Hydrocarbon C-H bond activation by a tungsten acetylene complex

Thermal activation of Cp*W(NO)(eta(2)-CPhCH2)(CH2SiMe3) (1) in neat hydrocarbon solutions transiently generates Cp*W(NO)(eta(2)-PhC=CH) (A), which subsequently activates solvent C-H bonds. For example, the thermolysis of 1 in benzene solution generates quantitatively Cp*W(NO)(eta(2)-CPhCH2)(Ph) (2) The thermolysis of 1 in solutions of methyl-substituted arenes such as toluene or p-, m-, or o-xylene provides mixtures of aryl and benzyl vinyl complexes of the general formulas Cp*W(NO)(eta(2)-CPhCH2)(aryl), Cp*W(NO)(eta(2)-CPhCH2)(eta(1)-benzyl), or Cp*W(NO)(eta(2)-benzyl)(eta(1)-CPh=CH2). Similarly, the thermolysis of 1 in (Me3Si)(2)O affords Cp*W(NO)(eta(2)-CPhCH2)(CH2SiMe2OSiMe3) (10). Mechanistic and kinetic studies support the proposal that the formation of A from 1 by elimination of silane is the rate-controlling process in these reactions. Intra- and intermolecular selectivity studies reveal that the strongest C-H bond (yielding the stronger M-C bond) is the preferred site of reactivity, as expected. Dual C-H bond activation of aliphatic hydrocarbons occurs during the thermal activation ;of I in solutions of these substrates. Consequently, metallacycles of the form Cp*W(NO)(eta(2)-CH(eta(2)-Ph)CH2CH(R)CH2) [(11)R = Pr-n; (12)R = Bu-n; (13)R = Bu-t; and (14)R = OEt] result from the dehydrogenation of n-pentane, n-hexane, 2,2-dimethylbutane, and diethyl ether, respectively. This dual C-H activation process displays a selectivity for substrates that contain an ethyl substituent. Dual C-H bond activation of 2,3-dimethyl-2-butene in the presence of 1 under thermolysis conditions regioselectively affords Cp*W(NO)(eta(3)-endo-CH2C(Me)C(Me)CH2(eta(1)-CPhMe)) (15), Attempts to trap acetylene complex A with PNMe3 result in the formation of the metallacyclopropane complex Cp*W(NO)(CH2SiMe3)(eta(2)-CH2CPh(PMe3)) (16). All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of compounds 11, 15, and 16 have been established by X-ray diffraction methods.