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

Organo-osmium­(II)
16-electron complexes [OsII(η6-arene)­(R-PhDPEN)]
(where η6-arene = para-cymene or
biphenyl) can catalyze the reduction of prochiral
ketones to optically pure alcohols in the presence of a hydride source.
Such complexes can achieve the conversion of pyruvate to unnatural d-lactate in cancer cells. To improve the catalytic performance
of these osmium complexes, we have introduced electron-donor and electron-acceptor
substituents (R) into the para (R1) or meta (R2) positions of the chiral R-phenyl-sulfonyl-diphenyl­ethyl­enediamine
(R-PhDPEN) ligands and explored the reduction of quinones, potential
biological substrates, which play a major role in cellular electron
transfer chains. We show that the series of [OsII(η6-arene)­(R-PhDPEN)] derivatives exhibit high turnover frequencies,
enantioselectivities (>92%), and conversions (>93%) for the
asymmetric
transfer hydrogenation (ATH) of acetophenone-derived substrates and
reduce duroquinone and menadione to their di-alcohol derivatives.
Modeling of the catalysis using density functional theory (DFT) calculations
suggests a mechanism involving formic acid deprotonation assisted
by the catalyst amine groups, phenyl-duroquinone stacking, hydride
transfer to OsII, possible CO2 coordination,
and tilting of the η6-arene ring, followed by hydride
transfer to the quinone. These findings not only reveal subtle differences
between Ru­(II) and Os­(II) catalysts, but also introduce potential
biological applications.