Enantioselective hydrogenation is one of the most versatile methods of asymmetric synthesis, with heterogeneous catalysis, using chiral modifiers, rapidly becoming an alternative to the “traditional” homogeneous methods. The role of modifiers in asymmetric hydrogenations is to enhance catalysis, with the bonding mode and geometry of adsorption being important, as well as the modifier concentration and the type and position of the substituent groups in the aromatic ring.

Ultrasonic irradiation (sonication) is known to be beneficial in catalytic asymmetric hydrogenations. Sonication removes catalyst surface impurities, and gives enhanced adsorption to the chiral modifiers.

Now a team from Michigan Technological University, Houghton, U.S.A. (S. C. Mhadgut, I. Bucsi, M. Török and B. Török, Chem. Commun., 2004, (8), 984–985; doi: 10.1039/b315244h) has revisited the Pd-catalysed, proline-modified, asymmetric hydrogenation of isophorone (3,3,5-trimethyl-2-cyclohexen-1-one (with a C=C bond)). They examined the catalyst, the modifier and the effects of sonication.

Pd/Al₂O₃ was found to give a better, though low, enantiomeric excess (ee) than Pd/C. Proline and its derivatives (isomeric hydroxyl-prolines, prolinols and proline esters) were tested as chiral modifiers for Pd/Al₂O₃. Proline was the best modifier, and both enantiomers gave ee ≤ 35%.

Presonication was found to enhance the enantioselectivity when both the Pd/Al₂O₃ catalyst and the proline modifier were present. “Modifier-free” presonication and the presence of substrate during pretreatment decreased the enantioselectivity.

The reaction was performed at 50 bar pressure and 25°C. Presonication for 20 minutes gave the highest optical yields, and increased optical yields across all the H₂ pressure range. Maximum ee occurred at a 1 : 2 isophorone : proline ratio, and with optimised conditions and presonication, the ee for the Pd/Al₂O₃-(S)-proline catalytic system was ≤ 85%.

Ultrasonic cleaning of the catalyst enhanced both the adsorption of the modifier and the modifier-induced surface restructuring of the Pd. The high ee was due to proline adsorption on the Pd surface. New catalysts that can strongly adsorb proline could thus become important in heterogeneous catalysis for C=C double bond hydrogenation of α,β-unsaturated carbonyl compounds.