Competing nickel-catalysed dimerisation and degenerate isomerisation of propene

Methods have been established for analysing the deuterium content of propene dimers by successive dihydroxylation and acetal formation, assaying isotope distribution in the dioxolane by {H-1}, {H-2} doubly decoupled C-13 NMR. When specifically deuterated propene, synthesised by a modified Grignard method, was subjected to catalytic dimerisation employing Et(2)Al(2)Cl(3) and chloro-eta(3)-propenyl(iPr(3)P)nickel, it was found that isotopic scrambling was complete within experimental error. This was traced to successful competition from an Ni-H promoted degenerate isomerisation of propene which is rapid on the time-scale of dimer formation. This points to a nickel hydride driven dimerisation mechanism in preference to alternatives involving metallocyclopentane intermediates.