Preformed Enolates and Equivalents

On the asymmetric point of view, it can be summarized that the level of enantioselectivity obtained is highly dependent on the nature of the substrate, 

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

Entry Silyl enol ethers Yield (%) ee (%) Configuration0 

Among the several screened cinchona derivatives, (DHQ)2AQN 6 was selected for a full optimization of the conditions (solvent and temperature). The reaction was carried out in DMF with a combination of benzoyl fluoride and ethanol as a latent source of HF. Enantioselectivities up to 92% ee were obtained with tetralones trimethylsilyl enol ethers Sa-d using 10 mol% of organocatalyst 6 (Table 7.1, entries 1-4). Indanone silyl enol ethers Se-g afforded the corresponding ketones 7e-g with lower selectivities (64—74% ee, Table 7.1, entries 5-7) while the 2,2,6-trimethyl-cyclohexanone was obtained with only 58% ee, the reaction being conducted at — 10 °C (data not shown in Table 7.1). 

Since the first report by Duhamel, most of the enantioseiective protonations were involving metal enolates and, therefore, the use of stoichiometric amounts of the chiral proton sources although catalytic versions are now emerging [7]. This subsection has summarized the three noticeable examples describing the 

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