Pyrido-Annulated 1,3-Azaphospholes: Synthesis of 1,3-Azaphospholo[5,4-b]pyridines and Preliminary Reactivity Studies

Pyrido-annulated sigma(2)-phosphorus heterocycles, 1,3-azaphospholo[5,4-b]pyridines 4 and 5, were synthesized by reduction of diethyl 2-aminopyridine-3-phosphonates 1 with LiAlH4 and cyclocondensation of the resulting 2-amino-3-phosphanylpyridines 2 with dimethylformamide and dimethylacetamide dimethyl acetal, respectively, via intermediate phosphaalkenes 3. The P=C-N heterocycles are stable in the presence of OH and NH compounds but add tBuLi at the P=C bond. Reaction with one equivalent of M(CO)(5)(thf) leads to eta(1)-P-coordinated (azaphospholo[5,4-b]pyridine)M(CO)5 complexes (M = Cr, Mo, W). Spectroscopic data are in accordance with the dominance of it-acceptor properties. X-ray crystal structure analyses reveal base-pairing of 2-amino3-phosphanylpyridine (2a) and NH-functional azaphospholopyridine 5a by N-H center dot center dot center dot N hydrogen bonds, and the competing formation of 1,3-diphosphetane 6c from the phosphaalkene intermediate.