Mechanistic Insights into C–H Amination via Dicopper Nitrenes

We
examine important reactivity pathways relevant to stoichiometric
and catalytic C–H amination via isolable β-diketiminato
dicopper alkylnitrene intermediates {[Cl2NN]­Cu}2(μ-NR). Kinetic studies involving the stoichiometric amination
of ethylbenzene by {[Cl2NN]­Cu}2(μ-NtBu) (3) demonstrate that the terminal nitrene
[Cl2NN]­CuNtBu is the
active intermediate in C–H amination. Initial rates exhibit
saturation behavior at high ethylbenzene loadings and an inverse dependence
on the copper species [Cl2NN]­Cu, both consistent with dissociation
of a [Cl2NN]Cu fragment from 3 prior to C–H
amination. C–H amination experiments employing 1,4-dimethylcyclohexane
and benzylic radical clock substrate support a stepwise H-atom abstraction/radical
rebound pathway. Dicopper nitrenes [Cu]2(μ-NCHRR′)
derived from 1° and 2° alkylazides are unstable toward tautomerization
to copper­(I) imine complexes [Cu]­(HNCRR′), rendering
1° and 2° alkylnitrene complexes unsuitable for C–H
amination.