Increase in Activity and Selectivity in Catalysis via Surface Modification with Self-Assembled Monolayers

Selectivity has become a critical consideration in the design of catalysts, but increases in selectivity are typically achieved at the expense of overall activity. Here we report on a unique case where the addition of alkyl thiol self-assembled monolayers to colloidal platinum nanoparticles leads to significant improvements in both activity and selectivity during the hydrogenation of a-keto esters with cinchonidine as a chiral modifier. These catalytic improvements may be explained by a kinetic effect in which the cinchonidine residence time on the surface is increased by the thiol self-assembled layers. More nuanced compromises between activity and selectivity are involved when using supported catalysts, but reasonable performances are still possible with Pt/Al2O3 catalysts treated with cinchonidine-derivatized thiols. Indeed, these all-heterogeneous catalysts can promote the a-keto ester hydrogenation reaction with almost the same enantioselectivity as the best naked supported Pt catalyst without the need to add cinchonidine to the reaction mixture. They are among the most enantioselective all-heterogeneous catalysts reported to date.