Evidence for π-allylic nickel complexes as intermediates in the reactions of nickelocene with allylic chlorides

Reactions of trans-1-chloro-2-butene and of 3-chloro-1-butene with nickelocene give mixtures of (1-methyl-2-propenyl)-, (trans-2-butenyl)-, and (cis-2-butenyl)-cyclopentadienes. The reaction between π-crotyl-π-cyclopentadienylnickel and 5-chlorocyclopentadiene yields identical products. In the presence of tetrahcloromethane, 5-(trichloromethyl)cyclopentadiene is formed. Mechanisms involving oxidative addition and π-allylic nickel complexes are discussed.