Access to a Cationic, Electron-Poor N‑Heterocyclic Carbene with a Quinazolinium Core by Postsynthetic Modification of Related Neutral Derivatives

Derivatives of cationic six-membered N-heterocyclic carbene G with a quinazolinium scaffold are easily obtained by abstraction of an alcoholate group from related neutral precursors which are easily accessible. On the basis of this route, chalcogene adducts and metal complexes of G, which is a benzo-anellated pyrimidiniumylidene, could be prepared. Evaluation of the ligand properties by IR and 77Se NMR spectroscopy indicate that cationic carbene G is a much more powerful π-acceptor (δ­(77Se): 768 ppm) and a significantly weaker overall donor (TEP: 2073 cm–1) than related neutral carbene 4 (361 ppm, 2054 cm–1). DFT calculations provide a rationale for these experimental observations. AuCl complexes of both carbenes were used as catalysts for the cyclization of a propargylamide to a methyleneoxazoline. The complex containing the cationic carbene featured a much higher activity.