Catalytic reductive [4 + 1]-cycloadditions of vinylidenes and dienes

Five-membered rings for two nickels The Diels-Alder reaction is widely used to make six-membered rings by adding four-carbon dienes to two-carbon alkenes. It would seem straightforward to likewise access five-membered rings from dienes and one-carbon sources, or carbenes, but that does not tend to work. Instead, the carbene adds to just half of the diene to form a cyclopropane. Zhou and Uyeda now show that a catalyst with two nickel centers can steer this reaction toward the cyclopentyl products (see the Perspective by Johnson and Weix). A chiral version of the catalyst rendered the reaction enantioselective in intramolecular cases. Science , this issue p.857 ; see also p.819