The Effects of Phosphorus Ligands on the Stereoselectivity of the Nickel-catalyzed Dimerization of Norbornadiene

The effects of phosphorus ligands on the stereoselectivity of the nickel-catalyzed 2+2 cyclodimerization of norbornadiene have been studied. The electronic factor, νCO(A1), of Ni(CO)3L (where L is the ligand) is correlated with the ratio of the exo-trans-exo (X-t-X) to the exo-trans-endo (X-t-N) isomers of the product, but no simple correlation is apparent between the steric parameter and the selectivity. A three-coordinated Ni(0) species is proposed as the intermediate. The values of ΔHX-t-X−ΔHX-t-N and ΔSX-t-X−ΔSX-t-N are large and negative.