Nucleophile-induced cyclisation of 2-diazo-2′-thiocyanatoacetophenone: 2-(cyanohydrazono)- and 2-(triphenylphosphoranylidenehydrazono)benzo[b]thiophen-3(2H)-one

Abstract

2-Diazo-2′-thiocyanatoacetophenone (Va) dissolves in cold dilute alkali and acidification of the solution provides 2-(cyanohydrazono)benzo[b]thiophen-3(2H)-one (IXa). The potassium salt of the latter, obtained by the interaction of the diazo-ketone and potassium cyanide, on acidic hydrolysis gives 2-semicarbazonobenzo[b]-thiophen-3(2H)-one(IXc), identical with that obtained by condensing 2,2-dibromobenzo[b]thiophen-3(2H)-one (VIIc) with semicarbazide. With triphenylphosphine the diazo-ketone gives the expected adduct, which on brief boiling with methanol suffers elimination of hydrocyanic acid to provide 2-(triphenylphosphoranylidene-hydrazono)benzo[b]thiophen-3(2H)-one (VIIIb). The potassium salt of 2-(cyanohydrazono)indane-1,3-dione (IIb) is obtained by an analogous reaction of potassium cyanide with 2-diazo-2′-(methoxycarbonyl)-acetophenone, the triphenylphosphine adduct of which, however, did not cyclise. It is suggested that these cyclisations proceed by nucleophilic attack at the terminal nitrogen atom of the diazo-function rather than by deprotonation of the diazo-carbon atom.