Synthetic Strategies for Adding Enantioselectivity to Metal-Based Solid Catalysts Using Cinchonidine As the Chiral Agent

Abstract

A number of synthetic protocols were developed to produce Ptbased catalysts with chiral characteristics, bestowed by cinchonidine (Cd) moieties in a variety of ways. Two general strategies were explored: (1) the tethering of the Cd modifiers, and (2) the creation of confined spaces above the catalytic sites. Tethering of Cd near the Pt nanoparticles (NPs) of Pt/SiO 2 or Pt/Al 2 O 3 catalysts could be optimized, mainly by adjusting the Cd/Pt ratio, to match the performance obtained by adding Cd to the reaction mixture in solution for the enantioselective promotion of the hydrogenation of ethyl pyruvate (EtPy). Encapsulating the Cd fragments adsorbed on Pt inside a thin silica shell proved to also be an excellent way to make an all-heterogeneous enantioselective catalyst. Systematically optimizing the diameter of the 1D pores of SBA-15, a silica mesoporous material with a well-defined structure, by growing thin films of various thicknesses using atomic layer deposition (ALD) was shown to improve the enantioselectivity of the addition of thiols to ketones promoted by tethered Cd. Other approaches were shown to be less successful but to nevertheless offer valuable lessons for the design of multifunctional solid catalysts.