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

Chiral, acyclic allylic alcohols 1 are epoxidized chemoselectively to the epoxy alcohols 2 by hydrogen peroxide, catalyzed by titanium-conta ining zeolites (TS-I, Ti-beta). For substrates with 1,3-allylic (A(1,3 )) strain, a high diastereoselectivity is observed with preference for the three isomer, while derivatives with 1,2-allylic (A(1,2)) strain or no allylic strain give a low threo diastereomeric excess. Compariso n of the diastereomeric ratios of the titanium-containing zeolites wit h those for meta-chloroperbenzoic acid shows a good correspondence whi ch suggests that the active species for the oxygen transfer in the epo xidations for zeolites is peracid- rather than peroxo-type. Comparison of the diastereomeric ratios achieved with the three-membered ring pe roxide oxidants dimethyldioxirane and MTO/UHP (metal peroxo complex) d isfavor the peroxo species since significantly lower three diastereose lectivities for substrates with 1,3-allylic strain were obtained. Dire ct coordination of the allylic alcohol through a metal alcoholate bond is unlikely because of the different diastereomeric ratios obtained f or the heterogeneous and homogeneous titanium species with allylic alc ohols that possess 1,2-allylic strain. Moreover, the number of coordin ation sites at the titanium atom in the zeolite framework is limited f or steric reasons and the constrained space around the active center i n the zeolite lattice presents severe geometrical problems for the ste reoelectronically controlled linear S(N)2-type alignment of the oxygen donor (metal-activated peroxide bond) and the acceptor (metal-alcohol ate-bonded substrate) during the epoxidation process.