An investigation of the reaction of bis(cyclopentadienyl)titanium dichlorides with trimethylaluminum. Mechanism of an α-hydrogen abstraction reaction

Article abstract of DOI:10.1021/om00080a010

The reaction of Me₃Al with Cp₂TiCl₂ and its derivatives has been studied. Synthetic procedures for the preparation of (C₅R₅)(C₅H₅)TiCl₂ from (C₅R₅)Li and (C₅H₅)TiCl₃ were developed, and sources of the major byproducts (C₅H₅)₂TiCl₂ and (C₅R₅)₂TiCl₂ are suggested. The derivatives studied were (C₅R₅) = C₅H₄SiMe₃, 1,2,4-trimethylcyclopentadienyl, (CH₃)₅C₅, indenyl, 1,3-Ph₂C₅H₃, and (CH₃)C₅H₄. All of these complexes reacted with 1 equiv of (CH₃)₃Al to produce Cp₂Ti(CH₃)Cl·Al(CH₃)₂Cl which would react further with a second equivalent of (CH₃)₃Al to yield a Cp₂TiCH₂AlMe₂Cl derivative. The isotope effect for the parent reaction was 2.9. Substituents on the ring slowed the rate of methylene formation. A mechanism for α-hydrogen abstraction from a Ti-(CH₃)Cl unit by an aluminum alkyl is proposed in which the Lewis acidic aluminum alkyl activates the titanium methyl group by complexation with the chlorine. The resulting aluminate then serves as a strong Lewis base to abstract the α-hydrogen.