UBC Theses and Dissertations
C-H activation by a tungsten trimethylsilylallyl complex Chow, Catherine
Thermolysis of Cp*W(NO)(Np)(η³-CH₂CHCHSiMe₃) generates a 16-electron allene intermediate complex that selectively activates hydrocarbons at their methyl groups. In the case of linear alkanes, only terminal activation is observed. This selectivity persists in the presence of an ether functionality, but not of other oxygen-containing substrates such as aldehydes and alcohols. With these latter substrates, oxidation of the complex to Cp*W(O)₂Np has been noted. The existence of the allene intermediate has been verified by two thermolytic experiments, and kinetic studies show that Cp*W(NO)(Np)(η³- CH₂CHCHSiMe₃) is consumed according to pseudo-first-order kinetics during C–H bond activation. The neopentyl ligand can be functionalized by reaction with CO, and the resulting acyl complexes undergo chemical exchange on a slow timescale. The congeneric Mo complex has also been synthesized, and although this complex is equally capable of generating the η²-allene intermediate, its preferred mode of reactivity is coupling of the allyl and alkyl ligands. As a result, the Mo complex is inferior to the W system for C–H activation. The thermolysis of Cp*W(NO)(Np)(η³-CH₂CHCHSiMe₃) in benzene has been studied since the major products of this reaction each contain a 1,3-disubstituted allyl ligand which might reduce reactivity at the allyl ligand in subsequent chemistry. In thermolytic conditions, the resulting disubstituted allyl hydride complex undergoes no apparent reaction with alkanes, but with deuterobenzene, deuterium incorporation into the allyl ligand is observed. In addition to H/D exchange, the hydride ligands in these complexes can also migrate onto the allyl ligand, forming an η²-olefin complex that can be trapped as the pyridine adduct. The activation of fluorobenzenes by Cp*W(NO)(Np)(η³-CH₂CHCHSiMe₃) was also studied, and in these substrates, exclusive activation of the C–H bond is observed. Migration of the newly formed aryl ligands onto the allyl ligand does not occur when there is a fluorine atom in the ortho position, which is probably due to the reduced nucleophilicity of the ligand relative to the unfluorinated phenyl ligand. Selectivity in the activation of C–H bonds in fluorobenzenes appears to be determined by sterics.
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