Nickel-catalyzed reaction of butadiene with strained ring olefins: Formation of a four-membered cyclic compound

Reaction of butadiene with strained ring olefins such as norbornene, dicyclopentadiene etc. Gives an exo-methylene- and methyl-substituted four-membered cyclic compound (II). The effective catalysts are (n-Bu₃P)₂NiBr₂/NaBH₄ or /alkoxide $\text{1}\text{1}$ syn-π-crotylbis(triethyl phosphite)nickel hexafluorophosphate (IV), and tetrakis(triethyl phosphite)nickel/CF₃COOH $\text{1}\text{1}$. π-Crotyl complex IV reacts with the strained ring olefins to give the corresponding product similarly. It is concluded that the active species for this catalytic reaction is a nickel hydride and that this reaction proceeds through π-crotyl intermediate.