Modular Synthesis of Tetracyclic Heteroarenes via Platinum‐Catalyzed Cyclization of Ethynylbiaryl Precursors

Polycyclic heteroarenes that contain internal thiophene, furan and pyrrole rings are well‐documented materials due to their presence in fossil fuels and their importance in organic optoelectronic applications. Accordingly, a variety of methods are available for their synthesis. In this report we prepared naphthannulated 4‐membered heteroarene isomers through the PtCl₂‐catalyzed 6‐endo cyclization of ethynylbiaryls. Isomers were accessible from two structurally different precursors that contained the ethynyl subunit in different positions. We synthesized two pairs of thiophene‐based ethynylbiaryls where each pair would lead to the same tetracyclic isomer and explored their reactivity in the Pt‐catalyzed transformation. The formation of a detectable amount of inseparable side‐product was observed when the alkyne was introduced onto the benzene ring of the biaryls. This is suggested to be an exo‐methylene group containing derivative that forms via a 5‐exo cyclization pathway. Similarly, naphthannulated benzofuran isomers were prepared following the same strategy and an exo‐methylene‐containing side product was observed when the alkyne unit was on the benzene ring of the biaryl. Finally, novel synthesis methods were described for benzo‐fused carbazoles from ethynylbiaryls containing an alkyne either on the heterocyclic ring or on the phenyl substituent attached to the indole ring. No side‐product formation was detected in this case.