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Published July 30, 2014 | Supplemental Material
Journal Article Open

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(CH_2SiMe_3)_2Me and (FI)Ti(CH_2CMe_3)_2Me (FI = phenoxy-imine), have been synthesized and structurally characterized by X-ray diffraction. (FI)Ti(CH_2SiMe_3)_2Me can be activated with 1 equiv of B(C_6F_5)_3 at room temperature to give the solvent-separated ion pair [(FI)Ti(CH_2SiMe_3)_2][MeB(C_6F_5)_3], which catalytically trimerizes ethylene or 1-pentene to produce 1-hexene or C_(15) olefins, respectively. The neopentyl analogue (FI)Ti(CH_2CMe_3)_2Me is unstable toward activation with B(C_6F_5)_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(CH_2CMe_3)_2]^+, (ii) α-elimination of neopentane to give the neopentylidene complex [(FI)Ti(═CHCMe_3)]^+, and (iii) subsequent conversion to the imido-olefin complex [(MeOAr_2N═)Ti(OArHC═CHCMe_3)]^+ 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(CMe_3)CH_2CH_2)]^+, resulting from addition of ethylene to the neopentylidene [(FI)Ti(═CHCMe_3)]^+. None of the complexes observed spectroscopically subsequent to [(FI)Ti(CH_2CMe_3)_2]^+ is an intermediate or precursor for ethylene trimerization, but notwithstanding these off-cycle pathways, [(FI)Ti(CH_2CMe_3)_2]^+ is a precatalyst that undergoes rapid initiation to generate a catalyst for trimerizing ethylene or 1-pentene.

Additional Information

© 2014 American Chemical Society. Received: June 3, 2014; Published: July 9, 2014. Dedicated in memory of Greg Hillhouse, a friend, colleague, and outstanding organometallic chemist. We thank Larry Henling and Michael Takase for assistance with crystallographic work, Mona Shahgholi and Naseem Torian for assistance with mass spectrometry, and Thomas S. Teets, Harry B. Gray, and Jay R. Winkler for helpful discussions. This work was funded by BP through the XC^2 program.

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Supplemental Material - ja5055687_si_005.pdf


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