Key mechanistic insights into the intramolecular C-H bond amination and double bond aziridination in sulfamate esters catalyzed by dirhodium tetracarboxylate complexes

摘要

Density Functional Theory was used to study the mechanisms of intramolecular C-H amination and olefin aziridination reactions of a variety of sulfamate esters. Particular emphasis is placed on the mechanism and factors governing amination of primary, secondary, tertiary and benzylic C-H bonds, the competition between tertiary and benzylic C-H amination, and the competition between allylic C-H amination and olefin aziridination. In these studies we used three different dirhodium paddlewheel catalysts, such as model (H2O)Rh-2(O2CH)(4) (I), (H2O)Rh-2(AcO)(4) (II), and (H2O)Rh-2(esp)(2) (III). In general, we found that all catalysts have a diamagnetic closed shell singlet state with a single Rh-Rh s-bond. Active catalytic species in the studied amination reactions are triplet state dirhodium-nitrene complexes with the Rh-Rh single bond and Rh-N double bond (with one s-bond and two "one-electron p-bonds"). From the active nitrenoid species, the C-H bond amination proceeds via triplet-to-singlet surface crossing and singlet state concerted C-H insertion mechanism. The calculated energy barriers correlate with the trend in homolytic bond dissociation energy of the activated C-H bonds. With the allylic substrate, the competing C = C double bond aziridination follows a stepwise pathway involving the formation of radical intermediate and radical coupling to produce singlet aziridination product. However, the allylic C-H bond amination occurs with a lower barrier which is consistent with experimental product distributions. (C) 2017 Elsevier B.V. All rights reserved.