Titanium-catalyzed cyclotrimerization of butadiene: Part II. The (C 6H 6)Ti II(AlCl 4) 2-Et xAlCl 3- x( x = 1–3) systems

Abstract

The cyclotrimerization of butadiene to ( Z, E, E)-1,5,9-cyclododecatriene (( Z, E, E)-CDT) is catalyzed by the (C 6H 6)Ti(AlCl 4) 2( Ia)-Et x AlCl 3- x ( x = 1–3) systems which contain the Ia, (C 6H 6)Ti(AlCl 4)(AlCl 3Et) ( Ia-1) or (C 6H 6)Ti(AlCl 3Et) 2 ( Ia-2) complexes as active species. The ethyl substituents bring about an increase in the initial reaction rate followed by an enhanced deactivation of the catalyst. In the highly ethylated systems, e.g. at a molar ratio of Et 2AlCl: Ia >/ 6, the initial, very rapid reaction rate greatly decreases in later stages, and the reaction continues at a rate about one order lower. During the initial rapid stage, a precipitate of TiCl 2 is formed via deactivation of Ia-2 and decomposition of a complex derived from the catalytically ineffective (C 6H 6)Ti(AlCl 3Et)(AlCl 2Et 2) ( Ia-3) complex. The continued formation of ( Z, E, E)-CDT in the low-activity stage probably proceeds on Ia-2, a low concentration of which is maintained by the reversible reaction between TiCl 2 and EtAlCl 2 present in a mixture with Et 2AlCl. The reaction order with respect to butadiene, as determined from the initial reaction rates, decreases from 1.9 for Ia to 1.5 for Ia-1 and to a value between 1.0 and 1.5 for Ia-2. The reaction order in the catalyst increases (probably to 1.0) with decreasing catalyst concentration and vice versa for all the systems; a value of 0.5–0.6 was obtained for [Ti] = 6.5 × 10 −4 − 3.9 × 10 −3M. The observed reversible interconversions of the (C 6H 6)Ti II complexes Ia-Ia-2 and the Ti(II) complexes containing ( Z, E, E)-CDT, 1,5-cyclooctadiene or butadiene and the transformation of all of them to the (HMB)Ti II complexes by reaction with hexamethylbenzene (HMB) support our suggestion that ( Z, E, E)-CDT is selectively formed on the Ti(II) complexes with retained trinuclear structure. The catalyst deactivation is assumed to be initiated by an electron transfer from Ti(II) to an organic ligand, and the deactivation products differ in their dependence on the nature and the content of Et x AlCl 3- x