Aryl C-O oxidative addition of phenol derivatives to nickel supported by an N-heterocyclic carbene via a Ni five-centered complex.

Phenol derivatives have been increasingly used as alternatives for aryl halides in cross-coupling reactions due to their lower toxicity and easier availability. Nickel complexes have been developed as efficient catalysts for aryl C-O bond activation of phenol derivatives. Herein, we performed a density functional study to investigate the aryl C-O bond oxidative addition of phenol derivatives on Ni-SIPr. For phenyl sulfonates and phenyl esters, the pathway via a five-centered transition state involving the interaction from sulfonyl/carbonyl O to Ni is preferred over that via a three-centered transition state. While the five-centered transition state with the interaction from sulfamoyl N to Ni is favored over the interaction from sulfamoyl O to Ni for phenyl sulfamate, the interaction from carbamoyl O to Ni is more favored for phenyl carbamate. For aryl ethers, the Lewis acid AlMe facilitates aryl C-O bond activation by forming a Lewis acid/base adduct with 2-methoxynaphthalene (NaphOMe) resulting in a lower free energy barrier than that in the absence of AlMe. While the free energy barriers for the aryl C-O bond oxidative addition of a NaphOMe/AlMeR (R = Me, Ph, OMe, and H) adduct via the "classical" three-centered transition state are all similar, the corresponding free energy barriers via the five-centered transition state, involving the interaction from the R group of AlMeR to Ni, can be lower and depend on the R group. Not only is the aryl C-O bond weakened, but the nucleophilicity of Ni is enhanced in the latter pathway. In fact, these key interactions are analogous to those via the five-centered transition states of phenyl sulfonate/sulfamate and phenyl ester/carbamate. Our results revealed that the pathway involving an additional electron donating interaction to Ni via the five-centered transition state facilitates the aryl C-O bond oxidative addition of phenol derivatives. Through this pathway, the appropriate use of organoaluminum can improve the efficiency of Ni catalysts for cross-coupling reactions of inert aryl ethers.