Synthesis of Enantioselective Solid Tandem Catalysts with Tethered Molecular Functionalities

Abstract

A synthetic protocol was developed to prepare a bifunctional heterogeneous catalyst with phosphonic acid and cinchonidine moieties tethered to a mesoporous silica support (SBA-15). The nature of the bond of the molecular groups to the surface was characterized by 29Si solid-state nuclear magnetic resonance (NMR), and the retention of the integrity of the tethered groups throughout the individual synthetic steps was characterized by 13C and 31P solid-state NMR. The ratio of acidic to basic sites could be controlled by tuning the intermediate acid-site removal procedure, which relies on the use of UV/ozonolysis and was quantified by a combination of the analysis of pore-size distribution data (extracted from N2 adsorption–desorption isotherms), acid–base titrations, and carbon and nitrogen elemental analysis. The performance of the resulting catalysts was tested for two processes, a deacetylation-Henry reaction sequence and a chiral cyano-ethoxycarbonylation reaction. In comparison with equivalent physical mixtures of two monofunctional SBA-15-catalysts with tethered phosphonic acid or tethered cinchonidine, respectively, the bifunctional catalysts displayed less activity but higher selectivity in the first system, with the surface silanol groups also playing an important role. In the second reaction, all activities were comparable, and enhanced enantioselectivity was observed with the new bifunctional catalyst. For the first reaction, a 1:1 ratio of coverages of the acidic and basic functionalities offered optimum performance, but in the second, more cinchonidine than phosphonic acid was needed. The catalysts also proved to be quite robust and to retain their activity after several recycling cycles.