Diastereoselective Catalytic Epoxidation of Chiral Allylic Alcohols by the TS-1 and Ti-β Zeolites: Evidence for a Hydrogen-Bonded, Peroxy-Type Loaded Complex as Oxidizing Species

Abstract

The similar diastereoselectivities as for peracids implicate a pentacoordinated metal peroxy species in the epoxidation of chiral allylic alcohols by the titanium-containing zeolites TS-1 and Ti-β. In the loaded complex, the allylic alcohol substrate is fixed through hydrogen bonding to the metal center, with A1,3 strain as the dominant conformational 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 of metal−alcoholate bonding. Neither does a peroxo-type titanium complex apply, as confirmed by comparing the diastereoselectivities with those observed for dioxiranes. With the help of tert-butyl-substituted allylic alcohols as substrates, which are not epoxidized by the TS-1 but by the Ti-β catalyst, it was unequivocally confirmed that the oxidation occurs inside of the zeolite and not on the outer surface.