Mechanistic aspects of the nickel‐catalyzed addition reaction of alkenyl‐zirconium reagents to cyclopropyl ketones

Nickel‐catalyzed addition of alkenylzirconium species to cyclopropyl ketones proceeds by an electron transfer mechanism. One‐electron reduction of the substrate leads to a ketyl which may either couple to give the 1, 2‐addition product or undergo a second one‐electron reduction, followed by rearrangement and coupling to give a β‐methyl, β‐alkenyl ketone. This rearrangement is postulated to occur via β‐hydride elimination and reinsertion in an organonickel intermediate. The ratio of 1, 2‐adduct to ring‐opened product formed is dependent on the concentration of nickel(I) catalyst present and suggests that two Ni(I) species, used successively, are required to open the three‐membered ring.