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
W. Adam et al., 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, Journal of organic chemistry, 62(11), 1997, pp. 3631-3637
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.