Highly Enantioselective Bioinspired Epoxidation of Electron-Deficient Olefins with H₂O₂ on Aminopyridine Mn Catalysts

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 [LM^II(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 Mn^VO 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 C_α–C_β single bond and the epoxide ring collapse