| Research Abstract |
The development of practical methods effecting stereoselective hydrogenation of simple ketones is highly desirable, since the existing homogeneous catalysts lack reactivity and/or stereoselectivity. A new Ru (II) complex, trans-RuCl_2 [(S)-xylbinap][(S)-daipen] or the R, R enantiomer acts as an excellent precatalyst for asymmetric hydrogenation of simple ketones (XylBINAP=2, 2'-bis (di-3, 5-xylylphosphino)-1, 1'-binaphthyl. DAIPEN=1, 1-dianisyl-2-isopropyl-1, 2-ethylenediamine). The reaction is conducted with a substrate/catalyst molar ratio of up to 100000 in 2-propanol containing alkaline base under 1-80 atm (mostly <10 atm) of H_2 at room temperature. A wide variety of prochiral ketones are hydrogenated with a high degree of enantioselection. Hydrogenation of open-chain α, β-unsaturated ketones proceeds selectively at the C=O linkage to afford chiral allylic alcohols with a high ee, solving a long-standing synthetic problem. Hydrogenation of cyclopropyl ketones gives the correspondi
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ng carbinols in high ee without skeletal change. Alkyl aryl ketones with various substituents are hydrogenated with a consistently high enantioselectivity. 2, 2, 2-Trifluoroacetophenone and the ring-substituted derivatives are also usable as substrates. The reaction is tolerant of aromatic halides and CF_3, OCH_2, COOCH(CH_3)_2, NO_2, and NH_2 groups. Various ortho-substituted benzophenones are hydrogenated to chiral benzhydrols with consistently high ee. No over-reduction forming to diarylmethanes are observed. The catalyst system is extremely effective for enantioselective hydrogenation of hetero-aromatic ketones. The substrates with furan, thiophene, pyrrole, thiazole, or pyridine rings are hydrogenated with the same asymmetric sense to give chiral secondary alcohols in consistently high ee. The electronic properties, Lewis basic tendency, and steric size of the heterocycles appear to have little effect on the extent of enantioselectivity. This procedure is particularly useful for a large-scale reaction because of the low cost of the catalyst, operational simplicity, and environmental consciousness. Less
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