Acid‐Catalyzed Cyclodimerizations via Vinyl Cations as Intermediates

The electrophilic addition of hydrogen chloride and bromide to allene and of hydrogen bromide to methylacetylene at −70 °C leads not only to the simple Markownikoff adducts but also to cis‐ and trans‐1,3‐dihalo‐1,3‐dimethylcyclobutanes. These cyclodimerizations, which evidently proceed via vinyl cations, have opened up new short routes for the synthesis of cyclobutane‐, cyclobutene‐, and bicyclobutane derivatives starting from substrates that are industrially readily accessible.