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C-H activation of hydrocarbons by tungsten alkylidene and related complexes Adams, Craig S.

Abstract

Thermolysis of Cp*W(NO)(CH₂C₆H₅)(CH₂CMe₃) (2) in various solvents generates neopentane and benzylidene Cp*W(NO)(=CHC₆H₅) (B) in situ. Complex B activates the C-H bonds of alkane solvents to yield alkene or allyl hydride complexes, and activates arene solvents to yield aryl or benzyl derivatives. The basic mechanistic features of the formation and reactivity of B, and the scope of B's activation chemistry are very similar to those of the previously-studied neopentylidene Cp*W(NO)(=CHCMe₃) (A) derived from Cp*W(NO)(CH₂CMe₃)₂ (1). However, the product distributions derived from the activation of substituted arenes by B are more abundant in the aryl products over the benzyl products than those obtained from A (toluene, p>xylene). Likewise, the aryl regioisomer distributions obtained from B favour the meta isomer over other isomers, more so than those obtained from A (α,α,α-trifluorotoluene, toluene). The mechanism of the thermal chemistry of 1 and 2 is re-examined for the possible involvement of hydrocarbon intermediates in the formation of the activation products. The observation of H/D scrambling in Cp*W(NO)(CD₂CMe₃)₂ (1-d₄) prior to neopentane elimination indicates that hydrocarbon intermediates do exist on the reaction coordinate. The near-unity values of the KIEs measured for benzene, tetramethylsilane and mesitylene indicate that the key step in the C-H activation of these substrates is coordination to the metal center, rather than substrate C-H bond scission. An in-depth experimental and theoretical investigation of the activation of toluene reveals that the aryl vs benzyl product distributions are controlled by the relative energetics of substrate coordination in two different fashions to the metal center. In contrast, the aryl regioselectivity is controlled by the relative energies of the aryl products. The product distributions obtained from the activation of other substituted arenes are controlled by these same factors, but with variations that depend on the substrate and the alkylidene complex utilized. The alkylidene systems can potentially be used to convert alkanes into homoallylic alcohols, via the allyl hydride products of C-H activation. Strategies for developing related Cp'M(NO)-based systems (Cp' = Cp or Cp*; M = Mo, W) are also described, along with preliminary investigations into the activation chemistry derived from CpMo(NO)(CD₂CMe₃)₂ and Cp*W(NO)(CH₂CMe₃)(η³-l,l-Me₂-C₃H₃).

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