Pyrrolo[1,2‑<i>c</i>]pyrimidin-1-ylidene:\nA Diamino Carbene Embedded in a Six-Membered Aromatic Scaffold

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