On the mechanism of the nickel-catalysed regioselective cyclodimerization of isoprene

A study of various Ni-ligand catalysed oligomerizations of isoprene has shown that with π-acidic P ligands the selectivity to cyclodimers amount to 97%. A new type of ligand is introduced, viz. fluoroalkyl phosphites having π-acceptor properties comparable to those of, e.g. PCl₃. With tris(hexafluoroisopropyl) phosphite the main product is 1,4-dimethy1-4-vinylcyclohexene. A detailed explanation based on a two-step mechanism is given. As to the first step, for a series of ligands having similar steric properties the changes in product distribution as a function of the electronic ligand parameter are explained in terms of a gradual change in HOMO-LUMO interactions between Ni and the olefins, with strong π-acidic ligands promoting the head-to-head coupling of the isoprene molecules. The second step, involving reductive elimination of a cyclodimer from the metal, shows an increasing selectivity towards substituted cyclohexenes for the head-to-head and tail-to-tail intermediates with increasing π-acidity of the ligand. The qualitative orbital treatment presented as an explanation is also applicable to reactions found for other metallacyclopentanes.