Pd‐Catalyzed Cyclization of Alkynyl Norbornene Derivatives for the Synthesis of Benzofused Heteroarenes

Modular approaches, which allow a systematic variation of heteroaromatic cores and substituents, are crucial for the development of heteroaromatic drug candidates and organic functional materials. A new strategy involving the cyclization of heteroarenes tethered with alkynes through a norbornene bridge was developed. The precursors were readily prepared by a three-component coupling process of heteroaryl halides, norbornadiene, and terminal alkynes. The Pd catalytic system derived from Pd(OAc)2 and 2-(pyrazol-1-yl)pyridine transformed a variety of five-membered heteroarenes to the corresponding benzofused products, including (di)benzothiophene, indazole, carbazole, indole, and benzofuran, with aryl and alkyl substituents at the C4(C7) position. During the cyclization process, the norbornene ring underwent a retro-Diels-Alder reaction, serving as an acetylene synthon. This approach was used to synthesize naphthalene derivatives from electron-rich arenes, demonstrating its versatility in the annulation of (hetero)aromatic rings.