Effective activation of the chiral salen/Ti(OiPr)₄ catalyst with achiral phenolic N-oxides as additives in the enantioselective cyanosilylation of ketones

The activation of chiral titanium(IV) complexes with phenolic N-oxides additives has been found to provide an alternative strategy for the asymmetric cyanosilylation of ketones. By using 10 mol % of chiral salen-titanium(IV) complex in combination with 1 mol % achiral phenolic N-oxide as an additive, aromatic, aliphatic and heterocyclic ketones have been converted into the corresponding cyanohydrin trimethylsilyl ethers in 58-96% yields with 56-82% ee. Several factors concerning the reactivity and enantioselectivity have been discussed. A catalytic cycle based on experimental phenomena and studies has been proposed to explain the origin of this activation and the asymmetric induction.