Cinchona methyl ethers as modifiers in the enantioselective hydrogenation of (E)-2,3-diphenylpropenoic acids over Pd catalyst

The enantioselective hydrogenation of (E)-2,3-diphenylpropenoic acids over Pd/Al2O3 modified by (R)C8single bond(S)C9 cinchona ethers resulted in the inversion of the sense of the enantioselectivity. To find the explanation of the phenomenon, the interaction of acids bearing different substituents with cinchona alkaloids was studied in solution by NMR spectroscopy and experiments using mixtures of modifiers were carried out. The prominent non-linear behaviour obtained revealed the altered adsorption of the cinchona methyl ethers when compared with the parent alkaloids. The investigations indicated that the interaction of the ether derivatives with the unsaturated acids is more flexible and the presence of the methyl group reshapes the chiral surface sites. The combination of these effects complemented by the bulkiness of the diaryl substituted acrylic acids may lead to the inversion of the docking preference of the substrates in the altered chiral pocket of the adsorbed modifier and consequently results in decrease in the enantioselectivity or even in the inversion of its sense. Novel evidence on the ligand-accelerated mechanism in the enantioselective hydrogenation of (E)-2,3-diphenylpropenoic acids over cinchona alkaloid-modified Pd in the presence of benzylamine was also presented.