Insights into the Intermolecular C鈥揌 Activations of Hydrocarbons Initiated by Cp*W(NO)(畏3-allyl)(CH2CMe3) Complexes

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• 作者：Guillaume P. Lef猫vre ; Rhett A. Baillie ; Diana Fabulyak ; Peter Legzdins
• 刊名：Organometallics
• 出版年：2013
• 出版时间：October 14, 2013
• 年：2013
• 卷：32
• 期：19
• 页码：5561-5572
• 全文大小：705K
• 年卷期：v.32,no.19(October 14, 2013)
• ISSN：1520-6041

文摘

Thermolyses of 18e Cp*W(NO)(畏³-allyl)(CH₂CMe₃) compounds (Cp* = 畏⁵-C₅Me₅) result in the intramolecular elimination of CMe₄ and the formation of 16e 畏²-diene and/or 畏²-allene intermediate complexes that effect a variety of intermolecular C鈥揌 activations of hydrocarbons. The outcomes of the reactions of the Cp*W(NO)(畏³-allyl)(CH₂CMe₃) compounds with both C(sp³)鈥揌 and C(sp²)鈥揌 bonds of hydrocarbons are dependent on the natures of the allyl ligands in ways that are not immediately obvious. In an effort to better understand the different selectivities of the various C鈥揌 activation processes, we have examined several of these transformations by DFT calculations. The results of these computational investigations have provided several interesting and useful insights into the mechanistic pathways involved. Specifically, they have established that geminal dialkyl substituents on the allyl ligands markedly stabilize the 畏²-allene intermediate complexes, whereas the absence of such substituents favors the formation of the 畏²-diene complexes. In the case of the analogous molybdenum systems, the 畏²-diene intermediate complexes undergo rapid isomerization to the 畏⁴-diene complexes and do not effect intermolecular C鈥揌 activations. In some instances involving the tungsten complexes, the initially formed 畏¹-hydrocarbyl product (which may or may not be isolable) isomerizes by intramolecular exchange of the newly formed hydrocarbyl ligand with a hydrogen atom on the allyl ligand or undergoes additional C鈥揌 activations and is converted to a new hydrido allyl compound. DFT methods indicate that a plausible mechanism for the latter transformation involves a 尾-hydrogen abstraction from the lateral alkyl chain by the allyl ligand. The rate-determining step of this process is thus the formation of a 16e 畏²-olefin complex with the olefin originating from the alkyl chain, and this process should be favored by relatively electron-rich Cp*W(NO)(畏³-allyl)(n-alkyl) complexes, as is experimentally observed. In all cases of benzene C(sp²)鈥揌 activations by the tungsten systems, the 畏²-allene intermediate complexes exhibit better reactivity than the 畏²-diene intermediates. However, theoretical considerations indicate that the stereochemical properties of the first-formed Cp*W(NO)(畏³-allyl)(Ph) products determine their differing thermal stabilities. If the aryl鈥揳llyl coupling product, Cp*W(NO)(畏²-allyl-Ph), contains an activatable C鈥揌 bond close to the tungsten center, then the thermodynamically favored intramolecular exchange of the phenyl ligand with a hydrogen atom on the allyl ligand occurs. Otherwise, it does not, and the Cp*W(NO)(畏³-allyl)(Ph) complexes persist.