3-Aryl-2<i>H</i>-azirines as annulation reagents in the Ni(II)-catalyzed synthesis of 1<i>H</i>-benzo[4,5]thieno[3,2-<i>b</i>]pyrroles

A high-yielding method for the synthesis of 3-arylbenzo[4,5]thieno[3,2-<i>b</i>]pyrroles has been developed via pyrrole ring annulation to the aromatic benzo[<i>b</i>]thiophene system, using 3-arylazirines as a N‒C=C synthon. The reaction is catalyzed by Ni(hfacac)₂ and proceeds through the azirine ring opening across the N=C3 bond. Azirines with both electron-donating and electron-withdrawing C3-aryl substituents tolerate the reaction conditions. The reaction of the <i>N</i>-methylindole analog also provides the annulation product but in moderate yield. The described synthesis is the first example of a dealkoxycarbonylative annulation reaction, in which 2<i>H</i>-azirines act as the annulation reagent.