Protonations, enolates, pivalic acid

Later, the same group showed that an asymmetric protonation of preformed lithium enolate was possible by a catalytic amount of chiral proton source 23 and stoichiometric amount of an achiral proton source [45]. For instance, when hthium enolate 44, generated from ketene 41 and -BuLi, was treated with 0.2 equiv of 23 followed by slow addition of 0.85 equiv of phenylpropanone, (S)-enriched ketone 45 was obtained with 94% ee (Scheme 4). In this reaction, various achiral proton sources including thiophenol, 2,6-di-ferf-butyl-4-methylphenol, H2O, and pivalic acid were used to provide enantioselectivity higher than 90% ee. The value of the achiral acid must be smaller than that of 45 to accomplish a high level of asymmetric induction. The catalytic cycle shown in Scheme 2 is the possible mechanism of this reaction. 

Regioselective Protonation of Enolates. In a study of the regioselectivity of protonation of allenyl enolates, pivalic acid demonstrated superior selectivity in the example shown (eq 2), allene 1, resulting from protonation at C-2, was formed exclusively when the intermediate enolate was quenched with pivalic acid at-80°C. ... 

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