Titanium-catalyzed cyclotrimerization of butadiene: I. Arenetitanium(II) chloro- and bromo-alane complexes

(η 6-Benzene)bis(dichloroalanedi-μ-chloro)titanium(II) ( Ia) and its bromo analogue (Ib) catalyze the cyclotrimerization of butadiene to ( Z,E,E)-1,5,9-cyclododecatriene (CDT) with an overall selectivity as high as 92% and 95%, respectively. The convenient catalytic properties of the complexes, i.e. the high catalytic stability of Ia and the high initial rate induced by Ib, have been optimally combined in mixed chloro-bromo complexes. Isothermal and isobaric kinetic measurements have revealed that the reaction order with respect to butadiene is close to 2.0 while the reaction order with respect to the catalyst approaches 1.0 at extreme dilution and decreases with increasing catalyst concentration. Deactivation of the catalyst results in the separation of insoluble titanium halides. The proposed kinetic scheme includes rapid equilibrium between arene- and bis(butadiene)-titanium(II) complexes, followed by the ratedetermining formation of the intermediate titanium complex with an openchain butadiene dimer and very fast cycloaddition of the third butadiene molecule. The formation of ( Z,E,E)-1,5,9-CDT is accounted for by the cycloaddition of butadiene in an s-( Z)- conformation to an intermediate, which is probably [1,2-bis( syn-η 3-allyl)ethane]bis(tetrahaloalanate)titanium-(IV) or s η 1-allyl analogue.