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Facile and Selective Aliphatic C−H Bond Activation at Ambient Temperatures Initiated by Cp*W(NO)(CH2CMe3)(η3-CH2CHCHMe)

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posted on 19.03.2008, 00:00 by Jenkins Y. K. Tsang, Miriam S. A. Buschhaus, Peter M. Graham, Christopher J. Semiao, Scott P. Semproni, Simon J. Kim, Peter Legzdins
Thermolysis of Cp*W(NO)(CH2CMe3)(η3-CH2CHCHMe) (1) at ambient temperatures leads to the loss of neopentane and the formation of the η2-diene intermediate, Cp*W(NO)(η2-CH2CHCHCH2) (A), which has been isolated as its 18e PMe3 adduct. In the presence of linear alkanes, A effects C−H activations of the hydrocarbons exclusively at their terminal carbons and forms 18e Cp*W(NO)(n-alkyl)(η3-CH2CHCHMe) complexes. Similarly, treatments of 1 with methylcyclohexane, chloropentane, diethyl ether, and triethylamine all lead to the corresponding terminal C−H activation products. Furthermore, a judicious choice of solvents permits the C−H activation of gaseous hydrocarbons (i.e., propane, ethane, and methane) at ambient temperatures under moderately elevated pressures. However, reactions between intermediate A and cyclohexene, acetone, 3-pentanone, and 2-butyne lead to coupling between the η2-diene ligand and the site of unsaturation on the organic molecule. For example, Cp*W(NO)(η31-CH2CHCHCH2C(CH2CH3)2O) is formed exclusively in 3-pentanone. When the site of unsaturation is sufficiently sterically hindered, as in the case of 2,3-dimethyl-2-butene, C−H activation again becomes dominant, and so the C−H activation product, Cp*W(NO)(η1-CH2CMeCMe2)(η3-CH2CHCHMe), is formed exclusively from the alkene and 1. All new complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of most of them have been established by X-ray crystallographic analyses. Finally, the newly formed alkyl ligands may be liberated from the tungsten centers in the product complexes by treatment with iodine. Thus, exposure of a CDCl3 solution of the n-pentyl allyl complex, Cp*W(NO)(n-C5H11)(η3-CH2CHCHMe), to I2 at −60 °C produces n-C5H11I in moderate yields.