Cinchonidine-Induced Restructuring of Pt/Al 2 O 3 During Enantioselective Hydrogenation

The intrinsic selectivity of cinchonidine-modified Pt/alumina is poor in the hydrogenation of 3,5-di-(trifluoromethyl)-acetophenone but stirring of the catalyst system in toluene under nitrogen prior to reaction more than triples the ee. SEM and TEM analysis revealed dramatic restructuring of the catalyst and the Pt particles. We propose that the interaction of the hydroxy methyl-quinoline fragment (“anchoring moiety”) of cinchonidine with Pt is responsible for the restructuring. Reductive pretreatment at elevated temperature as well as the attrition induced by stirring in toluene accelerate the process. The higher ee is attributed to morphological changes of Pt, leading to the development of a more “selective” surface. The chirality of cinchonidine is unimportant in the restructuring and no correlation between the size of Pt particles and the ee has been found.