ENANTIOSELECTIVE HYDROGENATION .4. HYDROGEN ISOTOPE-EXCHANGE IN 10,11-DIHYDROCINCHONIDINE AND IN QUINOLINE CATALYZED BY PLATINUM-GROUP METALS

Hydrogen isotope (H/D) exchange in the alkaloid 10,11-dihydrocinchonidine has been studied over 6.3% Pt/silica (EUROPT-1), 5% Ru/alumina, 5% Rh/alumina, and 5% Pd/alumina at 293 K using C2H5OD and D2 as solvent and deuterium source. Exchange was accompanied by hydrogenation. Over Pt, fast exchange occurred in the hydroxyl group followed by multiple exchange in which alkaloid molecules containing 2, 3, 4 and 5 deuterium atoms were formed simultaneously. Mass spectrometry and H-1 NMR showed that this multiple exchange occurred in the quinoline ring system and at C9, but not in the quinuclidine ring system. The pattern of exchange in Ru was similar. Over Rh extensive hydrogenolysis of the quinuclidine ring system occurred, and over Pd the quinoline ring system was rapidly hydrogenated. Quinoline exchange and hydrogenation were also studied at 293 K; relatively rapid exchange occurred over Pt, Ru, and Rh, particularly at the 2- and 8-positions, whereas hydrogenation without significant exchange occurred over Pd. 10,11-Dihydrocinchonidine is adsorbed on Pt and Ru via the quinoline ring system and the multiple nature of the exchange indicates that the quinoline moiety is adsorbed approximately parallel to the metal surface by multicenter pi-bonding. An additional interaction of the alkaloid molecule with the surface occurs at carbon atom C9, which may interpret the slower exchange in the alkaloid by comparison with that in quinoline. This study supports and enhances the model proposed to interpret the origin of enantioselectivity in pyruvate hydrogenation over Pt and Ir modified by cinchona alkaloids. The similarities of exchange over Pt and Ru suggest that enantioselective catalysis should be achievable over Ru. The failure of 10,11-dihydrocinchonidine to retain molecular integrity on adsorption on these particular Rh and Pd surfaces interprets our failure so far to achieve enantioselective pyruvate hydrogenation over these catalysts. (C) 1994 Academic Press, Inc.