Mechanistic Insights into the Versatile Reactivities of High Valent Nonheme Oxoiron(IV) Complexes

Abstract

Heme 과 nonheme iron monooxygenases 의 모방 합성 연구는 최근 생무기와 생화학 분야에서 많은 흥미를 끌어왔다. 더욱이 단핵 nonheme oxoiron(IV) 종의 생성과 특성에 대한 최근의 연구 결과는 nonheme iron 효소의 생체 모방 연구 분야의 이정표가 되었다. 이 논문은 nonheme ligand 를 가진 high-valent iron-oxo 종에 의한 p-X-MAs 기질의 산화반응 메커니즘을 분광학적 기술로 연구한 내용이다. 연구결과 nonheme oxoiron(IV) 종에 의한 p-Me-MA 기질의 산화반응에 의해 N,N’-dimethyl-N,N’-diphenylhydrazine 을 생성물로 얻을 수 있었다. 또한 para 위치에 치환기를 달리한 기질인 p-X-MAs 기질들의 ?p 값과 각 기질들의 반응속도의 관계를 통해 -1.96 의 작은 Hammett ??값을 얻을 수 있었다. 수행한 메커니즘의 연구와 생성물의 분석을 통해 aniline 의 질소원자에서 electron transfer 가 일어나는 것이 oxoiron(IV) 착물에 의한 p-Me-MA 기질의 산화반응에서의 반응속도 결정단계임을 제안하였다.;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 nonheme iron enzymes, oxoiron(IV) intermediates have recently been identified by spectroscopic methods in the catalytic cycle of TauD, 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 P450 have been studied for the past twenty years. However, two possible mechanisms, electron transfer followed by proton transfer and hydrogen transfer from ?-C-H bonds of the methyl group in N,N’-dimethylanilines, 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 mechanism-based 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 N-methylanilines 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)FeIV=O]2+ (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.