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.