Titanium silicates for carbon–carbon bond formation reactions

The activity of titanium silicate molecular sieves, TS-1 and aluminum-free Ti-β, has been studied for carbon–carbon bond formation reactions such as Mukaiyama-type aldol condensation and Michael addition under mild liquid-phase conditions. All the reactions were efficiently carried out in the absence of water by using dry solvents in the temperature range of 313–333 K. In the case of Mukaiyama aldol condensation, a variety of aldehydes successfully react with silyl enol ethers to give β-hydroxy esters/aldols in high yields under heterogeneous reaction conditions. Similarly in the case of Michael addition, silyl enol ethers undergo addition reactions with Michael acceptors such as acrylates and α,β-unsaturated ketones to give 1,4-addition products. The selectivity for aldol as well as Michael addition product is always 100%, regardless of conversion level, and no side products are formed. The facile formation of nucleophiles from silyl enol ethers appears to be the key step for successful carbon–carbon bond formation reactions over titanium silicates. The activity of other isomorphously substituted zeolites such as V-, Sn-, and Al-ZSM-5 as well as modified zeolites by ion exchange exhibits less activity than that observed over TS-1 and Ti-β in the aldol condensation. Among the various solvents investigated tetrahydrofuran (THF) shows the best activity. The observed catalytic activity is explained on the basis of “oxophilic Lewis acidity” of titanium silicate molecular sieves in the absence of H2O2 and H2O.