Thermal activation of hydrocarbon C-H bonds initiated by a tungsten allyl complex

Gentle thermolysis of the allyl complex, (CpW)-W-*(NO)(CH2CMe3)(eta(3)-H2CCHCMe2) (1), at 50 degreesC 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 (CpW)-W-*(NO)(CH2SiMe3)(eta(3)-H2CCHCMe2) (2), (CpW)-W-*(NO)(CH2C6H3-3,5-Me-2)(eta(3)-H2CCHCMe2) (3), and (CpW)-W-*(NO)(C6H5)(eta(3)-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 sp(2) C-H bonds. For example, C-H bond activation of p-xylene leads to the formation of (CpW)-W-*(NO)(CH2C6H4-4-Me)(eta(3)-H2CCHCMe2) (5) (26%) and (CpW)-W-*(NO)(C6H3-2,5-Me-2)(eta(3)-H2CCHCMe2) (6) (74%). Mechanistic and labeling studies indicate that the transient C-H-activating intermediates are the allene complex, (CpW)-W-*(NO)(eta(2)-H2C=C=CMe2) (A), and the eta(2)-diene complex, (CpW)-W-*(NO)(eta(2)-H2C=CHC(Me)=CH2) (B). Intermediates A and B react with cyclohexene to form (CpW)-W-*(NO)(eta(3)-CH2C(2-cyclohexenyl)CMe2)(H) (18) and (CpW)-W-*(NO)(eta(3)-CH2CHC)(Me)CH2CbetaH(C4H8)CalphaH (19), respectively, and intermediate A can be isolated as its PMe3 adduct, (CpW)-W-*(NO)(PMe3)(eta(2)-H2C=C=CMe2) (20). Interestingly, thermal reaction of 1 with 2,3-dimethylbut-2-ene results in the formation of a species that undergoes eta(3) –> eta(1) isomerization of the dimethylallyl ligand following the initial C-H bond-activating step to yield (CpW)-W-*(NO)(eta(3)-CMe2CMeCH2)(eta(1)-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 (CpW)-W-*(NO)(eta(3)-C6H9)(H) (22) with dimethylpropylcyclohexane being formed as a byproduct, and in methylcyclohexane it forms the two isomeric complexes, (CpW)-W-*(NO)(eta(3)-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.