Kinetics of (Porphyrin)manganese(III)‐Catalyzed Olefin Epoxidation with a Soluble Iodosylbenzene Derivative

Abstract

AbstractWe examined the kinetics of a well‐behaved system for homogeneous porphyrin‐catalyzed olefin epoxidation with a soluble iodosylbenzene derivative 1 as the terminal oxidant and Mn(TPFPP)Cl (2) as the catalyst. The epoxidation rates were measured by using the initial rate method, and the epoxidation products were determined by gas chromatography. The epoxidation rate was found to be first order with respect to the porphyrin catalyst and zero order on the terminal oxidant. In addition, we found the rate law to be sensitive to the nature and concentration of olefin substrates. Saturation kinetics were observed with all olefin substrates at high olefin concentrations, and the kinetic data are consistent with a Michaelis–Menten kinetic model. According to the observed saturation kinetic results, we propose that there is a complexation between the active oxidant and the substrate, and the rate‐determining step is thought to be the breakdown of this putative substrate–oxidant complex that generates the epoxidation products and the resting state porphyrin catalyst. Competitive epoxidations further indicate a reversible complexation of the active oxidant and the olefin substrate. The activation parameters ΔH‡ and ΔS‡ for the oxygen‐transfer process (k2) in the cis‐cyclooctene epoxidation were determined to be 12.3 ± 0.9 kcal mol–1 and–15.6 ± 3.2 cal mol–1 K–1, respectively. In addition, the Hammett constant ρ+ was measured for the epoxidation of para‐substituted styrenes, and the value of –0.27 ± 0.04 is too low to be consistent with the involvement of a discrete carbocation in the transition state. We also prepared a (porphyrin)manganese catalyst immobilized on silica support, and found the epoxidation of cis‐cyclooctene catalyzed by this heterogeneous catalyst proceeds at virtually the same turnover frequency as by the homogeneous porphyrin catalyst. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)