Enantioselective Cyanosilylation of Ketones Catalyzed by a Chiral Oxazaborolidinium Ion

The chiral oxazaborolidinium salt 1 (X = TfO) is an excellent catalyst for the cyanosilylation of
methyl ketones promoted by trimethylsilyl cyanide and diphenylmethyl phosphine oxide as co-reactants
(to generate Ph2MePOTMS(NC:) as a reactive intermediate). The face selectivity of this reaction parallels
that previously observed for the corresponding reaction of aldehydes. A unifying and rational mechanistic
explanation is provided for these enantioselective reactions. Evidence is presented to support the importance
of α-C−H···O hydrogen bonding, π,π-interaction of the complexed ketonic carbonyl with the mexyl group
of 1, and an early transition state for high enantioselectivity. The cyanosilylation reaction described herein
provides access to many useful chiral compounds.