A new class of "tethered" ruthenium(II) catalyst for asymmetric transfer hydrogenation reactions

Ruthenium dimer 4 is converted directly to monomeric asymmetric transfer hydrogenation catalyst 2 under the conditions employed for ketone reduction. Using 0.25 mol % of either 4 or 0.5 mol % of 2 in formic acid/triethylamine, it is possible to achieve ketone reduction in quantitative conversion and with ee's as high as 98%. Complex 2 is a robust "single-reagent" catalyst which offers significant scope for modification toward specific substrates. The synthesis and applications of an analogous complex derived from (1R,2S)-norephedrine are also described. Copyright © 2004 American Chemical Society.