Annulated boron substituted N-heterocyclic carbenes: theoretical prediction of highly electrophilic carbenes

Theoretical calculations were carried out to understand the effect of annulation on the electronic and ligand properties of boron substituted N-heterocyclic carbenes (B-NHCs). Annulation results in a decrease in stability as indicated by the calculated values of singlet-triplet separations and stabilization energies as well as HOMO-LUMO gaps. Annulated B-NHCs are found to be weaker sigma-donors but better pi-acceptors than the parent ones. The decrease in sigma-donation ability and the increase in pi-accepting ability are further supported by the calculated values of proton affinities, nucleophilicity and electrophilicity indices as well as P-31 NMR chemical shifts of the corresponding NHC-PPh adducts. Most of the annulated B-NHCs are found to have significantly enhanced electrophilicity than the other known carbenes.