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 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 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.