Allyl-Anion Isomerizations

TABLE 31. Relative energies (kcalmol ) of the cydopropyl-allyl anion transformation and the allyl anion isomerization ... 

Table 1. Relative energies (AAHf, kcal/mol) with respect to the cyclopropyl anion of the transition state of the conrotatory cyclopropyl-allyl anion rearrangement A, the allyl anion B, and the transition state of the allyl anion isomerization C.

It is evident from Table 1 that the activation energy for the allyl anion isomerization is much lower than for the conrotation of the cyclopropyl anion to give the allyl anion. Consequently, in order to verify the predicted conrotatory mode one has to trap the first formed allyl anion before it isomerizes to give the thermodynamically most stable isomer, e.g., in a cycloaddition reaction. Exactly this was possible with 2, 3 and 5. So far, however, a similarly fast reaction has not been found for allyl anions 13). 

The experimental probe of the conformational preferences of allyl anions is based primarily on the kinetics of base catalyzed isomerization reactions of olefins and many results have been summarized by Bank180). Kinetic data of base catalyzed isomerization of 1-butene181) show that the less stable cis isomer of 2-butene is formed through a thermodynamically more stable allylic anion ... 

Additional evidence for the greater stability of the cis conformation of allylic anions is provided by other base catalyzed isomerization studies of 1-butene and 1-pentene. It was found that the thermodynamically less stable cis isomers of 2-butene and 2-pentene were the major products of the reaction182-186). Furthermore, m-2-butene isomerizes, under the same conditions, faster than the tram isomer to give 1-butene. 

Not only can acids catalyze olefin isomerization, but strong bases can also effect isomerization. These base-catalyzed isomerizations proceed through proton abstraction of an allylic hydrogen atom followed by protonation of the allylic anion to regenerate either the original or the isomeric olefin ... 

As in the case of double bond isomerization of butenes, the double bond isomerization of VBH is considered to be initiated by abstraction of an allylic proton from a tertiary carbon atom to give an allylic anion that may be stabilized by metal ions, yielding the E- and Z-EBH isomers. 

Base-catalyzed (NaOMe) isomerizations of 1H- and AH-azepines to 3H-azepines are well known and most likely proceed via allylic anion intermediates (80TL595,72CB982). 

The exhaustive controlled-potential reduction of 6-chioro-l-phenylhex-l-yne at — 1.57 V in dimethylformamide containing tetrabutylammonium perchlorate gave a mixture of products. among which was ( >(2-phcnylvinyl)cyclobutane (9).11 It is probable that the mechanism involves initial isomerization of the acetylene to an allene 8 which is reduced at — 1.57 V to the radical anion. Protonation and further onc-clectron reduction then yield the allylic anion. An intramolecular nucleophilic substitution eventually gives the cyclobutane.11... 

Selectivities to various isomers are more difficult to predict when metal oxides are used as catalysts. ZnO preferentially produced 79% 1-butene and several percent of a -2-butene [624-64-6] (75). CdO catalyst produced 55% 1-butene and 45% ar-2-butene. It was also reported that while interconversion between 1-butene and or-2-butene was quite facile on CdO, cis—trans isomerization was slow. This was attributed to the presence of a 71-allyl anion intermediate (76). High or-2-butene selectivities were obtained with molybdenum carbonyl encapsulated in zeolites (77). On the other hand, deuteration using Th02 catalyst produced predominandy the 1,4-addition product, trans-l-bu. en.e-d2 with no isotope scrambling (78). 

A pentadienyl cation has the same number of ji-electrons as the allyl anion, and its electrocyclic reactions will be conrotatory. In terms of the Woodward-Hoffmann rule, it can be drawn 4.82 as an allowed [K4a] process. It has been shown to be fully stereospecific, with the stereo isomeric pentadienyl cations 4.83 and 4.85 giving the stereoisomeric cyclopentenyl cations 4.84 and 4.86 in conrotatory reactions, followed in their NMR spectra. 

Prototropic isomerization of the propene molecule in the presence of hydroxide ion has been studied using ab initio and DFT methods in the gas phase and in DMSO solution 152 the mechanism involves formation of an intermediate complex of the allyl anion with a water molecule. 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

The study of the spectra of living polymer systems is valuable from a more practical point of view and indicates that the term has some limitations. At room temperature all the polymer-lithium compounds in hydrocarbon solvents show spectra which are stable for considerable time intervals. At elevated temperatures spectral changes occur at least for polystyryllithium, which indicate that isomerization reactions are occurring 4). Most of them display instability in solvents containing appreciable amounts of more polar constituents such as tetrahydrofuran. This effect was first noticed for poly-sty rylsodium 11) and has been attributed to the elimination of sodium hydride, followed by a subsequent reaction to form the more stable substituted allyl anion 21). 

Tolbert [1-3] has summarized the various photochemical pathways open to allyl anions in general, including electron photoejection. Other pathways include ring closure to a cyclopropyl anion, E-Z isomerization, protonation and a-bond cleavage. With the availablity of a greater number of photochemical pathways for allyl anions in general, it is not surprising that electron photoejection is not an important pathway for aryl-stabilized allyl anions. One of the most studied... 

It was Smith and Showell who inferred that 195 is deprotonated to give the cyclopropyl anion 196 which isomerizes to the allyl anion 197 (the cyclopropyl allyl anion rearrangement is treated in detail in Section IV.C.). 

The problems with a-deprotonations of salts of carboxylic acids are also nicely exemplified by a study of Ford and Newcomb with the isomeric acids 218 and 222. Reaction of the cis, trans acid 218 with LDA in THF at 0°C for 30 min resulted in the desired allyl anion 220 which was protonated to give the two isomeric a-benzylcinnamates (221). 

The cyclopropyl-allyl anion case itself turned out to be more of a problem. Mulvaney and Savage reacted the rruns,rruns-l,2,3-triphenylcyclopropane (316) with n-butyllithium/TMEDA which led to one (or more) of the isomeric 1,2,3-triphenylallyl anions (317). 

The cyclopropyl anion 320 should be the intermediate both in the cis-trans isomerization to give 322 (NaOMe in MeOH) and in the ring-opening reaction (NaH in DMF) to give 321" whose stereochemistry is unknown. From these studies it seemed reasonable that cyclopropyl anions did indeed thermally isomerize to give allyl anions. 

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