DIASTEREOSELECTIVE CATALYTIC EPOXIDATION OF CHIRAL ALLYLIC ALCOHOLS BY THE TS-1 AND TI-BETA ZEOLITES - EVIDENCE FOR A HYDROGEN-BONDED, PEROXY-TYPE LOADED COMPLEX AS OXIDIZING SPECIES

The similar diastereoselectivities as for peracids implicate a pentaco ordinated metal peroxy species in the epoxidation of chiral allylic al cohols by the titanium-containing zeolites TS-1 and Ti-beta. In the lo aded complex, the allylic alcohol substrate is fixed through hydrogen bonding to the metal center, with A(1,3) strain as the dominant confor mational feature. By comparison of the diastereoselectivities to those for VO(acac)(2)/t-BuOOH, we exclude a peroxy-type complex with direct coordination of the allylic alcohol to the titanium center by means o f metal-alcoholate bonding. Neither does a peroxo-type titanium comple x apply, as confirmed by comparing the diastereoselectivities with tho se observed for dioxiranes. With the help of tert-butyl-substituted al lylic alcohols as substrates, which are not epoxidized by the TS-1 but by the Ti-beta catalyst, it was unequivocally confirmed that the oxid ation occurs inside of the zeolite and not on the outer surface.