Theoretical Investigation of Hydride Insertion into N-Heterocyclic Carbenes Containing N, P, C, O and S Heteroatoms.

The endocyclic ring expansion of N-heterocyclic carbene (NHC) rings containing one N atom and one P, N, C, O or S heteroatom has been investigated in a computational study. Ring expansion was determined to follow a common pathway, leading to a final expanded six-membered ring that was predicted to be thermodynamically stable for all heterocycles. However, reactivity is driven by kinetics with ring-expansion reactivity not expected for the P/NHC and C/NHC (cAAC) containing heterocycles due to large barriers for ring-expansion. In contrast, ring expansion is predicted to be feasible for N-heterocyclic carbenes containing an O (O/NHC) or S (S/NHC) heteroatom, with insertion into the O-C and S-C bond favoured over insertion into the N-C bond, respectively.