The reactivity of organophosphorus compounds. Part XXX. Iminophospholes and a new synthesis of benzofurazans via intramolecular rearrangement of 1-o-nitroarylimino-1,2,5-triphenylphospholes

Abstract

A series of N-substituted 1-imino-1,2,5-triphenylphospholes (2) have been synthesised. Reactions of aryl, methanesulphonyl, arylsulphonyl, ethoxycarbonyl, phenoxycarbonyl, and diphenylphosphinyl azides with 1,2,5-triphenylphosphole gave the corresponding N-substituted 1-iminophospholes [2; X = Ar, MeSO₂, ArSO₂, EtO₂C, PhO₂C, and Ph₂P(O)] in good yields by non-nitrene routes. The tosyliminophosphole (2; X = Tosyl) was also produced by reaction of anhydrous chloramine-T with the phosphole. Benzoyl azide reacted by decomposition followed by Curtius rearrangement rather than by reaction with the relatively weakly nucleophilic 1,2,5-triphenylphosphole (cf. Ph₃P). The electron deficient 4-nitrobenzoyl and 2,4-dinitrobenzoyl azides gave the corresponding 1-aroylimino-1,2,5-triphenylphospholes [2; X = C₆H₄NO₂-p and 2,4-(NO₂)₂C₆H₃] in 6 and 55% yields respectively. 5,7-Dimethyltetrazolo[1,5-a]pyrimidine (13) in the presence of copper gave the 1-[(4,6-dimethylpyrimidin-2-yl)imino]phosphole (14). Thermolysis of 5-phenyl- and 5-p-nitrophenyl-1,3,4-dioxazol-2-ones (7) with 1,2,5-triphenylphosphole at 150° gave the corresponding 1-aroylimino-1,2,5-triphenylphospholes (2; X = PhCO and p-NO₂·C₆H₄CO), but these decomposed at this temperature to give the corresponding aryl cyanides and the phosphole oxide. The use of copper-bronze reduced the decomposition point of the dioxazolidin-2-ones sufficiently for the iminophospholes to be isolated. Base catalysed decomposition of ethyl N-(p-nitrophenylsulphonyloxy)carbamate (4) in the presence of the phosphole gave 1-ethoxycarbonylimino-1,2,5-triphenylphosphole (2; X = CO₂Et). In the last two reactions the phosphole is probably acting as a nitrene trap as it is in the thermolysis of the 1-phthalimidobenzofuro[2,3-b]azirine (9) to give the 1-phthalimidoiminophosphole.

Thermolysis of 1-o-nitroarylimino-1,2,5-triphenylphospholes at 150° gives 1,2,5-triphenylphosphole oxide and the corresponding substituted benzofurazans; this reaction is shown to proceed via intramolecular attack by an oxygen atom of the neighbouring nitro-group on the phosphorus atom. The corresponding N-o-nitrophenyliminotriphenylphosphorane and triethyl phosphorimidate [Ph₃PNAr and (EtO)₃PNAr] are stable, pointing to a special effect of the phosphole ring, believed to involve ring strain, at least in part.