity [21] the normal metalpara distinction should be essentially absent and the kinetic parameters should vary systematically with the chain length. In the event, both situations ([20] and [21]) were revealed! For the m-nitrophenyl alkanoates (C2 to C,) the values of K,, hardly change, whereas those for the p-nitrophenyl esters vary more significantly with chain length (Table A5.9), particularly when points for the C8 and C I 2 esters, derived from the data of Bonora et al. (1985) in Table A5.8, are taken into consideration. The two types of behaviour are clearly seen in graphs of PK,,.~ plotted against the number of acyl carbon atoms (Fig. 6). The results show that the more efficient cleavage of m-nitrophenyl esters is maintained (at least to the Ch ester) so that aryl binding occurs in the transition state [20], even though alkyl binding is dominant for the substrate. On the other hand, for the p-nitrophenyl esters alkyl inclusion is dominant for the substrates and for the cleavage transition states [21] (Tee et al., 1990b). In contrast to the above results with nitrophenyl esters, it was found that the kinetic parameters for the esterolysis of 0-acyl derivatives of 4- and 5-chloroaspirin show virtually no dependence on the acyl chain length, at

0 . S . TEE

34

/ /

5.0

/’ 0

m-NO,

A

/

A

P-NO,

2.0

’

i 3

4

5

6

7

8

9 1 0 1 1 1 2

No of carbons in RCO

Fig. 6 Dependence of transition state stabilization (pKTs) on acyl chain length for the cleavage of m- and p-nitrophenyl alkanoates by a-CD (filled points) and p-CD (open points). Data from Table A.5.9, with points for the CHand CI2esters from Table A5.8.

least to Ch (Tee et al., 1990b). implying that aryl binding dominates in the initial state and in the transition state. While this seems at variance with what was found for the nitrophenyl esters, it probably is not. Chlorophenyl groups are more hydrophobic than nitrophenyl groups (Hansch, 1971; Leo et a l . , 1971; Hansch and Leo, 1979) and sufficiently so that the switch from aryl to alkyl binding need not occur until the acyl chain is longer than Ch (Tee ef al., 1990b). The studies just discussed also impinge on another important question. What do the observed kinetic constants represent when the substrate reacts through a substrate.catalyst complex other than that which gives rise to the observed saturation kinetics and hence to the experimental Ks? In other words, what is the relationship, if any, between the mode of substrate binding (reflected in K s ) and the mode of transition state binding (as reflected in KTS)? To address these questions, consider the situation where a substrate forms a second 1 : 1 complex with CD, in addition to that in ( 3 ) , which has a different geometry and reactivity (16). In this case (4) must be expanded

and replaced by (17a). Saturation kinetics will still be observed (17b), since the two CD-mediated processes ( 3 ) and (16) are kinetically equivalent.

TRANSITION STATE STABILIZATION

35

However, the apparent constants in (17b) are composite, having the forms: KSOPP = K s K Y ( K s KA) and kApp= ( k , K & + k h K s ) / ( K s KA). From these, the apparent second-order rate constant for the catalysis is given by (18).

+

+

+ +

kobsd - ( k , K s K & k , K&[CD]+ k: Ks[CD]) ( K s .Kk KA. [CD] K s * [CD])

+

The importance of (18) is that it shows that the value of kippobtained from measurements reflects the principal catalysed pathway [(3) or (16)], regardless of the dominant mode of substrate binding. More precisely, if the catalytic reaction proceeds largely through the pathway represented by ( 3 ) , then k,lKs>>k:/Ki and so k2PP = k , / K s . On the other hand, if the other process (16) is dominant (k,lKs << k : l K i ) , then kiPP = k f / K i . Correspondingly, from (9), KPrs = k , . K s / k , or k; Kilk:, depending on which reaction pathway is dominant and regardless of the principal mode of substrate binding ( K s or K i ) . Therefore, variations of KTs (and klpp) with structure may be used to diagnose the mode of binding of the transition state, independent of that of the substrate (Tee, 1989; Tee et al., 1990b). The above considerations were implicit in recent studies of the basic cleavage of the carboxynitrophenyl alkanoates [22] and [23] by a- and p-CD (Tee and Du, 1988, 1992). These substrates were chosen because the carboxyl groups, which are ionized in basic solution, improve the solubility of longer esters. Also, their aryl groups were expected to be more hydrophilic and less likely to bind in the CD cavity, so that the effects of binding the alkanoate chains should be emphasized. As it turned out, the carboxylate groups promote 2 : 1 binding, the consequences of which are greater with the longer acyl chains. The esters show a variety of kinetic behaviours, depending on the ester, its chain length, and the C D : acceleration (with or without saturation); retardation; acceleration and retardation; retardation and acceleration; two kinds of acceleration! Despite this diversity, simple reaction schemes suffice to explain the kinetics (Tee and Du, 1988, 1992).

0 . S. TEE

36

For the substrates [22] (C, to C,) (Table A5.10) reacting with a-CD there is normal cleavage through a 1 : 1 complex and, beyond C2, formation of non-productive 2 : 1 (CD : ester) complexes. The dissociation constants for 1 : 1 binding decrease systematically with the acyl chain length but those for 2: 1 binding do not. Thus, the first CD binds to the alkyl chain [24] and the second one to the aryl group [25]. Beyond C3. where steric effects on acyl transfer are no longer important (Guthrie, 1973; Tee and Enos, 1988), kinetic parameters vary regularly with chain length: there is a gradual change from retardation to acceleration (k,lk, = 0.48+ 3.4) and a large increase in ester reactivity ( k 2 = 1.7+ 150 M-' s-') due to improved transition state stabilization. Correspondingly, the KTs values decrease (16+ 0.15 mM) systematically with chain length (Table A5.10), indicating the importance of alkyl group binding in the transition state. In addition, a longer acyl chain may position the ester carbonyl higher in the C D cavity, closer to the attacking nucleophile, thereby facilitating reaction (Tee and Du, 1992).

X

For the substrates [22] reacting with p-CD through 1 : l complexes the features are broadly similar (Table A5.10). However, for the C6, C,, C8, 2-ethylhexanoate (2EtC6), and 4-methylpentanoate (4MeCS) esters, a cleavage process involving two molecules of CD is evident at high [p-CD] (Tee and Du, 1988, 1992). This process, which may be ascribed to attack of a second CD molecule on the 1 : 1 ester.CD complex (19), can be characterized by a rate constant, kc2. kc2

S.CD+CD + P

TRANSITION STATE STABILIZATION

CD-O-

37

(61

Values of kc2 increase with chain length, and that for the 4MeCs ester is comparable to that of the C6 ester (Tee and Du, 1992). Therefore, binding of the alkyl chains of these esters contributes significantly to stabilization of the transition state in (19). This may mean that a longer alkanoate chain positions the ester carbonyl group higher in the C D cavity, allowing easier access to a second C D molecule acting as the nucleophile [26]. However, as outlined below, the reaction may well take place within a discrete 2 : 1 complex [27]. From the rate constant kc2 and that for reaction of S - C D complex ( k c ) , application of the Kurz approach leads to K& = k,/kc2 for dissociation of the second C D molecule from the transition state in (19). Values of KkS are remarkably similar for the C6, C7, C8 and 4MeCS esters (45. 59, 49 and 48 mM, respectively), providing strong evidence that the second C D stabilizes the transition state by binding to the aryl group of an ester molecule already bound by its alkanoate chain to the first C D [27] (Tee and Du, 1992).

As discussed in more detail elsewhere (Tee and Du, 1992), the transition state [27] probably arises from a 2: 1 ternary complex (20). If such is the case, kC2= k6/K2 and if K 2 = 5 0 m M , then k;= k,, meaning that the 1 : 1 and 2 : 1 CDaester complexes have virtually the same reactivities. On the other hand, if K2 > 50 mM (as is quite likely) then kS > k,, and the 2 : 1 complexes would be more reactive. The finding that 2: 1 binding of [22] by a-CD does not lead to cleavage, whereas it seems to be productive for p-CD, must relate to subtle differences in the geometries of the 2 : 1 complexes formed by the two different CDs. More specifically, it probably reflects how tightly the esters [22] are held in the C D cavities and whether the solvent has sufficient access to the reaction centre for transition state solvation (Tee and Du, 1992).

38

0 . S.TEE

The more limited results for the four esters [23] (C2, C4, Ch and C,) show largely similar results to those for [22] except that a cleavage reaction involving two CD molecules is observed with both a- and p-CD. Again, the importance of this process increases with chain length and the ratios k,lkCz (= KG,) are fairly constant for a given CD, consistent with reaction through a 2 : 1 complex (Tee and Du, 1992). Other types of ester have been studied (Fendler and Fendler, 1975; Bender and Komiyama, 1978; Szejtli, 1982), though in much less detail. Brass and Bender (1973) studied the cleavage of two diaryl carbonates and three diaryl methylphosphonates in basic buffers (Table A5.11). For the carbonates, reacting with p-CD, introduction of p-nitro groups increases the acceleration ratio and worsens substrate binding, so that K,rs barely alters. More interesting are the results for the phosphonates in that the effects of nitro groups depend on their position and on the CD. With p-CD the presence of p-nitro groups does not affect k,lk, as both K s and K.rs are raised by a factor of 3, but for m-nitro groups the acceleration increases due to weaker substrate binding, KTs being unaltered. Binding of the methylphosphonate esters to a-CD is 6-30 times weaker and introduction of the p-nitro groups raises K,l.s, but does not affect K , ; with m-nitro groups these effects are reversed (Table A5.11). Thus, with both CDs transition state stabilization is unaffected by an rn-nitro substituent but weakened by p-nitro. Perhaps, this means that, in the transition state for cleavage, one of the phenyl groups is located more or less across the top of the C D cavity so that a p-nitro group interacts (repulsively) with the opposite side of the rim, while a phenyl bearing rn-nitro group can avoid such an interaction. This picture is similar to that suggested by Matsui et al. (1985) for the cleavage of phenyl acetates by CDs, and supported by subsequent studies (Tee and Hoeven, 1989), but there is a big difference. In those cases, mefa substitution generally improves transition state stabilization due to inclusion of the substituent in the C D cavity (see Figs 4 and 5 , and related discussion). Komiyama and Bender (1980) compared the cleavage of p-nitrophenyl acetate and its thiophenyl analogue by a- and p-CD (Table A5.12). These esters have virtually the same reactivity, since the loss of leaving group is not rate limiting, and so it is not surprising that their kinetic parameters are not very different. With each CD, k, is virtually the same for the two esters but for both CDs binding of the thio ester is 2-2.5 times stronger and transition state binding is about 3.7 times stronger. These relatively small preferences must reflect subtle differences of the geometry and solvation of the initial state and the transition state due to the substitution of sulphur for oxygen. In most cases of ester cleavage by CDs, an anion of the C D functions as a nucleophile (see Scheme 2). However, in a few instances the anion acts as a general base, assisting the attack of a molecule of water (e.g. [28]).

TRANSITION STATE STABILIZATION

39

Obviously, in such cases the CD is acting as a true catalyst in esterolysis. The basic cleavage of trifluoroethyl p-nitrobenzoate by a-CD occurs by both pathways: approximately 20% by nucleophilic attack; and approximately 80% by general base catalysis (GBC) (Komiyama and Inoue, 1980~).The two processes are discernible because only the former leads to the observable p-nitrobenzoyl-CD. For the ester, K s = 3.4 rnM and k,lk, = 4.4 for the GBC route (1.25 for the nucleophilic route), and so KTs = 0.77 rnM. For reaction within the ester.CD complex [28], it was estimated that the “effective molarity” of the CD hydroxyl anion was 21-210 M (for Bransted p = 0.4 to 0.6 for GBC). Such values are quite reasonable for intramolecular general base catalysis (Kirby, 1980).

At the start of this section the cleavage of meta- and para-substituted phenyl acetates by a- and p-CD was discussed in detail and a variety of evidence was cited that is consistent with the mechanisms A and B, in Scheme 2. Further support for the view that para-substituents tend to force the phenyl group out of the cavity (Scheme 2B) comes from the different effects that neutral additives (potential inhibitors) have on the cleavage of m- and p-nitrophenyl acetate (rnNPA and pNPA). In brief, species which bind to CDs, and inhibit the reaction of mNPA, do not necessarily inhibit that of pNPA (Tee and Hoeven, 1989; Tee el al., 1993b). Reactions proceeding through S * C D inclusion complexes should show competitive inhibition (Fersht, 1985) in the presence of additives which bind in the CD cavity. Such behaviour has been observed for the cleavage of mNPA by a-CD (VanEtten et al., 1967a) and by p-CD (Tee and Hoeven, 1989), supportive of the mechanism in Scheme 2A. In sharp contrast, with many potential inhibitors, the cleavage of pNPA is not retarded to the extent expected for competitive inhibition, and in a few cases slight rate enhancements are observed (Tee and Hoeven, 1989; Tee et al., 1993b). In those cases where the inhibition is significantly less than expected there must be a pathway in which the reaction of pNPA with the C D is mediated by the potential inhibitor (PI), one that compensates (partially or totally) for

0 . S. TEE

40

the inhibitory effects of PI.CD complexation. As a working hypothesis the reaction of PI with the pNPA.CD complex (21) was chosen, since this is relatively easy to analyse for. With this process, as well as those in (2) and

PI+S+CD

eP I + S - C D K,

k.,

+

products+PI

k""' = (k"b'd(Ks [CD]) - k,Ks}/[CD]

=

k,

+ k,[PI]

(23)

(3), (4) must be expanded to (22). The form of (22) is not particularly convenient for analysis, since it contains two concentration variables, but it can be rearranged to a more tractable, linear form, (23). This transformation amounts to correcting kobsd for the background reaction (2) and substrate binding, thereby isolating the contributions from the two reactions of the CD-bound ester: (3) and (21). According to (23), the "corrected" rate constants (kcor') should vary linearly with [PI], and the slope provides an estimate of the rate constant k , (Tee and Hoeven, 1989). Based on this approach, the effects of a large number of PIS (mainly alcohols, alkanoate ions and alkanesulphonate ions) on the rate of cleavage of pNPA by a- and p-CD have been examined and analysed (Tables A5.13 and A5.14) (Tee et ul., 1993b). In initial studies with p-CD it was noted that values of k , vary in inverse proportion to the inhibition constant, K I , suggesting that PI is bound in the CD cavity in the transition state (Tee and Hoeven, 1989). Therefore, the PI-mediated reaction is more reasonably viewed as being between the ester and the PIeCD complex. The third-order processes in (21) and (24) are kinetically equivalent ( k , = k,/Ks = kh/Kl), and so kb values are easily found from k , . Such values of kb show some variation with structure but they are quite similar for different PIS and not very different from k2 for the reaction of the CD with pNPA! For example, for pNPA reacting with 15 different alcohol.p-CD complexes values of kb span the range 10-95 M-'s-' (Table A5.14), close to k2 = 83 M - ' s - ' for the reaction of pNPA with p-CD alone. Similar behaviour was observed for other PIS (Table A5.14) and for a C D (Table A5.13), for which k2 = 26 M - ' s-'.

kh

PI+CD+S

K,

PI.CD+S

products+PI

(24)

These similarities in the reactivities of different PI * CD complexes and the CD towards pNPA are entirely consistent with the process in (24), provided that the ester moiety is outside the CD cavity in the cleavage transition state

TRANS IT1ON STATE STAB IL IZATlON

41

MeLL 02N-@-Or

?

0I

Scheme 3

so that something else (a PI) may occupy the cavity (Scheme 3) (Tee and Hoeven, 1989). To a first approximation it does not matter what the PI is, as long as it does not interfere with formation of the transition state. Full competitive inhibition of the reaction of pNPA with p-CD has been found with iodide ion, 1,6-hexandiol, and the dianions of suberic acid and adipic acid, which must mean that these species bind to the CD in such a way as to block the reaction. Likewise, the cleavage of pNPA by a-CD is fully inhibited by t-butyl and neopentyl alcohols, benzoate ion, and tosylate ion (Tee ef al., 1992). The mechanism outlined in Scheme 3, in which the ester moiety is not bound in the CD cavity in the transition state, is consistent with the analysis of steric effects by Matsui et al. (1985) and with’ the depiction in Scheme 2B, given earlier. Thus. although pNPA forms a substrate. CD complex, the latter is not on the reaction coordinate for acyl transfer and its formation is irrelevant except in giving rise to the observed saturation kinetics. Further analysis of the rate constants in Tables A5.13 and A5.14 can be made using the Kurz approach, particularly regarding the structural dependence of the transition state stabilization. For the PI-mediated reaction, we define K.l.s by (25), where now TS stands for the transition state in reaction (3) and TSePI is that in reaction (21) [or (24)]. As indicated in ( 2 5 ) , K.rs may also be derived from the rate constants for the second order process in (3 = 5 ) and the third-order process involving PI, since k2 = k c / K s and k3 = k , / K s [see (21)].

0 . S.TEE

42

Table 6 Correlations of the transition state binding (pKTs) of potential inhibitors in thc cleavage of pNPA by CDs with their ability to bind to CDs (pK,)."

CD

Slope

N

r

Alcohols

a

R-COT Alcohols %SO, R-COY

13 5

P P P

1.03 1.01 0.77 0.67 0.75

0.993 0.993 0.973 0.986 0.996 0.995

PI K-SO?

a

a

0.81

4

15 6 7

"Baaed on data from Tee et al. (1993b) (see Tables A5.13 and AS.14). N is the number of points; r is the correlation coefficients of pKrs against pK,.

The values of KTs in Tables A5.13 and A5.14 vary significantly with structure but they do so in a manner that strongly parallels K, for the PI C D complexation. In fact, for both a-CD and p-CD, and three series of PIS, there are good correlations between pKrs and pKI (Table 6); the data for aand p-CD and alcohols are shown in Fig. 7. The two correlations for alcohols are particularly noteworthy since each includes various structural types (linear, secondary, branched, cyclic, etc.). Thus, the abilities of PIS to bind (and stabilize) the transition state of the reaction of pNPA with CDs is firmly related to their abilities to bind in the C D cavities. The LFERs summarized in Table 6, with slopes approaching one, strongly suggest that the mode of binding of PI in the transition state {TS . PI} of (24) is not very different from that in the PI.CD complexes. However, whether this mode has the PI oriented with its hydrophilic group towards the wider, secondary rim of the CD or towards the narrower, primary rim is not yet known. The generally higher slopes, closer to 1 , of the correlations for a-CD (Table 5) probably reflect the more restrictive binding that results from the tighter fit of alkyl chains in the narrower cavity of a-CD (see Section 2 and discussion of Fig. 1). In contrast, p-CD has a wider cavity so that the fit is looser and the PIS may more easily adopt an orientation which does not interfere with the cleavage of pNPA. These proposals are consistent with the higher values of kb for p-CD than for a-CD, and they may be related to another difference between the behaviours of a- and P-CD: t-butyl and neopentyl alcohols inhibit the reaction of pNPA with a-CD but not that with p-CD (Tee et al., 1993b)! One more feature of the data in Table A5.14 is worthy of note. Values of k,, for the reaction in (24) vary from 7.8 to 9 5 ~ - ' s - l , compared to k2 = 83 M - * s-I for reaction (3). For the two cases (n-PrOH and i-PrOH) where kh > k2 the P I . C D complex is more reactive than p-CD alone, so that the occupancy of the CD cavity by either of these PIS in place of a few water molecules affords additional, albeit modest, transition state stabilization.

TRANSITION STATE STABILIZATION

43

3.0

2.0

i

A

3.0

2.0

m Y

n

v1

h?+ 0.

1.o

1 .o

3.0

2.0

0.0

PK,

Fig. 7 Correlation of transition state binding (pKTs) of alcohols mediating the cleavage of pNPA by a- and p-CD with their ability to bind to these CDs (pK,). The left and right scales are offset for clarity. Data from Tables A5.13 and A5.14.

Thus, the effects of simple alcohols on the cleavage of pNPA by a- and p-CD show a whole spectrum of behaviour from full inhibition (kh<< k 2 ) to modest retardation (kb< k 2 ) to actual catalysis (kb> k 2 ) . Even more surprising than the foregoing observations for the effects of PIS on pNPA cleavage, it has been found that the cleavage of p-nitrophenyl hexanoate (pNPH) by p-CD is not inhibited by many alcohols: instead it is catalysed (Tee and Bozzi, 1990). The observation is surprising since the normal cleavage ensues with acyl chain binding (Bonora et al., 1985; Tee et al., 1990b), as discussed earlier (see [21]), and so a PI would be expected to preclude such binding and inhibit the reaction. That such inhibition does not happen implies that the chain of pNPH does not penetrate deeply into the p-CD cavity during cleavage, allowing sufficient room for another, smallish molecule to bind as well and in a manner which affords some transition state stabilization.

S + CD + PI

4

S.CD + PI

k,

S.CD.PI + products

(26)

K,

Addition of small alcohols (C, to C,) brings about rate increases and saturation behaviour which is attributed to the formation of discrete 1 : 1 : 1 (pNPH :p-CD : ROH) complexes (Tee and Bozzi, 1990). Analysis of the data affords constants for the dissociation and reaction of these ternary complexes (Table A5.15), based on the model in (26). Values of k , for reaction within the ternary complexes are only 1 . 4 4 . 3 times larger than

0 . S. TEE

44

2.5

v)

Y

-

2.0 -

n

1.5-

1.0

-

0.5

1 .o

1.5

2.0

2.5

PK t

Fig. 8 Correlation of the transition state binding (pKTs) of alcohols catalysing the cleavage of p-nitrophenyl hexanoate by p-CD with their binding in the initial state ternary complexes (pK,) [see (26)]. Data from Table A5.15.

k, = 0.14 s-’ for reaction within the CD .pNPH complex, since the catalysis is relatively modest. For the binding of PI in the ternary complexes, K, is about twice K1, meaning that the binding of pNPH to C D - P I has a dissociation constant about twice Ks. For 12 alcohols of various types as PIS, the values of pK, for the ternary complexes and pKTs for the termolecular transition state in (26) vary linearly with pKI; pK, correlates with a slope of 0.90 (r = 0.994) and for pKTs the slope is 0.74 (r = 0.986) [see Fig. 2 in Tee and Bozzi (1990)]. Thus, it seems that the binding of the alcohol in the ternary complex and in the related transition state are not very different from that in the p-CD.ROH complexes. Also, of course, there is a good correlation between PKTS and pK, with a slope of 0.81 (r = 0.994) (Fig. 8), implying that the structure of the termolecular transition state is quite similar to that of the ternary complex, at least as far as the binding of the alcohol is concerned. However, as with pNPA cleavage, it is not yet known what is the actual geometry involved; conceivably, it might be as in [29] or as in [30], although the latter appears more likely. These findings for the effects of additives on the cleavage of pNPA and pNPH by CDs provide a paradigm for simple allosteric effects (Walsh, 1979; Page, 1984; Fersht, 1985) in that they provide examples where the binding and reactivity of a substrate with a catalyst can be improved by the concomitant binding of a third, inert species. This behaviour has been termed “spectator catalysis” (Tee and Bozzi, 1990). With catalysts, such as enzymes, having more complex binding regions, one can imagine that more

45

TRANSITION STATE STABILIZATION

ArO

2:

ti? X

ArokG

dramatic catalytic effects are possible with a “spectator” of the appropriate shape and structure to optimize the fit of the transition state to the catalyst. In such circumstances, the concentration level of the “spectator” may be used as a switch to turn the enzyme “on” or “off”, as required. AMIDE CLEAVAGE

The effects of cyclodextrins on this reaction, while potentially of great interest, have been rarely studied, presumably because amide hydrolysis is generally slow and so tedious to study. As a result, only two examples are considered here. Tutt and Schwartz (1971) studied the basic cleavage of some penicillin derivatives [31]+ [32] in the presence of p-CD. For acyl groups bearing alkyl and aryl substituents of various sizes and geometries, the accelerations (kc/kt,)vary less (31-89) than the spread of K s values (3.85-75 m M ) (Table A5.16). Approximately, therefore, the KTs values vary in parallel with K s , suggesting that the acyl group is involved in substrate and transition state binding to about the same extent. The basic hydrolysis of p-nitroacetanilide is retarded by a-CD, whereas that of trifluoroacetanilide and its rn-nitro derivative is modestly enhanced

L N H

0

“x”p$ - -02ci:”x H

I

0

co,

HN

I

COT

0 . S. TEE

46

(1.5-3.3 times); reaction of the p-nitro isomer (27) is catalysed 16-fold (Komiyama and Bender, 1977). For the last substrate, K s = 62mM and so KTS = 3.9 mM, indicating relatively weak binding of both the substrate and the transition state. These features, which contrast with those found in phenyl acetate cleavage (vide supra), may result from differences in mechanism; for aryl esters the rate-limiting step is formation of a tetrahedral intermediate but for anilides it is the loss of amine from the intermediate, assisted by general acid catalysis. p-NOZPhNHCOCF3

+ OH- -+

p-NO2PhNH2

+ CF3COO-

(27) The CD-mediated cleavage of p-N02C6H4NHCOCF3proceeds by acyl transfer to a-CD. Since the trifluoracetyl-CD, so produced, hydrolyses fairly quickly even at pH7, the overall reaction shows true catalysis. Thus, for the reaction in (27), a-CD behaves as a model enzyme and shows three of the features of chymotrypsin: (i) it provides a hydrophobic binding site; (ii) it catalyses the loss of leaving group; and (iii) the reaction proceeds through an acyl intermediate (Komiyama and Bender, 1977; Bender and Komiyama, 1978). DEPROTONATION

The anions of CDs may also function as simple basic catalysts towards acidic substrates included in their cavities. Such was observed by Daffe and Fastrez (1983) who studied the deprotonation and hydrolysis of oxazolones in basic media containing CDs. Also, in a paper dealing mainly with catalysis by amylose, it was noted that CDs catalyse the deprotonation of long chain p-keto esters in basic aqueous DMSO (Cheng et al., 1985); no saturation kinetics were found for CDs, indicating weak substrate binding under the conditions used. Prompted by this earlier work, the deprotonation of simple p-keto esters (RCOCH,COOR’) by CDs in wholly aqueous solution (pH = 10) was studied and saturation kinetics were found in most cases (Tee et al., 1993a; Tee, 1989). Changes in the kinetic parameters (Table A5.17) are more pronounced for variations of the alkoxyl group (R’O) than for those of the acyl group (RCO), suggesting that substrate binding and transition state binding both involve inclusion of the alkyl group of the ester function [33], rather than of the acyl moiety [34] (Tee, 1989; Tee et al., 1993a). 6 Other catalysts

In his pioneering paper, Kurz (1963) considered examples of catalysis by protons, water, hydroxide ion, and, to a limited extent, general acids and

TRANSITION STATE STABILIZATION

47

COOR'

I

bases. The focus of his attention was reactions having two pathways, with transition states differing by one proton. For such cases he estimated apparent transition state pK, values (py?), using the approach outlined in Section 3. His example, involving general acidbase catalysis, was that of the enolization of a ketone. This reaction is considered again here, but using some recent results for the reverse reaction (ketonization). For illustrative purposes some other reactions are discussed, along with applications involving different catalytic systems. ACIDS AND BASES

The enolization of aldehydes and ketones [35]-+ [36] is subject to both acid and base catalysis (Bell, 1973; Toullec, 1982; Albery, 1982). Although the kinetics of the reaction were first studied 90 years ago (Lapworth, 1904) and

0

\CH-C / - - + t / \

[351

,c=c /

\

OH

\

~361

there have been many subsequent mechanistic studies, great strides in our understanding have been made in the last decade through the judicious use of new approaches and techniques (Dubois et al., 1981; Tapuhi and Jencks, 1982; Guthrie et al., 1984). Of equal importance are the recent successes, mainly due to Capon, Kresge, and coworkers (Capon et af., 1988; Kresge, 1986, 1990; Chiang and Kresge, 1991), in generating transient enols and in studying their ketonization [36] -+ [35]. These elegant studies of the reverse reaction have consolidated and greatly expanded our knowledge of ketoenol chemistry (Keefe and Kresge, 1990). In acidic solution the enolization of simple ketones is general acid catalysed (Bell, 1973; Toullec, 1982), but measurements of inverse solvent

0 STEE

48

isotope effects (e.g. Albery and Gelles, 1982) have established that the catalysis arises from specific acid catalysis with general base catalysis. Thus, enolization at low pH occurs in two discrete steps: pre-equilibrium protonation of the ketone and rate-limiting proton abstraction (28). This mechanism requires that the ketonization under the same conditions should exhibit true general acid catalysis, which it does (Chiang et af., 1988). In more basic solution, enolization exhibits general base catalysis due t o rate-limiting deprotonation (29), and ketonization occurs by specific base with general acid catalysis (Pruszynski et al., 1986; Chiang and Kresge, 1991). n

+

A

keto zz12 ketoHf I enol

en* A

keto F=? enolate HA

HA Hi

enol

I(I,

A study of the ketonization of the enol of acetophenone by Kresge and coworkers (Chiang et al., 1988) affords data that can be treated by the Kurz approach. From the rate constants for general acid (kHA)and general base catalysis (= kbAK,F), referring to transition states differing by a proton on the enolic oxygen, one can estimate the apparent pK, of the transition state, pKi (Kurz, 1963, 1972) for acid-catalysed ketonization (Scheme 4). As seen in Table A6.1, values of pK1 are almost constant (3.33-3.93) for carboxylic acid catalysts, and remarkably similar for H 3 0 + and H3P04, even though the acids have a range of 6.5 pK, units and a 280-fold spread of reactivity in kHA. This near constancy of pKi values implies that the transition state structure in (30) does not vary appreciably in going from hydronium ion to propionic acid as the general acid delivering the proton to the enol, at least in the vicinity of the enolic OH. The enol of isobutyrophenone is 2000 times less reactive than that of acetophenone (Pruszynski et al., 1986), yet its pK2 values are very similar (3.34-3.84) (Table A6.1). It should be noted, however, that pKa does decrease somewhat as the pK, of H A increases. This is a consequence of the higher Bronsted a values for kHAthan for k;lA (0.50 and 0.32, respectively, for acetophenone enol) (Chiang et af., 1988). From the partial change in pK, in going from the enol of acetophenone (pK,; = 10.34) to the transition state (pKf = 3.6), relative to the overall change in pK, between the enol and the conjugate acid of acetophenone (pKcYH'= -4.16) (Cox et af., 1979), one can estimate a Leffler index (Leffler and Grunwald, 1963; Williams, 1984, 1992) of a = (10.34-3.6)/ (10.34 4.16) = 0.46, which is virtually the same as the measured Bransted a = 0.50 k 0.07 for catalysis (kHA)by carboxylic acids (Chiang et af., 1988). The closeness of these two independent indices for two different protons (enol O H and catalyst HA) suggests that bonding changes at the two extremes of the transition state in (30) occur almost synchronously, so that there is very little imbalance (see Bernasconi, 1987, 1992a,b).

+

TRANSITION STATE STABILIZATION

A-H

A-H

49

/OH

CHZ=C

\

Ph

/O-

CH2z C,

kHA

k;,

Ph

A-

CH3-C

A-

CH3-C

/OH+ ‘Ph

No

‘Ph

(30)

(31)

Scheme 4

Another observation that may be of relevance to the potential catalysis of enolization (and ketonization) is that the pK, for proton loss from the conjugate acid of acetophenone from carbon must be 3.80 (the sum of the pK of 7.96 for enol formation and pKFH’ = -4.16). Thus, protonation of the enol on carbon by a carboxylic acid (or its reverse) occurs with very little energy cost, consistent with the B r ~ n s t e da of 0.50. Furthermore, this observation, the finding that pK: for the enolic hydrogen atom is approximately 3.6, and the suggestion that bond making and bond breaking occur almost synchronously (see above), are all consistent with the fact that enolization is subject to simultaneous general acid and general base catalysis in strong carboxylate buffers (Hegarty and Jencks, 1975; Albery and Gelles, 1982; Hegarty and Dowling, 1991). The primary literature now contains a very large body of kinetic data for the catalysis of enolization and ketonization, not only of ketones and aldehydes but also of P-diketones, P-keto esters, and dienones, much of which could be treated by the Kurz approach. Also, data exist for third-order enolization, due to combined general acid and base catalysis, that could also be analysed. Such treatment is beyond the scope the present review. However, one study of metal ion catalysis of enolization is discussed later in this section. Another example of the use of transition state pK, values has been provided by Pollack (1978). From the rate constants for the decarboxylation of substituted a,a-dirnethylbenzoylacetic acids ([37]-+ [38]) and their anions, he calculated pK: for reaction of the acids (Table A6.2). The values vary significantly with the phenyl substituent ( p = +1.7), much more so than the pK, values of the substrate acids ( p = +0.2). This difference is consistent with the proton being much closer to the phenyl group in the transition state than in the initial state, and it may even denote a relatively “late” transition state (Pollack, 1978). However, from the pK, values of the reactant acids (approximately 3.4), the transition states (approximately 4.4), and the enol product (11.8) (Pruszynski et al., 1986), the Leffler index

0 . S. TEE

50

(Leffler and Grunwald, 1963; Williams, 1984, 1992) is only about 0.12, suggesting a very “early” transition state. Conceivably, there is transition state imbalance and charge separation (see Logue et al., 1975) which gives rise to this seeming conflict. Regardless of the finer details, the same basic situation must pertain to decarboxylation of the benzoylacetic acids studied by Straub and Bender (1972b), since reaction of these acids and their anions have quite different p values (+0.03 and +1.42, respectively) and the pK,, values of the parent acids have a very low p.

Transition state p K , values may also be estimated for reactions which involve nucleophilic attack by water and by hydroxide ion (Kurz, 1963, 1072). Such may be the case in the formation of pseudobases from quaternary heterocyclic cations (32a,b), a number of which have rates of

Q’(

+ OHz)

-

(32a)

k,

Q - OH

+ Hf

(32b)

equilibration which are amenable to measurement (Bunting, 1979). For many such systems, it has been noted that the ratio of hydroxide ion attack to water attack is fairly constant, with koHlkw = lo7 M - ’ , even for cations with quite different structures and reactivities (Bunting and Meathrel, 1973; Bunting and Norris, 1977; Tee et al., 1978; Tee and Paventi, 1981). In terms of the Kurz approach, comparing two transition states differing by one proton, K: = koHKw/kw (Kurz, 1963, 1972), and so the above ratio implies that the transition state for water attack ([39] + [40]), has a hydioxyl pKi = 7.

TRANSITION STATE STABILIZATION

51

Three main points emerge from this observation. Firstly, the virtual constancy of pK$ implies that the transition state structure does not vary greatly for cations of widely different structures and reactivities. Secondly, the pK$ value of about 7 may be related to the fact that water attack (33) can be catalysed by general bases (Bunting and Meathrel, 1973; Gravitz and Jencks, 1974a,b,c; Tee and Paventi, 1981), presumably because it can occur with no great energy cost (Guthrie, 1980). Thirdly, during water attack, the pK, of the hydroxyl protons goes from 16 (water) through about 7 (transition state) to about -3 (putative oxonium product), and so, with respect to these protons, the Leffler index a is about (16-7)/ (16 3) = 0.47, again consistent with general base catalysis. Also consistent with this value, the slope of a plot of -1ogkOH against pKR+ (for Q O H formation from Q') is 0.40-0.45 for various quinolinium and isoquinolinium cations (Bunting and Norris, 1977).

+

Q+ + OH^ + :B + Q-OH

+ HB+

(33)

The occurrence of reaction (33) raises further possibilities. If the attack of water can be catalysed by general bases, it is possible that the rate constant k , actually refers to water-assisted attack of water (see Kurz et al., 1986) and that koH is for hydroxide ion acting as the general base. In fact, a variety of evidence suggests that both water and OH- do react as general bases in (33) (Bunting and Meathrel, 1973; Bunting and Norris, 1977). If both these processes actually take place, then the pK: of about 7 refers to a proton further removed (by one water molecule) from the heterocyclic moiety. Also, the Bronsted p (=0.5) defined by the two species is essentially the same as the Leffler index (see above). The situation just discussed probably applies also to the attack of water on other kinds of stabilized carbocations. For example, some of the many transient carbocations studied in recent years by McClelland and Steenken, and their coworkers (e.g. Steenken et al., 1986; McClelland and Steenken, 1988), have relatively constant koH/k, ratios. For alkyldialkoxy cations [41], koHlk, = lo3 to lo4 M - ' and so pK$ = 10 to 11; for the trialkoxy analogues [42], kor,/k, = lo4 to lo6 M-' and pK2 = 8 to 10 (Table A6.3), suggesting a more acidic transition state for [42], due to the extra oxygen atom. Within each series there is a systematic variation of pK:, since logkoH correlates with logk,, with a slope of approximately 0.6 (McClelland and Steenken, 1988), rather than 1. Estimates of the Leffler indices for the two series of

Ro\ RO'

C+-R'

[411

Ro, RO'

C+-OR 1421

0 . S. TEE

52

cations are 0.30 and 0.35, consistent with early transition states. The lower value for the less stable, more reactive alkyldialkoxy carbocations is as expected from Hammond’s postulate (Hammond, 1955). The attack of water on the ions [41] and [42] may well be general base catalysed (34), with a Bronsted p = 0.3. Such appears highly likely, almost mandatory, since the loss of alkoxyl groups from orthoesters shows general acid catalysis with Bronsted a = 0.7 (Fife, 1972). A o H

R 0’ \O>\C’-R’ / /

H SR

-

0’I

A-H

R

’ 7o H 0-

-R’

(34)

\R

The hydration of simple ketenes (RCH= C = O + RCH,COOH) also shows relatively constant values of koHlk, which are quite low (100-1000) (Tidwell, 1990; Allen et al., 1992), implying pK2 = 11 to 12 for the transition state for water attack. Corresponding to this, the Leffler index and the Pnucare both about 0.25. Whether these low values really indicate an early transition state or arise because water and hydroxide ion react quite differently is not yet clear. However, it appears possible that water attack proceeds through a cyclic mechanism involving two (or more) water molecules (Allen et al., 1992) whereas hydroxide ion probably attacks conventionally as a nucleophile (Tidwell, 1990). Of course, any mechanism for the water reaction which is superior to simple nucleophilic attack will elevate k , and necessarily lead to low koHlk, ratios. METAL IONS

In principle, reactions which are subject to electrophilic catalysis by protons can be catalysed by metal ions also (e.g. Tee and Iyengar, 1988; Suh, 1992). However, metal ions may function in other ways, such as to deliver a hydroxide ion nucleophile to the reaction centre (e.g. Dugas, 1989; Chin, 1991), and it is often difficult to decide between kinetically equivalent mechanisms without resorting to extensive (and intensive) model studies. Use of the Kurz approach may help to resolve such ambiguities, as shown below.

Ph2P(0)OAr + -OEt(+M+) -+ Ph2P(0)OEt

+ -OAr(+M+)

(35)

Recently, Dunn and Buncel (1989) showed that the attack of ethoxide ion on p-nitrophenyl diphenylphosphinate in ethanol is catalysed by alkali metal ions (35). They found that the transition state stabilization afforded by metal

TRANSITION STATE STABILIZATION

53

Et

1431 ions follows their ability to form EtO- M+ ion pairs (Li+ > Na+ > K + ) , as seen in the data in Table A6.4(a). There is a fair correlation of pKTs with the pKi, for ethoxide ion binding, with slope of +2.8 and so the interaction of the metal ion with the transition state parallels that in the initial state but it is stronger and more sensitive to the nature of the ion. These observations can be taken as evidence that the metal ion stabilizes the transition state by simultaneously binding the incoming ethoxide ion and the phosphoryl oxygen of the ester [43]. In essence, therefore, the alkali metal ion facilitates the attack of EtO- by delivering the nucleophile and by providing electrophilic assistance. In a companion study, Buncel and coworkers (Pregel et al., 1990) looked at the analogous cleavage of p-nitrophenyl benzenesulphonate (36). With this reaction the findings were quite different; there is an inverse relation between the binding of the metal ion in the transition state and its ion-pairing ability [Table A6.4(b)], with Li+ actually being slightly inhibitory. Obviously, the alkali metal ions function very differently in this case and so a different transition state structure [44] was proposed, one in which the solvated metal ion affords stabilization by binding to two of the sulphuryl oxygen atoms, which implies that it assists nucleophilic attack by providing electrophilic assistance only. PhSOzOAr + -OEt(+M+) + PhS0,OEt

+ -OAr(+M+)

(36)

One other feature of the results of Buncel and coworkers warrants comment. For both (35) and (36), the behaviour of Cs+ ions seems to be out of line with that of the other ions (Table A6.4) (Pregel et al., 1990). This might be due to an inaccurate K,, (which was taken from the literature). However, the discrepancy appears to be resolved by considerations of ion size. For the phosphinate cleavage (35). transition state stabilization (expressed as a free energy of transfer) correlates inversely with the radius of the naked metal ion. In contrast, for sulphonate cleavage (36) the correlation is with the inverse of the radii of the solvated ions. Accordingly, the difference in the metal ion catalysis for the two reactions was ascribed to the extent of charge dispersal in the two transition states: for (35) the developing charge is localized largely on one oxygen [43] while in (36) the

0 . S. TEE

54

Me

WI

R = H , (OCH2CH2)40Me

~461

charge is largely delocalized over two oxygen atoms [44] (Pregel et al., 1990). Thus, the two transition states, [43] and [44], have different propensities for stabilization by metal ions. Ercolani and Mandolini (1990) studied the reaction [45] + [46] of methoxide ion with phenyl acetate and with a derivative bearing a poly(oxyethy1ene) side chain at the 2-position to provide a metal ion binding site. The reaction of phenyl acetate exhibited modest catalysis by Sr2+ and Ba2+ ions, but none by Na+ and K+. By contrast, cleavage of the functionalized ester showed significant catalysis by all four ions. Thus, the polyether functionality promotes metal ion binding in the transition state and improves metal ion catalysis, as was anticipated. On the basis of a treatment which differs from the Kurz approach in style, but not in substance, the following values of pKTs were obtained. For phenyl acetate: Sr2+, 2.45; and Ba2+, 2.21. For the functionalized ester: Na+, 1.52; K + , 1.97; Sr2+, 3.05; and Ba2+, 3.74. Accordingly, the polyether side chain improves the binding of Sr2+ and Ba2+ to the cleavage transition state by approximately 4 and 30 times, respectively, resulting in proportionate increases in catalysis and promoting Ba2+ over Sr2+. No conclusions about the details of the transition state structure were made; whether the metal ions function as electrophilic catalysts, as deliverers of the methoxide ion nucleophile, or in both ways, was not discernible. The effects of metal ions on the alcoholysis and hydrolysis of esters containing crown ether functionalities have also been studied (Cacciapaglia et al., 1989, 1992; Hedderwick et al., 1991a,b). In the former case, transition state binding was considered explicitly, in the same manner as did Ercolani and Mandolini (1990). One more example of metal ion catalysis will be considered briefly. In a now classic paper, Cox (1974) showed that the enolization of 2-acetylpyridine (but not 4-acetylpyridine) is catalysed by divalent transition metal ions. Proton abstraction by acetate ions is strongly accelerated by Zn2+,Ni2+ and Cu2+ ions and the transition state stabilization by these ions roughly parallels their abilities to bind to the substrate (Table A6.5). The three metal ions are significantly superior to the proton as electrophilic catalysts, no doubt because they can chelate to both the pyridine nitrogen and the

TRANS IT1ON STATE STAB ILlZATl ON

55

ketone oxygen weakly in the initial state, more strongly in the transition state, and most strongly to the enolate ([47] + [48]). In contrast, the proton binds to the basic nitrogen of 2-acetylpyridine (pK, = 2.64) in the initial state, whereas electrophilic catalysis of the enolization requires the much less favourable carbonyl 0-protonation [pK, = -6?; cf. acetophenone, pK, = -4.16 (Cox et af., 1979)l. The situation is different with the metal

ion catalysed debromination and enolization of dienones leading to salicylate ions (Tee and Iyengar, 1988); in these reactions proton catalysis is much more competitive with that by metal ions because the geometries of the reactants and products are such that H + can participate in chelation through hydrogen-bond formation. AMYLOSE

This starch derivative is a water-soluble polymer of D-glucose with a largely helical structure. Therefore, like cyclodextrins, it has the ability to bind alkyl chains and to catalyse reactions through the involvement of ionized hydroxyl groups in basic solution (Hui et af., 1982; Cheng e f af., 1985). A kinetic study of the cleavage of p-nitrophenyl alkanoates by amylose (Hui et uf., 1982) showed that some of the reaction parameters vary significantly with the acyl chain length ( N = 5 , 8, 12 or 16) (Table A6.6). Substrate binding increases with N , but transition state binding increases more steeply (KTS = lop4+ 2 x lo-’ M ) , as the acceleration also rises (k,lk, = 4.3+ 174). However, this is largely due to a decrease in k,,, since k , is almost constant. Overall, there are broad similarities to cleavage of the same type of esters by CDs (Bonora et af., 1985; Tee et af., 1990b), as discussed in Section 5 . MICELLES

Many organic reactions can be accelerated or retarded by incorporation of the reactants into micelles (Fendler and Fendler, 1970, 1975; Kunitake and Shinkai, 1980; Bunton, 1984; Bunton and Savelli, 1986; Bunton et af., 1991).

0 . S . TEE

56

The effects observed in a given case depend on the reaction type, its charge type, and the nature of the surfactant (cationic, anionic, or neutral) forming the micelles. One recent study of ester cleavage (37) in cationic micelles affords data that are amenable to treatment by the Kurz approach. C11H23COOChH4-X

+ 20H-

+ C11H23C00-

+ -0ChH4-X + H20

(37)

The base-induced hydrolysis of nine phenyl laurate esters catalysed by cetyltrimethylammonium bromide (CTAB) micelles showed a sensitivity to the phenoxide leaving group (PI, = -0.51 f 0.06) that is essentially the same as for the reaction in wholly aqueous solution (pis = -0.56f0.05) (Al-Awadi and Williams, 1990). Kinetic data, obtained with constant [ B Y ] to avoid dilution of the OH- in the Stern layer due to the ion exchange effect (Bunton and Savelli, 1986), were analysed with an equation of the same form as (4). The analysis provided constants (Keq) for dissociation of the esters from the micellar pseudophase which are constant, within experimental error. These observations mean that the transition state stabilization afforded by the micellar environment is essentially constant for esters with a 200-fold range of reactivity (Table A6.7). The lack of variation of K.rs with substituents on the phenoxide leaving group, and the fact that ljlpequals that for aqueous solution, suggests that for the micellar catalysis the reaction centre and the leaving group are in a largely or wholly aqueous environment in the transition state. Presumably, only the long alkyl chain of the laurate esters extend into the core of the micelle. This presumption could be tested by a comparable study of phenyl esters with different alkyl chain lengths for which one anticipates a strong correlation between pKrs and pK,,, values. CATALYTIC ANTIBODIES

One of the most exciting developments in bioorganic chemistry in recent years has been that of “catalytic antibodies” (Dugas, 1989). These proteins, which are sometimes called “abzymes”, are of interest as “synthetic” catalysts and because they may help in understanding proteinhgand binding and the nature of enzymic catalysis (Schultz, 1988, 1989a,b). Using techniques of molecular biology, several groups of researchers have induced, screened, and cloned antibodies which have enzyme-like properties. The main methodology has been to develop antibodies to a small protein coupled to a hapten having the structure of a “transition state analogue”, as envisaged by Jencks (1969). For example, antibodies which are specific for binding charged, tetrahedral phosphonate and phosphate esters can catalyse the hydrolysis of carboxylic and carbonate esters (Schultz, 1988, 1989a,b; Tramontano et al., 1988). In what follows, three examples of catalytic

TRANSITION STATE STABILIZATION

57

antibodies are presented to show how the Kurz approach can be applied to these new catalysts. Kraut (1988) has also discussed catalytic antibodies briefly in his review. Schultz and coworkers (Jackson et al., 1988) have generated an antibody which exhibits behaviour similar to the enzyme chorismate mutase. The enzyme catalyses the conversion of chorismate [49] to prephenate (501 as part of the shikimate pathway for the biosynthesis of aromatic amino acids in plants and micro-organisms (Haslam, 1974; Dixon and Webb, 1979). I t is unusual for an enzyme in that it does not seem to employ acid-base chemistry, nucleophilic or electrophilic catalysis, metal ions, or redox chemistry. Rather, it binds the substrate and forces it into the appropriate conformation for reaction and stabilizes the transition state, without using distinct catalytic groups.

OH

OH

PI

The conversion of [49] into [50] involves a Claisen rearrangement. Once this was realized it was less surprising that no specific catalytic groups on the enzyme are involved. Support for the Claisen-type mechanism comes from the inhibition shown by the bicyclic dicarboxylate [51], prepared by Bartlett and Johnson (1985) as an analogue of the presumed transition state [52]. This same structure [51], coupled through the hydroxyl group to a small protein, was used as a hapten to induce antibodies, one (out of eight) of which mimics the behaviour of chorismate mutase, albeit less efficiently (Table 7).

0 . S. TEE

58

Table 7 Chorismate mutase and a catalytic antibody mimic.“

Enzyme Antibody

3 x loh

10 000

18 260

6 x lo-’’ 3 x lo-*

“Based on data in Jackson el al. (1988).

The catalytic antibody accelerates reaction (38) by a factor of lo4, compared to 3 x lo6 for the enzyme. The superiority of the enzyme is due to the 5000-fold stronger binding of the transition state [52] by the enzyme, attenuated by a factor of 14 due to stronger substrate binding. Note that the inhibitor [51] binds to the enzyme ( K l = 1.5 X lo-’ M) (Bartlett and Johnson, 1985) almost as well as the transition state binds to the antibody. Presumably, therefore, to induce a more efficient abzyme would require a hapten with an even stronger structural resemblance than [5 11 to the transition state [52]. However, as pointed out recently by Wolfenden and Kati (1991), few antibodies have dissociation constants less than 10-“’ M because more efficient binding is not necessary for their natural function. Thus, in many instances it may not be feasible to obtain antibodies with truly enzyme-like transition state binding properties without chemical o r genetically induced modification to introduce additional catalytic functionality (Schultz, 1989a,b). Using the same transition state analogue [51] as the hapten, Hilvert and coworkers (Hilvert et al., 1988; Hilvert and Nared, 1988) have also generated a monoclonal antibody that catalyses the rearrangement of chorismate to prephenate (Table 8). Rate accelerations are relatively modest because of stronger substrate binding and weaker transition state binding than in the previous example (Table 7). The hapten [51] is bound to the antibody ( K I = 6 x lo-’ M ) with a similar strength to that of the transition state which again suggests that use of a better transition state analogue might elicit a more efficient abzyme. From the temperature dependence of K,rs, the free energy of transition state stabilization (AG!;.,) can be dissected into enthalpic and entropic contributions: AH& = 4.49 kcal mol-’; -TAS& = 4.48 kcal mol-’ (at 25°C). Thus, the transition state stabilization is equally enthalpic and entropic in origin. The abzyme also showed very good enantioselectivity, with a 90: 1 preference for the natural (-)-enantiomer of chorismate (Hilvert and Nared, 1988). An example of esterase behaviour is provided by a catalytic antibody developed by Tramontano et al. (1988), using a phosphonate transition state analogue [53] as the hapten. The antibody cleaves the carboxylic ester [54, R = Me] with enzyme-like efficiency (k,lk, = 6.25 x lo6; K M = 1.5 m M ;

TRANSITION STATE STABILIZATION

59

Table 8 Another catalytic antibody mimic of chorismate mutase."

Temp./"C 14.0 25.0 36.0

k,lk,

K d W

KTSIM

250 190 110

49 51 38

2.0 x 10-7 2.7 x 10-7 3.5 x 10-7

"Based on data in Hilvert el al. (1988).

KTs = 2.4 X 1 0 - ' " ~ ) . Interestingly, cleavage of the trifluoracetamido ester [54, R = CF3] is 50 times less efficient (k,lk, = 1.2 x lo5), due to 10-fold stronger substrate binding and to fivefold weaker transition state binding (K,rs = 1.25 x lo-' M), even though the hapten used to elicit the antibody bears a CF3 in the analogous position. Thus, a subtle change (CH3-+ CF3) in the substrate [54], at a site remote from the reaction centre, brings about significant changes in substrate and transition state binding, and in catalytic efficiency, reminiscent of the substrate selectivity often shown by enzymes. 0

CF3CONH

NHCO(CH2)4COOH

RCONH

NHCO(CH2)4COOH

In the last 5 years, catalytic antibodies have been generated for several reaction types, including the various types of hydrolysis, transesterification, amide bond formation, p-elimination, cycloreversion, transacylation, redox reactions, E-2 isomerization, epoxidation, and Diels-Alder reactions. For more information on these and other recent developments, such as semi-synthetic antibodies, site-directed mutagenesis, and the bait-and-switch strategy, the reader should consult the appropriate authorities (Schultz, 1988, 1989a,b; Benkovic et al., 1990; Janda et al., 1990, 1991; Janjic and Tramontano, 1990; Lerner et af., 1991).

0 . S. TEE

60

ENZYMES

As mentioned in the Introduction, various authors have been influenced (directly or indirectly) by the Kurz approach in their discussions of enzyme behaviour (e.g. Wolfenden, 1972; Lienhard, 1973; Jencks, 1975; Schowen, 1978; Fersht, 1985; Kraut, 1988; Wolfenden and Kati, 1991). Also, as noted earlier, the concepts of transition state binding and stabilization were crucial to the development of “transition state analogues” as enzyme inhibitors and hence as chemotherapeutic agents (Jencks, 1969; Wolfenden, 1972; Wolfenden and Frick, 1987; Wolfenden and Kati, 1991). The Kurz approach has already been applied to certain aspects of the behaviour of a-chymotrypsin by Schowen (1978). Here, to provide another example, the focus is on a different point: the binding of amino acid side chains adjacent to the carbonyl of the scissile peptide or ester bond (Fersht, 1985). For the cleavage of a series of eight N-acetyl-L-amino acid methyl esters by a-chymotrypsin at pH7.8 (39), introduction of larger hydrophobic side chains brings about substantial changes in the Michaelis-Menten parameters: kcat increases 1000-fold and K M drops by 1000 (Table A6.8). Thus, for the change from glycine to phenylalanine k,,,lKM, the measure of “enzyme specificity”, increases by a factor of 1 million. Correspondingly, KTS decreases from lo-’ to 10-” M , meaning that binding of the benzyl group of phenylalanine (relative t o that of a hydrogen) lowers the transition state energy by 8 kcal mol-I. AcNHCH(R)COOMe

+ OH- -+

AcNHCH(R)COO-

+ MeOH

(39)

As noted by the original authors (Dorovska et al., 1972), and cited by Fersht (1985), there is an excellent linear correlation between logkc,,lKM and the Hansch hydrophobicity parameters (T)of the side chains (Fig. 9, A), except for the two branched side chains (valine and isoleucine residues). However, since the k , values for the esters do vary somewhat (Table A6.8), the values of pK.rs do not correlate as strongly with v (Fig. 9, B). Moreover, the plot shows distinct curvature which probably indicates the onset of a saturation effect due to the physical limits of the S, binding pocket, adjacent to the enzyme’s active site. Still, the points for valine and isoleucine deviate below the others, suggesting that the pocket has a relatively narrow opening. Overall, the correlations with T provide strong evidence that the S I binding site of a-chymotrypsin is highly hydrophobic, giving rise to a high degree of selectivity for peptide bond cleavage next to aromatic amino acid residues (phenylalanine, tyrosine and tryptophan). Other serine proteases exhibit different selectivities due to differences in the S, site. For example, trypsin cleaves peptide bonds adjacent to lysine or arginine residues because the S , site contains an ionized Asp-189 carboxylate group, in place of the

TRANSITION STATE STABILIZATION

61

12.0 10.0

6.0

8.0 x+

4.0

6.0

2

n

2.0

4.0

m

9

0.0

0.0

0.5

1.0

1.5

2.0

2.5

.2.0

Hydrophobicity (n)

Fig. 9 Correlation of (A) the second order rate constants ( k 2 = k , , , / K M ) and ( B ) the transition stabilization ( ~ K Twith ~ ) the hydrophobicity (a)of the substituent of the amino acid residue for the cleavage of N-acetylamino acid methyl esters by u-chymotrypsin. The open symbols are for the points for two branched residues (valine and isoleucine). Data from Table A6.8.

Ser-189 of chymotrypsin, that favours binding of a positively charged side chain (Fersht, 1985). The efficacy of penicillin-type antibiotics is constrained by the ability of bacteria to induce enzymes for their destruction. In relation to this problem, Page and coworkers (Buckwell el a/., 1988a,b) have studied the hydrolysis of acylated penicillins [55] and cephalosporins [56] catalysed by a bacterial p-lactamase (Tables A6.9 and A6.10). It is noteworthy that the two series of substrates show quite different responses to changes in the length of the acyl side chain (C, to Clz). For the penicillins, which are cleaved much more efficiently, there is a broad maximum in the kinetic parameters around Cs to C , , whereas for the cephalosporins there is a linear increase in k,lKM and

H

I: ' " NOF &d O A cN /

0 . S. TEE

62

pKTs up to C12(Fig. 10). Conceivably, binding of the acyl chains to the enzyme is dissimilar for the two series of lactams since there is a large M for [55] and difference in transition state stabilization: KTS = KTS = lo-" M for [56]. Perhaps, because the transition state for cephalosporin cleavage is bound in a less than optimal orientation for ring-opening, there is greater (and different) sensitivity to the acyl side chain. The discussion of KTS values above is an attempt to show how they may be used to gain insights into transition state binding at or near the active sites of enzymes. For other examples of the explicit or implicit application of Kurz's ideas to enzymes, the reader is directed to the references cited at the start of this subsection and in the Introduction, particularly the reviews by Kraut (1988) and by Wolfenden and Kati (1991). 7 Future prospects

The object of this review was to show how Kurz's approach to quantifying transition state stabilization is useful in the discussion of the kinetic effects of cyclodextrins on organic reactions, while at the same time pointing out its comparable utility for various other types of catalyst. It is hoped that the approach gains wider acceptance and employment since it provides a framework for the discussion of factors affecting transition state stability in both catalysed and retarded reactions. This review will have been of service if it exposes the Kurz approach to a broader audience and particularly if it stimulates other researchers to utilize

16'0

-

r-----7

13'0

t

15.0 -

-

12.0

C

(I)

0. ?

0

w

w VI

?i

0.

c

n

(I)

- 11.0

14.0

2

4

6

8

10

y' n

12

Number of Acyl Carbons Fig. 10 Dependence of transition state stabilization (pKTs) on acyl chain length for the cleavage of 6-acylpenicillins and 7-acylcephalosporins by p-lactamase I . Data from Tables A6.9 and A6.10.

TRANSITION STATE STABILIZATION

63

the approach on an even wider range of catalytic systems. For instance, there appears to be no inherent reason why the approach could not be applied to gas phase reactions, and to catalysis at solid surfaces or other interfaces. However, such applications must wait for others more knowledgeable than the present author. There is room for further analysis in many traditional areas, as pointed out above during the discussion of enolization. Also, it is noted that the employment of transition state pK,f values is very close to the use of the proton activating factors and deprotonating factors, introduced by Stewart (Stewart and Srinivasan, 1978; Stewart, 1985). It is to be hoped that the two approaches can be consolidated in a common view of acid-base catalysis. Enzymes have evolved their awesome efficiency over billions of years. Mankind does not have that much time! In developing new, highly selective, and possibly totally synthetic catalysts, we must use whatever theoretical, practical, and heuristic tools are available to us. The concept of transition state stabilization is one such tool.

Acknowledgements

I sincerely thank my coworkers whose names appear in the references. Our studies have been supported by operating grants and scholarships from the Natural Sciences and Engineering Research Council of Canada. I am also grateful to Professor Toshio Fujita (Kyoto University) for providing the data necessary for the calculation of K.rs for the esters in Tables AS.2 to AS.4.

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TRANSITION STATE STABILIZATION

67

Kurz. J. L. (1972). Acc. Chem. Res. 5, 1 Kurz, J. L., Lee, J., Love, M. and Rhodes, S . (1986). J. Am. Chem. Soc. 108, 2960 Laidler, K. J. (1987). Chemical Kinetics, 3rd edn. Harper & Row, New York Lapworth, A. (1904). J . Chem. Soc. 30 Leatherbarrow, R. J. (1990). Trends Biochem. Sci 15, 455 Leatherbarrow, R. J. and Fersht, A. R. (1987). In Enzyme Mechanism (ed. M. I . Page and A. Williams), Chap. 6. Royal Society of Chemistry, London Lefflet, J. E. and Grunwald, E . (1963). Rates and Equilibria of Organic Reactions, pp. 156ff. Wiley, New York Lehn, J.-M. (1985). Science (Washington, D.C.) 227, 849 Lehn, J.-M. (1988). Angew. Chem. Int. Ed. Engl. 27, 89 Leo, A., Hansch, C. and Elkins, D. (1971). Chem. Rev. 71, 525 Lerner, R. A., Benkovic, S. J. and Schultz, P. G. (1991). Science (Washington, D.C.) 252, 659 Liebman, J. F. and Greenberg, A. (eds) (1988). Mechanistic Principles of Enzyme Activity. VCH, New York Lienhard, G . E . (1973). Science (Washington, D.C.) 180, 149 Logue, M. W., Pollack, R. M. and Vitullo, V. P. (1975). J . Am. Chem. Soc. 97, 6868 Matsui, Y. and Mochida, K. (1979). Bull. Chem. Soc. Jpn 52, 2808 Matsui, T., Nishioka, T. and Fujita, T. (1985). Topics Curr. Chem. 128, 61 McClelland, R. A. and Steenken, S . (1988). J . Am. Chem. SOC.110, 5860 Menger, F. M. (1985). Acc. Chem. Res. 18, 128 Menger, F. M. (1992). Biochemistry 31, 5368 Menger. F. M. and Ladika, M. (1987). J. Am. Chem. SOC. 109, 3145 Menger, F. M. and Venkataram, U. V. (1986). J. A m . Chem. SOC.108, 2980 Ono, K., Tokuda, M. and Murakami, K . (1979). Polymer Reprints, Jpn. 28, 1302 Page, M. I. (ed.) (1984). The Chemistry of Enzyme Action. Elsevier, Amsterdam Page, M. I. (1987). In Enzyme Mechanisms (ed. M. I . Page and A. Williams), Chap. 1. Royal Society of Chemistry, London Page, M. I . and Jencks, W. P. (1987). Gazz. Chitn. Ital. 117, 455 Page, M. I . and Williams, A. (eds) (1987). Enzyme Mechanisms. Royal Society o f Chemistry, London Pagington, J. S . (1987). Chem. Br. 455 Palepu, R. and Reinsborough, V. C. (1988). Can. J . Chem. 66, 325 Palepu, R., Richardson, J. E. and Reinsborough, V. C. (1989). Langmuir 5, 218 Pauling, L. (1946). Chem. Eng. News 24, 1375 Pollack, R. M. (1978). In Transition States in Biochemical Processes (ed. R. D. Gandour and R. L. Schowen), Chap. 12. Plenum, New York Pregel, M. J., Dunn, E. J. and Buncel, E. (1990). Can. J. Chem. 68, 1846 Pruszynski, P., Chiang, Y., Kresge, A. J., Schepp, N. and Walsh, P. A. (1986). J. Phys. Chem. 90, 3760 Ramamurthy, V. (1986). Tetrahedron 42, 5785 Ramamurthy, V. and Eaton, D . F. (1988). Acc. Chem. Res. 21, 300 Reichardt, C . (1988). Solvents and Solvent Effects in Organic Chemistry. V C H , Weinheim Ruasse, M.-F. (1990). Acc. Chem. Res. 23, 87 Ruasse, M.-F. (1992). Adv. Phys. Org. Chem. 28, 207 Ruasse, M.-F., Motabelli, S. and Galland, B. (1991). J. A m . Chem. Soc. 113, 3440 Saenger, W. (1980). Angew. Chem. Int. Ed. Engl. 19, 344. Sanemasa, I. and Akamine, Y. (1987). Bull. Chem. Soc. Jpn. 60,2059 Sanemasa, I . , Takuma, T. and Deguchi, T. (1989). Bull. Chem. SOC.Jpn 62, 3102 Sanemasa. I . , Osajima, T. and Deguchi, T. (1990). Bull. Chem. Soc. Jpn 63, 2814

68

0 . S. TEE

Satake, I., Ikenoue, T., Takeshita, T., Hayakawa, K. and Meda, T. (1985). Bull. Chem. SOC.Jpn 58, 2746 Satake, I., Yoshida, S., Hayakawa, K., Meda, T. and Kusomoto, Y. (1986). Bull. Chem. SOC.Jpn 59, 3991 Schowen, R. L. (1978). In Transition States in Biochemical Processes (ed. R. D . Gandour and R. L. Schowen), Chap. 2. Plenum, New York Schultz, P. G. (1988). Science (Washington, D.C.) 240, 426 Schultz, P. G. (1989a). Acc. Chem. Res. 22, 287 Schultz, P. G. (1989b). Angew. Chem. lnt. Ed. Engl. 28, 1283 Siegel, B. and Breslow, R. (1975). J. Am. Chem. SOC. 97, 6869 Sirlin, C. (1984). Bull. SOC.Chim. Fr. 11-5 Smith, R. H. (1972). Aust. 1. Chem. 25, 2503 Steenken, S., Buschek, J. and McClelland, R. A. (1986). J . Am. Chem. SOC.108, 2808 Sternbach, D. D. and Rossana, D. M. (1982). J . Am. Chem. SOC.104, 5853 Stewart, R. (1985). The Proton: Applications to Organic Chemistry. Academic Press, Orlando, FL Stewart, R. and Srinivasan, R. (1978). Acc. Chem. Res. 11, 271 Stoddart, J. F. (1987). In Enzyme Mechanisms (ed. M. I . Page and A. Williams), Chap. 3. Royal Society of Chemistry, London Stoddart, J. F. and Zarzycki, R. (1988). Recl. Trav. Chim. Pays-Bas 107, 515 Straub, T. S. and Bender, M. L. (1972a). J. Am. Chem. SOC. 94, 8875 Straub, T. S. and Bender, M. L. (1972b). J . Am. Chem. SOC.94, 8881 Suh, J. (1992). Acc. Chem. Res. 25, 273 Szejtli, J. (1982). Cyclodextrins and their Inclusion Complexes. Akademiai Kiado, Budapest Tabushi, I. (1982). Acc. Chem. Res. 15, 66 Tagaki, W. and Ogino, K. (1985). Topics Curr. Chem. 128, 143 Takasaki, B. K. and Tee, 0. S. (1989). Can. J . Chem. 67, 193 Tanaka, S . , Uekama, K. and Ikeda, K. (1976). Chem. Pharm. Bull. 24, 2825 Tanford. C. (1980). The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd edn, Chaps 2, 3 and 6. Wiley, New York Tapuhi, E. and Jencks, W. P. (1982). J . Am. Chem. SOC.104, 5758 Tee, 0. S. (1989). Carbohydr. Res. 192, 181 Tee, 0. S. and Bennett, J. M. (1984). Can. J. Chem. 62, 1585 Tee, 0. S. and Bennett, J. M. (1988a). J . Am. Chem. SOC.110, 269 Tee, 0. S. and Bennett, J . M. (1988b). J . Am. Chem. SOC. 110, 3226 Tee, 0. S. and Bozzi, M. (1990). J . Am. Chem. SOC. 112, 7815 Tee, 0. S. and Du, X.-X. (1988). J. Org. Chem. 53, 1837 Tee, 0. S. and Du, X.-X. (1992). J. Am. Chem. SOC. 114, 620 Tee, 0. S. and Enos, J. A. (1988). Can. J. Chem. 66, 3027 Tee, 0. S. and Hoeven, J . J. (1989). J . Am. Chem. SOC.111, 8318 Tee, 0. S. and Iyengar, N. R. (1985). J. Org. Chem. 50, 4468 Tee, 0. S. and Iyengar, N. R. (1988). Can. J . Chem. 66, 1194 Tee, 0. S. and Iyengar, N. R. (1990). Can. J. Chem. 68, 1769 Tee, 0. S. and Javed, B. (1993). Submitted for Publication Tee, 0. S. and Paventi, M. (1981). J. Org. Chem. 46,4172 Tee, 0. S. and Takasaki, B. K. (1985). Can. J. Chem. 63,3540 Tee, 0. S., Thackray, D. C. and Berks, C. G. (1978). Can. J . Chem. 56, 2970 Tee, 0. S., Iyengar, N. R. and Bennett, J. M. (1986). J . Org. Chem. 51, 2585 Tee, 0. S., Iyengar, N. R. and Takasaki, B. K. (1993a). Can. J . Chem., in press Tee, 0. S., Paventi, M. and Bennett, J. M. (1989). J . Am. Chem. SOC.111, 2233 Tee, 0. S . . Javed, B. and Mikkelesen, S. R. (1990a). Can. J . Chem. 68, 2119

TRANSITION STATE STABILIZATION

69

Tee, 0. S., Mazza, C. and Du, X.-X. (1990b). J . Org. Chem. 55, 3603 Tee, 0. S., Bozzi, M., Hoeven, J. J. and Gadosy, T. A. (1992). Manuscript in preparation Tidwell, T. T. (1990). Acc. Chem. Res. 23, 273 Toullec, J . (1982). Adv. Phys. Org. Chem. 18, 1 Trainor, G. L. and Breslow, R. (1981). J . A m . Chem. Soc. 103, 154 Tramontano, A,, Amman, A. A. and Lerner, R. A. (1988). J. A m . Chem. Soc. 110, 2282 Tutt, D. E. and Schwartz, M. A. (1971). J . A m . Chem. Soc. 93, 767 VanderJagt, D. L., Killian, F. L. and Bender, M. L. (1970). J . A m . Chem. Soc. 92. 1016 VanEtten, R. L., Sebastian, J. F., Clowes, G. A. and Bender, M. L. (1967a). J. Am. Chern. SOC. 89, 3242 VanEtten, R. L., Clowes, G. A., Sebastian, J. F. and Bender, M. L. (1067b). J. Am. Chem. Soc. 89, 3253 Van Hooidonk, C. and Breebart-Hansen, J. C. A. E. (1970). Rec. Trav. Chim. (Pays-Bas) 89, 289 Walsh, C. (1979). Enzymatic Reaction Mechanisms. W. H. Freeman, San Francisco Williams, A. (1984). In The Chemistry of Enzyme Action (ed. M. I. Page), Chap. 5. Elsevier, Amsterdam Williams, A. (1992). Adv. Phys. Org. Chem. 27, 1 Wolfenden, R. (1972). Acc. Chem. Res. 5 , 10 Wolfenden, R. and Frick, L. (1987). In Enzyme Mechanisms (ed. M. I. Page and A. Williams), Chap. 7. Royal Society of Chemistry, London Wolfenden, R. and Kati, W. M. (1991). Ace. Chem. Res. 24, 209

Note added in proof: The author has been made aware of several more references which treat transition state stabilization by metal ions in the Kurz manner (Rudakov er al., 1974; Illuminati et al., 1983; Ercolani et al., 1983; Mandolini and Masci, 1984; Galli and Mandolini, 1984) and that the Kurz approach was discussed briefly in the second edition of Hammett’s famous book (Hammett, 1970). In another area, a recent special issue of Accounts of Chemical Research (vol. 26, #8, pp. 389-453 (1993)), which is devoted to “Chemistry and Immunology”, has several articles on catalytic antibodies. Ercolani, G., Mandolini, L. and Masci, B. (1983). J . A m . Chem. SOC. 105, 6146 Galli, C. and Mandolini, L. (1984). J . Chem. SOC. Perkin Trans. 2 1435 Hammett, L. P. (1970). Physical Organic Chemistry, 2nd edn. McGraw-Hill, New York, pp. 13g-140 Illuminati, G., Mandolini, L. and Masci, B. (1983). J. Am. Chem. SOC. 105, 555 Mandolini, L. and Masci, B. (1984). J. Am. Chem. Soc. 106, 168 Rudakov, E. S., Kozhevnikov, I. V. and Zamashchikov, V. V. (1974). Russ. Chem. Rev. (Engl. Transl.) 43, 305

0 . S . TEE

70

Appendix

The following tables are organized and numbered in relation to the section of the main text in which the data are discussed, to facilitate their location and cross-referencing. Thus, Table A4.1 is the first table of data in this Appendix that is referred to in Section 4, and so on. For the most part reaction conditions are omitted since the reactions were carried out in wholly or largely aqueous solution at or near 25°C. With few exceptions, the original literature cited does not contain KTS values; they were calculated specifically for the present review.

Table A4.1 Catalysis of decarboxylation by /3-cyclodextrin.

Phenylcyanoacetate anions' 4-Me0 4-Me H 4-CI 4-CI" 4-C1" 4-Br 3-Me 2-Me 2-CI Benzoylacetic acids" H 2-Me 3-Me 4-Me 3-CI 4-CI 4-NO2

15.9 12.7 18.7 23.3 33.3 44.2 16.6 15.8 12.0 19.8

17.6 15.7 39.5 17.6 12.5 10.6 8.54 37.3 67.8 29.8

1.11 1.24 2.11 0.755 0.375 0.240 0.514 2.36 5.65 1.51

7.6 4.1 7.5 6.8 6.1 5.2 2.2

9.8 15 7.6 4.7 6.0 6.8 8.4

1.3 3.7 1.0 0.69 0.98 1.3 3.8

"From the data of Straub and Bender (1972a); at 60.4"C. "At 45.4"C. 'At 35.4"C. "Based on data from Straub and Bender (1972b); at 50.3"C and pH = 3 (25°C).

TRANS IT1ON STATE STAB ILlZATlON

71

Table A4.2 Catalysis of bromine attack on phenols and phenoxides by acyclodextrin."

Substrate

k Z u l ~ -s-''

k 3c 1M - Z s - l

KslrnM

KTSIrnM ~~

Phenols H 2-Me 2,6-diMe 2-Br 4-Me 4-t-BU 4-Br 4-COzEt 4-CN

4.1 x lo5 1.5 x loh 1.2 x 106 1.0 x lo4 6.6 x 10' 5.9 x 105 3900 1600 160

Phenoxides 2-NOl 2-Br 3-NO2 4-NOl 4-Br 4-CN

1.6 x 6.2 X 4.2 x 1.2 x 5.4 x 3.1 x

10' 10' 10' 10' 10' 10'

3.5 x 2.2 x 1.2 x 6.7 x 2.4 x 8.9 x 8.5 x 3.4 x 4.4 x 5.7 x 7.9 x 2.8 X 4.8 x 2.0 x 5.8 x

50 4.3 15 52 83 7.0 1.4 4.8 7.1

10"

10"' 10"' 107 10' 10' 10'

loh 10s

10'2 10'2 10"

10'2 10" 10'2

=40 -110 4.2 0.47 1.2 1.6

~

0.12 0.068 0.10 0.15 0.28 0.66 0.46 0.46 0.35 0.28 0.79 0.15 0.25 0.27 0.54

"Based on data from Tee and Bennett (1988a).

Table A4.3 Debromination of 4-alkyl-4-bromo-2,5-cyclohexadienones catalysed by a-cyclodextrin."

Alkyl

kck

KslrnM

KrsImM

Me Et i-Pr n-Pr t-Bu 3.4-diMe 4-Me, 2-COO-

78 3Y 23 12 28 29 170

4.8 2.9 2.4 0.75 2.3 3.6 15

0.062 0.074 0.10 0.063 0.083 0.12

0.088h

"Based on data from Tee and Bennett (1988b). From the rate constants (at fixed [ H i ] and [Br-1) for reaction of the dienone ( k , ) and the apparent rate constant for the dienone.CD complex ( k c ) ; K.rs = k , K s / k , . In actuality, the reaction is believed to take place between C D . B r ~ and S. for which the ratios k J k , are much higher ( 2 4 W 6 0 0 ) . in which case K,.s = k , K , l k , , where K , = 0.286 M is for the dissociation of the CD.Br- complex. hBased on data from Takasaki and Tee (1989).

0. S. TEE

72

Table A4.4 Catalysis of the attack of bromine by a-cyclodextrin."

Substrate

k Z u l M - ' s-

'

k&C2 s-'

KSImM

KTSImM

4-Fluorophenol 4-Chlorophenol 4-Bromophenol" 4-Iodophenol 5-Bromosalicylate 5-Sulphosalicylate 5-Nitrosalicylate 2-Pyridone N-Methyl-2-pyridone 4-Pyridone N-Met hyl-4-pyridone Furan-2-COOFuran-3-COOThiophene-2-COO2-MeOPhCOO4-MeOPhCOOPhOCH2COOPhO(CH3)CHCOOHCOOH HCOO-

4350 3600 3900 5500 1.5 X lo' 1.1 x 10' 1.6 x 104 1.8 x 104 3.45 x 104 9100 9500 2.3 x 104 5.2 x 104 1350 5500 180 1.6 x 104 3.0 x 104 0.94 32

1.1 x 107 8.4 X 10' 8.5 x lo6 1.9 x 107 7.3 x lox 1.8 x 10' 2.4 x 107 1.9 x 107 2.8 x 107 1.3 x lo7 1.5 x 107 2.1 x 107 7.3 x 107 4.5 x 10' 1.2 x 107 8.5 x los 4.7 x 107 5.8 x 107 5100 1.75 x 105

120 3.6 1.4 0.47 4.7 37 4.7 9.7 1.9 9.9 1.2 4.4 5.0 0.52 0.87 0.45 0.66 0.90 high high

0.40 0.43 0.46 0.29 0.21 0.61 0.67 0.95 1.2 0.70 0.63 1.1 0.71 0.30 0.46 0.21 0.34 0.52 0.18 0.18

"Bascd on data from Javed (1990). Tee et al. (1990a) and Tee and Javed (1993). The anions wcrc studied at pH values chosen to ensure that the correct form was reacting and in the light of earlier work. 'Same data as in Table A4.3. 'Reacting via the anion, at low pH.

Table A5.1

Basic cleavage of m- and p-X-phenyl acetates by cyclodextrin." ~~

Substituent

H p-Me p-t-Bu p-NOz p-CO, m-Me m-CI m-Et m-t-Bu tn-NO, m-COT P-NO2 m-CI m-Et tn-NOZ m-t-Bu p-t-Bu m-t-Bu

CD

k,lku

KslmM

KTS/mM

a a a a a

27 3.3 1.1 3.4 5.3 95 156 240 260 300 68 9.1 24 89 96 250 55 87

22 11 6.5 12 150 17 5.6 11 2.0 19 105 6.1 3.5 2.2 8.0 0.13 4.0 9.9

0.81 3.3 5.9 3.5 28 0.18 0.036 0.046 0.0077 0.063 1.5 0.67 0.15 0.025 0.083 0.00052 0.073 0.11

Ly

a a

a (Y

a

P P P P P Y Y

"Bascd o n the data of VanEtten et a/. (1967a).

TRANSITION STATE STABILIZATION

Table A5.2

X H Me Et n-Pr i-Pr n-Bu t-Bu n-Pen OMe OEt F CI Br I NO2 COMe COEt

73

Cleavage of p-X-phenyl acetates by P-cyclodextrin at pH 10.6." 1O4kUls-' 7.34 6.72 6.89 6.63 6.99 5.49 5.81 5.98 7.70 8.03 12.3 16.2 16.3 16.0 65.9 28.1 25.8

100kcls-'

k,lku

KSImM

KTSImM

1.0 0.65 0.32 0.27 0.16 0.31 0.089 0.55 0.41 0.22 2.1 2.2 1.7 0.87 9.0 1.3 0.9

14 10 5 4 2 6 2 9 5 3 17 14 10 5 14 5 3

7.2 3.2 1.5 0.70 1.3 0.24 0.14 0.16 3.5 2.9 7.8 3.2 2.1 0.99 7.5 6.8 4.8

0.53 0.33 0.32 0.17 0.57 0.042 0.091 0.017 0.66 1.1 0.46 0.24 0.20 0.18 0.55 1.5 1.4

"Based on data provided by Fujita (1988). See also Matsui el al. (1985).

Table A5.3 Cleavage of m-X-phenyl acetates by P-cyclodextrin at pH 9.7."

Hh Me Et n-Pr i-Pr n-Bu i-Bu t-Bu' OMe OEt Oi-Pr F CI Br I CN NO:! CHO COMe COEt

7.34 0.967 0.886 0.739 0.758 0.729 0.660 4.9 1.23 1.10 1.06 2.17 2.01 2.47 2.12 5.19 7.25 2.38 4.60 2.17

1.0 0.24 0.69 0.86 1.2 0.67 1.2 12.2 0.34 0.47 0.95 0.43 0.70 0.95 1.6 2.4 6.6 1.1 2.3 3.9

14 25 78 116 158 92 182 249 28 43 90 20 35 38 7.5 46 91 46 50 180

7.2 4.0 1.8 0.50 0.42 0.22 0.15 0.13 5.1 2.5 1.4 5.9 2.7 1.8 0.79 8.5 6.2 6.4 4.8 3.1

0.53 0.16 0.023 0.0043 0.0027 0.0024 0.00082 0.000.52 0.18 0.059 0.016 0.30 0.078 0.047 0.011 0.18 0.068 0.14 0.096 0.017

"Based on data provided by Fujita (1988). See also Matsui et al. (1985). 'Data at pH 10.6; taken from Table A5.2. 'Taken from Table AS.l.

0. S . TEE

74

Table A5.4

H Me Et n-Pr i-Pr n-Bu i-Bu S-BU t-Bu" OMe OEt Oi-Pr F CI Br I CN NO? CHO

COMe'

COEt

Cleavage of m-X-phenyl acetates by a-cyclodextrin at pH 9.7."

1.04 0.967 0.886 0.739 0.758 0.729 0.660 0.663 0.607 1.23 1.10 1.06 2.17 2.01 2.47 2.12 5.19 7.25 2.38 4.60 2.17

0.423 1.42 1.95 1.01 0.924 0.920 0.474 0.490 0.658 2.34 1.62 0.710 2.27 4.48 4.17 3.68 14.3 19.0 8.62 2.14 2.86

41 147 220 137 122 126 72 74 108 190 147 67 105 223 169 174 276 262 362 47 132

22.7 21.5 10.2 3.61 16.3 1.37 14.0 2.86 38.1 24.6 14.2 28.0 32.2 4.99 1.56 0.48 13.8 14.3 33.5 57.7 35.9

0.56 0.15 0.046 0.026 0.13 0.01 1 0.19 0.039 0.35 0.13 0.096 0.42 0.31 0.022 0.0092 0.0028 0.050 0.054 0.093 1.24 0.27

"Based on data provided by Fujita (1988). See also Matsui et al. (1985). "Values differ from those of VanEtten el al. (1967a) in Table AS.l. "Values for this substituent appear out of line.

TRANSITION STATE STABILIZATION

75

Table A5.5 Cleavage of aromatic esters in the presence of cyclodextrins." ~

Ester PhCOOEt P-NO2 p-CI p-Me PhCH =CHCOOEt m-Me p-Me m-NOz P-NOz m-CI p-CI p-CN

Ks/mM

P P P P

0.06 0.26 0.12 0.10

4.1 17 4.5 2.0

68 65 37 20

a a a a a a

0.09 0 0 0.48 0.22 0.3 0 0

5.3 5.5 3.7 6.7 11.8 8.0 10.0 7.8

59 Large Large 14 54 27 Large Large

0.54 0.43 0.45 1.47 2.01 0.62 0.65 1.34 1.47

8.5 4.2 9.6 13.4 6.0 4.5 5.8 7.0 4.0

16 9.9 21 9.1 3.0 7.3 8.9 5.2 2.7

0.10

8.1

81

0.53

6.5

12

0.46

1.95

a

P P P P P P P P P P P P

Ph(CH&COOEt Ph(CHz),COOEt "Based on the data of Tanaka

kAk"

a

PhCH=CHCOOEt m-Me p-Me m-NO2 P-NO2 m-Me0 p-Me0 p-CI p-CN PhCHzCOOEt

el

~~~~

CD

af. (1976).

KTS/mM

4.2

0 . S. TEE

76

Table A5.6

Effect of capping on the cleavage of esters by P-cyclodextrin."

Ester m-NO2C6H40Ach

P

rn-t-BuC6H40Ach

P

pNP Ad-propiolate"

P-(NMeCH0)7 P-(NEtCHO)T P-(NMeCH0)7

P

/3-(NMeCH0)7 pNP t-Bu-Ad-propiolate" P pNP Fc-propiolate" P pNP Fc-acrylate" P P-(NMeCH0)7 P-caP m-NO2C6H40Acd P P-S-Me P-S-t-BU P-cap' p-N02C6H40Acd P P-S-Me P-S-t-Bu /3-cap'

64 660 1140 365 3 300 2 150 14 000 15 000 1.4 x 10' 7.5 x 105 2.4 x 10' 10" 72 123 25 6.5 7.7 10.6 7.9 3.9

5.3 5.1 26 0.1% 0.46 0.30 2.7 1.8 5.0 7.0 2.0 7.5 6.1 6.6 0.82 0.11 4.8 4.7 0.33 0.012

8.3 x 7.7 x 2.3 x 5.3 x 1.4 x 1.4 x 1.9 x 1.2 x 3.6 x 9.3 x 9.8 x 7.5 x 8.5 x 5.4 x 3.3 x 1.7 x 6.2 x 4.4 x 4.2 x 3.2 x

10-j 10P 10-7 10-7 10-7 10-7 lo-' 10-9 lo-? lo-' 10-4

lo-' lo-'

"Abbreviations: pNP, p-nitrophenyl; Ad, adamantyl; Fc, ferrocenyl; p-cap, p-CD capped on the primary side by -OSO2C,H4OC6H,SO3-; p-cap', capping by -OS02C6H4CH2C,H.,S03-. "Emert and Breslow (1975); in aqueous solution. 'Breslow ef a/. (1980); in 60% (vlv) DMSO/water. at 30°C. "Fujita ef al. (1980); in aqueous solution.

Table A5.7

Cleavage of 4-carboxylphenyl alkanoates by a-cyclodextrin."

5.3 0.68 0.19 "Based on the data of VanEtten ef a/. (1967a).

150 12 1.1

28 18 5.8

TRANSITION STATE STABILIZATION

77

Table A5.8 Cleavage of p-nitrophenyl alkanoates by a- and P-cyclodextrin." Ester

CD

Acetate Butanoate Hexanoate Octanoate Dodecanoate Acetate Butanoate Hexanoate Octanoate Dodecanoate

ff

a ff ff ff

P P P P P

kJk,

KSImM

3.2 1.6 2.5 3.6 11 12.2 8.2 5.8 9.8 67

10.5 4.8 2.0 0.98 0.37 6.5 3.9 2.3 1.9 0.75

KTs/mM

3.3 3.0 0.80 0.27 0.035 0.53 0.48 0.40 0.19 0.011

"Based on the data of Bonora ef al. (1985).

Table A5.9

~

Cleavage of nitrophenyl alkanoates by

(Y-

and P-cyclodextrin."

~~

rn-Nitrophenyl Acetate Propanoate Butanoate Pentanoate Hexanoate

290 110 110 70 83

25 6.5 5.4 4.1 3.5

0.086 0.059 0.049 0.058 0.042

p-Nitrophenyl Acetate Propanoate Butanoate Pentanoate Hexanoate

2.8 110 110 70 82

10 6.5 5.4 4.1 3.5

3.6 6.0 2.6 1.6 0.94

rn-Nitrophenyl Acetate Propanoate Butanoate Pentanoate Hexanoate

62 43 34 24 27

12

3.7 2.4 1.8

0.19 0.12 0.11 0.10 0.067

7.8 5.2 2.7 2.0 1.3

0.96 1.10 0.57 0.53 0.35

p-Nitrophenyl Acetate Propanoate Butanoate Pentanoate Hexanoate "Data from Tee

8.1 4.7 4.7 3.8 3.7 ef

al. (1990b).

5.2

0 . S. TEE

78

Table A5.10

Cleavage of 4-carboxy-2-nitrophenyl alkanoates [22] by cyclodextrins.a

Acetate Propanoate Butanoate Pentanoate Hexanoate Heptanoate Octanoate 2-Ethylhexanoate 4-Methylpentanoate Acetate Propanoate Butanoate Pentanoate Hexanoate Heptanoate Octanoate 2-Ethylhexanoate 4-Methylpentanoate

1.8 1.2 0.48 0.77 1.4 2.4 3.4 1.9 0.69 2.9 0.66 0.27 0.14 0.28 0.55 1.2 0.09 0.18

a

a a

a a a a a

a

P P P P P P P P P

9.6 17 7.7 2.1 1.4 1.1 0.50 2.3 1.2 6.5 5.5 1.5 0.76 0.38 0.27 0.79 0.45 0.26

5.3 14 16 2.8 1.o 0.46 0.15 1.2 1.7 2.2 8.3 5.6 5.4 1.4 0.49 0.66 4.8 1.4

"Data from Tee and Du (1992). K s = K , for 1 : 1 binding. With a-CD, 2: 1 binding was also observed. With p-CD, a second-order process (19) intrudes at high [CD] which may result from 2 : 1 binding (see main text p. 36 et seq).

Table A5.11 Basic cleavage of diaryl carbonates and diaryl methylphosphonates by cyclodextrins.'

(PhO);?CO (y-N02ChH40)2CO (PhO),P(O)Me

P P P

(p-N02C6H40)2P(0)Me

P

(rn-N02ChH40)2P(0)Me

P

a a

a

"Based on the data of Brass and Bender (1973).

2.3 7.5 16 35 19 8.4 41 66

7.3 15 1.4 38 4.6 31 3.5 95

3.2 2.0 0.088 1.1 0.24 3.7 0.085 1.4

TRANSITION STATE STABILIZATION

Table AS.12 trins."

79

Cleavage of p-nitrophenyl acetate and its thio analogue by cyclodex-

a a

P P

3.5 5.4 9 17

12 4.8 6.1 3.0

3.4 0.89 0.67 0.18

"Based on the data of Komiyama and Bender (1980).

Table AS.13 Constants for the cleavage of p-nitrophenyl acetate by a-CD in the presence of potential inhibitors (PI)."

PI

Kl/niM

Alcohols i-Pr n-Pr S-BU i-Bu c-Pen c-Hex i-Pen n-Bu 2-Pen n-Pen 2-Hex n-Hex n-Hep Alkanesulphonate ions C4 Ci Ch C7

CH Alkanoate ions C3

C4

C5 Ch

k , l M - ' s-'

k&-I

s-'

KTSImM

200 43 38 36 22 15 13 11 7.4 3.1 2.8 1.1 0.44

0.33 1.8 1.9 1.5 2.2 2.5 4.5 6.0 9.1 22 28 75 145

6.7 7.4 7.1 5.2 4.8 3.8 6.1 6.7 6.7 6.7 7.9 8.3 6.3

800 150 140 180 120 110 59 44 29 12 9.5 3.6 1.8

22.9 6.37 2.64 1.48 0.93

1.28 5.90 13.2 26.2 29.2

2.89 3.72 3.45 3.83 2.68

209 45.3 20.2 10.2 9.14

596 90.5 16.3 3.80

0.190 1.81 2.85 12.1

11.2 16.2 4.60 4.56

1410 148 93.7 22.1

"Data of Tee el ul. (1YY3b). Values of k, are for reaction (21); values of k b ( = k ; K , / K , ) are for reaction (24). Values of KTs = k,/k, for (25), where k , = 0.267s-'.

0. S . TEE

80

Table A514 Constants for the cleavage of p-nitrophenyl acetate by p-CD in the presence of potential inhibitors (PI)."

Alcohols n-Pr i-Pr S-Bu n-Bu s-Pen i-Bu t-Bu n-Pen s-Hex c-Pen i-Pen n-Hex c-Hex neo-Pen n-Hep Alkanesulphonate ions c4

c 5

Ch

c 7

CH

CI,, Alkanoate ions c 4 c 5

C6 CX c 7

cs CH

269 263 65 60 32 24 21 16 10.5 8.3 5.6 4.6 2.0 1.74 1.41

2.8 2.9 6.1 6.3 7.7 10 13 15 22 26 48 39 81 83 58

89 16.7 5.6 2.3 0.97 0.24

3.0 11.5 33 60 90 260

260 74 16 15 5.9 4.6 2.3

2.2 5.2 21 25 46 64 82

94 95 50 48 31 31 34 30 29 27 34 22 21 18 10

240 230 110 105 86 66 51 44 30 25 14 17 8.1 8.0 11

34 24 24 17 11 7.8

220 57 20 11 7.3 2.5

74 49 43 46 35 37 24

300 130 31 26 14 10 8.0

"Data of Tee ef al. (1993b). Values of k , are for reaction (21); values of kh ( = k ; K , / K , ) are for reaction (24). Values of KTs = k J k , for (25). where k , = 0.66s-'. C,* is 4methylpentanoate ion; C: is the cyclohexanecarboxylate ion.

TRANSITION STATE STABILIZATION

81

Table A5.15 Constants for basic cleavage of p-nitrophenyl hexanoate in presence of /3-cyclodextrin and alcohols, R-OH."

270 260 65 60 32 24 21 16 8.3 4.6 2.0 1.7

n-Pr i-Pr S-BU n-Bu 2-Pen i-Bu t-Bu n-Pen c-Pen n-Hex c-Hex neo-Pen

370 415 116 86 81 44 45 38 16 9.8 5.8 3.3

0.51 0.60 0.41 0.43 0.30 0.36 0.34 0.39 0.37 0.24 0.28 0.20

1.4 1.4 3.6 5.1 3.7 8.2 7.7 10 23 25 48 61

100 95 38 27 37 17 18 14 6.0 5.6 2.9 2.3

"Data from Tee and Bozzi (1990). Values of K , and k , are for reaction (26). The other constants are: k , = k , / K , (for S.CD + PI+ P) and K . , , = k J k , = k,K,ik,.

Table A5.16

Basic cleavage of penicillins [31] by /3-cyclodextrin."

Substituent, R Me n-Pen n-Non PhCHz Ph2CH PhX 1-Np' 2-Np' 4-PhC6H4 2-PhChH4

kJk, 37 66 47 77 34 40 31 89 54 63

KslmM

33 41 21 43 4.7 3.85 16 75 38 13

"Based on data of Tutt and Schwartz (1971); at 31.5"C. 'Np, naphthyl.

KpJmM 0.89 0.62 0.45 0.56 0.14 0.096 0.52 0.84 0.70 0.21

0 . S.TEE

82

Table A5.17

Catalysis of the deprotonation of P-keto esters by cyclodextrins."

Substrate MeCOCH2CO2Me MeCOCH2COzEt MeCOCH2COzallyl EtCOCH2C02Me EtCOCH2CO2Et PrCOCHZC02Et i-PrCOCH2C02Et' Cyclopentanone-2-COOEt MeCOCH2C02Et EtCOCHZCO2Et PrCOCH2C02Et i-PrCOCH2C02Et

CD

kclku

KslrnM

a

-h

-h

7.5 8.4 -b 6.7 12 4.6 18 2.9 1.9 4.9 2.9

3.1 0.22 -h 1.8 3.4 1.1 11 6.3 2.4 11 6.6

a

a a a a

a

a

P P

P P

KrsImM 5.8' 0.41 0.026 3.Xh 0.26 0.29 0.25' 0.61 2.2 1.3 2.2 2.3

"Data from Tee el ul. (1993a). hSaturation kinetics were not observed; the plot of k"h'dvs. [CD] was linear. Thus, kz was obtained from the slope and K,, from k,,/kz (9). 'The entry for this compound in Tee (1989) was incorrect due to a transcription error.

Table A6.1 Catalysis of the ketonization of two enols." HA

PKHA

Acetophenone enolh H,O+ HW4 NCCHZCOOH ClCHzCOOH MeOCHzCOOH HCOOH CHTCOOH CHTCHZCOOH

-1.74 2.15 2.47 2.87 3.57 3.75 4.76 4.88

lsobutyrophenone enoF HJO+ NCCHZCOOH ClCIIzCOOH MeOCHzCOOH HCOOH CHJCOOH CHJCH~COOH

-1.74 2.47 2.87 3.57 3.75 4.76 4.88

k H A 1 M - I S-'

1250 462 90.1 77.7 25.0 18.8 4.53 9.84 2.14 0.0599 0.0445 0.0109 0.0124 0.0030 0.0027

10-'k;lA/M-' 3800 4240 234 214 91.9 109 46.2 40.9 303 5.17 4.57 2.42 3.37 0.773 0.739

s-'

PfG 3.86 3.38 3.93 3.90 3.77 3.58 3.33 3.72 3.63 3.84 3.77 3.43 3.37 3.37 3.34

"The pK; values are calculated from K i = k i t , . K F l k H A .where K: is for the enol. "Based o n the data of Chiang el al. (1988); enol pKF = 10.34. 'Based on the data of Pruszynski et ul. (1986); enol pKF = 11.78.

TRANSITION STATE STABILIZATION

Table A6.2

83

Decarboxylation of a,a-dimethylbenzoylaceticacids.O

Substituent

PK,

P g

4-Me0 H 4-CI 4-NO2

3.43 3.40 3.38 3.24

4.78 4.44 4.02 3.16

"Taken from Pollack (1978); the pK, refers to substrate acid.

Table A6.3

Transition state pK, for the attack of water on carbocations."

Alkyldialkoxy cations Et(Me0)2C+ Me(MeO)zC+ i-Pr(MeO)&+ Et (Et0)2C+ Me(Et0)2C+ Me(i-PrO)zC+

50 000 30 000 67 000 28 000 28 000 2 000

1 800 2 850 1940 4 290 3 290 9 000

10.75 10.55 10.71 10.37 10.48 10.05

Trialkoxy cations CF,CH,O(EtO)zC+ (Me0)C EtO(Me0)2C+ i-PrO(Me0)2C+ (Et0)3C+ (i-PrO),C+

10 000 1400 400 98 67 1

15 000 40 700 143 000 184 000 143 000 360 000

9.82 9.39 8.85 8.74 8.84 8.44

"Based on the data of Steenken ef al. (1986) and McClelland and Steenken (1988)

Alkali metal ion catalysis of ethoxide attack on: (a) p-nitrophenyl diphenylphosphinate;" and (b) p-nitrophenyl benzenesulphonate.h

Table A6.4

M+

Ki,/mM

Li Na+

4.72 9.80 11.1 8.26

+

K+

cs+

KTSIrnM

0.193 0.829 2.88 3.76b

7.51 3.52 2.32 2.44

"Dunn and Buncel (1989); Kipis for dissociation of EtO-M+ ion pairs in anhydrous ethanol; KTs is for dissociation of M* from the transition state. 'Pregel et al. (1990).

0 . S . TEE

84

Table A6.5 Electrophilic catalysis of the enolization of 2-acetylpyridine."

MI1+

H+ Zn2

NiZ+

+

cuz+

KrsIM

k2clk2u

KshM

207 4 470 10 500 20 300

2.29 28 2.04 1.33

1.11 x 6.27 X 1.94 x 6.55 x

lo-' lo-" 10-7 lo-'

"Based on the data of Cox (1974). For acetate attack on the substratc k Z a = 7.25 x 10 M I s '; for acetate attack on the substrate bound to the ion, kzC = 0.00150. 0.0324, 0.076 and 1 . 4 7 ~ - ' s - I ,for the catalysts in order.

'

Table A6.6 Basic cleavage of p-nitrophenyl alkanoates by amylose.u

Valerate Caprylate Laurate Palmitate

5 8 12 16

"Based on the data of Hui

Table A6.7

x 2-NO2-4-CI 4-NO2 2-NO2 4-CN 3-NO2 3-CI 4-CI H 4-Me

CI

0.45 0.22 0.073 0.036

4.3 5.5 11.8 174

1.0x 10-4 4.0 X lo-' 6.2 x 2.1 x 10-7

al. (1982). N is the number of acyl carbon atoms.

Basic cleavage of X-phenyl laurates in CTAB micelles.' 103kOH/M-'s-' k,,,/[oH-]/~-' s-' 46 18 14 13 6.3 1.8 1.0 0.39 0.22

5.8 4.8 4.5 3.3 0.55 0.30 0.28 0.101 0.088

"Based on the data of Al-Awadi and Williams (1990); K.,,

0.39 0.41 0.41 0.40 0.43 0.38 0.39 0.40 0.42 =

kOHK,,[OH~]/k,,,.

3.1 1.5 1.3 1.6 4.9 2.3 1.4 1.5 1.1

TRANSITION STATE STABILIZATION

a5

Table A6.8 Cleavage of N-acetylamino acid methyl esters by a-chymotrypsin." Residue GlY Ala aminoBut norVal norLeu Phe Val isoLeu

R

?T

H Me Et Pr Bu Bz i-Pr S-BU

0.00 0.50 1.00 1.so 2.00 2.63 1.30 1.80

KMlrnM

kCat/KM/M-'s - I

862

0.126 1.78 21.1 355 3 000 104000 1.97 2.47

-

66.7 14.3 5.37 0.93 87.7 -

KTS/~ 1.2 x 4.6 x 2.1 x 1.0 x 1.1 x 1.2 x 5.1 x 2.3 x

lo-' 10-8 10-9 10-1" 10-11 lo-' lo-'

"Based on data in Berezin et al. (1971) and Dorovska ei al. (1972) obtained at pH 7.8. Values of K.rs = k,, KM/k,,,, where k,, = koHIOH.'] = k o l , . 10 '.'. The Hansch hydrophobicity parameters (T)are from Leo ei al. (1971).

Table A6.9

Hydrolysis of 6-acylpenicillins catalysed by p-lactamase I."

N ~

2 3 4 5 7 8 9 10

0.138 0.116 0.1.09 0.116 0.119 0.126 0.125 0.132

1.21 0.66 1.46 1.03 1.30 0.56 2.30 1.98

77 1 1110 4070 3530 4690 1490 1580 2010

________

~-

6.37 16.8 27.9 34.3 36.1 26.6 6.87 10.2

~~~

2.17 0.69 0.39 0.34 0.33 0.47 1.82 1.30

"From the data of Buckwell ei al. (1988a); obtained at pH 7. N is the number of acyl carbon atoms. Values of K.r-s = k , KM/k,. where k,, = k o l l . lo-'.

Table A6.10 Hydrolysis of 7-acylcephalosporins catalysed by P-lactamase I."

2 3 4 5 6 7 8 9 10 12

0.140 0.129 0.130 0.131 0.133 0.128 0.131 0.134 0.131 0.065

3.93 3.25 1.49 4.91 1.92 1.68 1.29 0.85 0.99 0.35

2.90 2.00 1.50 6.00 7.40 4.80 5.20 5.60 7.20 7.10

738 615 1010 1220 3850 2860 4030 6590 7270 20 300

1.90 2.10 1.29 1.07 0.35 0.45 0.32 0.20 0.18 0.032

"From the data of Buckwell er al. (1988b); obtained at pH 7. N is the number of acyl carbon atoms. Values of K-,s = k , K M / k , . where k , = kc,,,. lo-'.