A Versatile Tripodal Cu(I) Reagent for C-N Bond Construction via Nitrene-transfer Chemistry: Catalytic Perspectives and Mechanistic Insights on C-H Aminations/amidinations and Olefin Aziridinations
A CuI 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.
V. Bagchi and P. Paraskevopoulou and P. K. Das and L. Chi and Q. Wang and A. Choudhury and J. S. Mathieson and L. Cronin and D. B. Pardue and T. R. Cundari and G. Mitrikas and Y. Sanakis and P. Stavropoulos, "A Versatile Tripodal Cu(I) Reagent for C-N Bond Construction via Nitrene-transfer Chemistry: Catalytic Perspectives and Mechanistic Insights on C-H Aminations/amidinations and Olefin Aziridinations," Journal of the American Chemical Society, vol. 136, no. 32, pp. 11362-11381, American Chemical Society (ACS), Jan 2014.
The definitive version is available at http://dx.doi.org/10.1021/ja503869j
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