Highly Enantioselective Bioinspired Epoxidation of Electron-Deficient Olefins with H₂O₂on Aminopyridine Mn Catalysts

Abstract

The asymmetric epoxidation of various electron-deficient olefins with H2O2 in the presence of a novel family of chiral bioinspired bipyrrolidine-derived aminopyridine manganese(II) complexes [LMII(OTf)2] is reported. High enantioselectivities (up to 99% ee) and epoxide selectivities (up to 100%), unprecedented for catalysts of this type, have been achieved; the catalysts perform up to 8500 catalytic turnovers. The presence of electron donors in the catalyst structure substantially enhances the enantioselectivity. Isotopic (18O) labeling studies provide evidence of the formation of the oxomanganese(V) active species. Hammett analysis suggests that the enantioselective epoxidation is rate-limited by the transfer of an electron to the MnVO intermediate, to form a short-lived acyclic (carbocationic) intermediate. In effect, the epoxide stereoconfiguration may be affected by the competition between the rotation around the Cα–Cβ single bond and the epoxide ring collapse.