G. Bond et Pb. Wells, ENANTIOSELECTIVE HYDROGENATION .4. HYDROGEN ISOTOPE-EXCHANGE IN 10,11-DIHYDROCINCHONIDINE AND IN QUINOLINE CATALYZED BY PLATINUM-GROUP METALS, Journal of catalysis, 150(2), 1994, pp. 329-334
Hydrogen isotope (H/D) exchange in the alkaloid 10,11-dihydrocinchonid
ine 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 deuteri
um atoms were formed simultaneously. Mass spectrometry and H-1 NMR sho
wed that this multiple exchange occurred in the quinoline ring system
and at C9, but not in the quinuclidine ring system. The pattern of exc
hange in Ru was similar. Over Rh extensive hydrogenolysis of the quinu
clidine ring system occurred, and over Pd the quinoline ring system wa
s 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 wit
hout significant exchange occurred over Pd. 10,11-Dihydrocinchonidine
is adsorbed on Pt and Ru via the quinoline ring system and the multipl
e nature of the exchange indicates that the quinoline moiety is adsorb
ed approximately parallel to the metal surface by multicenter pi-bondi
ng. An additional interaction of the alkaloid molecule with the surfac
e occurs at carbon atom C9, which may interpret the slower exchange in
the alkaloid by comparison with that in quinoline. This study support
s and enhances the model proposed to interpret the origin of enantiose
lectivity in pyruvate hydrogenation over Pt and Ir modified by cinchon
a alkaloids. The similarities of exchange over Pt and Ru suggest that
enantioselective catalysis should be achievable over Ru. The failure o
f 10,11-dihydrocinchonidine to retain molecular integrity on adsorptio
n on these particular Rh and Pd surfaces interprets our failure so far
to achieve enantioselective pyruvate hydrogenation over these catalys
ts. (C) 1994 Academic Press, Inc.