Effects of the electronic structure of five-membered N-heterocyclic carbenes on insertion of silanes and boranes into the NHC C–N bond

Abstract

The effect of varying the N-heterocyclic carbene (NHC) ligand on ring expansion and endocyclic C–N activation of NHCs by silanes and boranes has been investigated with theoretical methods. Five common NHCs were considered, including unsaturated, saturated, 3,4-methyl, 3,4-chloro substituted five-membered NHCs and a fused benzimidazole NHC. The substrates considered were Ph₂SiH₂ (with two hydrides migrating to the carbene carbon), H₂BMe (two hydrides migrating), HBMe₂ (hydride migration, followed by methyl migration) and BMe₃ (two methyls migrating). Consistent with experimental observations, it was found that the kinetic barrier is lowest for the saturated NHC. The barrier is also quite low for the benzimidazole NHC. It was determined that the partial positive charge on the central carbene carbon in the initial adduct is inversely proportional to the magnitude of the activation barrier.