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 MnO2 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.