Thermal Activation of Hydrocarbon C−H Bonds Initiated by a Tungsten Allyl Complex

Gentle thermolysis of the allyl complex, Cp*W(NO)(CH2CMe3)(η3-H2CCHCMe2) (1), at 50 °C in neat hydrocarbon solutions results in the loss of neopentane and the generation of transient intermediates that subsequently activate solvent C−H bonds. Thus, thermal reactions of 1 with tetramethylsilane, mesitylene, and benzene effect single C−H activations and lead to the exclusive formation of Cp*W(NO)(CH2SiMe3)(η3-H2CCHCMe2) (2), Cp*W(NO)(CH2C6H3-3,5-Me2)(η3-H2CCHCMe2) (3), and Cp*W(NO)(C6H5)(η3-H2CCHCMe2) (4), respectively. The products of reactions of 1 with other methyl-substituted arenes indicate an inherent preference of the system for the activation of stronger arene sp2 C−H bonds. For example, C−H bond activation of p-xylene leads to the formation of Cp*W(NO)(CH2C6H4-4-Me)(η3-H2CCHCMe2) (5) (26%) and Cp*W(NO)(C6H3-2,5-Me2)(η3-H2CCHCMe2) (6) (74%). Mechanistic and labeling studies indicate that the transient C−H-activating intermediates are the allene complex, Cp*W(NO)(η2-H2CCCMe2) (A), and the η2-diene complex, Cp*W(NO)(η2-H2CCHC(Me)CH2) (B). Intermediates A and B react with cyclohexene to form Cp*W(NO)(η3-CH2C(2-cyclohexenyl)CMe2)(H) (18) and Cp*W(NO)(η3-CH2CHC)(Me)CH2CβH(C4H8)CαH (19), respectively, and intermediate A can be isolated as its PMe3 adduct, Cp*W(NO)(PMe3)(η2-H2CCCMe2) (20). Interestingly, thermal reaction of 1 with 2,3-dimethylbut-2-ene results in the formation of a species that undergoes η3 → η1 isomerization of the dimethylallyl ligand following the initial C−H bond-activating step to yield Cp*W(NO)(η3-CMe2CMeCH2)(η1-CH2CHCMe2) (21). Thermolyses of 1 in alkane solvents afford allyl hydride complexes resulting from three successive C−H bond-activation reactions. For instance, 1 in cyclohexane converts to Cp*W(NO)(η3-C6H9)(H) (22) with dimethylpropylcyclohexane being formed as a byproduct, and in methylcyclohexane it forms the two isomeric complexes, Cp*W(NO)(η3-C7H11)(H) (23a,b). All new complexes have been characterized by conventional spectroscopic methods, and the solid-state molecular structures of 2, 3, 4, 18, 19, 20, and 21 have been established by X-ray crystallographic analyses.