Insights into the Intermolecular C–H Activations of Hydrocarbons Initiated by Cp*W(NO)(η³‑allyl)(CH₂CMe₃) Complexes

Thermolyses of 18e Cp*W­(NO)­(η³-allyl)­(CH₂CMe₃) compounds (Cp* = η⁵-C₅Me₅) result in the intramolecular elimination of CMe₄ and the formation of 16e η²-diene and/or η²-allene intermediate complexes that effect a variety of intermolecular C–H activations of hydrocarbons. The outcomes of the reactions of the Cp*W­(NO)­(η³-allyl)­(CH₂CMe₃) compounds with both C­(sp³)–H and C­(sp²)–H 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–H 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 η²-allene intermediate complexes, whereas the absence of such substituents favors the formation of the η²-diene complexes. In the case of the analogous molybdenum systems, the η²-diene intermediate complexes undergo rapid isomerization to the η⁴-diene complexes and do not effect intermolecular C–H activations. In some instances involving the tungsten complexes, the initially formed η¹-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–H activations and is converted to a new hydrido allyl compound. DFT methods indicate that a plausible mechanism for the latter transformation involves a β-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 η²-olefin complex with the olefin originating from the alkyl chain, and this process should be favored by relatively electron-rich Cp*W­(NO)­(η³-allyl)­(n-alkyl) complexes, as is experimentally observed. In all cases of benzene C­(sp²)–H activations by the tungsten systems, the η²-allene intermediate complexes exhibit better reactivity than the η²-diene intermediates. However, theoretical considerations indicate that the stereochemical properties of the first-formed Cp*W­(NO)­(η³-allyl)­(Ph) products determine their differing thermal stabilities. If the aryl–allyl coupling product, Cp*W­(NO)­(η²-allyl-Ph), contains an activatable C–H 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)­(η³-allyl)­(Ph) complexes persist.