Lewis Acid Promoted Titanium Alkylidene Formation: Off-Cycle Intermediates Relevant to Olefin Trimerization Catalysis

Two new precatalysts for ethylene and α-olefin trimerization, (FI)­Ti­(CH₂SiMe₃)₂Me and (FI)­Ti­(CH₂CMe₃)₂Me (FI = phenoxy-imine), have been synthesized and structurally characterized by X-ray diffraction. (FI)­Ti­(CH₂SiMe₃)₂Me can be activated with 1 equiv of B­(C₆F₅)₃ at room temperature to give the solvent-separated ion pair [(FI)­Ti­(CH₂SiMe₃)₂]­[MeB­(C₆F₅)₃], which catalytically trimerizes ethylene or 1-pentene to produce 1-hexene or C₁₅ olefins, respectively. The neopentyl analogue (FI)­Ti­(CH₂CMe₃)₂Me is unstable toward activation with B­(C₆F₅)₃ at room temperature, giving no discernible diamagnetic titanium complexes, but at −30 °C the following can be observed by NMR spectroscopy: (i) formation of the bis-neopentyl cation [(FI)­Ti­(CH₂CMe₃)₂]⁺, (ii) α-elimination of neopentane to give the neopentylidene complex [(FI)­Ti­(CHCMe₃)]⁺, and (iii) subsequent conversion to the imido-olefin complex [(MeOAr₂N)­Ti­(OArHCCHCMe₃)]⁺ via an intramolecular metathesis reaction with the imine fragment of the (FI) ligand. If the reaction is carried out at low temperature in the presence of ethylene, catalytic production of 1-hexene is observed, in addition to the titanacyclobutane complex [(FI)­Ti­(CH­(CMe₃)­CH₂CH₂)]⁺, resulting from addition of ethylene to the neopentylidene [(FI)­Ti­(CHCMe₃)]⁺. None of the complexes observed spectroscopically subsequent to [(FI)­Ti­(CH₂CMe₃)₂]⁺ is an intermediate or precursor for ethylene trimerization, but notwithstanding these off-cycle pathways, [(FI)­Ti­(CH₂CMe₃)₂]⁺ is a precatalyst that undergoes rapid initiation to generate a catalyst for trimerizing ethylene or 1-pentene