Cobalt‐Catalyzed Formal [4+2] Cycloaddition of α,α′‐Dichloro‐ ortho ‐Xylenes with Alkynes

A formal [4+2] cycloaddition of α,α′‐dichloro‐ ortho ‐xylenes with various alkynes has been developed using a low‐valent cobalt catalyst. The transformation has a wide substrate scope and high functional‐group tolerance and led to 1,4‐dihydronaphthalenes. The formed cycloadducts were easily aromatized with MnO 2 under air. A mechanistic investigation suggests that the transformation proceeds through a benzyl cobaltation of alkyne, not the classical Diels–Alder reaction of ortho ‐quinodimethanes. This methodology provides a straightforward and streamlined access to linearly expanded π‐conjugated aromatics.