Diastereoselective epoxidation of allylic alcohols with hydrogen peroxide catalyzed by titanium-containing zeolites or methyltrioxorhenium versus stoichiometric oxidation with dimethyldioxirane: Clues on the active species in the zeolite lattice

Abstract

Chiral, acyclic allylic alcohols 1 are epoxidized chemoselectively to the epoxy alcohols 2 by hydrogen peroxide, catalyzed by titanium-containing zeolites (TS-1, Ti-beta). For substrates with 1,3-allylic (A 1,3) strain, a high diastereoselectivity is observed with preference for the threo isomer, while derivatives with 1,2-allylic (A 1,2) strain or no allylic strain give a low threo diastereomeric excess. Comparison of the diastereomeric ratios of the titanium-containing zeolites with those for meta-chloroperbenzoic acid shows a good correspondence which suggests that the active species for the oxygen transfer in the epoxidations for zeolites is peracid- rather than peroxo-type. Comparison of the diastereomeric ratios achieved with the three-membered ring peroxide oxidants dimethyldioxirane and MTO/UHP (metal peroxo complex) disfavor the peroxo species since significantly lower thrio diastereoselectivities for substrates with 1,3-allylic strain were obtained. Direct coordination of the allylic alcohol through a metal alcoholate bond is unlikely because of the different diastereomeric ratios obtained for the heterogeneous and homogeneous titanium species with allylic alcohols that possess 1,2-allylic strain. Moreover, the number of coordination sites at the titanium atom in the zeolite framework is limited for steric reasons and the constrained space around the active center in the zeolite lattice presents severe geometrical problems for the stereoelectronically controlled linear S N2-type alignment of the oxygen donor (metal-activated peroxide bond) and the acceptor (metal-alcoholate-bonded substrate) during the epoxidation process.