Inversion of the enantioselectivity in the hydrogenation of (E)-2,3-diphenylpropenoic acids over Pd modified by cinchonidine silyl ethers

Natural cinchona alkaloids and their derivatives were applied as chiral modifiers in the enantioselective hydrogenation of (E)-2-(2-methoxyphenyl)-3-(4-fluorophenyl)propenoic acid over Pd/Al2O3 catalyst. The effect of the modifier structure on enantioselectivities and reaction rates was investigated. The natural cinchonine and its methyl ether resulted in opposite product enantiomers in excess. However, in the cinchonidine series larger substituents were needed to obtain inversion, such as the tert-butyl-dimethylsilyl group. To find an explanation of the phenomenon, stabilities of cinchona alkaloid derivatives under the reaction conditions were investigated by electron spray ionization mass spectrometry, and the modifiers relative adsorption strengths were studied using mixtures of cinchona alkaloids and sequentially added modifiers Decrease in the interaction strength of the cinchona ether derivatives with the acid and the catalyst surface can tentatively explain the observed decrease in the enantioselectivity and the eventual inversion of its sense. Results of these studies suggested the gradual alteration of the shape of the surface chiral sites by increasing the size of the substituent. The presence of benzylamine always increases the amount of the enantiomer which is formed in excess over the parent cinchona alkaloids and accelerates the desorption of the modifier, suggesting the participation of the additive in the surface intermediate. Occasionally, cinchona alkaloid mixtures provided enantioselectivities above or under the values obtained with both, sole modifiers, which is suggested to be due to the mutual interaction of the two cinchona derivatives on the surface.