Studies on Asymmetric Hydrogenation

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The development of practical methods effecting stereoselective hydrogenation of simple ketones is highly desirable, since the existing homogeneous catalysts lack reactivity and/or stereoselectivity. New Ru (II) complexes having a formula of trans-RuCl_2 (phosphine) _2 (1,2-diamine) or trans-RuCl_2 (diphosphine) (1,2-diamine) in 2-propanol containing alkaline base effect facile hydrogenation of a wide variety of simple ketones. For example, hydrogenation of acetophenone with a substrate/catalyst molar ratio of 2,400,000 under 45 atm at 30'C gives 1-phenylethanol quantitatively. The turnover frequency (TOF), defined as the moles of product per mol Ru catalyst per hour, approaches 228,000. The reaction proceeds at a reasonable rate even under atmospheric pressure of hydrogen.This mixed-ligand catalyst effects chemoselective hydrogenation of a carbonyl function in the presence of an olefinic or internal acetylenic bond. The reaction using an equimolar mixture of heptanal and 1-octene displays a carbonyl/olefin selectivity as high as 1500. Furthermore, a wide range of ketones and aldehydes possessing a carbon*carbon multiple bond are hydrogenated preferentially at the carbonyl group, leading to unsaturated alcohols. Both conjugated and unconjugated enals or enones can be used.Highly enantioselective hydrogenation of simple ketones is achievable when the Ru catalyst is modified by 2,2'-bis (diphenylphosphino)-1,1'-binaphthyl (BINAP) and certain chiral 1,2-diamines such as 1,2-diphenylethylenediamine and 1-alkyl-2,2-bis (p-methoxyphenyl)-1,2-ethylenediamine. The hydrogenation of alkyl aryl ketones and linear alpha, beta-unsaturated ketones under 1-8 atm of H_2 at room temperature affords the corresponding chiral alcohols in near 100% yield and in up to 99% ee.This procedure is particularly useful for a large-scale reaction because of the low cost of the catalyst, operational simplicity, and environmental consciousness.