• Phosphorus heterocycles;
  • P ligands­;
  • Annulation;
  • Tungsten


The acid-catalyzed reactions of 5-methyl-2-phosphanylaniline (1) with dialdehydes were studied. Whereas the reaction with glyoxal provides a mixture of two 1H-1,3-benzazaphospholes, 2 and 3, by concomitant reduction of a CHO group and C–C bond cleavage, respectively, the reaction with o-phthalic dicarbaldehyde provided in excellent yield the tetracyclic planar benzazaphosphole 4, which was characterized by crystal structure analysis. The active hydrogen atoms, delivered by aromatization of a dihydrobenzazaphosphole intermediate, forms an N–CH2 bridge by reductive N-alkylation. Pyridine-2,6-dicarbaldehyde reacts analogously but not chemoselectively and, thus, gives two isomers 5 and 6. Condensation with pyridine-2,6-dicarbaldehyde afforded the bis(benzazaphosphole)pyridine pincer ligand 7, but along with 2-(benzazaphospholyl)-6-tolylpyridine (8) by partial P–C bond cleavage. The high upfield 31P NMR chemical shift of 4 compared to those of normal benzazaphospholes indicates it to be a particularly π-rich σ2P ligand.