Comparison of the Mechanism of Borane, Silane, and Beryllium Hydride Ring Insertion into N‑Heterocyclic Carbene C–N Bonds: A Computational Study

A computational investigation has been carried out on the mechanism and energetics of the experimentally observed insertion/ring expansion of N-heterocyclic carbenes (NHCs) by boranes (H₂BNHR, BH₃; R = Me, Ph) and beryllium hydrides (BeH₂) in comparison with silanes (SiH₂R₂; R = Me, Ph). The results suggest that the ring insertion mechanisms are similar for boranes, beryllium hydrides, and silanes. The principal mechanism components are (1) hydrogen atom migration to the carbene carbon, (2) C–N bond expansion of the NHC with insertion of the main-group hydride into the ring, and (3) migration of a second hydrogen atom to the carbene carbon. The synthetically important NHC·BH₃ adduct is also predicted to be thermodynamically unstable with respect to this transformation but is kinetically stabilized with a high barrier to the first hydrogen atom migration. The BeH₂ insertion product provides a rare example of a Be–N π interaction