ABSTRACT
Platinum nanocolloids with different average particle size were prepared using cinchonidine (CD) or cinchonine (CN) as stabilizing agents. These colloids were used as catalysts in enantioselective hydrogenation of ethyl pyruvate (EtPy) in various solutions. Two forms of the alkaloids were distinguished: (i) the stabilizing form ((CD)st or (CN)st), and (ii) the excess form ((CD)ex or (CN)ex), i.e., the amount of alkaloids added into the liquid phase. In acetic acid-methanol mixture high rates and ee values around 80 % were obtained. The ee values slightly decreased with the Pt particle size, however the activity decrease was very pronounced. This behavior was attributed to the increase of the unreduced forms of Pt and the presence of Cl ions in small Pt nanoclusters below 2.8 nm. The performance of this type of Pt nanocolloids was strongly affected by the concentration of both (CD)ex and acetic acid. Kinetic evidences were obtained with respect to the exchange between the stabilizing and the excess forms of the alkaloids. Results of molecular modeling showed that only the "shielded" substrate-modifier complex could be accommodated on the plain surfaces of these small Pt nanocolloids. It is considered that the results provided further support for our "shielding effect model" suggested earlier.
