High-valent metal-oxo compounds are often used to mediate or catalyze oxidations of organic substrates. Nolin et al. have pursued the counterintuitive approach of using a rhenium-oxo complex to catalyze reduction. Rather than transferring oxygen to a hydrocarbon, the Re=O group facilitates hydride transfer from a silane to an imine. Unlike most catalysts used for this type of reaction, the Re complex is already in a high oxidation state and therefore fully stable in the open atmosphere. By appending a chiral bis(oxazoline) derivative to the Re center, the authors achieved enantioselective reductions of a broad range of aromatic imines under air at room temperature, with yields of 50 to 90% and very high enantiomeric excesses (92 to >99%). The imine nitrogens were protected with phosphinyl groups that could then be removed hydrolytically after reduction. The catalyst selectively reacts with the imine group even in the presence of esters and olefins, leading to a variety of chiral amines of potential use as pharmaceutical precursors. -- JSY
J. Am. Chem. Soc. 10.1021/ja050831a (2005).
