Cyclic(alkyl)(amino) Carbene-Promoted Ring Expansion of a Carbodicarbene Beryllacycle

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 C-carbene-Be bond to date at 1.8.56(4) angstrom. All of the compounds were fully characterized by X-ray crystallography, Fourier transform infrared spectroscopy, and H-1, C-13, and Be-9 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)).