American Chemical Society
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Intermolecular Activation of Hydrocarbon C−H Bonds under Ambient Conditions by 16-Electron Neopentylidene and Benzyne Complexes of Molybdenum

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posted on 2003-05-14, 00:00 authored by Kenji Wada, Craig B. Pamplin, Peter Legzdins, Brian O. Patrick, Irina Tsyba, Robert Bau
Cp*Mo(NO)(CH2CMe3)2 (1), a complex with α-agostic CH···Mo interactions, evolves neopentane in neat hydrocarbon solutions at room temperature and forms the transient 16-electron alkylidene complex, Cp*Mo(NO)(CHCMe3), which subsequently activates solvent CH bonds. Thus, it reacts with tetramethylsilane or mesitylene to form Cp*Mo(NO)(CH2CMe3)(CH2SiMe3) (2) or Cp*Mo(NO)(CH2CMe3)(η2-CH2C6H3-3,5-Me2) (3), respectively, in nearly quantitative yields. Under identical conditions, 1 in p-xylene generates a mixture of sp2 and sp3 CH bond activation products, namely Cp*Mo(NO)(CH2CMe3)(C6H3-2,5-Me2) (4, 73%) and Cp*Mo(NO)(CH2CMe3)(η2-CH2C6H4-4-Me) (5, 27%). In benzene at room temperature, 1 transforms to a mixture of Cp*Mo(NO)(CH2CMe3)(C6H5) (6) and Cp*Mo(NO)(C6H5)2 (7) in a sequential manner. Most interestingly, the thermal activation of 6 at ambient temperatures gives rise to two parallel modes of reactivity involving either the elimination of benzene and formation of Cp*Mo(NO)(CHCMe3) or the elimination of neopentane and formation of the benzyne complex, Cp*Mo(NO)(η2-C6H4). In pyridine, these intermediates are trapped as the isolable 18-electron adducts, Cp*Mo(NO)(CHCMe3)(NC5H5) (8) and Cp*Mo(NO)(η2-C6H4)(NC5H5) (9), and, in hydrocarbon solvents, they effect the intermolecular activation of aliphatic CH bonds at room temperature to generate mixtures of neopentyl- and phenyl-containing derivatives. However, the distribution of products resulting from the hydrocarbon activations is dependent on the nature of the solvent, probably due to solvation effects and the presence of σ- or π-hydrocarbon complexes on the reaction coordinates of the alkylidene and the benzyne intermediates. The results of DFT calculations on these processes in the gas phase support the existence of such hydrocarbon complexes and indicate that better agreement with experimental observations is obtained when the actual neopentyl ligand rather than the simpler methyl ligand is used in the model complexes.