Reagent addition sequence and equivalent in N‐heterocyclic carbene‐catalyzed nonpolar inversion enable conversion from aldimine to benzoxazole

Abstract

The umpolung reaction catalyzed by N-heterocyclic carbenes (NHCs) has been widely studied and well recognized, but their role in the nonpolar inversion reaction under oxidative condition has been rarely reported. In this paper, the mechanism of the oxidative nonpolar inversion reaction catalyzed by NHCs to produce benzoxazole is investigated in detail. The reaction occurs through five processes. For oxidation in the second process, two successive tautomerizations followed by oxidation are energetically more favorable than the other two pathways. The rate-determining step is the oxidation by 3,3 0 -5,5 0 -tetra-tert-butyl-4,4 0 -diphenoquinone. Mechanism calculations of the uncatalyzed reaction reveal that the very highly exothermic nature of the initial step is the main reason for the extremely high energy barrier in the following step. With the participation of NHC, this unfavorable transformation can be deftly prevented according to the specific sequence and equivalent of reagent addition, enabling the reaction to occur under mild conditions. K E Y W O R D S density functional theory, mechanism and roles of catalyst, NHC catalysis, nonpolar inversion, oxidation

Recent Advances in Theoretical Studies on Transition-Metal-Catalyzed Carbene Transformations

“…The diverse carbene-related transformations highlight the indispensable role of carbene in organic synthesis, which also implies that the contributions from computational studies are highly appreciated. ,− Previous computational studies of metal carbene-mediated X–H insertion (X = C, N, O, etc. ) − and olefin metathesis − have shown that theoretical calculations can enable a better mechanistic understanding of carbene chemistry. − A summary of recent advances in theoretical studies would provide critical guidance for metal carbene formation and enlighten the rational design of new metal carbene transformation reactions. This account mainly focuses on our recent computational studies of the transition-metal-catalyzed carbene transformation with nucleophiles.…”

Oxidative N‐Heterocyclic Carbene Catalysis

“…Recently, an in-depth DFT study was conducted relative to this catalytic cycle, in order to substantiate it by verification of both rationality and feasibility (Scheme 22). [41] Scheme 19. Detailed mechanism for NHC-catalyzed oxidative acetalization of benzoic acid.…”

“…Recently, an in‐depth DFT study was conducted relative to this catalytic cycle, in order to substantiate it by verification of both rationality and feasibility (Scheme 22 ). [41] …”