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C-H bond activation of hydrocarbons by a tungsten allene complex Stephen H. K., Ng

Abstract

Gentle thermolysis of the 18e alkyl-allyl complex, Cp*W(NO)(CH₂CMe₃)(η³-1,1-Me₂C₃H₃) (1.1), generates a reactive 16e allene intermediate, Cp*W(NO)(η²-H₂C=C=CMe₂) (A), by a first-order process with concomitant evolution of neopentane via hydrogen abstraction from the dimethylallyl ligand. Intermediate A has been structurally characterized as its PMe₃ adduct, and is capable of effecting both single and multiple intermolecular C-H bond activations of hydrocarbon solvents to form alkyl-allyl and allyl-hydrido complexes. The products of reactions of A with methyl-substituted arenes (i.e. sp² C-H vs sp³ C-H) indicate an inherent preference for the activation of stronger arene C-H bonds, however steric effects are also relevant as indicated by the exclusive formation of benzylic C-H activation products in mesitylene. Thermolyses of 1.1 in alkane solvents generate products resulting from three successive C-H bond-activation reactions. Preliminary studies suggest that allyl-hydrido complexes are formed along with an organic product; for example, in cyclohexane, 1,1- dimethylpropylcyclohexane results from coupling of a solvent molecule and a fragment derived from the precursor dimethylallyl ligand. The allyl-hydrido complexes have potential synthetic utility, since studies conducted on structurally and electronically related CpMo complexes show that they may be capable of ultimately producing homoallylic alcohols.

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