Silyl organocatalytic protonation

Scheme 7.4 Simplified organocatalytic protonation of a silyl enol ether [6].

Recently, Levacher and coworkers developed the first organocatalytic enantioselective protonation of silyl enol ethers S using readily available cinchona alkaloids [5]. 

Table 7.1 Levacher s organocatalytic enantioseiective protonation of silyl enol ethers [5]...

Poisson T, Dalla V, Marsais F, Dupas G, Oudeyer S, Levacher V. Organocatalytic enantioselective protonation of silyl enolates mediated by cinchona alkaloids and a latent source of HP. Angew. Chem. Lnt. Ed. 2007 46 7090-7093. 

This chapter covers organocatalytic processes where the enantio-determining step involves a proton transfer. Most of the organocatalytic processes outlined herein share a key step in common, i.e. the enantioselective protonation of an enolate or enol intermediate species obtained in situ from various precursors. The main organocatalytic approaches reported in the literature may be classified according to the nature of these precursors (Scheme 3.1). Special emphasis will be giveu to decarboxylation of malonates, addition of protic nucleophiles (NuH) to keteues or to a,P-unsaturated carbonyl compounds. We will also focus on tautomerisation of enols formed in situ via photodeconjugation of a,P-unsaturated esters and on the protonation of silyl enolates. Finally, a last section will be devoted to other miscellaneous substrates. 

Quite surprisingly, whereas the chemistry of silyl enolates is considered as one of the cornerstones in organic synthesis, only a few papers have been devoted to organocatalytic enantioselective protonation of this class of substrates. The first organocatalyzed process was reported by Levacher et al. [33] by making use of cinchona alkaloids as catalysts and a latent source of hydrogen fluoride. The mechanism postulated by the authors is illustrated in Scheme 3.38. Basically, the reaction between benzoyl fluoride and ethanol in the presence of a catalytic amount of... 

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