Abstract:
A Cu-I catalyst (1), supported by a framework of strongly basic
guanidinato moieties, mediates nitrene-transfer from PhI=NR sources to a
wide variety of aliphatic hydrocarbons (C-H amination or amidination in
the presence of nitriles) and olefins (aziridination). Product profiles
are consistent with a stepwise rather than concerted C-N bond formation.
Mechanistic investigations with the aid of Hammett plots, kinetic
isotope effects, labeled stereochemical probes, and radical traps and
clocks allow us to conclude that carboradical intermediates play a major
role and are generated by hydrogen-atom abstraction from substrate C-H
bonds or initial nitrene-addition to one of the olefinic carbons.
Subsequent processes include solvent-caged radical recombination to
afford the major amination and aziridination products but also
one-electron oxidation of diffusively free carboradicals to generate
amidination products due to carbocation participation. Analyses of
metal- and ligand-centered events by variable temperature electrospray
mass spectrometry, cyclic voltammetry, and electron paramagnetic
resonance spectroscopy, coupled with computational studies, indicate
that an active, but still elusive, copper-nitrene (S = 1) intermediate
initially abstracts a hydrogen atom from, or adds nitrene to, C-H and
C=C bonds, respectively, followed by a spin flip and radical rebound to
afford intra- and intermolecular C-N containing products.
