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.