Efficient olefin epoxidation by robust re4 cluster-supported MnIII complexes with peracids: Evidence of simultaneous operation of multiple active oxidant species, Mnv=O, MnIV=O, and MnIII-OOC(O)R

Abstract

Two new tetranuclear chalcocyanide cluster complexes, [(Mn(SaloPh)H 2O}4Re4Q4(CN)12] ·4CH3OH· 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 [Mn III(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 MnV=O, Mn IV=O, and MnIIIOOC(O)R. Evidence in support of this interpretation has been derived from reactivity and Hammett studies, H 2 18Oexchange experiments, and the use of peroxyphenylacetic acid as a mechanistic probe. Moreover, it has been observed that the participation of MnV= O, MnIV=O, and Mn III-OOC(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. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA.