Novel nickel catalysts based on perfluoroalkyl-β-diketone ligands for the selective dimerization of propylene to 2,3-dimethylbutenes

The propylene dimerization to 2,3-dimethylbutenes (DMB) by novel homogeneous catalysts prepared in situ by two alternative routes was described. The former synthetic procedure was based on the oxidative addition of hexafluoro-2,4-pentandione (hfacac) to bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)(2)] in the presence of an ancillary phosphine ligand and an organoaluminium compound. The latter route consisted in the reaction of bis(perfluoroalkyl-beta -diketonato)nickel(II) complexes with the organoaluminium compound in the presence of the phosphine ligand. In this context the novel bis(tetradecafluoro-4, 6-nonandionato)nickel(II)(.)2 dimethylformamide complex [Ni(tdfnd)(2)(dmf)(2)] was synthesized and its crystal and molecular structures were determined. The catalytic systems obtained with both procedures resulted highly active in the oligomerization of propylene (TOF > 20000 h(-1)). In particular, when Ni(hfacac), was combined with the bulky and basic tricyclohexylphosphine (PCy3) and MAO was adopted as co-catalyst a very high regioselectivity to DMB within the C-6 cut was achieved (ca. 90%). The use of Ni(tdfnd),(dmf), in the place of Ni(hfacac), gave similar results under the same reaction conditions. Changing the type of phosphine ligand and the nature of the organoaluminium co-catalyst it was possible to deeply modify both regio- and chemo-selectivity of the process, thus allowing to orientate the reaction towards specific target oligomeric products. (C) 2001 Elsevier Science B.V. All rights reserved.