Palladium-Catalyzed Ring Enlargement of Aryl-Substituted Methylenecyclopropanes to Cyclobutenes

Methylenecyclopropanes 1 could be converted to the corresponding cyclobutenes 2 under the catalysis of palladium acetate in the presence of metal bromide in 1,2-dichloroethane under mild conditions. A plausible reaction mechanism has been proposed on the basis of deuterium labeling experiment.