C-H Bond Activation Process in Alkyl-Titanium Complex: Over the Barrier or Tunnel through the Barrier?

Recently, an Alkyl-Titanium complex, [Cp₂TiCH₂CHMe(SiMe₃)]⁺(I) which exists in equilibrium between its β-agostic and γ-agostic isomer was synthesized. EXSY studies show that the β-H atom of (I) undergoes facile exchange with the three hydrogen atoms of the β-methyl group (β-H/γ-H exchange) but not with the α-H atom. This exchange process is very different from that involving deuterium atom redistribution suggesting a significantly high kinetic isotope effect (KIE) because it is completely shut down when [Cp₂TiCH₂CH(CD₃)(SiMe₃)]⁺ is used. On the basis of such a large kinetic isotope effect, it is proposed that quantum mechanical proton tunnelling is involved during the β-H/γ-H exchange and possibly via allylic CH bond activation. In this study, we employed DFT calculation to obtain the electronic structural properties of the reaction. Additionally, in order to obtain a better insight of the tunnelling effect we carried out quantum tunnelling calculations.