Comparison of the Mechanism of Borane, Silane, and
Beryllium Hydride Ring Insertion into N‑Heterocyclic Carbene
C–N Bonds: A Computational Study
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Abstract
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 (H<sub>2</sub>BNHR, BH<sub>3</sub>; R = Me, Ph) and beryllium hydrides (BeH<sub>2</sub>) in
comparison with silanes (SiH<sub>2</sub>R<sub>2</sub>; 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·BH<sub>3</sub> 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 BeH<sub>2</sub> insertion product provides a rare example of a Be–N
π interaction
