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

Jablonkai, István and Bogner, Marcell Márk and Zsignár-Nagy, Barnabás and Simon, József and London, Gábor (2025) Modular Synthesis of Tetracyclic Heteroarenes via Platinum-Catalyzed Cyclization of Ethynylbiaryl Precursors. EUROPEAN JOURNAL OF ORGANIC CHEMISTRY. No. e202401114. ISSN 1434-193X

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Abstract

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 exomethylene group containing derivative that forms via a 5-exo cyclization pathway. Similarly, naphthannulated benzofuran isomers were prepared following the same strategy and an exomethylene-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.

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