Manganese-porphyrins and -azaporphyrins as catalysts in alkene epoxidations with peracetic acid. Part 2. Kinetics and mechanism1

Abstract

cis-Stilbene (cSt) and 1,1-diphenyl-2-picrylhydrazine (DPPH) were used as substrates for kinetic investigations of the catalytic system based on Mn^III-porphyrins and peracetic acid in CH₃CN. Catalysts employed were tetra-(2,6-dichlorophenyl)-porphyrinatomanganese chloride (TDCPPMnCl) 5, octanitrophthalocyaninatomanganese chloride (NO₂PcMnCl) 6 and tetra(tert-butyl)-tetraazaporphyrinatomanganese chloride (TAPMnCl) 7. It was found that for all these catalysts the first step of the reaction mechanism is the formation of an adduct ‘A' between the catalyst and AcOOH in a reversible way (k₁/k_-1), followed by an irreversible stage (k₂) for the formation of Mn^V-oxo species. The oxidative capability of the adduct ‘A' was found to be dependent on the electronic structure of the catalyst, while the reactivity of Mn-oxo species is only slightly influenced by catalyst structure. The formation of the high-valent Mn-oxo species is the rate-determining step of alkene epoxidations as demonstrated by the same k₂ value obtained with catalyst 6 in the epoxidation of cSt and trans-stilbene (tSt). Catalyst stability was found to be dependent on solvent polarity, CH₃CN being the best reaction medium.