Competitive intramolecular TiC versus AlC alkene insertions: Examining the role of Lewis acid cocatalysts in Ziegler-Natta alkene insertion and chain transfer reactions

Abstract

Mechanistic aspects of Ziegler-Natta olefin insertion, which include catalyst/cocatalyst interactions, chain propagation, and chain termination, have been examined for systems which model the Cp 2Ti(Cl)R/RAlCl 2 and Cp 2Ti(Cl)R/MgX 2 catalyst complexes. The reaction of (2-butyl-6-hepten-1-yl)titanocene chloride with (2-propyl-6-hepten-1-yl)aluminum dichloride:diethyl etherate produced 78% cyclization of the titanocene ligand, while less than 2% of the ligand originating on aluminum cyclized. In a complementary experiment, the reaction of (2-propyl-6-hepten-1-yl)titanocene chloride and (2-butyl-6-hepten-1-yl)aluminum dichloride:diethyl etherate again produced only intramolecular insertion of the titanium ligand (58%). Based on these results, equilibration of ligands through transmetallation between titanium and aluminum did not occur under these reaction conditions, and selective insertion into the titanium-carbon bond was confirmed for this process. Similarly, ligand cyclization with Cp 2Ti(Cl)R/MgX 2 also occurred through insertion into the titanium-carbon bond. The product distribution generated by the MgX 2 was highly solvent dependent. Cyclization in CH 2Cl 2 was very efficient, while reaction in toluene generated numerous products. Included in the toluene reaction mixture were compounds that resulted from ligand transposition/chain transfer of the titanium ligand.