404578-27-4Relevant articles and documents
Coplanar tetracyclic π-excess σ2P ligands
Niaz, Basit,Iftikhar, Fatima,Kindermann, Markus K.,Jones, Peter G.,Heinicke, Joachim
supporting information, p. 4220 - 4227 (2013/09/12)
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. Aromatization-driven acid-catalyzed condensations of the N,P-diprimary compound 1 with dialdehydes are coupled with hydrogen transfer reactions. With glyoxal and pyridine-2,6-dicarbaldehyde this gives rise to side products, but with o-C6H4(CHO)2, high yields of 4 are obtained. π-Delocalization through the coplanar aryl rings causes high π-density at the phosphorus atom.
