Shape-Selective Enantioselective Hydrogenation on Pt Nanoparticles

The structure sensitivity of enantioselective hydrogenations on chirally modified metals was investigated using Pt nanoparticles of different shapes. All three samples had an average particle size of 10 rim, but the fraction of dominantly cubic, cubooctahedral, and octahedral particles varied with decreasing {100} and increasing {111} faces in the same order. In the absence of chiral modifier the hydrogenation of ethyl pyruvate was independent of the shape of the Pt nanoparticles; variation of the specific reaction rates did not exceed the experimental error on all self-prepared catalysts and on a commercial Pt/Al2O3 used as reference. Addition of cinchonidine or quinine induced a significant rate enhancement by a factor of 4-15, and the rate was always higher with quinine. Also, 72-92% ees were achieved, and the reaction was shape selective: both the rate and the ee increased with increasing Pt{111}/Pt{100} ratio. A similar correlation in the hydrogenation of ketopantolactone confirmed that decarbonylation or aldol-type side reactions of ethyl pyruvate were not the reason for structure sensitivity. A combined catalytic and theoretical study revealed that the probable origin of the particle shape dependency of enantioselective hydrogenation is the adsorption behavior of the cinchona alkaloid. DFT studies of cinchonidine interacting with Pt(100) and Pt(111) terraces indicated a remarkably stronger interaction on the former crystallographic face by ca. 155 kJ/mol. The higher adsorption strength on Pt(100) was corroborated experimentally by the faster hydrogenation of the homoaromatic ring of the alkaloid, which fragment interacts the strongest with Pt during its adsorption. Thus, an ideal catalyst for the hydrogenation of activated ketones contains dominantly Pt{111} terraces, which crystallographic face is more active and affords higher enantioselectivity, combined with the higher stability of the modifier.