Chiral vanadyl Schiff base complex anchored on silicas as solid enantioselective catalysts for formation of cyanohydrins: optimization of the asymmetric induction by support modification

A series of vanadyl Schiff base complexes having a terminal carbon–carbon double bond pending alkyl chains of various lengths attached to the para position of the salen ligand have been prepared and anchored on three large surface area silicas, namely amorphous silica, ITQ-2, and MCM-41 through mercaptopropysilyl groups. The resulting solids having vanadium content around 0.04 mmol/g were tested as enantioselective catalysts for the reaction of aldehyde with trimethylsilyl cyanide and low ee values compared to solution were found. To optimize the enantioselectivity of the solid catalysts, silylation of the free silanol groups, variation of the linker length, and screening of the solvent were studied. The optimized enantioselective catalyst was found to be that in which the vanadyl salen complex is anchored to amorphous silica with the longest alkyl chain of the series (11 C) and in which the residual silanol groups were masked with trimethylsilyl groups. It was found that under optimal conditions (CHCl3 as solvent and 0°C) the activity of these solid catalysts is very close to that of the analogous complex in solution. Thus, for the reaction of benzaldehyde (1.64 mmol) with trimethylsilyl cyanide (3 mmol) in the presence of vanadium salen complex anchored on silica (100 mg) using chloroform as solvent at 0°C the enantiomeric excess was 85% as compared to the 90% measured for the homogeneous catalyst. The solid catalyst can be reused by simple filtration up to three times, retaining a large part of the activity of the fresh catalyst.