Preparation of new chiral dioxomolybdenum complexes heterogenised on modified USY-zeolites - Efficient catalysts for selective epoxidation of allylic alcohols

A series of dioxomolybdenum(VI) complexes were synthesised by reaction of Mo(VI) complexes with new chiral ligands derived from (2S,4R)-4-hydroxyproline. The complexes bearing a Si(OEt)(3) group were heterogenised to a modified USY-zeolite by covalent bonding. All catalysts, homogeneous and heterogenised, are active and regioselective in the epoxidation of allylic alcohols at room temperature using tert-butyl hydroperoxide as terminal oxidant. The catalytic activity of the heterogenised complexes is generally comparable with the corresponding homogeneous ones, yielding epoxyalcohols with excellent yields and selectivity and moderate enantioselectivity. Life time of heterogenised catalysts has been examined by repeated use of the complexes leading similar rates and yields of epoxide, whilst no appreciable loss of metal has been observed over several runs.