Cu(I)-catalyzed intramolecular C-C coupling of activated methylene compounds with vinyl halides: efficient synthesis of functionalized alkylidenecyclobutanes.

With the catalysis of CuI/L-proline, a number of 2-(3-bromobut-3-enyl)malonates underwent efficient intramolecular C-vinylation leading to the synthesis of functionalized alkylidenecyclobutanes. Competition experiments revealed that this four-membered ring closure is fundamentally preferred over the corresponding five-membered ring closure.