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The functionalization of methane and other hydrocarbons mediated by organometallic nitrosyl complexes of tungsten

University of British Columbia

The novel family of Cp*W(NO)(H)(η³-allyl) complexes has been synthesized and characterized. Upon thermolysis these compounds generate 16e Cp*W(NO)(η²-alkene) intermediate complexes that have been trapped as their corresponding 18e PMe₃ adducts. In neat alkane solutions, these Cp*W(NO)(η²-alkene) compounds effect three successive C-H activations of the alkane to produce a new Cp*W(NO)(H)(η³-allyl) complex in which the allyl ligand is derived from the alkane solvent. In the presence of CO, the Cp*W(NO)(H)(η³-allyl) complexes effect the regiospecific generation of saturated unsymmetrical ketones from hydrocarbons and CO gas via C-H activation and the formation of two new C-C bonds. For instance, the complex Cp*W(NO)(H)(η³-CH₂CHCMe₂) effects the conversion of benzene and mesitylene into the ketones 4-methyl-1-phenylpentan-1-one and 1-(3,5-dimethylphenyl)-5-methylhexan-2-one, respectively. The organometallic product of these reactions is Cp*W(NO)(CO)₂ which can be converted into the Cp*W(NO)(H)(η³-allyl) reactant in three steps, thereby completing a complete cycle with respect to tungsten. The conversion of methane into the β,γ-unsaturated ketone, 5-methylhex-4-en-2-one, is initiated by the complex Cp*W(NO)(CH₂CMe₃)(η³-CH₂CHCMe₂). In a cyclohexane solution, the complex effects the C-H activation of methane to produce Cp*W(NO)(CH₃)(η³-CH₂CHCMe₂) which has been fully characterized. Under CO pressure, the methyl ligand is converted into an acyl ligand in Cp*W(NO)(C(=O)CH₃)(η³-CH₂CHCMe₂), and then the acyl and allyl ligands couple to give Cp*W(NO)(CO)(η²-CH₂=CHCMe₂C(=O)CH₃) which contains an η²-bound ketone ligand. Finally, the ketone is released at 170 °C under CO pressure and Cp*W(NO)(CO)₂ is obtained which again can be converted back into the starting organometallic compound. Each of these organometallic complexes has been isolated and fully characterized, thereby allowing the stepwise transformation of methane through C-H activation and C-C bond-forming reactions to be definitively established. In a similar fashion, the complex Cp*W(NO)(CH₂CMe₃)(η³-CH₂CHCHPh) converts methane and ethane to the corresponding η²-bound ketone complexes, Cp*W(NO)(CO)(η²-PhCH=CHCH₂C(=O)CH₃) and Cp*W(NO)(CO)(η²-PhCH=CHCH₂C(=O)Et). The Cp*W(NO)(CH₂CMe₃)(η³-CH₂CHCHPh) complex also effects the terminal C-H activation of other alkanes and heteroatom-containing hydrocarbons. The carbonylation of ligands derived from a long-chain alkane and ether has been performed. In addition, the activation of methane, ethane, and propane has been optimized for the compound Cp*W(NO)(CH₂CMe₃)(η³-CH₂CHCHMe). Finally, the bound ketone ligands of complexes Cp*W(NO)(CO)(η²-PhCH=CHCH₂C(=O)(alkyl)) have been photolytically released as the cis and trans β,γ-unsaturated ketone products.

Text

2016-04-13

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/57567

Chemistry

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/