posted on 2019-08-04, 20:03authored byJacob E. Walley, Akachukwu D. Obi, Grace Breiner, Guocang Wang, Diane A. Dickie, Andrew Molino, Jason L. Dutton, David J. D. Wilson, Robert J. Gilliard
Recent synthetic
efforts have uncovered several bond activation pathways mediated by
beryllium. Having the highest charge density and electronegativity,
the chemistry of beryllium often diverges from that of its heavier
alkaline earth metal congeners. Herein, we report the synthesis of
a new carbodicarbene beryllacycle (2). Compound 2 converts to 3 via an unprecedented cyclic(alkyl)(amino)
carbene (CAAC)-promoted ring expansion reaction (RER). While CAAC
activates a carbon–beryllium bond, N-heterocyclic carbene (NHC)
coordinates to beryllium to give the tetracoordinate complex 4, which contains the longest carbeneC–Be
bond to date at 1.856(4) Å. All of the compounds were fully characterized
by X-ray crystallography, Fourier transform infrared spectroscopy,
and 1H, 13C, and 9Be NMR spectroscopy.
The ring expansion mechanism was modeled with both NHC and CAAC using
density functional theory calculations. While the activation energy
for the observed beryllium ring expansion with CAAC was found to be
14 kJ mol–1, the energy barrier for the hypothetical
NHC RER is significantly higher (199.1 kJ mol–1).