Synthesis of new chiral Mn(iii)–salen complexes as recoverable and reusable homogeneous catalysts for the asymmetric epoxidation of styrenes and chromenes

Abstract

New chiral Mn(III)–salen complexes 1a–e and 2a–e were synthesized from the reaction of C₂-symmetric chiral salen ligands and Mn(CH₃COO)₂·4H₂O under an inert atmosphere followed by aerobic oxidation. These complexes were obtained in 91–96% yields and characterized by HRMS, FT-IR, UV-visible spectroscopy, TGA, and elemental analysis. The chiral Mn(III)–salen complexes 1a–e and 2a–e were evaluated in the asymmetric epoxidation of styrene using NaOCl as an oxidant in ethyl acetate as a green solvent. The chiral Mn(III)–salen complexes 1b and 2b (2 mol%) catalyzed the asymmetric epoxidation of substituted styrenes and chromenes to afford the corresponding epoxides in 95–98% yields with 29–88% ee's. The catalysts 1b and 2b were recovered and reused for up to 2 and 3 runs, respectively, in the asymmetric epoxidation of styrene, and the yield of styrene oxide gradually decreased but the ee was consistent.