Lewis acid-containing mesoporous molecular sieves as solid efficient catalysts for solvent-free Mukaiyama-type aldol condensation

The activity of the titanium mesoporous molecular sieve Ti-MCM-41 and amorphous Ti-silicate materials for carbon-carbon bond formation in the Mukaiyama aldol-type reaction between benzaldehyde and methyl trimethylsilyl dimethylketene acetal was studied and the reaction network established. Only tetrahedral TiIV is catalytically active, where TiVI and highly dispersed TiO2 are not active or only slightly active for the Mukaiyama reaction and have no effect on selectivity. Whereas in Ti-zeolites only a some of the active sites are accessible to reactants, all of them are accessible in Ti-MCM-41. The structured Ti-MCM-41 offers a clear catalytic advantage over amorphous Ti-silicalites. Water has a negative effect on catalytic activity and selectivity, and catalyst preactivation and hydrophobicity play an important role in the final catalytic behavior. The influence of different solvents on conversion was explored.