Catalysis by electron transfer in organic chemistry

Journal of Molecular Catalysis, 20 (1983)

CATALYSIS

BY ELECTRON

27

27 - 52

TRANSFER

IN ORGANIC

CHEMISTRY

LENNART EBERSON Division of Organic Chemistry S-220 07 Lund (Sweden)

3, Chemical

Center,

University of Lund,

P.O.B.

740,

Summary Like a proton or a base, an electron or a positive hole can act as a very efficient catalyst in an organic reaction that in itself does not involve a change in oxidation level in going from substrate to product. If an electron is added to or removed from a neutral molecule, a radical anion (cation) is formed (eqn. i), and can undergo reactions at the new oxidation level (eqn. ii). At a later stage of the reaction sequence, a return to the original oxidation level can be achieved by a chain transfer step, i.e., a chemically different radical anion (cation) can reduce (oxidize) a new molecule of the starting material (eqn. iii). + or-e-

A ->

A- ‘(+‘) B-‘(+‘)+

A-‘(+‘)

(i)

one or several steps

A

__f

B

+

, B-‘(+‘)

(ii)

A-‘(+‘)

(iii)

A familiar example of this reaction type is the SRNl reaction (developed and studied by Bunnett, Komblum, Russell and Saveant in 1965 1980) which enables otherwise extremely slow aromatic nucleophilic substitution reactions to take place under mild conditions. Other examples, to be discussed in this paper, include the SoEl and SON2 reactions, cyclodimerizations and cycloreversions, cycloadditions in general, rearrangements, oligomerizations and ligand exchange reactions. In general, catalytic effects of the order of 10 - 20 kcal/mol can be attained by electron transfer catalysis.

Introduction:

the f&l mechanism

Every chemist is familiar with the concept of acid-base catalysis since his/her first acquaintance with organic reactions. The addition or removal 0304-5102/83/$3.00

0 Elsevier Sequoia/Printed

in The Netherlands

28

of a proton to or from an organic molecule (eqns. 1 and 2) brings about profound reactivity changes and promotes rate accelerations of many orders of magnitude. It is considerably less known that holes and electrons can play RH + BH+ e

RH2+ + B:

(1)

RH+B:GR-+BH+

(2)

an analogous catalytic role, as illustrated in eqns. 3 and 4. By removal or addition of an electron from or to a neutral organic molecule, respectively, RH - e- e RH+e-

e

RH+’

(3)

RH-’

(4)

we can create a hole-carrying radical cation or an electron-carrying radical anion, respectively. By appropriate manipulation of the rich, but largely unexplored, chemistry of these species [l, 21 it is possible to develop truly catalytic processes in which the hole/electron moves along a regularly repeating series of intermediates, thus setting up a chain reaction with a net chemical change as a result [ 31. However, the big difference between acids/bases and holes/electrons is that the latter cannot be kept neat in bottles but demand special arrangements with respect to storage and ‘packaging’ materials. Thus holes/electrons can be stored in batteries (or generated in power stations for instant consumption) and supplied to solutions, then called electrolytes, via electrodes, holes being transferred at the anode and electrons at the cathode. Perhaps more familiar as storage materials are metal ions, metals and certain types of organic compounds, highly substituted with I-, E-substituents [4, 51. Examples are given below: Storage of electrons

Storage of holes

K, Na, Li Sn, Fe Cr(II), Co(I)

Fe(III), Ag(II), Cu(II)/(III) Ce(IV), Co(II1) 2,3-dichloro-4,5-dicyanobenzoquinone

A different mode of creating holes/electrons exists in the photochemical generation of excited states of suitable organic molecules [6]. An excited state can act as a sink/source of electrons toward other species present, depending on the donor/acceptor properties of the latter. Generally, excited states can be considered strong reductants/oxidants in the proper context. The best known case of electron transfer chain (ETC) catalysis is no doubt the SRNl reaction [7 - 91 which formally looks exactly like an ordinary substitution reaction (eqn. 5), with no change of oxidation state between substrate and product. However, when we realize that R can be a RX+Nu--RNu+X-

(5)

29

nonactivated aryl group it is obvious that something special is required to make the reaction go at a reasonable rate. And this is injection of electrons, performed cathodically, via addition of an alkali metal or by photolysis. The mechanism has by now been established in its broad outlines (eqns. 6 -9) as a chain reaction where the propagation steps depend on the ease RX+e----tRX-’

(6)

RX- * ---‘R’+X-

(7)

R’+Nu--+RNu-’

(8)

RNu-‘+RX-RNu+RX-’

(9)

of cleavage of RX- into R’ and X- and the differing bond strengths of the R-X-’ and R-Nu-’ bonds. Normally, the catalytic efficiency is moderate to low, reflecting the propensity for electrons and radicals to react rather indiscriminately and thus to create many possibilities for chain terminating processes. The best catalytic yields so far recorded, a quantum yield of up to 50, appears to be in the photochemical initiation of the reaction between iodobenzene and diethyl phosphite [ lOa], and in the electrochemical initiation of the reaction between 2chloroquinoline and diethyl phosphite ion [lob] where the chain length was cu. 100. Another reaction [lOc] which in principle should be capable of showing very long chains is the electrochemically induced rearrangement of S,Sdiarylbenzene-1,2dicarbothioates to the isomeric 3,3-bis(arylthio)phthalides (an intramolecular SRN1 reaction). This reaction is initiated by a short period of electrolysis (3 min, with a charge dose of 0.05 F mol-‘) after which no more initiation is needed; the reaction runs to completion by itself for 1 - 2 h. As described here the chain length is 20 but it should be possible to improve this number considerably by alternative modes of operation, e.g. pulse electrolysis. l

The Sozl reaction The SRNl mechanism is now a well-established concept in physical organic chemistry, thanks to the efforts of Bunnett [7a], Kornblum [7b], Russell [8,11] and Saveant [9], and will be used here as the solid bridgehead in the territory of similar, albeit so far much less well documented and corroborated, chemistry. Alder paved the way for this development in a 1980 paper [ 121 in which the ETC concept was extended to include a number of previously unrelated phenomena [see also 3b], inspired by the finding [13] that oxidants (S20s2-, VO+, ArsN+‘) strongly catalyze the inside protonation of the bicyclic amine 1 in 70% sulfuric acid/water (eqn. 10). If the reaction is conducted in deuteriosulfuric acid/deuterium oxide instead, inside protonation occurs, showing that the inside proton originates from a position CYto nitrogen, and in addition extensive exchange of a

30

hydrogens (but no others) takes place. The exchange mechanism was proposed to be of the ETC type, and was denoted the SoEl mechanism (electrophilic substitution, catalyzed by an oxidant and with a crucial dissociative step = eqn. 12), outlined in eqns. 11 - 14. This mechanism is perfectly

(10) 1 -

RH-e‘

RH+’

(11)

RH+’ + B: \ -R’+BH+

(12)

R’ + BD+ e

(13)

RD+’ + B:

RD+’ + RH e

RD + RH+’

(14)

RH = R;NCH,-H in accordance with the known acidic properties of many radical cations. Aminylium radical cations especially would be expected to be strong acids, in view of the unusually weak a-C-H bond that already exists in their parent compounds [14] (see below). The mechanism has been further strengthened by CIDNP experiments, showing fast proton exchange between aminylium radical cations and protic species [ 151.

The SON2reaction Another oxidatively initiated mechanism proposed by Alder, the SON2 mechanism (nucleophilic Substitution, catalyzed by an oxidant and with a crucial associative step = eqn. 16) is shown in eqns. 15 - 18; the net _ ArX A ArX+’ (15) ArX+’ + Nu- Ar(X)Nu -

Ar(X)Nu

(16)

ArNu+’ + X-

(17)

ArNu+’ + ArX -

ArNu + ArX+’

(IS)

reaction is identical to that of the SRNl mechanism (eqn. 5) with R equal to an aryl group. It turned out that several examples of this reaction were already described in the literature [16 - 181, although its possible chain nature was only hinted at once [ 161. Equation 19 shows the case studied 4-MeOC6H4F + AcO- =4-MeOCGHaOAc

+ F-

(19)

31

most extensively so far, acetoxyl/fluorine exchange in 4-fluoroanisole. The reason for the emphasis on this particular reaction, apart from the fact that ipso attack is predominant and thus provides a clearcut case, rests upon fluoride ion being the leaving group in eqn. (17). In principle, other leaving groups are feasible, but could be mechanistically ambiguous due to the fact that step 17 might be formulated as an oxidative step if X cor;esponds to an oxidizable X-. Thus bromine might for example leave from Ar(X)Nu in a bromine atom transfer process which obviously is a chain breaking step. Fluorine is not subject to this source of ambiguity, since a fluorine atom transfer step is very unlikely. The SON2 reaction of eqn. (19) can be initiated by the anode [ 18,191 (with maximally cc. 300% current yield of ArOAc at low conversion), by Ag(II) [2Ol,CoWI)Wn% 5- [211, and photochemically [ 161. Moreover, a direct entry into the second propagation step (eqn. 17) was found in eqn. 20, where benzoyloxyl radical from the thermal decomposition of benzoyl ArF + PhCOO’ -+

Ar(F)OCOPh

(29)

peroxide is used to initiate the chain process from eqn. (17) and onwards [22]. The only difference between eqns. 17 and 20 is that the hole has been transferred between the reactants (and acetoxyl changed for benzoyloxyl, but this is only for experimental convenience). The first cycle thus gives ArOCOPh, whereas with acetate ion present as Nu- the following cycles will of course produce ArOAc. From the ArOAc/ArOCOPh ratio a chain length of 7 - 8 cycles was inferred. Without acetate ion present the yield of benzoate was unchanged but the aryl acetate yield was drastically curtailed (by a factor of more than 10). A literature search revealed that a fair number of SoN2-type reactions have been reported previously [18] and sometimes commented upon as ‘interesting’. Table 1 lists representative cases, together with reactions for which the first pieces of mechanistic evidence have been assembled (Nos. 6, 10 - 12, 16). It should be noted that the list of formally feasible SON2 reactions can be extended considerably if we also allow for leaving groups for which step 17 can, at least in principle, be replaced by an atom or radical transfer step. As already mentioned above, the leaving group would then be oxidized off and the oxidant would be used stoichiometrically. This would however not constitute a chain reaction but a ‘conventional’ oxidative substitution process [ 361. Few of the reactions of Table 1 exhibit yields characteristic of chain reactions, but then it should be remembered that little experimentation has so far been guided by the novel concept of the SON2mechanism. Moreover, as often encountered in oxidation processes, the product is more easily oxidized than the starting material and in some cases one actually marvels at the fact that any product at all survives the harsh conditions (from the oxidation point of view; e.g. PhOH in the presence of Cu(II1)). A third factor making the SON2 reaction experimentally ,difficult to detect is that chloro- and fluoroaromatic radical cations, as most other aromatic radical

32

TABLE 1 Representative cases of SoNX-type reactions Reaction Substrate No

Initiator/Nu-

Product (WY

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

anode/CFsCOOCo(III)/CF3COOPb(IV)/CFsCOOSzOsZ--Fe(II)-Fe(III)/HaO Cu(III)/CFsCOOH-Hz0 Cu( III)/CFsCOOH-Ha0 Cu(III)/CF&OOH-H, 0 S20s2--Cu(II)-Fe(II)/H?O Cu( II)-02/OHhu/CN- or anode hv/CNanode/AcOhv/AcOanode/CF$OOH

PhOCOCFs (2) PhOCOCFa (6) PhOCOCFs (trace) PhOH (2) PhOH ( 35)b 4-ClC6H40H ( 132)b PhOH ( 44)b PhOH ( 30)b 4-MeC6H40HC 4-MeOCsH4CN ( 95)d 4-MeOCeH4CN (89)d 4-MeOC6H40Ac ( 270)b 4-MeOC6H40Ac (20) 4-MeOC6H40COCFa

23 24 25 26 19, 27 27 19, 27 28 29 16 16 l&l9 16 30

Rh( III)

alI=

31

Cu(I1) or AraN+’ Cu( II)/AcOanode/AcO-

Ph2C=CPh2 (4000)b acetoxyferrocene no reactione

PhCl PhCl PhCl PhCl PhCl 1,4-Cl&H4 PhF PhF 4-MeC6H4Cl 4-MeOC6H4F 4-MeOCsHJCl 4-MeOC6H4F 4-MeOC6H4F 4-MeOCeH4Br

15 16 17 18

PhCN2 chloroferrocene 4-MezNCsH4F

0

(>350)b

32 33,34 33

Y)rdinary chemical yields, unless otherwise stated. bBased on the amount of initiator. CWithout oxygen present, a switch from an SoN2-type to an aryne mechanism takes place. dQuantum yield > 1. eThe radical cation has been reported to be relatively stable [ 351.

cations, have their positive charge distributed over the ring positions, thus making normal oxidative substitution (eqn. 20) competitive. This sidereaction is stoichiometric in holes and thus detracts part of the oxidant ArH + Nu- -

ArNu + H+ + 2e-

(20)

from acting in a catalytic fashion. Finally, one must acknowledge the fact that the oxidant sometimes can be depleted by oxidizing the nucleophile(s) (NW and/or X-) present in the system (e.g. SO,’ us. Cl-, reactions 4 and 8). It is therefore not surprising that the chain nature of an SON2 reaction is obscured by these practical difficulties. The SRNl reaction is actually in the same predicament, the initiator being often used in stoichiometric amount or even in excess. The persulfate-initiated hydrolyses (Nos. 4 and 8 of Table 1) of chloroand fluorobenzene are particularly interesting, at least from a principal point of view. They take place in neutral aqueous medium at room temperature and thus the reaction conditions differ vastly from those usually employed

33

(~350 'C, 5 -10 M aqueous NaOH) to hydrolyze, for example, chlorobenzene. The catalytic effect can be approximately estimated to involve a factor of lo5 in going from 5 M NaOH to water and a factor of 233 = lo9 in going from 350 to 20 ‘C, or combined, cu. 1014. This amounts to a lowering of the activation energy by ~20 kcal mol-‘. The Cu(II1) initiated hydrolysis of chloro- and fluoroaromatic compounds also proceeds under mild conditions (reflux in CF,COOH) and gives remarkably high yields of phenols. One can make here the reasonable assumption that protonation of the phenol formed to give ArOH,+ protects it from further oxidation by the very strong oxidant, Cu(II1) (E” > 1.8 V*). Reaction 17, acetoxyl/chlorine exchange in chloroferrocene, was first believed to be an ideal case of SON2 behaviour, but it soon appeared that chloroferricinium ion was completely unreactive toward acetate ion (i.e., the step of eqn. 16 failed) under the conditions used and even more strenuous ones [34]. The low reactivity is presumably due to the fact that the positive charge of chloroferricinium ions is localized on the metal to an extent that the ring positions are very weakly electrophilic. Reaction 18, again an expected next to ideal case, is hampered by the same effect: The positive charge of the radical cation is localized on the nitrogen atom, so that the ring position next to fluorine bears too little positive charge to be reactive enough toward the nucleophile. This radical cation was earlier described as relatively stable [ 351. Reaction 9 was part of a very thorough investigation of the aryne mechanism of 4chlorotoluene hydrolysis by sodium hydroxide at very high temperatures [ 291. The presence of oxygen led to an immediate switch to an ipso substitution mechanism at a somewhat lower temperature; it is tempting to suggest that this is mediated by Cu(III), formed directly from oxygen and Cu(I1) under the extreme conditions, in an SoN2-like reaction sequence. Reaction 15 is an intramolecular version (which should be denoted SONi) of the SON2 reaction [31]. Attempts to cyclize 2fluorophenylacetic acid failed, mainly due to the higher reactivity of COO- relative to the 2-fluorophenyl moiety with respect to oxidation [34]. Only the COOgroup appears to be attacked, and the product is that of decarboxylation to give the benzylic radical, further oxidation and solvolysis (eqn. 21). ArCH,COO- ““, -co2

ArCH; -=% AcO-

ArCH,OAc

(21)

ETC catalyzed cycloadditions Cycloaddition reactions of the [2 + 21 type were shown to be catalyzed by electron transfer oxidants as early as 1965. Thus Ledwith and coworkers [37, 381 found that Fe(II1) salts (nitrate, perchlorate, but not chloride) in *All potentials given in this paper are referred to the normal hydrogen electrode.

34

methanol at 25 “C converted N-vinylcarbazole into bins-1,2dic~b~olylcyclobutane - in principle a thermally forbidden process - in a fast reaction and with a catalytic efficiency of >800% (eqn. 22). Fe(m)

\

R

P

*

R = carbazolyl

(2%

R

Cerium(IV) complexes could also be used as catalysts; At about the same time, the same dimer had been isolated in low yield from the anodic oxidation of N-vinylcarbazole in acetonitrile [39], and its photosensitized cyclodimerization (qu~~rn yield >l) was also realized [ 401. More detailed studies demonstrated that 2,2’-bipyridyl in 3-fold excess over Fe(III) slowed down the reaction but increased the yield of the cyclobutane [38]. A second dimeric compound (eqn. 23) was isolated and demonstrated to be a product of oxidative dimerization. This reaction type is well known from organic electrochemistry where it has some generality f41]. A few other aromatic enamines underwent similar reactions upon treatment with electron transfer oxidants, among which Ag(1) and Cu(I1) could also be included [37b]. To complete the picture an organic electron transfer oxidant, tris(4-bromophenyl)ammoniumyl (of the type Ar,N+’ ) could be used to oxidize N-vinylcarbazole quantitatively to the oxidative

dimerization product (eqn. 23) [ 421, whereas another type of radical cation,

ck--&J -

-2e

C&OH

RCHCH~~H*CHR

I\

b

Me

R = carbazolyl

(23)

t, Me

in nitro(perylene)+’ , catalyzes the polymer~ation of ~-v~ylc~b~ole methane, causing an 84% conversion to a polymer of molecular weight 210 000 [43]. Kinetic experiments [38] demonstra~ that the rate of cyclodimerization was always six to seven times greater than the rate of formation of the reduced form of the catalyst, indicating the chain nature of the reaction (eqns. 24 - 26). RCH=CH2 =

[RCH=CHJ+’

(24)

[RCH=CHJ +* + RCH=CH2 R

m R

+ RCH=CH, --+

R’-J

+ [RcH=cH,]+*

(26)

On closer scrutiny the photosensitized cyclodimerization of N-vinylcarbazole was found to proceed via a chain mechanism. For all sensitizers

35

tried, the quantum yield was > 1, and in fact as high as 14 with chloranil as sensitizer. The same mechanism as in eqns. 24 - 26 was inferred, the oxidizing agent being the singlet state of the sensitizer. Oxygen, being essential for cyclodimerization to occur, played the role of reoxidizing the sensitizer radical anion. Ferrocene (E” = 0.55 V), a very easily oxidizable compound, acted as a strong retarder for the cyclodimerization by virtue of its great efficiency as fluorescence quencher toward N-vinylcarbazole. Aromatic emu-nines are very easily oxidizable compounds and thus excellent model compounds for studying chain mechanisms. High reducing power is however not necessary for cyclodimerization to occur, since both indene (El,z = 1.77 V) and 1,ldimethylindene (E1,2 = 1.68 V) undergo electron transfer catalyzed cyclodimerization (photochemically or Fe(N) initiated) to give the anti-head-to-head dimer (eqn. 27) [ 441. (27) Again compounds of lower oxidation potential (e.g., 1,2-dimethoxybenzene, 1.57 V; 1,2,4trimethoxybenzene, 1.36 V) than the substrate inhibited the formation of the cyclodimer. The same reaction could also be initiated anodically with 5,6-dimethoxy- and 1-t-butyl-5,6dimethoxyindene (El,* of both = 0.81 V) [ 451. Indene dimers could be cleaved photochemically (quantum yield 8.2) via an ETC mechanism [46a]. Another [2 + 21 ETC cycloaddition process is the anodically and tris(4bromophenyl)ammoniumyl-catalyzed addition of oxygen to the somewhat unusual olefin, adamantylideneadamantane, to give the corresponding oxetane. Chain lengths of up to 20 - 25 were reported [ 46b, c]. The electron transfer - induced cycloreversion of cyclobutanes has recently [47] been studied in some detail with the cage compound shown in eqn. 28. This reaction can be initiated at room temperature in the dark by weak electron transfer oxidants of the quinone type (p-chloranil, E” = 0.25 V; 2,6dichlorobenzoquinone, E” = 0.1 V) if the aryl group is activated by a methoxy group (El,z of cage compound is then 1.09 V) but not when Ar = Ph (El,* of cage compound = 1.41 V). On irradiation in the presence of the above-mentioned sensitizers and a host of others, the diphenyl derivative underwent cycloreversion in quantitative substance yield and very Ar

(28) high quantum yields. Added good electron donors, such as triethylamine (El,a = 0.82 V) quenched the reactions completely, in agreement with a radical cation chain mechanism similar to that in eqns. 24 - 26. The Diels-Alder reaction ([2 + 41 cycloaddition) is also subject to electron transfer catalysis, as was suggested around 1970 on the basis of a

36

radiation chemical study of the dimerization of 1,3-cyclohexadiene [48]. This phenomenon has recently been developed for synthetic purposes by Bauld et al. [ 49 - 511. To take 1,3-cyclohexadiene as an example, its thermal [2 + 41 dimerization (eqn. 29) proceeds in 30% yield at 200 “C for 20 h.

0\

Ar,N+ -/ CH,CI, 0°C

+ exo &

(29)

In contrast, the reaction gives a 70% yield after 15 min in dichloromethane at 0 “C in the presence of catalytic amounts (3 - 5%) of tris(6bromophenyl)ammoniumyl hexachloroantimonate, the endo/exo selectivity being slightly greater than that observed in the uncatalyzed reaction (5:l us. 4:l). This catalytic effect, representing a lowering of the transition state energy by 10 - 11 kcal mol-‘, is most clearly visible for sterically hindered dienes, such as 2,5dimethyl-2,4-hexadiene (eqn. 30); this reaction has no thermal

(30) counterpart. The suprafacial stereospecificity, so synthetically useful and characteristic of the uncatalyzed Diels-Alder reaction, was retained in the radical cation-catalyzed version and actually turned out to be somewhat better. From the detailed analysis of these results, it was concluded that the catalyzed reaction must be described as the symmetry-allowed reaction between the dienophile (DP) radical cation and the diene (D), i.e., as a [4 + l] cycloaddition, rather than the forbidden [ 3 + 21 cycloaddition of dienophile and diene radical cation. Thus the chain reaction was depicted as in eqns. 31 - 33 [ 501 DP + Ar,N+’ DP+’ + D h

Ar,N + DP+’ A+’

A+‘+DP---,A+DP+’

(31) (32) (33)

A recent report has dealt with the photosensitized (by 9,10dicyanoanthracene) version of eqn. 29 which gives the cycloaddition products in good yield and represents a useful synthetic technique for obtaining DielsAlder adducts from electron-rich dienes and olefins [ 521. In principle, many types of pericyclic processes should be subject to catalysis by redox reagents, especially oxidants, but very little is known about these possibilities. From the cases discussed above, this area should be a very promising one for the development of new synthetic methods. The photosensitized [4 + 41 cycloreversion of the anthracene dimer (eqn. 34) in a limiting quantum yield of 0.75 was recently described [53], and no doubt many other examples will be revealed in the future.

37

(34)

ETC catalyzed decomposition

of diazoalkanes

The decomposition of aliphatic diazo compounds to give alkenes (eqn. 35) is efficiently catalyzed by electron transfer oxidants [32] or by BR,C=N=N =

R&=CR2 + 2Nz

(35)

the anode [54]. The reaction actually represents a formal change of the oxidation state of the carbon atom originally bonded to nitrogen, but in essence the reaction is of the ETC type just as, e.g., the SoN2-like photochemically or anodically stimulated replacement of fluorine by cyan0 in 4fluoroanisole (see Table 1). A mechanistic study in acetonitrile showed that diphenyldiazomethane (E,,, = 1.19 V) is decomposed by copper(I1) perchlorate (oxidation potential around 1.0 V) or tris(4-bromophenyl)ammoniumyl perchlorate (El,* = 1.28 V) to give predominantly tetraphenylethylene (El,* = 1.56 V) and small or trace amounts of benzophenone azine (E1,Z = 1.92 V) and trace amounts of products resulting from reactions with residual water (diphenylmethanol, benzophenone, and benzpinacol). There was practically no difference between the two initiators, and the catalytic efficiency was very high in both cases, 5 000 - 10 000%. The reaction was first order in oxidant and diazo compound. The suggested mechanism (eqns. 36, 37) is formally identical to the R,C=N=N -“7

R,&r;=N

R,C-&=N

+ R,C=N=N

-

(36) R&-&N

R,C--&N R2C--r;S=N I

I

rlr A*

-2Nz

1

R,&R,

+

1

-N2

R2C=N--Pj’=CR2

~cav, ?I! R2C=CR2 + R&=N-N=CR*

+ R&N,+’

(37)

38

SON2 mechanism (eqns. 15 - 18). The radical cation of Ph,CN, attacks a parent molecule in the rate-determining step either at the diazo carbon (leading to the alkene) or the terminal nitrogen atom (leading to the azine), as shown in eqn. 37. One should note that eqns. 36 and 37 seemingly represent a thermodynamically uphill process, if we only look at the change in oxidation potential in going from substrate to product (maximally 1.92 - 1.19 = 0.73 V = 17 kcal mol-’ in the case of the azine of eqn. 37). Of course this is not so; the overall reaction is strongly exothermic (for R = H the formation of the alkene + nitrogen from two molecules of substrate is exothermic by ca. 125 kcal mol-‘) due to the high thermodynamic stability of molecular nitrogen, and this is more than enough to produce what is formally a higher energy species as a result of eqns. 36 and 37.

ETC catalyzed reactions of small rings Small ring species, being highly strained and easily oxidizable [ 551, are expected to be susceptible to ETC catalysis under suitable conditions. Two such cases have been reported recently by Simonet et al. [56, 571, the tetramerization of N-benzylaziridine by the anode, t&(4-bromophenyl)ammoniumyl, or thianthrene radical cation (eqn. 38), and isomerization of epoxides to ketones by the anode (eqn. 39). In both cases a chain reaction

R-N3

&j-y-

R-N ’

‘li C”) (38)

cNJ-R

I!

R&,-,CR,’ 0

L R,R’CCOR’ > 1000%

(39)

was postulated, initiated by formation of the substrate radical cation. For eqn. 38, the chain reaction steps shown in eqns. 40 - 43 were invoked (S = substrate, S’ = ring-opened substrate, T = tetramer). The alternative mechanism, that anodically generated H+ catalyzes the tetramerization process, as suggested for e.g., the anodic ring-opening of N-acetylaziridine (eqn. 44) [ 581, was ruled out by the far greater catalytic power of the -es-

(46)

s+*

s+* (s’)+’ -%

(s’)+’

(S’),+’ 7

(41) T+’

(42)

39

T+’ + S h

T + S+’

CH,CON -=-+ 3 CH30H

(43)

CH,CONHCH2CH,0CH,

(44)

anode (room temperature in methanol or dichloromethane) as compared to homogeneous proton catalysis (reflux for several h in ethanol) and the fact that radical cations could be used as initiators in indirect electrochemical procedures.

ETC catalyzed rotation around bonds and ligand exchange An intriguing suggestion of electron transfer promotion of rotation around double bonds in acetone hydrazones was made a few years ago [ 591 (eqn. 45). The reaction rate was strongly influenced by the addition of

(45)

polyhalogenated compounds (CCb, CBr4), and this was interpreted in terms of a radical cation isomerization mechanism (eqn. 46, S = substrate, S” = rotamer of substrate). s + cc1 4 es+*

+ CCL+--’ ti

s” + ccl,

(46)

This mechanism is however not compatible with the Marcus theory since the oxidizing power of a polyhalogenated compound, although a reasonably good electron transfer oxidant in other contexts [60], is too low to allow for the ‘stoichiometric’ use in the oxidation of an acetone hydrazone (E” values can be estimated to be cu. 0 and at least 1.2 V, respectively). On the other hand, if a chain mechanism is assumed, this obstacle can be removed if the chain is long enough. Although presently almost entirely speculative, the possibility of an ETC mechanism for rotation around double bonds should be easily tested by looking at the effect of chain-terminating additives, other types of initiators, etc. Finally, in our list of established or suggested ETC mechanisms it should be remembered that this concept is not limited to organic reactions but can be equally well applied to organometallic and inorganic ones, such as ligand exchange reactions, on which excellent reviews are available [3a, b] . A recent and well-corroborated example is the electrocatalytic (anodically) substitution of carbonylmetal complexes, such as the tungsten complex of eqn. 47 [61]. This complex is stable to substitution by triphenylphosphine at 25 “C. It was proposed and experimentally verified that the

40

cis(MeCN),W(C0)4 + Ph,P -

anode at

0.2 V vs. SCE

Isolated yield cu. 100% Catalytic efficiency ca. 2000%

cis(MeCN)(Ph,P)W(CO),

+ MeCN (47)

anodically produced radical cation undergoes very fast ligand substitution and the new radical cation then transfers the hole to a new substrate molecule.

Theoretical treatment of ETC reactions Since we are dealing with chain reactions, certain general requirements must be met for an ETC reaction to be feasible. Firstly, all steps, except possibly the initiating redox reaction, must be very fast to allow for long reaction chains to build up [ 621. This means that the overall reaction cannot be endothermic but should preferably be exothermic (unless it can be run with a large excess of reagent under equilibrium conditions, such as the SoEl reaction; eqns. 11 - 14). Heats of reaction for models of the reaction discussed above are given in Table 2. It should be noted that these model reactions presumably represent extremes of exothermicity and that substituents will decrease heats of reaction considerably but not to the extent that the reactions become endothermic. With this reservation in mind, we still find that most of the reactions discussed above should be exothermic and thus fulfill the first criterion for sustaining a chain reaction. Next the individual steps will have to be considered in some detail. The initiating step need not necessarily be fast, even if in practice it often is. The non-bonded electron transfer between two species is governed by the Marcus theory [67 - 711 which views the reactants as two spheres (of radii rl and r2) immersed in a continuous dielectric of dielectric constant D. The two spheres diffuse together (rate constant kd) and form an encounter complex with a distance between the two electron exchange centers of rl + r2. The transition state is reached from the encounter complex by bond reorganization in the reactants (with bond reorganization energy = Xi) and solvent reorganization around them (with solvent reorganization energy = A,) so as to make the energy levels of the exchanging electron in each reactant match each other exactly. This is a consequence of the Franck-Condon principle which in essence tells us that transfer of an electron between two species takes place much faster than any change in the nuclear coordinates. If it is assumed that no reorganization takes place and the electron then were to be transferred between unperturbed reactants, a perpetuum mobile of the first kind would in principle be feasible, and hence we see the need for this formulation of the electron transfer transition state. The total reorganization energy h = hi + X0 can either be estimated from extrakinetic parameters or determined experimentally. It is one consequence

TABLE 2 Heats of reaction of model ETC processes* Reaction typeb

Model reaction (solvent or phase)

SRN1

PhBr + SH- -

-18

PhSH + Br- (H,O)

SRNl

PhBr + NH2- ---+

sON2

PhF + Hz0 -

SON2

PhCl + HZ0 -

So~2-like

2CHz=N=N

[2 + 21 CA

2CHz=CHz -

[4 + 21 CA

$

[4 + 21 CA

2 1,3-cyclohexadiene

<-40

PhNHz + Br- (H,O)C

-14

PhOH + HF (H,O)

-14

PhOH + HCl (HzO) -

-130

CHZ=CH2 + 2Nz (gas) cycle-C4H,

-18

(gas)

-38

*II-0

rearrangement of epoxides

oligomerization of aziridines

AH” (kcal mol-‘)

-

WA

-----f dicyclohexadiene

(liq)

CH&OCH3(gas)

c”u N k

*Thermodynamic data were obtained from standard tables and textbooks bCA = cycloaddition. CObviously an entirely constructed example.

-29 -29

<-40

[ 63 - 661.

of the Marcus treatment that X for a heteronuclear redox process (eqn. 48) is approximately equal to the mean value of the reorganization energies of the two self-exchange reactions into which in principle every redox process can be dissected (eqns. 49 - 51). From a determination of the selfexchange rate constant, AAiA-. and An+.,n can be calculated by the Eyring equation. A+D

=A-’

A+A-‘e

+ D+’ A-‘+A

(43) (49)

D+’ + D ti~+~+*

(59)

h An = 0.5(h,,,

(51)

-. + L+*/n)

h is a parameter that varies considerably with the electronic characteristics of the reactants. A few values of X for self-exchange reactions are given in Table 3; in general, compounds with possibilities of extensive r-delocalization of the electron/hole to be exchanged tend to have low X values, whereas those where the electron/hole must be localized in one bond predominantly have high X values. Moreover, solvation shells, especially those of hydration, and ion pairing of ionic species increase X [71].

42 TABLE 3 Experimentally found h values for selected self-exchange reactions of organic and inorganic species [ 70, 711 Reactiona

SoIventb Fkcal mol-‘)

dibenzo-p-dioxin/CRa phenothiazine/CR (4-MeCsH4)sN/CR ferrocene/ferricinium benzene/AR toluene/AR naphthalene/AR naphthalene/AR naphthalene/AR naphthalene/AR anthracene/AR perylene/AR 3-N02CsH4CN/AR (CN)IC=C(CN),/AR PhNOz/AR PhNOz/AR Ph&‘/PhsC+

CHsCN CHsCN CH$N PrOH/H*O (1 :l) DME (ion pair) DME (ion pair) DME DME (ion pair) DMF HMPA DME DME DMF CH&N DMF DMF/H20 (9:l) TFA/AcOH (3 :7)

Co( III)/Co( II) Fe(CN)e3-12Fe( phen)j+14+ IrC162--/3Mn04 1-/2Ce( IV)/Ce(III)

8.8 5.4 11.8 23.1 18.8 18.3 8.2 16.3 11.9 12.7 8.8 8.3 15.1 7.6 19.2 30 15.7

H2O H2O H2O H2O H2O I.320

<

57 44 19 31 41 57

WR = cation radical, AR = anion radical. hTemperature range 20 - 25 “C.

Armed with these definitions and concepts, we are ready to write down the Marcus’ expression of &, the observed rate constant for nonbonded electron transfer between two species, using the simple kinetic model introduced above (eqn. 52), where AGO’ is the standard free energy

change of the reaction under the actual conditions of solvent, temperature, etc., corrected by a term (AG,) to take account of the electrostatic effect of transferring the electron in the transition state (this term is calculated by a simple Coulomb expression, see eqn. 53; sometimes it is necessary AG, = 331.22,Z2/D(r,

+ r2)

43

to multiply by a factor involving the ionic strength of the medium (2, and 2, are the charges of the reactants)). In logarithmic form, this equation generates a parabola, the shape and maximum of which is determined by the value of X. A greater h moves the maximum toward lower AGO’ and flattens the parabola. We can now proceed to scrutinize an example of an initiation step of a typical ETC reaction, and choose the S ON2 reaction of eqn. 19, initiated by the 12-tungstocobalt(III)ate ion (E” = 1.00 V), for this purpose [21]. The reaction takes place in acetic acid/O.5 M KOAc at ca. 110 “C. 4-Methoxyfluorobenzene has an E” value of 2.10 V which means that AGO for the initial ET step is -23.06(1.0 - 2.1) = 25.4 kcal mol-‘. In principle, this value should be corrected by an electrostatic term, but in practice and especially in the particular example at hand (low dielectric constant, high ionic charge) this involves a rather delicate and difficult matter of judgement. If we assume that rl + r2 in the transition state is 8 a and use D = 6.2 for acetic acid, eqn. 53 gives AGc = -6 X 331.2/6.2/8 = -40 kcal mol-’ for a situation in which ZJ, changes from (0) X (-5) = 0 to (+l) X (-6) = -6 upon transfer of the electron. But we now have neglected to multiply AG, by the factor expressing the effect of ionic strength (p) = 10-(2i*s[r, + ‘21 J/J/DV referred to above, and the difficulty lies in estimating this factor. Acetic acid is a low-dielectric constant medium and it is not possible to give any reliable estimate of p. In the absence of other information, we shall assume that this factor is 0.1 (corresponding to /J being 0.075 in this case) thus reducing AG, to -4 kcal mol-‘. AGO’ is then 25.4 - 4.0 = 21.4 kcal mol-i. Next we need an estimate of A for the ET step. For the 12-tungstocobalt(III)ate5-‘6self-exchange reaction h is known to be ca. 25 kcal mol- l under slightly different conditions [72], and that of 4-methoxyfluorobenzene can be estimated from X values of analogous compounds (see Table 3) to be 12 kcal mol-‘. This gives A for the reaction under study = 0.5(12 + 25) = 18.5 kcal mol-‘, and h,, can then be calculated from eqn. 52 to be 2.6 X low2 M-l s-l, a rate constant fully compatible with experimental findings. The above example illustrates well the difficulties and uncertainties involved in applying the Marcus treatment to real organic systems in solvents of low D. Nevertheless, it is currently the best simple approach that we have available, and with further experimentation the unknown factors should easily be brought under control. Table 4 gives a selection of examples of estimated rate constants for the initiation steps of different types of ETC reactions. Due to the uncertainties involved in finding accurate AGO’ values in most of the solvents employed, these rate constants should be viewed with great care. We do note, however, that some reactions seem to have extremely slow initiation steps, and it is of interest to discuss if they are indeed feasible at all. The first reaction, the SRNl reaction of iodobenzene with pinacolone enolate ion [ 731, is one of the few SRN1 reactions that can be run without

Ph2CN2 + (4-BrC6H4)sN+’

R2N-N=CMe2

SoN2-iike

rotation

+ CBr4 [ 591

PhN02 (120)

CH3CN (25)

(0)

‘332Clz

SON2 [2 + 21 CA [4 + 21 CA [ 321

(25) (110) (40) (25)

DMSO HOAc HOAc MeOH

Phi + t-BuCOCH2- [ 73 3 4-MeOChH4F + [CO(III)W~~O~~]~- [21] 4-MeOCbH4F + Ag(II)(bpy), [ 201 N-vinylcarbazole + Fe( III) [ 37 ] 1,3-cyclohexadiene + (4-BrC6H4)3N+’ [49]

sON2

SRNl

Solvent (temp./C)

Assumed initiation step

Reaction type

20

15

40 12 12 10 12

X1

Estimated rate constants for the initiation step of different examples of ETC reactions [ 71]

TABLE 4

47

12

40 25 44 57 12

Xz

33.5

13.5

40 18.5 28 33.5 12

(X, + h2)/2

26.2

-2.5

28.6 21.4 10.2 15.4 24.0

AC”

10-s x 10-11

x lo-ii x lo--’

s-l 1

8 x 10-s

1.1 x 109

1.3 2.6 14 4 x 1.2

k (M-1

45

external stimulation and yet is reasonably fast. The nature of the initiation step was assumed to be ET between the enolate ion and iodobenzene, and now we can see that this leads to a calculated ET rate constant which is very small and would require extremely long chains to be compatible with the observed rates (and this is of course a prediction that can be checked experimentally). A snag is the uncertainty of the E” value of the carbanion; here the oxidation potential (-0.2 V) of benzyl methyl ketone enolate ion was used as the closest analogy [ 741. The relation of this oxidation potential to the E” value is not known, except that it is probable that it contains a term involving overpotential. Hence E” could be > -0.2 V and the predicted rate constant even smaller. Another estimated, very slow initiation step is the ET reaction between 1,3-cyclohexadiene and tris( 4-bromophenyl)ammoniumyl [ 491. The same source of uncertainty as above applies to the oxidation potential of 1,3cyclohexadiene, in that its E” value probably is lower than the experimental El,, value (2.34 V) due to an overpotential term. In this case, such a correction would lead to a larger predicted ET rate constant. Still, this reaction is one in which to look for experimental evidence for unusually long reaction chains. The last reaction of Table 4, decreased rotational barriers around bonds in acetone hydrazones in the presence of polyhalides, is a curious and interesting one [ 591. The oxidation potentials of aliphatic hydrazones are known for 2,4dinitrophenylhydrazones only [75] and lie in the region between 1.5 and 1.6 V, and that of acetone hydrazones can be estimated to be slightly lower, 1.2 V. The E” of CBr&Br,‘Brwas put equal to that of CCI,/CCls’ Cl- [60], but this is an uncertain assumption. It may be that this E” value is greater, and then the predicted ET rate constant will increase. This reaction should be amenable to several types of experimental tests, e.g., by looking for possibly strong inhibition by added nucleophiles (which would intercept the radical cation and thus break the chain). The second step of any of the ETC reactions discussed is the crucial one, and involves a dissociative (SRN 1, S,,l, small ring cleavage) or associative (SON2 or CA) reaction. It is thus determined entirely by the reactivity of the particular radical ion formed in the initiation step, and the theoretical problems involved at this stage of an ETC process are part of a far larger problem area, namely that of radical ion reactivity in general [ 76 - 791. Unfortunately, our knowledge of the rules of radical ion behaviour is limited, and the controversies presently raging around these problems [SO, 811 indicate that they will not be easily resolved because of the high degree of complexity of radical ion processes [ 301. However, for the purpose at hand it will suffice to ascertain that radical ion reactions normally are extremely facile, an important requirement for a step in a radical chain reaction. Quantitatively, it is to be expected that radical ions should be very reactive, firstly because of their charge which strongly enhances nucleophilicity (basicity) or electrophilicity (formation of a radical ion from a neutral molecule is the simplest act of

46

Umpolung [ 821 imaginable!), secondly because of the well-known property of odd-electron species to possess considerably weaker bonds than their parent closed-shell compounds. To take two examples of bond weakening, the enthalpy of cleavage of the (x C-H bond of toluene in the gas phase decreases by 28 kcal mol-’ (eqns. 52 and 53) by removal of an electron, and the barrier of rotation around the C=C bond in stilbene decreases by cu. 25 kcal mol-’ upon addition of an electron [83]. An even more drastic effect is seen in the pK difference between toluene and (toluene)+‘, estimated to be >60 pK, units [ 791 in acetonitrile solution! PhCH3 -

PhCH;

PhCH,+’ -

+ H’

PhCH2’ + H+

(52) (53)

Table 5 shows selected examples of rate constants for actual or model cases of the second dissociative/associative step of certain ETC reactions. Few of the rate constants of the second step of authentic ETC processes are known, except for the l&l variety where the second step, cleavage of aryl halide radical anions, has been subjected to a few kinetic studies [84 - 871. We therefore have to rely on model systems. Because of the scarcity of rate data for the reaction between simple monocyclic aromatic radical cations and nucleophiles (they are extremely fast and thus difficult to measure) the radical cations of perylene and 9,10-diphenylanthracene have been quoted as models for such reactions. We then can be sure that the corresponding reactions of monocyclic compounds are much faster than those of these two relatively stable radical cations. Unfortunately, it was not possible to find any reported kinetic results for models of olefin radical cation/neutral molecule reactions (the second -step of cycloaddition processes). The second step of the SON2 reaction, attack at the ipso position of an aryl halide radical cation, is expected to be an extremely fast process, as judged by the model rate constants for nuclear attack of nucleophiles upon aromatic radical cations (Table 5). Also estimates based on a previously published thermochemical method (which in essence predicts that radical cation reactivity is determined mainly by the relative oxidation potentials of the parent molecule of the radical cation and the nucleophile) leads to the same conclusion [76]. It is, however, important to note that we here run into a fundamental problem of radical ion reactivity in particular and organic chemistry in general, namely that competition between two types of elementary processes becomes possible (for examples, see Table 5). Apart from forming a covalent bond to the nucleophile, the radical cation can oxidize it in an electron transfer step (eqns. 54 and 55), a dichotomy inherent in all situations where a donor (D) and acceptor (A) species can interact, irrespective of charge and oxidation state (eqns. 56 and 57) [71]. RH+’ + Nu- -

k(Nu)H

(54)

RH+’ + Nu- -

RH + Nu’

(55)

47 TABLE

5

Rate constants

of examples

or models

of the second

Reaction

step of ETC reactions Solvent (temp./“C)

k (s-i or M-is-‘)

Ref.

(l-ClCi,,H,)-’

-

l-Ci,,H,’

+ Cl-

MezSO

(25)

5 x 10’

84

(l-BrC1cH7)-•

-

1-C1eH7’

+ Br-

MezSO

(25)

3 x 10s

84

(9~C1Ci4H9)--’

-

9-(&Ha’

+ Cl-

MezSO

(25)

1.5 x 102

84

(9-C1Ci4H9)-

-

9-C&,Ha’

+ Cl-

DMF (25)

1.2 x 102

85

(9-BrCi4H9)-’

---+

9-Ci4H9’

+ Br-

MezSO

3 x 10s

84

(9.BrC14H9)-’

-

9-C14H9’

+ Br-

DMF (25)

2.5 x 10s

85

CH3CN

(25)

5 x 10s

84 86a

( 4-CIC6H4CN)-’

-

4-CNC6H4’

+ Cl-

(25)

(4-ClC6H4COPh)-’

-

4-PhCOC6H4’

+ Cl-

CH3CN

(22)

25

(4-BrCsH4COPh)-’

-

4-PhCOChH4’

+ Br-

CH3CN

(---29)

1 x 103

86a

DMF (20)

5

86b

DMF (56)

0.5

86b

DMF (20)

>2 x 10s

87

DMF (2)

1.3 x 103

87

DMF (20)

>2 x 10s

87

Hz0

(23)

>2 x 10’

88

(23)

5 x 10s

88

Hz0

(23)

1 x 106

88

Hz0

(23)

1 x 103

88

(4-IC6H4N02)-

-

(4-BrC6H4N02)-’

4-NO&H4’ -

( 2-C1CgH6N)-’

2-C,H6N’

b -

(4-CICsHsNz)-

c -

4-CsH,N*’ -

+ Hz0

(mesitylene)+’

+ Br-

+ Cl-

5-Ci3H,N2’

C6HsCHa +’ + Hz0 (p-xylene)+’

4-N02C6H4’

a-

( 5-CIC1sH,Nz)-

+ I-

+ Cl+ Cl-

H(HO)C6H,CH3 ---+

+ Hz0

+ H+

H(HO)&H3(CH3),

-

H(HO)&H2(CHs)s

(isodurene)+’

+ Hz0

-

H(HO)&H(CHs)4

(isodurene)+’

+ HO-

-

H( HS))&H(

(perylene)+’

+ Cl-

--+

(perylene)+’

+ I- -

(perylene)+’

+ CN- -

(perylene)+’

+ AcO-

+ H+

H( Cl)C2cH1 CzoHi2

+ H+

CHa)4

1

+ I’

H(NC)C2eH,, -

H( AcO)C2cH1

DPA + I’

+ H+

1

Hz0

Hz0 (23) MeCN (25)

1.2 x 109

88

6.3 x lo5

89

MeCN

(25)

2.1 x 10’0

89

MeOH

(25)

4.6 x 10’

89

MeOH

(25)

7.5 x 104

89

MeCN (25)

1.5 x 107

90

MeCN (25)

7 x 10s

90

DPA+’

+ I- -

DPA+’

+ Br- -

DPA+’

+ CN-

-

DPA + CN’

MeCN (25)

6.3 x lo6

90

DPA+’

+ Hz0

-

DPA(OH)

MeCN (25)

0.13

90

DPA+’

+ py -

MeCN

3.7 x 104

90

DPA + Br’

DPA( py)”

+ H+

(25)

A+D---,A-D

(56)

A+D-A-‘+D+’

(57)

This situation can partly be analyzed in terms of the Marcus theory, particularly if the reaction is strongly endergonic; in this case we can be reasonably sure that electron transfer is far too slow a process to compete with the associative radical cation/nucleophile bond-forming step. However,

48

for less strongly endergonic and sometimes even exergonic reactions (in the electron transfer sense) this distinction becomes blurred, and we need only consider two cases in Table 5 to realize that there is an interesting reactivity problem hidden here: Why does DPA+’ react with Br- with electron transfer (AGO for ET = 6.2 kcal mol-‘) whereas (isodurene)+’ reacts with HO- to form a C-OH bond (AGO for ET < -10 kcal mol-l)? There are presently no ready answers to this and many other similar questions, so the whole problem of dual radical ion reactivity should be a fertile area for theoretical and experimental studies. The third step of an ETC mechanism (eqns. 8, 17, 25, 32) brings the system back to the radical ion level; in the forward direction a new radical ion appears whereas trivially in the reverse direction the starting species are obtained back. For these elementary processes we have even fewer quantitative data, the SaNl reaction again being the most - and now only -studied case. Table 6 gives rate constants for some Ar’ and Nu- pairs, and it is seen that they are all in the very fast range of 10’ - lo9 M-’ s-l [91,92]. For the SON2 mechanism no quantitative data for the third step are available, but a qualitative insight was obtained by a thermochemical estimate of the relative rates of the two competing reactions (eqn. 58) in the case of acetoxylation of fluorobenzene and 4-methoxyfluorobenzene PhF+’ + AcO- -

Pli(F)OAc -

(PhOAc)+’ + F-

(58)

[ 181. These calculations showed that the loss of fluoride from the fluoroacetoxycyclohexadienyl radical should be favoured over that of acetate ion by about 10 kcal mol-‘. With the introduction of a substituent which makes the starting fluoride easier to oxidize (e.g., 4-methoxy), the rate of loss of the leaving ion should increase, still with F- strongly preferred over AcO-. That this prediction cannot be pursued in the extreme is clearly shown by the lack of SON2 reactivity exhibited by chloroferrocene and 4-fluoro-N,N-dimethylaniline (see above). Qualitatively, the predicted behaviour of chloride ion as a leaving group relative to acetate ion is analogous. The third step of an ETC cycloaddition reaction, intramolecular reaction between a carbonium ion centre and a radical (eqn. 59) is identical to R’ + R+ -

(R-R)+’

(59)

the San1 third step (eqn. 8), except for the difference in charge type. To the author’s knowledge, no solution rate data are known for this kind of reaction, but there is no reason to assume that it should be slower than its anionic counterpart. After the third step, the new radical ion must fulfill a final requirement in that it must be capable of oxidizing/reducing a molecule of the starting material in a fast step in order to obtain efficient chain transfer. Generally, this should present no serious problem, since one safely can tolerate a difference in E” value of GO.3 V in the endergonic sense without encountering unreasonably low rates for the chain transfer step. Exemplifying with X

49

TABLE 6 Rate constants for certain cases of the third step of the Sri,,,

mechanism Ref.

Solvent (temp./C)

k

4-NOzC6H4’ + Cl-

DMF (25)

1.4 x 10’

91

4-N02CeH4’

+ Br-

DMF (25)

91

+ I-

DMF (25)

1.3 x 10s 2.0 x 109 2.3 x 10’ 1.7 x 10s

92

1.4 x 10’ 1.8 x 10’

93 93

4.5 x 10’

93

Reaction step

4-NOzC6H4’ l-C,eH,’ l-C,&’

+ PhS-

NH3 (-40)

+ PhS-

DMSO (20)

1-quinolyl’ + PhS1-quinolyl’ + (EtO)zPO1-quinolyl’ + PhCOCH2-

NH3 (-40) NH3 (-40) NH3 (---40)

(M-i s-l)

91 92

values of 10 and 20 kcal mol-‘, this corresponds to rate constants 23 X lo5 and 21 X lo4 M-’ s-l, respectively, according to the Marcus treatment (eqn. 52). Since the substrate generally is present in high concentration, the chain transfer step should be a very fast one. Moreover, the nature of the new radical ion often is such as to produce a stronger oxidant/reductant than the initiating species, implying diffusion-controlled rate constants for the chain transfer step. Table 7 lists examples of chain transfer reactions and their energetics; they are all weakly endergonic, thermoneutral or exergonic, except for one case, PhF/(PhOH)+‘. This reaction is far too endergonic to be a feasible chain transfer step and hence we must either surmise a different, more strongly oxidizing intermediate, such as (PhOH),++‘, or a different type of mechanism. TABLE 7 Energetics of chain transfer steps in ETC reactions Reaction type

Substrate

New radical ion

AGO of chain transfer step (kcal mol-’ )

SRN1

Phi

(PhSPh)-’

-31

SRN1

PhBr


sOE1

RH

(PhCH,CN)-’ RD+’

sON2

PhF

(PhOH)+’

>20

sON2

PhF

(PhOHa)++’ (4-MeOCsH40Ac)+’ (4-MeOCeH4CN)+’

ca. 0 10
sON2 SON2

SoNa-like CA CA

4-MeOC6H4F 4-MeOC6H4F Ph2CN2 ethylene 1,3-cyclohexadiene

(Ph2C=CPhi) (cycle bu tane)+’ (dicyclohexadiene)+’

0

>o co. 0

50

Termination reactions in ETC mechanism can be of two kinds, namely either those well-known ones available to neutral radicals, or those available to radical ions (coupling, fragmentation, irreversible electron exchange with species taking part in the chain reaction or impurities). To take an example, the R’ intermediate in the Sa,l mechanism (eqn. 7) should readily enter into radical reactions with the solvent (SH), e.g., hydrogen atom abstraction [94], and the newly formed radical S’ would then not be productive in the San1 sense. In the So,2 mechanism a likely termination process is the irreversible electron transfer oxidation of the nucleophile competing with the chain transfer step, e.g., the radical cation oxidizing acetate ion to carbon dioxide and methyl radical. Generally, the high redox reactivity of radical ions allows for deplorably efficient termination pathways.

References 1

2 3

4 5 6 7

8 9 10

11 12 13 14 15 16 17 18 19

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