Allylic and benzylic carbanions

AUyl transfer reactions, 73, 1 Allylic alcohols, synthesis from epoxides, 29, 3 by Wittig rearrangement, 46, 2 Allylic and benzylic carbanions, heteroatom-substituted, 27, 1 Allylic hydroperoxides, in... 

For a review of allylic and benzylic carbanions substituted by hetero atoms, see Biellmann Duccp Org. React. 1982, 27. 1-344. 

Many interesting and important synthetic applications of 1,1-diphenylethylene and its derivatives in polymer chemistry are based on the addition reactions of polymeric organolithium compounds with 1,1-diphenylethylenes. Therefore, it is important to understand the scope and limitations of this chemistry. In contrast to the factors discussed with respect to the ability of 1,1-dipheny-lalkylcarbanions to initiate polymerization of styrenes and dienes, the additions of poly(styryl)lithium and poly(dienyl)lithium to 1,1-diphenylethylene should be very favorable reactions since it can be estimated that the corresponding 1,1-diphenylalkyllithium is approximately 64.5kJ/mol more stable than allylic and benzylic carbanions as discussed in Sect. 2.2 (see Table 2). Furthermore, the exothermicity of this addition reaction is also enhanced by the conversion of a tt-bond to a more stable a-bond [51]. However, the rate of an addition reaction cannot be deduced from thermodynamic (equilibrium) data an accessible kinetic pathway must also exist [3]. In the following sections, the importance of these kinetic considerations will be apparent. 

However, like carbocations, allylic and benzyllic carbanions are lower than ordinary carbanions due to resonance stabilization. 

Certain functional groups may be protected from reduction by conversion to anions that resist reduction. Such anions include the alkoxides of allylic and benzylic alcohols, phenoxide ions, mercaptide ions, acetylide ions, ketone carbanions, and carboxylate ions. Except for the carboxylate, phenoxide, and mercaptide ions, these anions are sufficiently basic to be proton-ated by an alcohol, so they are useful for protective purposes only in the... 

However, as in the preceding case, the hydrogen (both allylic and benzylic) in P-position with respect to the carbanionic site is acidic and metalation occurs readily at the expense of another living polymer molecule. Therefore, the fraction of the macromolecules bearing active carbon-carbon double bonds at a chain end is 0.5. 

Selenium-stabilized carbanions behave as excellent nucleophiles and react with primary alkyl bromides or iodides, allylic and benzylic bromides, epoxides, oxeta-nes, disulfides, trialkylsilyl chlorides, aldehydes, ketones, carbon dioxide, dime-thylformamide, acid chlorides or alkyl chloroformates. With conjugated enones, in the presence of HMPA as cosolvent, the 1,4-addition product is essentially obtained. 

Sufficient reactivity towards the added electrophiles below the temperatures of carbanion decomposition or racemization. Usually, aldehydes, methyl iodide, allylic and benzylic bromides, and trialkylsilyl chlorides are the least problematic electrophiles in this context... 

Carbanions, like carbocations, may be stabilized by resonance delocalization the allyl and benzyl anions are relatively stable. The enolate anion, 4.77, is even more so, since in one of the resonance forms, the negative charge is located on the electronegative oxygen atom. The stability of carbanions will be very important to our discussions of the values for organic compounds in Chapter 8. 

Radical stability is, generally, related to carbocation stability. Thus, tertiary radicals are the most stable, and primary ones the least so. As with carbanions and carbocations, resonance delocalization increases stability the allylic and benzylic radicals are more stable than alkyl radicals. Notice that in 4.80 and 4.81, we use single-headed arrows to move one electron at a time in generating the resonance forms. 

The primary allylic carbanion apparently predominates and reacts with aromatic to yield the alkenylbenzene and regenerate the benzylic carbanion [Reaction (27)]. 

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