PC–N-Heterocycles: synthesis of biaryl-type 1,3-benzazaphospholes with ortho-substituted phenyl or 2-heteroaryl groups

Abstract

A facile synthesis of functionally substituted 2-(hetero)aryl 1,3-benzazaphospholes vianickel- or palladium-catalyzed phosphonylation of N-acyl-2-bromoanilides 1a–k with triethyl phosphite is presented. Anilidophosphonates 2a–g with naphthoyl-, o-substituted phenyl, furoyl- or thenoyl groups allow direct reductive cyclization with LiAlH₄ to benzazaphospholes 3. The reaction of the o-bromoderivative 2d proceeds with concomitant replacement of bromine by hydrogen, whereas the electron-withdrawing pyridyl group of 2h prevents the synthesis of 3h by this short route. An alternative synthesis of 2-pyridylbenzazaphosphole 3hvia anilidophosphonates succeeded starting from Fmoc-anilinophosphonate 2kvia selective cleavage of the N-protecting group, reduction of the resulting phosphonoaniline to phosphinoaniline and cyclization with pyridine-2-carboxaldehydevia a dihydrobenzazaphosphole 8. N-Substituted pyridylmethylbenzazaphosphole 9 was detected as a side product. The structure elucidation of the new compounds is based on multinuclear NMR data and X-ray crystal structure analyses of a phosphonoanilide, underlining the dominance of N–H⋯OP hydrogen bonds over N–H⋯OC type hydrogen bonds, of 3h and a supramolecular associate of 3b and its unprecedented air oxidation product 10.