C-H Amination Mediated by Cobalt Organoazide Adducts and the Corresponding Cobalt Nitrenoid Intermediates.
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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 CoIII iminyl (ArL)CoBr(N(C6H4-p-tBu)) or CoIV 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 CoII analogues expel N2 only above 60 C, affording paramagnetic intermediates that convert to the corresponding Co-imine complexes via -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.
