Modular Synthesis of Phenanthro[9,10‐ c ]thiophenes by a Sequence of CH Activation, Suzuki Cross‐Coupling and Photocyclization Reactions

A total number of 15 different 3,4‐diarylthiophenes were synthesized, which bear a chlorine atom in ortho ‐position of one of the aryl substituents. One aryl group was introduced by an oxidative cross‐coupling reaction, involving a CH activation at C4(3) of the thiophene core. The other aryl group was in most cases introduced by a Suzuki cross‐coupling reaction, which succeeded the oxidative cross‐coupling step. Photocyclization reactions of the 3,4‐diarylthiophenes were performed in a solvent mixture of benzene and acetonitrile (50:50 v/v) at λ =254 nm and proceeded to the title compounds in yields of 60–82 %. The selectivity of the photocyclization was determined at the ortho ‐chloro‐substituted aryl ring by the position of the chlorine substituent. At the other ring, a single regioisomer was observed for phenyl and para ‐substituted phenyl groups. For 2‐naphthyl and ortho ‐substituted phenyl rings a clear preference was observed in favor of a major regioisomer, while meta ‐substitution in the phenyl ring led to a about 1:1 mixture of 5‐ and 7‐substituted phenanthro[9,10‐ c ]thiophenes. Mechanistically, the photocyclization is likely to occur as a photochemically allowed, conrotatory [(4n+2)π] process accompanied by elimination of HCl. It was shown for two phenanthro[9,10‐ c ]thiophene products that they can be readily brominated in positions C1 and C3 (74–77 %), which in turn allows for further functionalization at these positions, for example, in the course of halogen–metal exchange and polymerization reactions.