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A comparison of skeletal rearrangement reactions of even-electron anions in solution and in the gas phase
ELSEVIER
International Journal of Mass Spectrometry and Ion Processes 133 (1994) 1-12
Review
A comparison of skeletal rearrangement reactions of even-electron anions in solution and in the gas phase Peter C.H. Eichinger,
Suresh Dua, John H. Bowie*
Department of Chemistry, The University of Adelaide, Adelaide, S.A. 5005, Australia
(Received 3 December 1993; accepted 3 January 1994)
Abstract There is significant correspondence between certain skeletal rearrangement processes of close-shell organic anions in the condensed and gas phases. Examples of such correspondence include the acyloin, acyl oxyacetate, anionic oxy Cope, anionic Wolff, benzilic acid, Dieckmann, Lossen, Smiles and Wittig rearrangements. In contrast, there are some rearrangements observed in the condensed phase, which are either minor or do not occur at all in the gas phase. The Favorskii, Tiemann and Carroll rearrangements fall into this category. Finally, there are some gas phase rearrangements which have no condensed phase analogy: for example the negative ion pinacol/pinacolone and Beckmann rearrangements. These, and related processes are discussed in this Review. Key words:
Anion rearrangement;
Gas phase versus solution
1. Introduction During the last decade, we have investigated the basic collision induced fragmentations of even electron anions (M - H)- derived from organic molecules (M). The parent ions are formed either (i) by deprotonation of the neutral molecule by a strong base (e.g. HO- or NH;) in the chemical ionisation source of a reverse sector VG ZAB 2HF mass spectrometer [l], or (ii), in a minority of cases, by the fast atom bombardment technique [2]. The ions so formed generally have low energies, and dissociation is effected by allowing the (M - H)- ions to proceed through the magnet into a collision cell containing helium, and the consequent product anions formed by collision induced dissociation are moni-
tored by scanning the electric sector of the mass spectrometer. The spectrum so produced is called a collision induced mass spectrum (or MS-MS). The basic fragmentations of (M - H)- ions are often simple: these have been reviewed [3-71. In certain cases simple fragmentation is energetically unfavourable. In such a situation, the first formed (M - H)- ion may undergo skeletal rearrangement prior to or during fragmentation. The present article details the skeletal rearrangement processes of closed-shell organic anions in the gas phase, and reports significant correspondence between these reactions and those which occur in solution.
2. Skeletal rearrangement processes
* Corresponding author. 0168-l 176/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168-I 176(94)03953-W
Anionic
and base catalysed
rearrangements
are
2
P.C.H. Eichinger et al./Int. J. Mass Spectrom. Ion Processes 133 (1994) l-12
generally less well known than carbenium ion rearrangements in the condensed phase, and as a consequence, are often thought to be few in number. There are, in fact, many examples of such reactions occurring in solution. In order to emphasise this point, we list just a few of the better known examples, e.g. (A) 1,2_rearrangements including the acyloin (o-ketol) [8], anionic pinacol [9], Baker-Venkataraman [lo], benzilic acid [ll], Favorskii [ 121, Fritsch-Buttenberg-Weichell [ 131, Grovenstein-Zimmerman [ 141, Hofmann [ 151, Lossen [16], Neber [17], Payne [18], RambergBacklund [19], and Wittig [20] rearrangements; (B) [3,3] rearrangements including the anionic ortho Fries [21], anionic Carroll [22], and anionic oxy-Cope [23]; (C) ipso rearrangements including Smiles [24], and Truce-Smiles [25]; and (D), a variety of anionic cyclisation reactions including the Dieckmann condensation [26]. A search of the literature has led us to conclude, rather surprisingly, that little comparative experimental data are available concerning solvent and counter ion effects for such rearrangement reactions occurring in the condensed phase. What is known suggests that a change in solvent may affect the rate of reaction but seldom the overall reaction pathway. A particular example of this has been described for the Claisen rearrangement [27]. The only instances that we know of changes in products resulting specifically from a change of solvent or counter ion are those involving particular examples of the Wittig [28-301, Favorskii [31], and Lossen [32] rearrangements. If a change in solvent does not play a major role in the mechanism of an anionic intramolecular rearrangement, it could be argued that we should expect correspondence between such rearrangements in the gas and condensed phases. In general terms this expectation is realised. There are exceptions however; some are spectacular, others are quite subtle. Let us now consider examples of individual rearrangement reactions. 2.1. 1,banionic rearrangements Many of the rearrangements that we have studied are formally classified as 1,2_rearrangements, i.e. where an incipient anion migrates to a directly adjacent position.
lR1-(RCHQI
I$--CH-O-R
/
\
\ \
[R;
(R-CH-0-N ’
(R)(RKHO f
Scheme 1.
2.1.1. The Wittig rearrangement The Wittig rearrangement is the classical example of a 1,2 anionic condensed phase rearrangement. It occurs for a wide range of benzylic and allylic ethers which may be deprotonated on a carbon a to the ether oxygen atom, with subsequent rearrangement of that carbanion to ultimately form an alcohol [20]. The reaction could, in principle, involve either of the intermediates shown in Scheme 1 [33], but since the migratory aptitudes in condensed phase reactions are in the order of free radical stabilities, the radical pair mechanism is suggested to be the more likely ]341. The Wittig rearrangement also occurs following collisional activation in the gas phase. For example, product ion’ and peak width measurements show the spectra of Ph-CHOMe and Ph(Me)CHO- to be identical with the exception of a single peak corresponding to the loss of Me’ from the former ion [35]. Similar studies indicate that the gas phase Wittig rearrangement occurs for a variety of ethers: the systems studied are listed in Table 1. It seems that the actual structure of the reactive intermediate in such a reaction pathway depends upon a number of factors including the relative electron affinities of R; and RCHO. In some cases, structures intermediate between the two extremes shown in Scheme 1 are likely. The data shown in Table 1 indicate a general correspondence between gas phase and condensed phase rearrangements in these systems. An exception occurs for ally1 alkyl ethers where there is apparently a more facile decomposition channel than that of the expected Wittig rearrangement. ’ Product ion studies involve a comparison of the collision induced tandem negative ion mass spectra (MS-MS-MS) (and sometimes the charge reversal (positive ion) spectra) of the product ion under study, with those of a known species prepared by an independent route (cf. e.g. Refs. 35 and 38).
3
P.C.H. Eichinger et al./Int J. Mass Spectrom. Ion Processes 133 (1994) 1-12
;;c 0
-3
-CH$H$HO
-
b
This is of particular interest in the case of deprotonated ally1 ethyl ether, where labelling and product ion studies indicate the operation of the elimination reaction outlined in sequence (1); i.e. loss of ethene to form homoenolate ion a, which rearranges through the cyclopropyloxy anion b to the stable acetone enolate ion [38].
Q I
Ph,N--CHPh
(1)
(MeCOCH2)
a
H
-
Table 1 Gas phase Wittig rearrangements Compound
Phi.+
.H .a’
\ Ph
(2) PhN--CHPh,
The nitrogen analogue of the condensed phase Wittig rearrangement is called the Stephens rearrangement [40]. As an example, treatment of benzyldiphenylamine with n-butyllithium gives benzhydrylaniline in high yield: the proposed mechanism involves the intermediacy of the spiroazacyclopropane species shown in sequence (2) [41]. Product ion and labelling studies show that this interconversion also occurs to completion upon collisional activation in the gas phase [42]. Whether the reaction is concerted or stepwise (cf. sequence (2)) is not known.
Ref.
Gas phase
Condensed
Major Minor Major No Major Major Minor
Major Minor Major Major Major Major Minor
phase 35 36 37 38 36 38 39
studies establish that this facile rearrangement also occurs following collisional activation in the gas phase, although it is not known whether the interconversion is stepwise (cf. sequence (3)) or concerted [44]. Certain of the fragmentations of product ion c (Scheme 2) also formally involve 1,2 anionic rearrangement. For example, the characteristic loss of CO occurs as summarised in sequence (4). Such fragmentations have also been observed in other systems: for example deprotonated pyruvates fragment as shown in Eq. (5), with the position of CO loss being established by 13C labelling [45].
2.1.2. The (acyloxy)acetate rearrangement The base catalysed (acyloxy)acetate-acylhydroxy acetate reaction is a 1,2-anionic rearrangement which is known to proceed through an oxirane intermediate (sequence (3) (R = alkyl) (Scheme 2)) [43]. 13C Labelling and product ion
%‘C,)CO .
ethers
Wittig rearrangement?
type
Benzyl alkyl ether Benzyl ally1 ether Diallyl ether Ally1 alkyl ether Ally1 phenyl ether Diphenylmethyl ether Vinyl alkyl ether
I
of deprotonated
2.1.3.
The negative ion pinacol rearrangement
rearrangement pinacol-pinacolone The (sequence (6) (Scheme 3)) is one of the oldest and most famous of all acid catalysed rearrangements
2R
-
RCOC(R)(O-)(C02R)
(3)
C
W -
MCOCOR) RO-C-=0]
[(CH$O)
-
Me0C=O] Scheme 2.
RCOC(R)(RO)(O-)
-
-(CH$O2Me)
+
co
(4)
+ co
(5)
P.C.H. Eichinger et al./Inf. J. Mass Spectrom. Ion Processes I33 (1994) I-12
H+ Me&(OH)C(OH)Me, -
-HZ0 Me&(OH)C(+OHdMe, -
Me+C(OH)CMes-)
-
Me$(OH)+CM%
(6)
MeCOCMes + H+
[(k@CCOMe) HO-1-
(Me$COCH&
+
H20
(7)
(8) + MeOH Scheme 3.
qz- p - 6 +MeoH
(9)
H 1
I
-0Me
Scheme 4.
[9,46]. The reaction involves a simple Whitmore 1,2-methyl shift [47], and the driving force for the reaction is the stabilization of the final carbenium ion intermediate by loss of a proton to form the neutral product. Base catalysed analogues of the pinacol rearrangement are not common: those that have been reported either effect release of ring strain [48], or involve P-chlorohydrins [49]. The anticipated gas phase negative ion pinacol/
pinacolone (sequence (7)) does not occur since loss of water is not observed in the collisional induced mass spectrum of deprotonated pinacol[50]. However, the deprotonated form of the cognate ,LI-methoxyhydrin undergoes pronounced loss of methanol, but product ion studies together with a deuterium isotope effect investigation show that this reaction involves the epoxide mechanism shown in sequence (8) [50]. This reaction occurs
P.C.H. Eichinger et al./Int J. Mass Spectrom. Ion Processes 133 (1994) I-12
P Ph#.Z(OH)C(O-)Ph,
-
Ph
“x-r: Ph 0 ,O_Ph ‘H’
-
(PhCOCPh3 - H) + H,O
to the exclusion of the pinacol rearrangement when (i) the substituents are alkyl, and (ii) there is no conformational restraint on the reacting groups. However, epoxide formation is unlikely to occur when the two oxygenated substituents cannot approach an anti orientation. Thus although deprotonated trans-2-methoxycyclohexanol loses methanol by an epoxide mechanism (sequence (9) (Scheme 4)), the cis isomer is unable to effect such cyclisation and is forced to undergo the pinacol rearrangement (cf. sequence (10)) [50]. Although these reactions have no direct analogy in the condensed phase, base catalyses of cis-2-chlorocyclohexanols are known to yield pinacol products [49]. In marked contrast to the above behaviour, deprotonated phenyl ethylene glycols undergo facile pinacol type rearrangements [51]. For example, deprotonated benzpinacol undergoes the collision induced rearrangement summarised in sequence (11). It is proposed that the ease of this reaction is primarily a function of two major features, viz (i) the rearrangement proceeds from an eclipsed or near eclipsed conformer which is stabilized by a strong intramolecular hydrogen bond (see sequence (1 l)), and (ii) the enhanced migratory aptitude of Ph- in comparison to those of incipient alkyl anions [51]. 2.1.4. The acyloin and benzilic acid rearrangements These reactions both involve 1,2 rearrangement during base catalysis of cu-hydroxyketo systems in the condensed phase. Both alkyl and aryl migration occur during the acyloin rearrangement [52]. We first noticed this reaction in the gas phase when
PhCOCOPh
+ HO- -
deuterium labelling indicated that the simple fragmentations of MeCOC(O-)Me2 were preceded by statistical scrambling of the three methyl groups [53]. This scenario also pertains for analogous systems, for example the collision induced mass spectra of the two isomers shown in Eq. (12) (Scheme 5) are identical; thus equilibration precedes fragmentation [53]. The benzilic acid rearrangement has been known for almost seventy years [54]. Treatment of benzil with potassium hydroxide gives the potassium salt of benzilic acid, and the reaction is thought to proceed as shown in sequence (13). The reaction between HO- and benzil also occurs under high pressure conditions in the chemical ionization source of an MS 50 instrument [55]. 2.1.5. The Favorskii rearrangement The Favorskii rearrangement in solution involves base catalysed conversion of a suitably substituted a-haloketone to a carboxylic acid or derivative thereof [56]. This exothermic process is initiated by cyclisation through the carbanion centre of an ambident enolate precursor to form the Favorskii cyclopropanone intermediate as shown in route A (Scheme 6). A competing process may occur through O- (route B, Scheme 6). The relative distribution of products depends on many factors, including the natures of solvent and substrate, i.e. a protic solvent should preferentially solvate the O- centre of the ambident precursor, thus favouring the Favorskii reaction. The ambident behaviour of enolate anions in bimolecular gas phase reactions is well known
Ph
Scheme 5.
Ph$(O-)C02H
(13)
P.C.H. Eichinger et al./Int. J. Mass Spectrom. Ion Processes 133 (1994) I-12
Me-CO-
Me, F X A
gi
Me$CONu
-X-
c
B
‘A$
MeCOC(Nu)@le)2
Scheme 6.
activation, fragments (by endothermic reactions) through Favorskii intermediate e or by competitive reactions through f. This question cannot be answered for X = Cl, since there are no characteristic fragmentations which enable identification of the reactive intermediate. However for X = MeS and MeO, reaction proceeds exclusively through intermediate f, i.e. the “harder” [58] nucleophilic centre, reacts at the “hard” electrophilic centre. This has been demonstrated by deuterium isotope effect and product ion studies [53]. For example, -[CH2COC(Me)2(OMe)] fragments through f (X = MeO) to yield -[CH&OC(Me) = CH2] and MeOH.
IO
0
I(
II<
1 x-1
0
#Y
I(
e
> Xl
f Scheme 7.
[57], but such reactivity in intramolecular reactions is not well understood. In this particular intramolecular situation, the question is whether enolate ion d (Scheme 7) under conditions of collisional
2.1.6. The Lossen and Wolff rearrangements There are several well known condensed phase
RCONHOH
+ H,O
-HO
HO-
[(PhNCO) HO-] w
-
PhNH-
+
(14)
PhNHCO,-
(15)
- co,
0
PhCON--OH*
RNJ32
R-N=C=O -
R- -N--OH c
PhN--C02* CO2
- N2
R-CO-CH=+N=N-
-
RT~H
-
RCH=C=O
(16)
-
PhC=CO
(17)
OL
-N2
PhCO--C=+N=N-
n
phv Scheme 8.
P.C.H. Eichinger et aLlInt J. Mass Spectrom. Ion Processes 133 (1994) I-12
1,Zrearrangements which proceed via nitrene or carbene intermediates. The Lossen and Wolff rearrangements are two such examples. Hydroxamic acids (or their acyl derivatives) yield isocyanates when treated with base. This is known as the Lossen rearrangement, and the mechanism involves the nitrene rearrangement shown in sequence (14) (R = alkyl or aryl (Scheme 8)) [59]. The Lossen rearrangement also occurs in the gas phase for deprotonated hydroxamic acids [60,61]. There are many collision induced fragmentations of these systems which proceed through Lossen intermediates: the most striking is the formation of PhNH- from PhCON--OH (see sequence (15)) [61]. The Wolff rearrangement in the condensed phase is a neutral reaction involving decomposition of a diazoketone into a carbene plus nitrogen, with the carbene then rearranging into a ketene derivative (sequence (16), (R = alkyl or aryl, Scheme 8)) [62]. Product ion studies indicate that collisional activation of deprotonated diazoketones in the gas phase also results in loss of nitrogen with concomitant Wolff rearrangement [63]. For example, a deprotonated phenyldiazoketone yields PhQOby the process shown in sequence (17).
2.1.7. The Beckmann and Tiemann rearrangements The Beckmann rearrangement is an acid catalysed rearrangement which involves the conversion of a protonated oxime (or suitable derivative) to an amide (see sequence (18) (Scheme 9) for a simple example) [64]. The group which migrates to N is often that trans to OH, but isomerisation prior to rearrangement is also known [65]. The rearrangement also occurs for protonated oximes in the gas phase [66]. There is no base catalysed counterpart in solution: presumably the reaction is energetically unfavourable because HO- is a poor leaving group. The collisional activation mass spectra of deprotonated oximes contain a base peak resulting from elimination of water, an unusual fragmentation in the negative ion mode. Labelling and product ion studies suggest that this loss is associated with a Beckmann type rearrangement, a reaction which has no direct analogy in solution [67]. The rearrangement is summarised in sequence (19) (Scheme 9) for a prototypical example. Although the Beckmann rearrangement is a major process for simple oximes, it is only a minor process in more complex systems where more favourable reaction channels are available. For example, (i)
+H,O
-Hz0
MeC+=NMe -MeCONHMe
Me$=N(+OHF
Me&=NO-
+ H+
-
TH2(Me)C=NOH -
[(CHpC=NMe)
-
(CHpC=NMe - H)- +
Hz0
MqC=NNHCONH-
I
-
[M@=NNH-
(18)
HO-]
(19)
(O=C=NH)]
OCN- + Me$=NNH*
(20)
0 R
RC02N=C‘Me -
Scheme9.
RC02- + MeCN
(21)
P.C.H. Eichinger et aLlInt. J. Mass Spectrom. Ion Processes 133 (1994) l-12 0) base
R(NH&=NOS02Ph
R(NH-)C=NOH -
NH=C(R)--NOH
(ii) Hz0
RNHCONH,
[(HN=C=NR) -OH] e
+
[(RCN) -NHOHj -
(22)
(RN=C=NH - H)‘ + H,O
(23)
(RCN- H>-+ NH@
(24)
Scheme 10.
the major decomposition of semicarbazones involves a simple cleavage/deprotonation process shown in sequence (20) [68], and (ii) deprotonated cr-ketoximes undergo the rearrangement shown in sequence (21); a reaction favoured by the formation of the stable carboxylate product anion [69]. The Tiemann rearrangement is a base-catalysed solution reaction that is very similar to the proposed gas phase negative ion Beckmann rearrangement. The reaction was first reported over one hundred years ago and is summarised in sequence (22) (Scheme 10 (R = alkyl or aryl)) [70]. It is perhaps ironic that the gas phase Tiemann rearrangement (sequence (23)) is a very minor reaction when deprotonated amidoximes are subjected to collisional activation: the major fragmentation of such systems involves the loss of hydroxylamine shown in sequence (24), a process confirmed by *H and 15N labelling together with product ion studies [71].
2.2. [3,3] Rearrangements 2.2.1. The anionic oxy Cope, Claisen and Carroll rearrangements
The anionic oxy Cope [72] and Claisen (together with the [731 rearrangements associated Carroll [22] rearrangement) are well known solution rearrangements which occur by [3,3] processes through six membered transition states. Our studies of these rearrangements have provided some surprises. Three examples follow. Treatment of diallyl ether with KNH2-NH3 gives hexadien-3-01 (g to h, sequence (25) (Scheme 11)) by a Wittig rearrangement (cf. Scheme 1)
[72,74]. This alcohol is stable to further base treatment, but on heating it is transformed to hex-5-en-l-al by a (neutral) oxy Cope rearrangement [75]. A modification of this scenario occurs in the gas phase. The collisional activation mass spectra of g, h and i (Scheme 11) are all identical: product ion studies indicate that the spectra are those of the anionic oxy Cope product i [76]. Thus in the gas phase, deprotonated diallyl ether first forms the Wittig product which then transforms to the oxy Cope product ion, indicating, at least in this case, that the anionic oxy Cope rearrangement is more facile in the gas phase than in solution. Analogous oxy Cope rearrangements are also observed during bimolecular gas phase reactions [77]. The behaviour of deprotonated ally1 vinyl ether provides an interesting example of a system which undergoes different rearrangements in the condensed and gas phases. Base catalysis of j (Scheme 11) produces the Wittig rearrangement product k [78]. In contrast, product ion studies show that the Wittig product is not formed in the gas phase: instead, the Claisen product 1 is formed exclusively [79]. The third example is that of deprotonated benzyl ally1 acetate. It undergoes the Claisen ester enolate rearrangement (sequence (27)) [80] in both the condensed and gas phases [81], while the associated Carroll rearrangement (sequence (28)) [22] occurs in solution, but not in the gas phase [81]. The non-occurrence of the Carroll rearrangement in the gas phase is shown by the observation that the major fragmentation (loss of ally1 alcohol) from the parent ion does not involve equilibration of the two oxygens. Ph-CHC(0)‘*0C3H5 + (PhCH =C=O) (i.e. C3H5’*O- -+ PhC20-+ C3H5’*0H).
9
P.C.H. Eichinger et aLlInt J. Mass Spectrom. Ion Processes 133 (1994) l-12
the collision induced mass spectra of both deprotonated I-vinylcyclobutanol and its (expected) [ 1,3] rearrangement product (deprotonated cyclohexanone (see sequence (30)) are identical. Product ion studies demonstrate that the characteristic fragmentations are exclusively those of deprotonated cyclohexanone [83].
h
A
$- - QJ I
k
j
(26)
0
P
0
P
*
\ 4
0
-
t
/
0
2.4. The Dieckmann condensation The Dieckmann condensation (e.g. sequence (31)) is a well known reaction of synthetic applicability [84], and it is the only example of this type of cyclisation that we have studied in depth. A number of groups [85-871 have confirmed that this process occurs in the gas phase. For example, product ion studies substantiate the operation of reaction (3 1). MeocO
lvleK0 0 (27)
-
Meole 0
-0
cr
+
MeOH
2.5. Ipso rearrangements
(28) 2.3. [I,31 Anionic rearrangements [ 1,3] Rearrangements (cf. sequence (29) 12), R = alkyl or aryl) are quite rare in densed phase [82]. The rearrangement rarer in the gas phase. It is only a major when release of ring strain is involved. For
yR
_YR
Scheme 12
(Scheme the conis even process example
The Smiles rearrangement is the classical (condensed phase) example of an aromatic ipso nucleophilic substitution reaction (see sequence (32) (Scheme (13)) [24]. This nucleophilic reaction normally requires an activating electron-withdrawing group (e.g. Z = nitro, sulphonyl or halogen) in either the ortho or para positions of the aromatic ring [88]. Generally X is a good leaving group and Y a strong nucleophile [88]. ZC,H,XCH$H,Y-
_c
ZC,H,YCH,CH,X
(32) PhOCH,CH,%
(29)
_t
L
Ph180CH 2CH 2O-
(33) The Smiles rearrangement shown in sequence (33) occurs upon collisional activation in the gas phase without the requirement of an additional electron withdrawing substituent on the ring [89]. ‘*O Labelling (sequence (33)) shows statistical equilibration of the two oxygens prior to the formation of the sole fragmentation product, the
10
P.C.H. Eichinger ef al./Int. J. Mass Spectrom. Ion Processes 133 (1994) I-I2
phenoxide anion. 13CLabelling at Ci of the phenyl ring indicates an ipso (rather than e.g. an ortho) attack, since the product phenoxide anion exclusively fragments by loss of 13C0 upon collisional activation.
3. Concluding remarks The present article demonstrates a significant correspondence between the skeletal rearrangement processes of closed-shell organic anions which occur in both condensed and gas phases. Examples that we have studied which show this general correspondence are: acyloin, acyl oxyacetate, anionic oxy Cope, anionic Wolff, benzilic acid, Dieckmann, Lossen, nitrogen Wittig, Smiles and Wittig rearrangements. Indeed, the evidence available indicates that some of these processes are more facile in the gas phase than in solution, e.g. the oxy Cope and Smiles rearrangements. There are also some cases where we have found reactions occurring in the gas and condensed phases to be quite different. This is presumably because there is a well defined reaction channel of lower energy than that of the expected rearrangement. Examples are: two “Wittig” exceptions (sequences (1) and (26)), the Tiemann rearrangement (sequences (23) and (24)) and the Carroll rearrangement (sequence (28)). Finally, there are those gas phase anionic rearrangements which have no condensed phase anionic counterpart, but which are analogous to the cognate acid catalysed reaction. The negative ion “pinacol/pinacolone” and Beckmann reactions are illustrative of this class of rearrangement. 4. References [l] M.B. Stringer, J.H. Bowie and J.L. Holmes, J. Am. Chem. Sot., 108 (1966) 3888. [2] M. Barber, R.S. Bordoli, G.J. Elliott, R.D. Sedgewick and A.N. Tyler, Anal. Chem. A, 42 (1982) 645. [3] J.H. Bowie, in Mass Spectrometry Specialist Reports, The Chemical Society, London, Vol. 10, 1989, p. 145. [4] J.H. Bowie, Mass Spectrom. Rev., 9 (1990) 349. [5] P.C.H. Eichinger and J.H. Bowie, Int. J. Mass Spectrom. Ion Processes, 110 (1991) 123.
[6] J.H. Bowie, Org. Mass Spectrom., in press. [7] J. Adams, Mass Spectrom. Rev., 9 (1990) 141; M.L. Gross, Int. J. Mass Spectrom. Ion Processes, 118/119 (1992) 137. [8] M. Rentzea and E. Hecker, Tetrahedron Lett., (1982) 1785 (and references cited therein). 191R. Fittig, Justus Liebigs Ann. Chem., 110 (1859) 17; 114 (1860) 54. VOI S. Saxena, J.K. Marandi and S.K. Grover, Synthesis (1985) 697 (and references cited therein). illI H. Gowal, A. Spiess, M. Ballenegger, L. Due, H. Moll, H.P. Schunke and H. Dahn, Helv. Chim. Acta, 68 (1985) 2132 (and references cited therein). WI D.H. Hunter, J.B. Stothers and E.W. Warnhoff, in P. de Mayo (Ed.), Rearrangements in Ground and Excited States, Vol. 1, Academic Press, New York, 1980, p. 391. 1131 B.A. Shainyan and A.N. Mirskova, Zh. Org. Khim., 19 (1983) 1344 (and references cited therein). 1141A. Maercker and M. Passlack, Chem. Ber., 116 (1983) 710 (and references cited therein). [lY E.S. Wallis and J.F. Lane, Org. React., 3 (1946) 267. P61 H.L. Yale, Chem. Rev., 33 (1943) 209. [I71 J.A. Hyatt, J. Org. Chem., 46 (1981) 3953 (and references cited therein). [18] C.H. Behrens, Y.S. Ko and K.B. Sharpless, J. Org. Chem., 50 (1985) 5687. [19] L. Paquette, Org. React., 25 (1977) 1. [20] H.E. Zimmerman in P. de Mayo (Ed.), Molecular Rearrangements, Interscience, New York, 1963. J.A. Marshall, T.M. Jenson and B.S. Detloft, J. Org. Chem., 51 (1986) 4316 (and references cited therein). 121 T. Calogeropoulou, B. Hammond and D.F. Wiemer, J. Org. Chem., 52 (1987) 4185 (and references cited therein). P2 M.F. Carroll, J. Chem. Sot., (1940) 1266. W. Kimel and A.C. Cope, J. Am. Chem. Sot., 65 (1943) 1992. [23] L.A. Paquette, F. Pierre and C.E. Cottrell, J. Am. Chem. Sot., 109 (1987) 5731 (and references cited therein). [24] L.A. Warren and S. Smiles, J. Chem. Sot., (1930) 956; (1930) 1327. [25] D.M. Snyder and W.E. Truce, J. Am. Chem. Sot., 101 (1979) 5432. [26] G. Boche, Top. Curr. Chem., 146 (1988) 1. [27] W.N. White and E.F. Wolfarter, J. Org. Chem., 35 (1970) 2196. [28] V. Schijllkopf and M. Rizk, Justus Liebigs Ann. Chem., 734 (1970) 106. V. Rautenstrauch, J. Chem. Sot., Chem. Commun., (1970) 4. [29] B.B. Snider, N.J. Haib and L. Fuzesi, J. Am. Chem. Sot., 98 (1976) 7115.
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11
[51] S.K. Dua, M.J. Alexander and J.H. Bowie, Org. Mass. Spectrom., 28 (1993) 1155. [52] S. Danilov and E. Venus-Danilov, Chem. Ber., 67 (1934) 24. H.E. Stavely, J. Am. Chem. Sot., 62 (1940) 489. [53] S.K. Dua, A.P. Pollnitz and J.H. Bowie, J. Chem. Sot., Perkin Trans 2, in press. [54] G. Scheuing, Chem. Ber., 56 (1923) 252. [55] R.N. Hayes, P.C.H. Eichinger and J.H. Bowie, Rapid Commun. Mass Spectrom., 4 (1990) 283. [56] A. Favorskii, J. Russ. Phys. Chem. Sot., 26 (1894) 559. A.S. Kende, Org. Reactions, 11 (1960) 261. [57] S. Ingemann, N.M.M. Nibbering, S.A. Sullivan and C.H. DePuy, J. Am. Chem. Sot., 104 (1982) 6520. I.L. Freriks, L.J. deKonig and N.M.M. Nibbering, J. Am. Chem. Sot., 113 (1991) 9119. [58] I. Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley, New York, 1977. [59] H. Lossen, Liebigs Ann. Chem., 150 (1869) 314; 161 (1872) 347. H.L. Yale, Chem. Rev., 33 (1943) 209. [60] M. Decouzon, 0. Exner, J.-F. Gal and P.-C. Maria, J. Org. Chem., 55 (1990) 3980. [61] G.W. Adams, J.H. Bowie and R.N. Hayes, J. Chem. Sot., Perkin Trans 2, (1991) 689. [62] L. Wolff, Justus Liebigs Ann. Chem., 325 (1902) 129; 394 (1912) 25. R. Eistert, in Newer Methods of Preparative Organic Chemistry, Vol. 1, Interscience, New York, 1948, p. 513. [63] A.T. Lebedev, R.N. Hayes and J.H. Bowie, J. Chem. Sot., Perkin Trans 2, (1991) 1127. A.T. Lebedev, V.A. Bakulev, R.N. Hayes and J.H. Bowie, Rapid Commun. Mass Spectrom., 5 (1991) 234. [64] E. Beckmann, Chem. Ber., 20 (1887) 1507. [65] G. Donamura and W.Z. Heldt, Org. Reactions, 11 (1960) 1. P.T. Lansbury and N.R. Mancuso, Tetrahedron Lett., (1965) 2445. [66] A. Maquestiau, R. Van Haverbeke, R. Flammang and P. Meyrant, Org. Mass Spectrom., 15 (1980) 80 (and references cited therein). [67] G.W. Adams and J.H. Bowie, J. Chem. Sot., Perkin Trans 2, (1989) 2159; (1990) 1279. [68] G.W. Adams and J.H. Bowie, Rapid Commun. Mass Spectrom., 4 (1990) 275. [69] G.W. Adams, J.H. Bowie and R.N. Hayes, J. Chem. Sot., Perkin Trans 2, (1991) 1809. [70] F. Tiemann, Chem. Ber., 24 (1891) 4162; L.G. Donamura and W.Z. Heldt, Org. Reactions, 11 (1946) 1. [7 l] G. W. Adams, J.H. Bowie, R.N. Hayes and M.L. Gross, J. Chem. Sot., Perkin Trans 2, (1992) 897. [72] J.A. Benson and M. Jones, J. Am. Chem. Sot., 86 (1964) 5017; 86 (1964) 5019.
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International Journal of Mass Spectrometry and Ion Processes 133 (1994) 1-12
Review
A comparison of skeletal rearrangement reactions of even-electron anions in solution and in the gas phase Peter C.H. Eichinger,
Suresh Dua, John H. Bowie*
Department of Chemistry, The University of Adelaide, Adelaide, S.A. 5005, Australia
(Received 3 December 1993; accepted 3 January 1994)
Abstract There is significant correspondence between certain skeletal rearrangement processes of close-shell organic anions in the condensed and gas phases. Examples of such correspondence include the acyloin, acyl oxyacetate, anionic oxy Cope, anionic Wolff, benzilic acid, Dieckmann, Lossen, Smiles and Wittig rearrangements. In contrast, there are some rearrangements observed in the condensed phase, which are either minor or do not occur at all in the gas phase. The Favorskii, Tiemann and Carroll rearrangements fall into this category. Finally, there are some gas phase rearrangements which have no condensed phase analogy: for example the negative ion pinacol/pinacolone and Beckmann rearrangements. These, and related processes are discussed in this Review. Key words:
Anion rearrangement;
Gas phase versus solution
1. Introduction During the last decade, we have investigated the basic collision induced fragmentations of even electron anions (M - H)- derived from organic molecules (M). The parent ions are formed either (i) by deprotonation of the neutral molecule by a strong base (e.g. HO- or NH;) in the chemical ionisation source of a reverse sector VG ZAB 2HF mass spectrometer [l], or (ii), in a minority of cases, by the fast atom bombardment technique [2]. The ions so formed generally have low energies, and dissociation is effected by allowing the (M - H)- ions to proceed through the magnet into a collision cell containing helium, and the consequent product anions formed by collision induced dissociation are moni-
tored by scanning the electric sector of the mass spectrometer. The spectrum so produced is called a collision induced mass spectrum (or MS-MS). The basic fragmentations of (M - H)- ions are often simple: these have been reviewed [3-71. In certain cases simple fragmentation is energetically unfavourable. In such a situation, the first formed (M - H)- ion may undergo skeletal rearrangement prior to or during fragmentation. The present article details the skeletal rearrangement processes of closed-shell organic anions in the gas phase, and reports significant correspondence between these reactions and those which occur in solution.
2. Skeletal rearrangement processes
* Corresponding author. 0168-l 176/94/%07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0168-I 176(94)03953-W
Anionic
and base catalysed
rearrangements
are
2
P.C.H. Eichinger et al./Int. J. Mass Spectrom. Ion Processes 133 (1994) l-12
generally less well known than carbenium ion rearrangements in the condensed phase, and as a consequence, are often thought to be few in number. There are, in fact, many examples of such reactions occurring in solution. In order to emphasise this point, we list just a few of the better known examples, e.g. (A) 1,2_rearrangements including the acyloin (o-ketol) [8], anionic pinacol [9], Baker-Venkataraman [lo], benzilic acid [ll], Favorskii [ 121, Fritsch-Buttenberg-Weichell [ 131, Grovenstein-Zimmerman [ 141, Hofmann [ 151, Lossen [16], Neber [17], Payne [18], RambergBacklund [19], and Wittig [20] rearrangements; (B) [3,3] rearrangements including the anionic ortho Fries [21], anionic Carroll [22], and anionic oxy-Cope [23]; (C) ipso rearrangements including Smiles [24], and Truce-Smiles [25]; and (D), a variety of anionic cyclisation reactions including the Dieckmann condensation [26]. A search of the literature has led us to conclude, rather surprisingly, that little comparative experimental data are available concerning solvent and counter ion effects for such rearrangement reactions occurring in the condensed phase. What is known suggests that a change in solvent may affect the rate of reaction but seldom the overall reaction pathway. A particular example of this has been described for the Claisen rearrangement [27]. The only instances that we know of changes in products resulting specifically from a change of solvent or counter ion are those involving particular examples of the Wittig [28-301, Favorskii [31], and Lossen [32] rearrangements. If a change in solvent does not play a major role in the mechanism of an anionic intramolecular rearrangement, it could be argued that we should expect correspondence between such rearrangements in the gas and condensed phases. In general terms this expectation is realised. There are exceptions however; some are spectacular, others are quite subtle. Let us now consider examples of individual rearrangement reactions. 2.1. 1,banionic rearrangements Many of the rearrangements that we have studied are formally classified as 1,2_rearrangements, i.e. where an incipient anion migrates to a directly adjacent position.
lR1-(RCHQI
I$--CH-O-R
/
\
\ \
[R;
(R-CH-0-N ’
(R)(RKHO f
Scheme 1.
2.1.1. The Wittig rearrangement The Wittig rearrangement is the classical example of a 1,2 anionic condensed phase rearrangement. It occurs for a wide range of benzylic and allylic ethers which may be deprotonated on a carbon a to the ether oxygen atom, with subsequent rearrangement of that carbanion to ultimately form an alcohol [20]. The reaction could, in principle, involve either of the intermediates shown in Scheme 1 [33], but since the migratory aptitudes in condensed phase reactions are in the order of free radical stabilities, the radical pair mechanism is suggested to be the more likely ]341. The Wittig rearrangement also occurs following collisional activation in the gas phase. For example, product ion’ and peak width measurements show the spectra of Ph-CHOMe and Ph(Me)CHO- to be identical with the exception of a single peak corresponding to the loss of Me’ from the former ion [35]. Similar studies indicate that the gas phase Wittig rearrangement occurs for a variety of ethers: the systems studied are listed in Table 1. It seems that the actual structure of the reactive intermediate in such a reaction pathway depends upon a number of factors including the relative electron affinities of R; and RCHO. In some cases, structures intermediate between the two extremes shown in Scheme 1 are likely. The data shown in Table 1 indicate a general correspondence between gas phase and condensed phase rearrangements in these systems. An exception occurs for ally1 alkyl ethers where there is apparently a more facile decomposition channel than that of the expected Wittig rearrangement. ’ Product ion studies involve a comparison of the collision induced tandem negative ion mass spectra (MS-MS-MS) (and sometimes the charge reversal (positive ion) spectra) of the product ion under study, with those of a known species prepared by an independent route (cf. e.g. Refs. 35 and 38).
3
P.C.H. Eichinger et al./Int J. Mass Spectrom. Ion Processes 133 (1994) 1-12
;;c 0
-3
-CH$H$HO
-
b
This is of particular interest in the case of deprotonated ally1 ethyl ether, where labelling and product ion studies indicate the operation of the elimination reaction outlined in sequence (1); i.e. loss of ethene to form homoenolate ion a, which rearranges through the cyclopropyloxy anion b to the stable acetone enolate ion [38].
Q I
Ph,N--CHPh
(1)
(MeCOCH2)
a
H
-
Table 1 Gas phase Wittig rearrangements Compound
Phi.+
.H .a’
\ Ph
(2) PhN--CHPh,
The nitrogen analogue of the condensed phase Wittig rearrangement is called the Stephens rearrangement [40]. As an example, treatment of benzyldiphenylamine with n-butyllithium gives benzhydrylaniline in high yield: the proposed mechanism involves the intermediacy of the spiroazacyclopropane species shown in sequence (2) [41]. Product ion and labelling studies show that this interconversion also occurs to completion upon collisional activation in the gas phase [42]. Whether the reaction is concerted or stepwise (cf. sequence (2)) is not known.
Ref.
Gas phase
Condensed
Major Minor Major No Major Major Minor
Major Minor Major Major Major Major Minor
phase 35 36 37 38 36 38 39
studies establish that this facile rearrangement also occurs following collisional activation in the gas phase, although it is not known whether the interconversion is stepwise (cf. sequence (3)) or concerted [44]. Certain of the fragmentations of product ion c (Scheme 2) also formally involve 1,2 anionic rearrangement. For example, the characteristic loss of CO occurs as summarised in sequence (4). Such fragmentations have also been observed in other systems: for example deprotonated pyruvates fragment as shown in Eq. (5), with the position of CO loss being established by 13C labelling [45].
2.1.2. The (acyloxy)acetate rearrangement The base catalysed (acyloxy)acetate-acylhydroxy acetate reaction is a 1,2-anionic rearrangement which is known to proceed through an oxirane intermediate (sequence (3) (R = alkyl) (Scheme 2)) [43]. 13C Labelling and product ion
%‘C,)CO .
ethers
Wittig rearrangement?
type
Benzyl alkyl ether Benzyl ally1 ether Diallyl ether Ally1 alkyl ether Ally1 phenyl ether Diphenylmethyl ether Vinyl alkyl ether
I
of deprotonated
2.1.3.
The negative ion pinacol rearrangement
rearrangement pinacol-pinacolone The (sequence (6) (Scheme 3)) is one of the oldest and most famous of all acid catalysed rearrangements
2R
-
RCOC(R)(O-)(C02R)
(3)
C
W -
MCOCOR) RO-C-=0]
[(CH$O)
-
Me0C=O] Scheme 2.
RCOC(R)(RO)(O-)
-
-(CH$O2Me)
+
co
(4)
+ co
(5)
P.C.H. Eichinger et al./Inf. J. Mass Spectrom. Ion Processes I33 (1994) I-12
H+ Me&(OH)C(OH)Me, -
-HZ0 Me&(OH)C(+OHdMe, -
Me+C(OH)CMes-)
-
Me$(OH)+CM%
(6)
MeCOCMes + H+
[(k@CCOMe) HO-1-
(Me$COCH&
+
H20
(7)
(8) + MeOH Scheme 3.
qz- p - 6 +MeoH
(9)
H 1
I
-0Me
Scheme 4.
[9,46]. The reaction involves a simple Whitmore 1,2-methyl shift [47], and the driving force for the reaction is the stabilization of the final carbenium ion intermediate by loss of a proton to form the neutral product. Base catalysed analogues of the pinacol rearrangement are not common: those that have been reported either effect release of ring strain [48], or involve P-chlorohydrins [49]. The anticipated gas phase negative ion pinacol/
pinacolone (sequence (7)) does not occur since loss of water is not observed in the collisional induced mass spectrum of deprotonated pinacol[50]. However, the deprotonated form of the cognate ,LI-methoxyhydrin undergoes pronounced loss of methanol, but product ion studies together with a deuterium isotope effect investigation show that this reaction involves the epoxide mechanism shown in sequence (8) [50]. This reaction occurs
P.C.H. Eichinger et al./Int J. Mass Spectrom. Ion Processes 133 (1994) I-12
P Ph#.Z(OH)C(O-)Ph,
-
Ph
“x-r: Ph 0 ,O_Ph ‘H’
-
(PhCOCPh3 - H) + H,O
to the exclusion of the pinacol rearrangement when (i) the substituents are alkyl, and (ii) there is no conformational restraint on the reacting groups. However, epoxide formation is unlikely to occur when the two oxygenated substituents cannot approach an anti orientation. Thus although deprotonated trans-2-methoxycyclohexanol loses methanol by an epoxide mechanism (sequence (9) (Scheme 4)), the cis isomer is unable to effect such cyclisation and is forced to undergo the pinacol rearrangement (cf. sequence (10)) [50]. Although these reactions have no direct analogy in the condensed phase, base catalyses of cis-2-chlorocyclohexanols are known to yield pinacol products [49]. In marked contrast to the above behaviour, deprotonated phenyl ethylene glycols undergo facile pinacol type rearrangements [51]. For example, deprotonated benzpinacol undergoes the collision induced rearrangement summarised in sequence (11). It is proposed that the ease of this reaction is primarily a function of two major features, viz (i) the rearrangement proceeds from an eclipsed or near eclipsed conformer which is stabilized by a strong intramolecular hydrogen bond (see sequence (1 l)), and (ii) the enhanced migratory aptitude of Ph- in comparison to those of incipient alkyl anions [51]. 2.1.4. The acyloin and benzilic acid rearrangements These reactions both involve 1,2 rearrangement during base catalysis of cu-hydroxyketo systems in the condensed phase. Both alkyl and aryl migration occur during the acyloin rearrangement [52]. We first noticed this reaction in the gas phase when
PhCOCOPh
+ HO- -
deuterium labelling indicated that the simple fragmentations of MeCOC(O-)Me2 were preceded by statistical scrambling of the three methyl groups [53]. This scenario also pertains for analogous systems, for example the collision induced mass spectra of the two isomers shown in Eq. (12) (Scheme 5) are identical; thus equilibration precedes fragmentation [53]. The benzilic acid rearrangement has been known for almost seventy years [54]. Treatment of benzil with potassium hydroxide gives the potassium salt of benzilic acid, and the reaction is thought to proceed as shown in sequence (13). The reaction between HO- and benzil also occurs under high pressure conditions in the chemical ionization source of an MS 50 instrument [55]. 2.1.5. The Favorskii rearrangement The Favorskii rearrangement in solution involves base catalysed conversion of a suitably substituted a-haloketone to a carboxylic acid or derivative thereof [56]. This exothermic process is initiated by cyclisation through the carbanion centre of an ambident enolate precursor to form the Favorskii cyclopropanone intermediate as shown in route A (Scheme 6). A competing process may occur through O- (route B, Scheme 6). The relative distribution of products depends on many factors, including the natures of solvent and substrate, i.e. a protic solvent should preferentially solvate the O- centre of the ambident precursor, thus favouring the Favorskii reaction. The ambident behaviour of enolate anions in bimolecular gas phase reactions is well known
Ph
Scheme 5.
Ph$(O-)C02H
(13)
P.C.H. Eichinger et al./Int. J. Mass Spectrom. Ion Processes 133 (1994) I-12
Me-CO-
Me, F X A
gi
Me$CONu
-X-
c
B
‘A$
MeCOC(Nu)@le)2
Scheme 6.
activation, fragments (by endothermic reactions) through Favorskii intermediate e or by competitive reactions through f. This question cannot be answered for X = Cl, since there are no characteristic fragmentations which enable identification of the reactive intermediate. However for X = MeS and MeO, reaction proceeds exclusively through intermediate f, i.e. the “harder” [58] nucleophilic centre, reacts at the “hard” electrophilic centre. This has been demonstrated by deuterium isotope effect and product ion studies [53]. For example, -[CH2COC(Me)2(OMe)] fragments through f (X = MeO) to yield -[CH&OC(Me) = CH2] and MeOH.
IO
0
I(
II<
1 x-1
0
#Y
I(
e
> Xl
f Scheme 7.
[57], but such reactivity in intramolecular reactions is not well understood. In this particular intramolecular situation, the question is whether enolate ion d (Scheme 7) under conditions of collisional
2.1.6. The Lossen and Wolff rearrangements There are several well known condensed phase
RCONHOH
+ H,O
-HO
HO-
[(PhNCO) HO-] w
-
PhNH-
+
(14)
PhNHCO,-
(15)
- co,
0
PhCON--OH*
RNJ32
R-N=C=O -
R- -N--OH c
PhN--C02* CO2
- N2
R-CO-CH=+N=N-
-
RT~H
-
RCH=C=O
(16)
-
PhC=CO
(17)
OL
-N2
PhCO--C=+N=N-
n
phv Scheme 8.
P.C.H. Eichinger et aLlInt J. Mass Spectrom. Ion Processes 133 (1994) I-12
1,Zrearrangements which proceed via nitrene or carbene intermediates. The Lossen and Wolff rearrangements are two such examples. Hydroxamic acids (or their acyl derivatives) yield isocyanates when treated with base. This is known as the Lossen rearrangement, and the mechanism involves the nitrene rearrangement shown in sequence (14) (R = alkyl or aryl (Scheme 8)) [59]. The Lossen rearrangement also occurs in the gas phase for deprotonated hydroxamic acids [60,61]. There are many collision induced fragmentations of these systems which proceed through Lossen intermediates: the most striking is the formation of PhNH- from PhCON--OH (see sequence (15)) [61]. The Wolff rearrangement in the condensed phase is a neutral reaction involving decomposition of a diazoketone into a carbene plus nitrogen, with the carbene then rearranging into a ketene derivative (sequence (16), (R = alkyl or aryl, Scheme 8)) [62]. Product ion studies indicate that collisional activation of deprotonated diazoketones in the gas phase also results in loss of nitrogen with concomitant Wolff rearrangement [63]. For example, a deprotonated phenyldiazoketone yields PhQOby the process shown in sequence (17).
2.1.7. The Beckmann and Tiemann rearrangements The Beckmann rearrangement is an acid catalysed rearrangement which involves the conversion of a protonated oxime (or suitable derivative) to an amide (see sequence (18) (Scheme 9) for a simple example) [64]. The group which migrates to N is often that trans to OH, but isomerisation prior to rearrangement is also known [65]. The rearrangement also occurs for protonated oximes in the gas phase [66]. There is no base catalysed counterpart in solution: presumably the reaction is energetically unfavourable because HO- is a poor leaving group. The collisional activation mass spectra of deprotonated oximes contain a base peak resulting from elimination of water, an unusual fragmentation in the negative ion mode. Labelling and product ion studies suggest that this loss is associated with a Beckmann type rearrangement, a reaction which has no direct analogy in solution [67]. The rearrangement is summarised in sequence (19) (Scheme 9) for a prototypical example. Although the Beckmann rearrangement is a major process for simple oximes, it is only a minor process in more complex systems where more favourable reaction channels are available. For example, (i)
+H,O
-Hz0
MeC+=NMe -MeCONHMe
Me$=N(+OHF
Me&=NO-
+ H+
-
TH2(Me)C=NOH -
[(CHpC=NMe)
-
(CHpC=NMe - H)- +
Hz0
MqC=NNHCONH-
I
-
[M@=NNH-
(18)
HO-]
(19)
(O=C=NH)]
OCN- + Me$=NNH*
(20)
0 R
RC02N=C‘Me -
Scheme9.
RC02- + MeCN
(21)
P.C.H. Eichinger et aLlInt. J. Mass Spectrom. Ion Processes 133 (1994) l-12 0) base
R(NH&=NOS02Ph
R(NH-)C=NOH -
NH=C(R)--NOH
(ii) Hz0
RNHCONH,
[(HN=C=NR) -OH] e
+
[(RCN) -NHOHj -
(22)
(RN=C=NH - H)‘ + H,O
(23)
(RCN- H>-+ NH@
(24)
Scheme 10.
the major decomposition of semicarbazones involves a simple cleavage/deprotonation process shown in sequence (20) [68], and (ii) deprotonated cr-ketoximes undergo the rearrangement shown in sequence (21); a reaction favoured by the formation of the stable carboxylate product anion [69]. The Tiemann rearrangement is a base-catalysed solution reaction that is very similar to the proposed gas phase negative ion Beckmann rearrangement. The reaction was first reported over one hundred years ago and is summarised in sequence (22) (Scheme 10 (R = alkyl or aryl)) [70]. It is perhaps ironic that the gas phase Tiemann rearrangement (sequence (23)) is a very minor reaction when deprotonated amidoximes are subjected to collisional activation: the major fragmentation of such systems involves the loss of hydroxylamine shown in sequence (24), a process confirmed by *H and 15N labelling together with product ion studies [71].
2.2. [3,3] Rearrangements 2.2.1. The anionic oxy Cope, Claisen and Carroll rearrangements
The anionic oxy Cope [72] and Claisen (together with the [731 rearrangements associated Carroll [22] rearrangement) are well known solution rearrangements which occur by [3,3] processes through six membered transition states. Our studies of these rearrangements have provided some surprises. Three examples follow. Treatment of diallyl ether with KNH2-NH3 gives hexadien-3-01 (g to h, sequence (25) (Scheme 11)) by a Wittig rearrangement (cf. Scheme 1)
[72,74]. This alcohol is stable to further base treatment, but on heating it is transformed to hex-5-en-l-al by a (neutral) oxy Cope rearrangement [75]. A modification of this scenario occurs in the gas phase. The collisional activation mass spectra of g, h and i (Scheme 11) are all identical: product ion studies indicate that the spectra are those of the anionic oxy Cope product i [76]. Thus in the gas phase, deprotonated diallyl ether first forms the Wittig product which then transforms to the oxy Cope product ion, indicating, at least in this case, that the anionic oxy Cope rearrangement is more facile in the gas phase than in solution. Analogous oxy Cope rearrangements are also observed during bimolecular gas phase reactions [77]. The behaviour of deprotonated ally1 vinyl ether provides an interesting example of a system which undergoes different rearrangements in the condensed and gas phases. Base catalysis of j (Scheme 11) produces the Wittig rearrangement product k [78]. In contrast, product ion studies show that the Wittig product is not formed in the gas phase: instead, the Claisen product 1 is formed exclusively [79]. The third example is that of deprotonated benzyl ally1 acetate. It undergoes the Claisen ester enolate rearrangement (sequence (27)) [80] in both the condensed and gas phases [81], while the associated Carroll rearrangement (sequence (28)) [22] occurs in solution, but not in the gas phase [81]. The non-occurrence of the Carroll rearrangement in the gas phase is shown by the observation that the major fragmentation (loss of ally1 alcohol) from the parent ion does not involve equilibration of the two oxygens. Ph-CHC(0)‘*0C3H5 + (PhCH =C=O) (i.e. C3H5’*O- -+ PhC20-+ C3H5’*0H).
9
P.C.H. Eichinger et aLlInt J. Mass Spectrom. Ion Processes 133 (1994) l-12
the collision induced mass spectra of both deprotonated I-vinylcyclobutanol and its (expected) [ 1,3] rearrangement product (deprotonated cyclohexanone (see sequence (30)) are identical. Product ion studies demonstrate that the characteristic fragmentations are exclusively those of deprotonated cyclohexanone [83].
h
A
$- - QJ I
k
j
(26)
0
P
0
P
*
\ 4
0
-
t
/
0
2.4. The Dieckmann condensation The Dieckmann condensation (e.g. sequence (31)) is a well known reaction of synthetic applicability [84], and it is the only example of this type of cyclisation that we have studied in depth. A number of groups [85-871 have confirmed that this process occurs in the gas phase. For example, product ion studies substantiate the operation of reaction (3 1). MeocO
lvleK0 0 (27)
-
Meole 0
-0
cr
+
MeOH
2.5. Ipso rearrangements
(28) 2.3. [I,31 Anionic rearrangements [ 1,3] Rearrangements (cf. sequence (29) 12), R = alkyl or aryl) are quite rare in densed phase [82]. The rearrangement rarer in the gas phase. It is only a major when release of ring strain is involved. For
yR
_YR
Scheme 12
(Scheme the conis even process example
The Smiles rearrangement is the classical (condensed phase) example of an aromatic ipso nucleophilic substitution reaction (see sequence (32) (Scheme (13)) [24]. This nucleophilic reaction normally requires an activating electron-withdrawing group (e.g. Z = nitro, sulphonyl or halogen) in either the ortho or para positions of the aromatic ring [88]. Generally X is a good leaving group and Y a strong nucleophile [88]. ZC,H,XCH$H,Y-
_c
ZC,H,YCH,CH,X
(32) PhOCH,CH,%
(29)
_t
L
Ph180CH 2CH 2O-
(33) The Smiles rearrangement shown in sequence (33) occurs upon collisional activation in the gas phase without the requirement of an additional electron withdrawing substituent on the ring [89]. ‘*O Labelling (sequence (33)) shows statistical equilibration of the two oxygens prior to the formation of the sole fragmentation product, the
10
P.C.H. Eichinger ef al./Int. J. Mass Spectrom. Ion Processes 133 (1994) I-I2
phenoxide anion. 13CLabelling at Ci of the phenyl ring indicates an ipso (rather than e.g. an ortho) attack, since the product phenoxide anion exclusively fragments by loss of 13C0 upon collisional activation.
3. Concluding remarks The present article demonstrates a significant correspondence between the skeletal rearrangement processes of closed-shell organic anions which occur in both condensed and gas phases. Examples that we have studied which show this general correspondence are: acyloin, acyl oxyacetate, anionic oxy Cope, anionic Wolff, benzilic acid, Dieckmann, Lossen, nitrogen Wittig, Smiles and Wittig rearrangements. Indeed, the evidence available indicates that some of these processes are more facile in the gas phase than in solution, e.g. the oxy Cope and Smiles rearrangements. There are also some cases where we have found reactions occurring in the gas and condensed phases to be quite different. This is presumably because there is a well defined reaction channel of lower energy than that of the expected rearrangement. Examples are: two “Wittig” exceptions (sequences (1) and (26)), the Tiemann rearrangement (sequences (23) and (24)) and the Carroll rearrangement (sequence (28)). Finally, there are those gas phase anionic rearrangements which have no condensed phase anionic counterpart, but which are analogous to the cognate acid catalysed reaction. The negative ion “pinacol/pinacolone” and Beckmann reactions are illustrative of this class of rearrangement. 4. References [l] M.B. Stringer, J.H. Bowie and J.L. Holmes, J. Am. Chem. Sot., 108 (1966) 3888. [2] M. Barber, R.S. Bordoli, G.J. Elliott, R.D. Sedgewick and A.N. Tyler, Anal. Chem. A, 42 (1982) 645. [3] J.H. Bowie, in Mass Spectrometry Specialist Reports, The Chemical Society, London, Vol. 10, 1989, p. 145. [4] J.H. Bowie, Mass Spectrom. Rev., 9 (1990) 349. [5] P.C.H. Eichinger and J.H. Bowie, Int. J. Mass Spectrom. Ion Processes, 110 (1991) 123.
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