Efficient Olefin Epoxidation by Robust Re₄ Cluster-Supported MnIII Complexes with Peracids: Evidence of Simultaneous Operation of Multiple Active Oxidant Species, MnVO, MnIVO, and MnIIIOOC(O)R

摘要

Two new tetranuclear chalcocyanide cluster complexes, [{Mn(saloph)H₂O}₄Re₄Q₄(CN)₁₂]4 CH₃OH 8 H₂O (saloph=N,N′-o-phenylenebis(salicylidenaminato), Q=Se (1-Se), Te (2-Te)), have been synthesized by the diffusion of a methanolic solution of [PPh₄]₄[Re₄Q₄(CN)₁₂] into a methanolic solution of [Mn(saloph)]+. The structure of 2-Te has been determined by X-ray crystallography. These rhenium cluster-supported [MnIII(saloph)] complexes have been found to efficiently catalyze a wide range of olefin epoxidations under mild experimental conditions in the presence of meta-chloroperbenzoic acid (mCPBA). Olefin epoxidation by these catalysts is proposed to involve the multiple active oxidants MnVO, MnIVO, and MnIIIOOC(O)R. Evidence in support of this interpretation has been derived from reactivity and Hammett studies, H₂18O-exchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. Moreover, it has been observed that the participation of MnVO, MnIVO, and MnIIIOOC(O)R can be controlled by changing the substrate concentration. This mechanism provides the greatest congruity with related oxidation reactions that employ certain Mn complexes as catalysts.