ja037076q_si_005.cif (14.39 kB)
Thermal Activation of Hydrocarbon C−H Bonds Initiated by a Tungsten Allyl Complex
dataset
posted on 2003-12-10, 00:00 authored by Stephen H. K. Ng, Craig S. Adams, Trevor W. Hayton, Peter Legzdins, Brian O. PatrickGentle 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-H2CCCMe2) (A), and the η2-diene complex, Cp*W(NO)(η2-H2CCHC(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-H2CCCMe2) (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.