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The Hydrogenation/Transfer Hydrogenation Network:  Asymmetric Hydrogenation of Ketones with Chiral η6-Arene/N-Tosylethylenediamine−Ruthenium(II) Catalysts

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posted on 12.07.2006, 00:00 authored by Takeshi Ohkuma, Noriyuki Utsumi, Kunihiko Tsutsumi, Kunihiko Murata, Christian Sandoval, Ryoji Noyori
Chiral η6-arene/N-tosylethylenediamine−Ru(II) complexes, known as excellent catalysts for asymmetric transfer hydrogenation of aromatic ketones in basic 2-propanol, can be used for asymmetric hydrogenation using H2 gas. Active catalysts are generated from RuCl[(S,S)-TsNCH(C6H5)CH(C6H5)NH2](η6-p-cymene) in methanol, but not 2-propanol, or by combination of Ru[(S,S)-TsNCH(C6H5)CH(C6H5)NH](η6-p-cymene) and CF3SO3H or other non-nucleophilic acids. This method allows, for the first time, asymmetric hydrogenation of simple ketones under acidic conditions. Hydrogenation of base-sensitive 4-chromanone and its derivatives with the S,S catalyst proceeds in methanol with a substrate-to-catalyst molar ratio of 1000−3000 (10 atm) to 7000 (100 atm), giving (S)-4-chromanols with 97% ee quantitatively. The reaction can be achieved even on a 2.4 kg scale. The mechanistic rationale for the catalytic efficiency is presented.