Heterogeneous asymmetric reactions Part 17. Asymmetric hydrogenation of 2-methyl-2-pentenoic acid over cinchona modified Pd/Al₂O₃ catalysts

The enantioselective hydrogenation of 2-methyl-2-pentenoic acid (MPA) over a cinchona modified 5% Pd/Al₂O₃ catalysts is described. The main experimental variables studied were the MPA/catalyst and the cinchonidine (CD)/MPA ratios, the effect of ultrasonic irradiation and the modifier structure. Low MPA/catalyst ratio was found to be advantageous for obtaining enhanced enantioselectivity, while higher CD/MPA ratios resulted in increasing enantioselective according to a saturation type curve. The ultrasonic pretreatment (similarly to α-ketoesters) also slightly increased the enantiomeric excesses. These modifications of the CD-Pd/Al₂O₃ system, resulted in enantioselectivities up to 66% ee (0°C, 10 min sonication, CD/MPA=0.5, 50 bar H₂ pressure) for (S)-2-methyl-pentanoic acid. The variation of the modifier structure provided unique information about the nature of the cinchona-substrate interaction. It has been revealed that the OH group of the alkaloid should be involved in the substrate-modifier interaction which more likely occurs in the liquid phase. An intermediate with refined structure is proposed and the mechanism is suggested to be of stoichiometric nature.