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.