Cationic oxorhenium chiral salen complexes for asymmetric hydrosilylation and kinetic resolution of alcohols

High valent transition metal oxo complexes are ubiquitous in catalytic oxidation and oxygen atom transfer reactions, while their application in catalytic reductions is only being realized in the past few years. Reaction of ReOCl(3)(PPh(3))(2) with appropriate chiral salen ligands, followed by oxo abstraction with a silylium cation, afforded several cationic oxorhenium(V)-saldach complexes 1-2, including a rare trinuclear rhenium cluster (2), all of which have been fully characterized. These complexes are effective catalysts for hydrosilylation of carbonyl compounds and silane alcoholysis, although the enantioselectivity for both reactions is rather low. An intermediate, featuring a partially reduced saldach ligand (a salen/salan hybrid), was identified in reaction of 1a with Et(3)SiH. The reaction mechanism is discussed in light of results from a crossover experiment. (C) 2007 Elsevier B.V. All rights reserved.