Regio- and Diastereoselective Epoxidation of Chiral Allylic Alcohols Catalyzed by Manganese(salen) and Iron(porphyrin) Complexes

Abstract

Chiral acyclic allylic alcohols 1 have been chemo-, regio-, and diastereoselectively epoxidized to the corresponding epoxy alcohols 2 by catalysis with the achiral manganese(salen) and iron(porphyrin) complexes 3 and 4 and iodosyl benzene as oxygen donor. The threo diastereoselectivities establish that hydrogen bonding accounts for the observed hydroxy-group directivity. The dramatically reduced stereoselection in the protic methanol and the opposite sense in the diastereoselectivity (erythro instead of threo) for the ether and ester derivatives 5 of the allylic alcohol 1f confirm that hydrogen bonding between the allylic alcohol and the oxo−metal complex operates in this oxygen-transfer process, with 1,3-allylic strain as the conformationally controlling feature. That metal−alcoholate bonding does not apply is displayed by the regioselectivities obtained in the epoxidation of 1-methylgeraniol (1h). By comparison of the diastereo- and regioselectivities with those of mechanistically defined catalytic and stoichiometric metal and nonmetal oxidants, we propose likely transition-state structures A and B for the present epoxidation.