C-H Amination Mediated by Cobalt Organoazide Adducts and the Corresponding Cobalt Nitrenoid Intermediates

Abstract

Treatment of (^ArL)CoBr (^ArL = 5-mesityl-1,9-(2,4,6-Ph₃C₆H₂)dipyrrin) with a stoichiometric amount of 1-azido-4-(tert-butyl)benzene N₃(C₆H₄-p-^tBu) furnished the corresponding four-coordinate organoazide-bound complex (^ArL)CoBr(N₃(C₆H₄-p-^tBu)). Spectroscopic and structural characterization of the complex indicated redox innocent ligation of the organoazide. Slow expulsion of dinitrogen (N₂) was observed at room temperature to afford a ligand functionalized product via a [3 + 2] annulation, which can be mediated by a high-valent nitrene intermediate such as a Co^III iminyl (^ArL)CoBr(^•N(C₆H₄-p-^tBu)) or Co^IV imido (^ArL)CoBr(N(C₆H₄-p-^tBu)) complex. The presence of the proposed intermediate and its viability as a nitrene group transfer reagent are supported by intermolecular C-H amination and aziridination reactivities. Unlike (^ArL)CoBr(N₃(C₆H₄-p-^tBu)), a series of alkyl azide-bound Co^II analogues expel N₂ only above 60 °C, affording paramagnetic intermediates that convert to the corresponding Co-imine complexes via α-H-atom abstraction. The corresponding N₂-released structures were observed via single-crystal-to-crystal transformation, suggesting formation of a Co-nitrenoid intermediate in solid-state. Alternatively, the alkyl azide-bound congeners supported by a more sterically accessible dipyrrinato scaffold ^tBuL (^tBuL = 5-mesityl-(1,9-di-tert-butyl)dipyrrin) facilitate intramolecular 1,3-dipolar cycloaddition as well as C-H amination to furnish 1,2,3-dihydrotriazole and substituted pyrrolidine products, respectively. For the C-H amination, we observe that the temperature required for azide activation varies depending on the presence of weak C-H bonds, suggesting that the alkyl azide adducts serve as viable species for C-H amination when the C-H bonds are (1) proximal to the azide moiety and (2) sufficiently weak to be activated.