The synthesis and reactions of 2-carboxythiophene-3-diazonium chloride

Abstract

The potential thiophyne-precursor 2-carboxythiophene-3- diazonium chloride was prepared by diazotization of 3-amino- 2-thenoic acid with isoamyl nitrite and hydrochloric acid. Decomposition of this diazonium salt in the presence of different aryne traps gave no indication of the intermediate formation of thiophyne. Instead, products were obtained indicating that nitrogen was lost homolytically and carbon dioxide, when it was not retained in the products, was lost heterolytically. The diazonium chloride arylated aromatic compounds to give 3-arylthiophenes in isomer ratios consistent with radical substitution. Small amounts of 3-aryl-2-thenoic acids were also found. These were not decarboxylated under the reaction conditions. In ethanol reduction occurred giving thiophene and 2-thenoic acid. The Sandmeyer reaction gave only 3-chloro-2-thenoic acid, while decomposition in bromotrichloromethane gave both 3-bromothiophene and 3-bromo-2-thenoic acid. Self- I condensation to give azo compounds was observed in chlorinated hydrocarbons and ethanol. ln situ nitrosation of 3-acetamidothiophene with isoamyl nitrite gave reactions very similar to the diazonium carboxylate salt. Again no aryne was indicated.