Quinone Reduction by Organo-Osmium Half-Sandwich Transfer Hydrogenation Catalysts

Organo-osmium(II) 16-electron complexes [OsII(?6-arene)(R-PhDPEN)] (where ?6-arene = para-cymene or biphenyl) can catalyze the reduction of prochiral ketones to optically pure alcohols in presence a hydride source. Such achieve conversion pyruvate unnatural d-lactate cancer cells. To improve catalytic performance these osmium complexes, we have introduced electron-donor and electron-acceptor substituents (R) into para (R1) meta (R2) positions chiral R-phenyl-sulfonyl-diphenylethylenediamine (R-PhDPEN) ligands explored quinones, potential biological substrates, which play major role cellular electron transfer chains. We show that series derivatives exhibit high turnover frequencies, enantioselectivities (>92%), conversions (>93%) for asymmetric hydrogenation (ATH) acetophenone-derived substrates reduce duroquinone menadione their di-alcohol derivatives. Modeling catalysis using density functional theory (DFT) calculations suggests mechanism involving formic acid deprotonation assisted by catalyst amine groups, phenyl-duroquinone stacking, OsII, possible CO2 coordination, tilting ring, followed quinone. These findings not only reveal subtle differences between Ru(II) Os(II) catalysts, but also introduce applications.