Solvent effects in enantioselective hydrogenation of 1-phenyl-1,2-propanedione

Solvent effects in enantioselective hydrogenation of 1-phenyl-1,2-propanedione (A) were investigated in a batch reactor over a cinchonidine modified Pt/Al2O3 catalyst. The effect of different solvents, binary solvent mixtures and solvent dielectric constant on regio- and enantioselectivity as well as on the hydrogenation rate were studied. The hydrogen solubility in different solvents and the dielectic constants of solvent mixtures were measured. The highest enantiomeric excesses (ee) of (R)-l-hydroxy-l-phenylpropanone (B) (65%) were obtained in toluene. The ee decreased non-linearly with an increasing solvent dielectric constant being close to zero in methanol. The role of the reactant conformation in different solvents was evaluated by quantum chemical calculations and the role of the Open(3) conformer of the modifier, cinchonidine was discussed. The dependence of ee on the dielectric constant could not solely be attributed to the abundance of the Open(3) conformer of cinchonidine in the liquid phase. A possible involvement of additional factors was proposed and discussed. The non-linear dependence of the ee on the dielectric constant was included in a kinetic model to describe quantitatively the variation of the ee in different solvents. (C) 2002 Elsevier Science B.V. All rights reserved.