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

Treatment of ( Ar L)CoBr ( Ar L = 5-mesityl-1,9-(2,4,6-Ph 3 C 6 H 2 )dipyrrin) with a stoichiometric amount of 1-azido-4-( tert -butyl)benzene N 3 (C 6 H 4 - p - t Bu) furnished the corresponding four-coordinate organoazide-bound complex ( Ar L)CoBr(N 3 (C 6 H 4 - p - t Bu)). Spectroscopic and structural characterization of the complex indicated redox innocent ligation of the organoazide. Slow expulsion of dinitrogen (N 2 ) 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 ( Ar L)CoBr( • N(C 6 H 4 - p - t Bu)) or Co IV imido ( Ar L)CoBr(N(C 6 H 4 - p - t Bu)) 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 ( Ar L)CoBr(N 3 (C 6 H 4 - p - t Bu)), a series of alkyl azide-bound Co II analogues expel N 2 only above 60 °C, affording paramagnetic intermediates that convert to the corresponding Co-imine complexes via α-H-atom abstraction. The corresponding N 2 -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 t Bu L ( t Bu L = 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.