Enantioselective hydrogenation of alpha,beta-unsaturated carboxylic acids over cinchonidine modified palladium: Nature of modifier-reactant interaction

The mechanism of enantiodifferentiation in the hydrogenation of alkenoic ac ids over cinchona-modified Pd has been investigated using the tiglic acid - -> 2-methyl-butanoic acid transformation as test reaction. Application of s imple derivatives of cinchonidine, modified at the (C-9)-OH and/or the quin uclidine nitrogen, proved that both functional groups are involved in the e nantiodiscriminating step. Addition of a strong base (1,8-diazabicyclo[5.4. 0]undec-7-ene, DBU) to tiglic acid prior to hydrogenation revealed that one cinchonidine molecule interacts with a dimer of tiglic acid on the metal s urface. Ab initio calculations corroborate the existence of an energeticall y favored acid dimer-cinchonidine intermediate stabilized by hydrogen bondi ng, involving both the OH and the quinuclidine nitrogen of cinchonidine. (C ) 1999 Academic Press.