Transition metal-catalyzed [4+2+2] cycloadditions of bicyclo[2.2.1]hepta-2,5-dienes (norbornadienes) and bicyclo[2.2.2]octa-2,5-dienes

The transition-metal-catalyzed [4 + 2 + 2] cycloadditions of norbornadienes , bicyclo[2.2.2]octa-2,5-diene, and benzobarrelene with 1,3-butadienes proc eed in excellent yields using cobalt-based catalytic systems. Two key disti nctions between these [4 + 2 + 2] cycloadditions and the corresponding tran sition-metal-catalyzed [2 + 2 + 2] reactions of norbornadiene are the requi rement of a bimetal catalytic system with a bisphosphine ligand for the for mer and exclusive regioselectivity in the [4 + 2 + 2] reaction of 2-substit uted norbornadienes to produce 1-substituted adducts. These distinctions ma y indicate two distinct mechanisms for the [4 + 2 + 2] and [2 + 2 + 2] reac tions.