Other Carbanion Rearrangements

It is possible to enhance the rate of a Claisen rearrangement, especially in the enolate Claisen reaction. Denmark et al. showed that other carbanionic centers accelerate the Claisen rearrangement (as in Table 11.23).467 Generation of the anion of sulfone 631 (sec. 8.6.A) with various bases led to acceleration of the reaction relative to the thermal reaction of 631 and also influenced the syn/anti ratio (632/633). In general, a donor group at the allyl position accelerates the rate and the presence of an amino stabilizing group increases the rate even more. 

The mechanism of this rearrangement has been investigated by a number of workers, showing it to be a carbanion mechanism similar to that found in other prototropic rearrangements. 

It is generally felt that other Kharasch reactions involve radical intermediates, since 2,3-dimethyl-2,3-diphenylbutane is formed when cumene is present, and rearrangement products are observed in the neophyl chloride system, although some of the latter might arise from the carbanion rearrangement reactions described in Section II, D. 

Solanesol and other prenyl alcohols are important as metabolites in mulberry and tobacco leaves and in the synthesis of isoprenoid quinones. Hence, Sato and collaborators107 have developed a stereoselective synthesis of all-trans-polyprenol alcohols up to C50. Construction of the requisite skeletons was accomplished by the alkylation of a p-toluenesulphonyl-stabilized carbanion, followed by reductive desulphonylation of the resulting allylic sulphonyl group. This was achieved most efficiently by the use of a large excess of lithium metal in ethylamine (equation (43)), although all reaction conditions led to mixtures. The minor product results from double bond rearrangement. 

This mechanism is exactly analogous to the allylic rearrangement mechanism for nucleophilic substitution (p. 421). The UV spectra of allylbenzene and 1-propenylbenzene in solutions containing NH2 are identical, which shows that the same carbanion is present in both cases, as required by this mechanism. The acid BH protonates the position that will give the more stable product, though the ratio of the two possible products can vary with the identity of BH". It has been shown that base-catalyzed double-bond shifts are partially intramolecular, at least in some cases. The intramolecularity has been ascribed to a conducted tour mechanism (p. 766) in which the base leads the proton from one carbanionic site to the other ... 

Carbanions can take part in most of the main reaction types, e.g. addition, elimination, displacement, rearrangement, etc. They are also involved in reactions, such as oxidation, that do not fit entirely satisfactorily into this classification, and as specific—ad hoc—intermediates in a number of other processes as well. A selection of the reactions in which they participate will now be considered many are of particular synthetic utility, because they result in the formation of carbon-carbon bonds. 

Simple 1,2-shifts of alkyl, from carbon to carbon, that are carbanionic in character are essentially unknown. Examples are known, however, in which alkyl is involved in a 1,2-shift from other atoms such as N and S to a carbanion atom—the Stevens rearrangement ... 

Other reactions involving the addition of carbanions are reactions like Perkin s reaction, Claisen condensation, benzilic acid rearrangement and Michael addition. 

Carbanions play critical roles in a wide variety of reaction pathways. As stated in the Introduction, this chapter will not focus on the synthetic utility of carbanions, but will instead focus on their mechanistic significance. In this section, a sample of important reaction mechanisms that involve transient or relatively short-lived car-banion intermediates will be introduced. As you will see, the key element in these mechanisms is the ability to form a carbanion that is reasonably stable, and often the kinetics of the reactions are dominated by carbanion stability. The role of carbanion intermediates in elimination reactions will be presented in some detail as a way to illustrate some of the methods that have been developed to probe for carbanion intermediates in reaction mechanisms. Other processes including additions and rearrangement reactions will be presented in less detail, but the role of carbanion stability in these reactions will be outlined. 

Carbanions also appear as intermediates in rearrangement processes. In some cases, this involves the rearrangement from one carbanion to another, but in other cases. 

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