Ethylene oligomerization catalyzed by a unique phosphine-oxazoline palladium(II) complex. Propagation and chain transfer mechanisms

Palladium(II)-catalyzed ethylene oligomerization reactions using a new [((PN)-N-and)Pd(CH3)(NCArF)][SbF6] complex, 2b ((PN)-N-and = phosphine-oxazoline, Ar-F = 3,5-(CF3)(2)C6H3), are described. Analysis of the X-ray crystal structure of the ((PN)-N-and)Pd(CH3)Cl precatalyst, 2a, reveals a unique axial Pd center dot center dot center dot H interaction (2.33 A) with a ligand C-sp3-H bond. Exposure of this complex to ethylene in methylene chloride produces a Schulz-Flory distribution of alpha-olefins ranging from C-4 to C-24. Effects of varying ethylene pressure and reaction temperature on oligomerizations are reported. The turnover frequency exhibits saturation behavior as ethylene pressure is increased; carrying out a Lineweaver-Burk analysis provides a barrier of 17.5 kcal/mol for migratory insertion of the palladium alkyl ethylene species. The invariance of the Schulz-Flory constant with added quantities of nitrile suggests that chain transfer occurs through a process best described as chain transfer to monomer. The catalyst resting state is shown to vary as a function of ethylene pressure and nitrile concentration. Oligomerization experiments performed with a related phosphine-oxazoline complex lacking Pd center dot center dot center dot H-C-sp3 axial interaction lead only to ethylene dimerization.