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

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.