Synthesis of 10H-di(1-benzothiopheno)[2,3-b:3′,2′-d]pyrrole: re-investigation of the Cadogan cyclization route

Abstract

Abstract The reaction of 2-nitro-3,3′-bi(1-benzothiophene) bearing ester groups at 7- and 7′-positions under the Cadogan cyclization conditions (PPh3, o-C6H4Cl2, refl.) afforded the targeted 10H-di(1-benzothiopheno)[2,3-b:3′,2′-d]pyrrole-1,8-dicarboxylate derivative in a low yield accompanied with a byproduct. On the other hand, the Cadogan reaction of 2-nitro-3,3′-bi(1-benzothiophene) without ester groups did not provide the corresponding pyrrole but gave a mixture of thiophene-ring-opening products, from which dibenzothiophenothiopyrans were isolated. The structures of these ring-opening products were fully characterized by spectroscopic and X-ray analyses and the generation mechanism giving the byproducts was deduced. Pristine 10H-di(1-benzothiopheno)[2,3-b:3′,2′-d]pyrrole could be prepared via another route, namely nitrosation followed by cyclization and reduction.