Homoenolates tautomerism

The proposed catalytic cycle is shown in Scheme 35 and begins with the imida-zolylidene carbene adding to the enal. Proton transfer provides acyl anion equivalent XLVII, which may be drawn as its homoenolate resonance form XLVIII. Addition of the homoenolate to aldehyde followed by tautomerization affords L the precursor for lactonization and regeneration of the carbene. 

The first step of this unprecedented umpolung reaction is identical to the standard MBH mechanism. Only the tautomerization event differentiates between a- (enolate) and -functionalization (homoenolate). 

This chapter describes the chemistry of carbanionic species having a carbonyl function at the -posi-tion, relative newcomers among synthetically useful carbanionic species. The chemistry of homoenolates is complicated by the problem of tautomerism between oxyanionic and carbanionic isomers through a process that formally involves homoconjugation. The synthetic problem caused by this tautomerism is much more severe in homoenolate chemistry (Scheme 1) than in enolate chemistry (Scheme 2), which also has a similar problem, since the carbanionic tautomer (1 Scheme 1) once formed often undergoes rapid and irreversible cyclization to the oxyanionic tautomer (2), and rarely acts as a carbon nucleophile. Until recently, therefore, chemists have not been able to make use of carbanionic homoenolates for organic syntheses. " However, a large number of useful homoenolate reactions have recently been discovered, and are described in this chapter. 

The Scheidt group reported a highly diastereo- and enantioselective NHC-catalyzed reaction of a,p-unsaturated aldehydes with nitrones to afford y-amino esters. It is postulated that a rare six-membered heterocycle is generated as the initial product of the reaction, which gives the final y-amino ester product upon the addition of an alcohol. The mechanism for this reaction involves the addition of the homoenolate equivalent to the nitrone as the stereochemical-determining step, and catalyst turnover is promoted by an intramolecular acylation after the tautomerization of enol to acyl azolium (Scheme 7.60). 

A-heterocyclic carbenes (NHC) are also efficient organocatalytic tools for generating homoenolate equivalents from a,P-unsaturated aldehydes. These reactive intermediates display a versatile reactivity in a number of catalytic transformations attesting to an important synthetic potential [38]. Recently, Scheldt et al. [39a] accomplished the first enantioselective protonation of a homoenolate species generated by a chiral NHC precursor 93 in the presence of DIE A and an excess of ethanol as the achiral proton source (Scheme 3.46). The suggested mechanism involves an initial addition of NHC 93 to the enal 89 followed by a formal 1,2-proton shift resulting in the formation of the chiral homoenolate equivalent 91. A diastereose-lective P-protonation/tautomerization sequence leads to the acyl triazolinium inter-... 

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