Catalytic Asymmetric Hydrogenation of Ketones Using non-Phosphine-Based N4 Ligands

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Homogeneous asymmetric hydrogenation of ketones using chiral diphosphine-Ru(II) complexes has been established as one of the most reliable strategies for the synthesis of optically active secondary alcohols since our Ru(OCOCH-3)-2(binap)/HCl combined catalyst made its debut in 1987. Having apparently started a trend with BINAP. Ru systems, a tremendous number of hydrogenation-active Ru complexes with trivalent phosphorus ligating atoms have been reported. The effectiveness has been examined and compared with the" privileged" BINAP system and its derivatives. In comparison, asymmetric hydrogenation using non-phosphine-based Ru complexes is still in the developmental stage. We have designed and synthesized Ph-BINAN-H-Py(3, 3'-Ph, Ph-N2, N2'-bis(pyridin-2-ylmethyl)-1, 1'-binaphthyl-2, 2'-diamine) which is characterized by a) inside sp^3N/outside sp^2N, b) donor-acceptor bifunctional ability, and c) cis-alpha-selective formation of octahedral metal complexes. The utility of ligands has been demonstrated in asymmetric hydrogenation of aromatic ketones in combination with a Ru-π-allyl precursor. The reaction quantitatively proceeds with the S/C ratio of over 10, 000 with up to 99 : 1 enantioselectivity. The success using non-phosphine ligand has made a new trail to asymmetric hydrogenation chemistry.