Imidazo[1,5-a]pyridin-3-ylidenes as π-accepting carbene ligands: substituent effects on properties of N-heterocyclic carbenes.

Various 1-substituted-imidazo[1,5-a]pyridin-3-ylidenes were prepared and characterized. The fundamental character and the effects of substituents on the imidazo[1,5-a]pyridine backbone on the electronic character of carbenes were evaluated using rhodium complexes and selenium adducts. C NMR chemical shifts of carbene carbons in imidazo[1,5-a]pyridin-3-ylidene and H and C NMR spectra of olefin moieties trans to the carbene in Rh(nhc)(cod)Cl complexes shifted relatively downfield compared to those for the corresponding complexes bearing conventional N-heterocyclic carbenes (NHCs).