Homoenolates reaction

In 1977, an article from the authors laboratories [9] reported an TiCV mediated coupling reaction of 1-alkoxy-l-siloxy-cyclopropane with aldehydes (Scheme 1), in which the intermediate formation of a titanium homoenolate (path b) was postulated instead of a then-more-likely Friedel-Crafts-like mechanism (path a). This finding some years later led to the isolation of the first stable metal homoenolate [10] that exhibits considerable nucleophilic reactivity toward (external) electrophiles. Although the metal-carbon bond in this titanium complex is essentially covalent, such titanium species underwent ready nucleophilic addition onto carbonyl compounds to give 4-hydroxy esters in good yield. Since then a number of characterizable metal homoenolates have been prepared from siloxycyclopropanes [11], The repertoire of metal homoenolate reactions now covers most of the standard reaction types ranging from simple... 

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. 

Scheme 7,63 NHC-catalyzed asymmetric homoenolate reaction of enals with nitroalkenes reported by Liu.

In order to separate structural effects from the electronic differences of these two catalyst classes. Bode synthesized chiral imidazolium salt 57 (Scheme 14.28). This allowed direct comparison of imidazolium versus triazolium precatalysts across a number of different reaction manifolds including those involving the catalytic generation of homoenolate equivalents, ester enolate equivalents, and acyl anions. These studies conclusively demonstrated that imidazolium-derived catalysts are superior for homoenolate reactions with less reactive electrophiles, while the triazolium-derived pre-catalysts are preferred for all other reactions. Interestingly, from the currently published body of the work, it does not appear to be any effects from the counterion of the azolium pre-catalysts in the presence of bases. 

---