Hydrocarbon C−H Bond Activation by a Tungsten Acetylene Complex

Thermal activation of Cp*W(NO)(η 2 -CPhCH 2 )(CH 2 SiMe 3 ) ( 1 ) in neat hydrocarbon solutions transiently generates Cp*W(NO)(η 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)(η 2 -CPhCH 2 )(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)(η 2 -CPhCH 2 )(aryl), Cp*W(NO)(η 2 -CPhCH 2 )(η 1 -benzyl), or Cp*W(NO)(η 2 -benzyl)(η 1 -CPh CH 2 ). Similarly, the thermolysis of 1 in (Me 3 Si) 2 O affords Cp*W(NO)(η 2 -CPhCH 2 )(CH 2 SiMe 2 OSiMe 3 ) ( 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 1 in solutions of these substrates. Consequently, metallacycles of the form Cp*W(NO)(η 2 - C H(η 2 -Ph)CH 2 CH(R) C H 2 ) [( 11 ) R = n Pr; ( 12 ) R = n Bu; ( 13 ) R = t Bu; 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)(η 3 - endo -CH 2 C(Me)C(Me)CH 2 (η 1 -CPhMe)) ( 15 ). Attempts to trap acetylene complex A with PMe 3 result in the formation of the metallacyclopropane complex Cp*W(NO)(CH 2 SiMe 3 )(η 2 -CH 2 CPh(PMe 3 )) ( 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.