On the mechanism of epoxidation of alkenes with hypochlorite, catalysed by manganese(III) tetraarylporphyrins
Authors: van der Made A. W. , Nolte R. J. M. , Drenth W. 
Published: September 2010

A detailed mechanistic study of the epoxidation of alkenes by the cytochrome‐P‐450 model sodium hypochlorite/manganese(III) tetraarylporphyrin is presented. From the zero‐order rate dependence on alkene concentration and low‐temperature‐trapping experiments with the hydrogen‐atom donor 1,1‐diphenyl‐2‐picrylhydrazine (DPPH), it is concluded that the formation of the active [(P)MnO]+ species from a (P)MnOCl complex is the rate‐determining step of the reaction. The trapping experiments also show that the reverse reaction is possible. Pyridine or imidazole derivatives as axial ligands accelerate the rate‐determining step by electron donation. Imidazoles, however, are destroyed under the reaction conditions. A general base function of pyridine or imidazole derivatives is excluded on the basis of experiments with porphyrin catalysts containing covalently bound pyridine. Large amounts of pyridine or imidazole retard the epoxidation reaction by blocking both sides of the catalyst, and thus preventing the hypochlorite anion to coordinate. In spite of the zero‐order‐in‐alkene concentration, different alkenes are expoxidized at different rates. This is explained by the occurrence of dimerization phenomena, i.e., a reaction of the [(P)MnO]+ species with MnIII, which eventually leads to catalyst destruction. Electron‐rich substrates effectively prevent this destruction. In the case of sterically hindered catalysts, i.e., catalysts bearing bulky substituents on the ortho positions of the phenyl rings, no dimerization occurs. With this class of catalysts, however, reaction rates are influenced by steric interactions between catalyst and substrate. Electron‐withdrawing substituents on the 4‐positions of the phenyl rings of the catalyst increase the epoxidation rate. This is a result of increased electrophilicity of the [(P)MnO]+ species and decreased electron density at the meso positions making the catalyst less susceptible to destruction. Lowering the pH of the aqueous phase from 13.5 to 12.7 increases the rate of epoxidation, because, at lower pH, a neutral HCl molecule must be expelled from a [(P)MnHOCl]+ complex instead of a chloride anion, in order to form the active species. Alcohols increase the reaction rate by providing better solvation of the leaving group (Cl or HCl). Aldehydes and ketones, which are formed as by‐products in epoxidation, accelerate the reaction in the case of sterically not hindered catalysts. These compounds rapidly trap the active species, thus preventing unfavourable dimerization processes occurring. A carbonyl oxide species is produced, which in turn acts as an active epoxidizing agent. The reaction between the [(P)MnO]+ species and an alkene is likely to proceed via a non‐concerted process by interaction of the LUMO of the active species with the HOMO of the alkene in an asymmetrical way. With this mechanism, the formation of trans‐epoxides from cis‐alkenes as well as the formation of aldehydes and ketones as by‐products are easily explained. Pyridines increase the stereoselectivity of the reaction by pulling the metal centre into the porphyrin plane. This creates increased steric hindrance to rotation of the oxo‐substrate intermediate.