Regio- and Diastereoselective Catalytic Epoxidation of Acyclic Allylic Alcohols with Methyltrioxorhenium: A Mechanistic Comparison with Metal (Peroxy and Peroxo Complexes) and Nonmetal (Peracids and Dioxirane) Oxidants

Abstract

Geraniol and its 1-methyl derivative (regiochemical probes) and a set of methyl- and tert-butyl-substituted chiral allylic alcohols (stereochemical probes) have been used to elucidate the mechanism of the MTO-catalyzed epoxidation of allylic alcohols. The regiochemical probes are preferentially epoxidized at the unfunctionalized double bond by these MTO-based oxidants, which establishes that MTO/UHP and MTO/H(2)O(2)/pyridine mainly operate through hydrogen bonding. Metal-alcoholate binding does not apply, which is in contrast to the transition-metal oxidants VO(acac)(2)/t-BuOOH, Mo(CO)(6)/t-BuOOH, MoO(2)[PhCON(Ph)O](2)/t-BuOOH, MoO(O(2))[PhCON(Ph)O](2), and H(2)WO(4)/H(2)O(2). For the stereochemical probes, the diastereoselectivity data show a good correspondence between the MTO-catalyzed systems (MTO/UHP and MTO/H(2)O(2)/pyridine) and the perhydrate-type oxidant Ti-beta/H(2)O(2) and the peracid m-CPBA. Conformational control through 1,3-allylic strain results in a high threo diastereoselectivity, in which hydrogen bonding between the hydroxy functionality and the rhenium catalyst is the decisive electronic feature. All these selectivity data are consistent with the rhenium peroxo complex as the active oxidant.