Synthesis, characterization and catalytic activities of nonheme manganese(III) complexes: Preferential formation of cis olefin oxide owing to steric hindrance

Three mononuclear nonheme Mn^III(salophen) complexes, 1a-1c, with tetradentate ligands containing two deprotonated phenolates ([(X₂-tert-butyl-salophen)Mn(OAc)(H₂O)] (tert-butyl-salophen = N,N′-bis(6-di-tert-butylsalicylidene)-1,2-phenylenediaminato, 1a for X = Cl, 1b for X = H, and 1c for X = CH₃)) were synthesized and characterized using ¹H NMR, ¹³C NMR, elemental analysis and ESI-Mass spectrometry. These Mn(III) complexes were used to efficiently catalyze the epoxidation reactions of diverse aliphatic, aromatic and terminal alkenes to form the corresponding epoxides with MCPBA (m-chloroperoxybenzoic acid) as an oxidant under mild conditions. Notably, catalysts 1a-1c preferably react with the cis-alkene because of the steric hindrance between the reactive intermediate Mn^III-OOC(O)R and the trans-type substrate. A Hammett study and product analysis using PPAA (peroxyphenylacetic acid) as a mechanistic indicator suggested that the peracid reacted with the Mn(III) complex to generate the Mn^III-OOC(O)R intermediate, which underwent both homolysis and heterolysis to form Mn^IV=O or Mn^V=O. The reactive Mn^V=O might participate in the alkene epoxidation with good stereospecificity, whereas the Mn^IV=O species might trigger radical-type oxidation to produce non-stereospecific by-products, such as ketones and aldehyde. On the other hand, Mn^III-OOC(O)R (2) could oxidize the reactive cyclohexene to the epoxide, whereas it was unable to epoxidize the poorly reactive 1-octene.