Catalytic linear oligomerization of ethylene to higher α-olefins:: Insight into the origin of the selective generation of 1-hexene promoted by a cationic cyclopentadienyl-arene titanium active catalyst

A detailed theoretical investigation is presented of crucial elementary steps of the selective linear ethylene oligomerization to 1-hexene by the cationic [(mu(5)-C(5)H(4)-(CMe(2))-bridge)-C(6)H(5))-Ti(IV)(CH(3))(2)](+) precatalyst, employing a gradient-corrected DFT method. The essential aspects of the originally proposed mechanism have been confirmed and supplemented by novel insights into how the selective ethylene oligomerization operates. This includes the examination of the ability of titana(IV)cycle intermediates to grow and/or to decompose affording alpha-olefins as a function of their size, the prediction of the favorable route for precatalyst activation, and the exploration of the cycloalkane production as a possible side process. After the Ti(IV)-Me(2) precatalyst is smoothly converted into the active Ti(II)-(ethylene)(2) catalyst complex, the two ethylene moieties readily undergo oxidative coupling to afford first the titana(IV)cyclopentane species. Metallacycle growth through bimolecular ethylene uptake and subsequent insertion displays very similar structural and energetic characteristics for five- and seven-membered titana(IV)cycles. Decomposition of titana(IV)cycles to alpha-olefins preferably takes place via a concerted transition-metal-assisted beta-H transfer for conformationally flexible metallacycles beginning with the titana(IV)cycloheptane, with very similar barriers having to be overcome. This decomposition path, however, is kinetically inaccessible for the rigid five-membered titana(IV)cyclopentane. Instead, the stepwise mechanism via a metastable Ti(IV)-alkenyl-hydride species is found to be operative in this case. A significantly raised activation barrier is connected with the stepwise path, which makes the growth of the titana(IV)cyclopentane to the seven-membered cycle the more favorable process than its decomposition to 1-butene. Cycloalkanes are less likely to be formed, due to a kinetically handicapped reductive CC elimination. On the basis of the detailed insights into the ability of titana(IV)cycles to undergo either growth or decomposition to alpha-olefins, the thermodynamic and kinetic aspects for the selectivity control of the linear ethylene oligomerization have been rationalized. The crucial role played by the hemilabile arene ligand for the selective oligomerization process has also been analyzed.