Organophosphorus compounds. XIX. Synthesis of 2,3-Dihydro-1 H-1,2-benzazaphosphole 2-oxides, variously substituted on nitrogen and phosphorus, by N-P cyclization of zwitterionic intermediates

Abstract

2-Phenyl-2,3-dihydro-1H-1,2-benzazaphosphole 2-oxide (7a) was prepared by thermolysis of the corresponding zwitterionic amino phosphinic acid (4), or its hydrochloride salt (1). Thermolysis of methyl (2-aminobenzy1)phenylphosphinate (5a) was accompanied by intermolecular O→N transmethylation to give, after cyclization, l-methyl-2-phenyl-2,3-dihydro-1H-1,2-benzazaphosphole 2-oxide (9a); similarly, the ethyl ester (5b) gave the N-ethyl heterocycle (9b).

Reaction of 2-phthalimidobenzyl bromide (13a) with diethyl methylphosphonite (14b) gave ethyl (2-phthalimidobenzy1)methylphosphinate (15a). Hydrolysis of (15a) afforded (2-aminobenzyl)- methylphosphinic acid (16), and thermolysis of this produced 2-methyl-2,3-dihydro-lH-1,2-benzaza- phosphole 2-oxide (18a). 1-Methyi-2-methoxy-2,3-dihydro-1H-l,2-benzazaphosphole 2-oxide (24) was synthesized in an analogous manner.

Base catalysed N-alkylation of the benzazaphosphole derivatives (7a) and (18a) was readily achieved, and the interconversion of 2-oxides and 2-sulfides was accomplished by conventional methods.