Enantioselective hydrogenation of 1-phenyl-1,2-propanedione

Enantioselective hydrogenation of a diketone, 1-phenyl-1,2-propanedione was studied in a pressurized reactor at 5 bar and at 0-25 degreesC in different solvents: ethanol, ethyl acetate, and dichloromethane over platinum catalysts. Both in situ modification (simultaneous addition of the reagent and the modifier) and pre-modification (preadsorption of the modifier prior to the reagent) of the catalyst were investigated using cinchonidine as catalyst modifier. Racemic hydrogenation proceeded with nearly the same rate as the selective hydrogenation in the presence of the catalyst modifier. The kinetic results revealed that the hydrogenation of the carbonyl group attached to the phenyl ring was preferred, the main product being 1-hydroxy-1-phenylpropanone; the ratio between 1-hydroxy-1-phenylpropanone and 2-hydroxy-1-phenylpropanone was about 11. The most effective and enantioselective catalyst was obtained by in situ modification in dichloromethane yielding in 67 mol% of (R)-1-hydroxy-1-phenylpropanone, corresponding to the enantiomeric excess of 64%. The enantiomeric excess was independent of the reactant conversion. In the second hydrogenation step the main product among diols was (1R,2S)-1-phenyl-1,2-propanediol. (C) 2001 Elsevier Science.