Pyrrolo[1,2-c]pyrimidin-1-ylidene: A Diamino Carbene Embedded in a Six-Membered Aromatic Scaffold

Abstract

N-heterocyclic carbenes (NHC) derived from the pyrrolo[1,2-c]pyrimidine skeleton have been synthesized. They are constitutional isomers of the well-known imidazo[1,5-a]pyridin-3-ylidenes (ImPy), but differ in the position where the nonbridgehead nitrogen is located; formally, it has been transferred from the five-membered ring in ImPy structures to the six-membered ring in pyrrolo[1,2-c]pyrimidin-1-ylidenes (PyPy). As consequence, the resulting diamino-stabilized carbene unit is embedded in an aromatic, six-membered, pyrimidine ring. IR data of the corresponding Rh-carbonyl derivatives evidence that the members of this new NHC family are exceptionally strong σ-donors; they even outperform ImPy or more traditional imidazole-2-ylidenes. On the other hand, Ganter′s selenourea method characterizes them as strong π-acceptor. Au-complexes bearing PyPy ligands have been also prepared, and their performance evaluated in the cyclization of N-(3-iodoprop-2-ynyl)-N-tosylanilines 21a–c into 1,2-dihydroquinolines of different substitution pattern 23a–c. The analysis of the product ratios obtained agrees with the donor ability rank established by the spectroscopic methods.