Gc. Bazan et al., Catalytic insertion of ethylene into Al-C bonds with pentamethylcyclopentadienyl-chromium(III) complexes, ORGANOMETAL, 20(10), 2001, pp. 2059-2064
Addition of ethylene to mixtures of Cp*CrMe2(PMe3) (1) and excess methylalu
minoxane (MAO), followed by aqueous workup of the resulting solutions, resu
lts in the formation of a distribution of odd-carbon alkanes. When B(C6F5)(
3) is added to 1, followed by THF, one obtains dark crystals. A single-crys
tal X-ray diffraction study revealed that the product of the reaction is [C
p*CrMe(PMe3)(THF)][MeB(C6F5)(3)] (2). Polyethylene is obtained when using 1
and 2 equiv of B(C6F5)(3) The reaction of ethylene with a mixture of 1 + 2
B(C6F5)(3) + 380 "AlMe3" produces a distribution of odd-carbon alkanes. If
triethylaluminum is used, instead of trimethylaluminum, the product is a d
istribution of even-carbon alkanes. The product distribution shifts to lowe
r molecular weight product, and lower activities are attained with increasi
ng triethylaluminum concentration. Use of more dichoromethane instead of to
luene or hexane increases the ethylene consumption. Triisobutylaluminum and
trioctylaluminum result in considerably lower activities. A mechanism for
the oligomerization of ethylene involving fast transmetalation reactions be
tween chromium and aluminum is proposed.