Nonheme manganese(III) complexes for various olefin epoxidation: Synthesis, characterization and catalytic activity

Three mononuclear imine-based non-heme manganese(III) complexes with tetradentate ligands which have two deprotonated phenolate moieties, ([(X₂saloph)Mn(OAc)(H₂O)], 1a for X = Cl, 1b for X = H, and 1c for X = CH₃, saloph = N,N-o-phenylenebis(salicylidenaminato)), were synthesized and characterized by ¹H NMR, ¹³C NMR, ESI-Mass and elemental analysis. Mn^III complexes catalysed efficiently various olefin epoxidation reactions with meta-chloroperbenzoic acid (MCPBA) under the mild condition. Mn^III complexes 1a and 1c with the electron-withdrawing group -Cl and electron-donating group –CH₃ showed little substituent effect on the epoxidation reactions. Product analysis, Hammett study and competition experiments with cis- and trans-2-octene suggested that Mn^IV = O, Mn^V = O, and Mn^III-OOC(O)R species might be key oxidants in the epoxidation reaction under this catalytic system. In addition, the use of PPAA as a mechanistic probe demonstrated that Mn-acylperoxo intermediate (Mn^III-OOC(O)R) 2 generated from the reaction of peracid with manganese complexes underwent both the heterolysis and the homolysis to produce Mn^V = O (3) or Mn^IV = O species (4). Moreover, the Mn^III-OOC(O)R 2 species could react directly with the easy-to-oxidize substrate to give epoxide, whereas the species 2 might not be competent to the difficult-to-oxidize substrate for the epoxidation reaction.