Asymmetric Epoxidation of Alkenes in Fluorinated Media, Catalyzed by Second-Generation Fluorous Chiral (Salen)manganese Complexes

Abstract

The synthesis of sterically hindered chiral (salen)manganese complexes bearing perfluoroalkyl ponytails and their use in asymmetric epoxidation reactions are described. For better understanding of the relative influences of steric and electronic effects on the enantioselectivity of the fluorous catalysts, the epoxidation of 1,2-dihydronaphthalene and benzosuberene was first studied under homogeneous conditions. It was shown that the presence of sterically demanding tert-butyl groups and, to a lesser degree, the displacement of the electron-withdrawing perfluoroalkyl substituents from the ligand core provide ees higher than those attainable with first generation fluorous chiral (salen)manganese complexes featuring perfluoroalkyl substituents in the key positions (3,3′ and 5,5′) in the ligand. Second generation catalysts (Mn-6)C7F15COO and (Mn-7)C7F15COO were successfully employed in the fluorous biphase epoxidation of alkenes with PhIO as the oxidant and pyridine N-oxide as an additive. Epoxide yields (68−98%) and ees (50−92%) were similar to those obtained with the same oxidizing system and standard (salen)manganese complexes under homogeneous conditions. When the reaction was complete, the fluorous layer in which the catalyst was immobilized was easily recoverable by simple phase separation at room temperature and could be used up to three times before significant decline in yield and enantioselectivity was observed.