Oxidation of N-Methylanilines by a Nonheme Iron(IV)-Oxo Complex

Abstract

High-valent iron-oxo species have been frequently employed in heme and nonheme iron enzymes to carry out catalytic oxygenation reactions. In heme-based systems, biological and synthetic oxoiron(IV) porphyrin π-cation radicals are believed to carry out the catalytic oxidations of organic substrates. In the nonheme iron enzymes, oxoiron(IV) intermediates have recently been identified by spectroscopic methods in the catalytic cycle of taurine dioxygenase, and synthetic nonheme oxoiron(IV) species bearing tetradentate and pentadentate ligands were also isolated and characterized. In addition, it has been demonstrated that the nonheme oxoiron(IV) species were capable of conducting the conversion of diverse organic subsatrates such as PPh3, thioanisole, alcohol, and alkanes. The molecular mechanisms of oxidative N-demethylation of N,N-dimethylanilines by heme enzymes such as peroxidases and cytochrome P-450 have been studied for the past twenty years. However, two possible mechanisms, electron transfer followed by proton transfer and hydrogen transfer from α-C-H bonds of the methyl group in N,Ndimethylanilines, are still under debate. Recently, we also reported the oxidative N-dealkylation of N,N-dialkylanilines and demonstrated that the oxidative N-dealkylation reactions occurred via a proton-coupled electron transfer process. Meanwhile, secondary amine substrates bearing a cyclopropyl group have been used in the relation to a mechanismbased inactivation of heme enzymes, and the formation of a highly reactive carbon-centered radical was reported in the oxidation of heteroatom-containing cyclopropyl substrates. Relatively, a few studies for the oxidation of Nmethylanilines as substrate were reported using iron, cobalt, and copper complexes. However, no detailed mechanistic study in the oxidation of secondary amines with nonheme Fe(IV)-oxo species has been reported until now. Presented in this study is a kinetic examination of the oxidation of N-methylanilines with [(tmc)Fe=O] (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), which yields N,N'-dimethyl-N,N'-diphenylhydrazine formed by the coupling of two N-methylaniline molecules. By determining the rate constants for the step of electron transfer from N-methylaniline to the iron(IV)-oxo species, we suggest that the oxidation of N-methylaniline occurs via an electron transfer. Experimental Procedure