Solvent effects in the enantioselective hydrogenation of ethyl benzoylformate

Solvent effects in the enantioselective hydrogenation of ethyl benzoylformate (EBF) to (R)-ethyl mandelate over (-)-cinchonidine (CD)-modified Pt/Al₂O₃ catalyst were studied in a semi-batch reactor. Solvents of different nature were used: protic (2-propanol, 1-propanol, 1-octanol and ethanol), aprotic polar (methyl acetate, acetone, ethyl acetate and tetrahydrofuran) and apolar solvents (methyl cyclohexane and toluene). The effects of pure solvents and binary solvent mixtures on hydrogenation rates and enantioselectivity were investigated. The highest enantiomeric excess (ee) of 72 % was obtained in ethyl acetate, decreasing nonlinearly with increasing dielectric constant (ε) being close to 20 % in ethanol. The highest value of the initial hydrogenation rate was obtained in the apolar solvents (21 mmol dm^-3 min^-1g _cat ^-1) while the lowest one was observed in tetrahydrofuran (2 mmol dm^-3 min^-1 g _cat ^-1). A kinetic model was proposed for the enantioselectivity dependence on dielectric constant based on Kirkwood treatment. The non-linear dependence of ee on (ε) was included in the model to describe quantitatively the variation of ee in different solvents. The results showed a good fit for ee as a function of ε.