Uthayopas Chayapat, Surawatanawong Panida
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand.
Dalton Trans. 2019 Jun 14;48(22):7817-7827. doi: 10.1039/c9dt00455f. Epub 2019 May 9.
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.
由于酚类衍生物毒性较低且易于获得，它们在交叉偶联反应中越来越多地被用作芳基卤化物的替代品。镍配合物已被开发为用于酚类衍生物芳基C-O键活化的高效催化剂。在此，我们进行了密度泛函研究，以研究酚类衍生物在Ni-SIPr上的芳基C-O键氧化加成。对于苯磺酸酯和苯酯，通过涉及磺酰基/羰基O与Ni相互作用的五中心过渡态的途径优于通过三中心过渡态的途径。对于苯氨基磺酸酯，来自氨磺酰基N与Ni相互作用的五中心过渡态优于来自氨磺酰基O与Ni的相互作用；而对于苯氨基甲酸酯，来自氨基甲酰基O与Ni的相互作用更受青睐。对于芳基醚，路易斯酸AlMe通过与2-甲氧基萘（NaphOMe）形成路易斯酸/碱加合物来促进芳基C-O键活化，从而导致比不存在AlMe时更低的自由能垒。虽然通过“经典”三中心过渡态的NaphOMe/AlMeR（R = Me、Ph、OMe和H）加合物的芳基C-O键氧化加成的自由能垒都相似，但通过涉及AlMeR的R基团与Ni相互作用的五中心过渡态的相应自由能垒可能更低，并且取决于R基团。在后一种途径中，不仅芳基C-O键被削弱，而且Ni的亲核性增强。事实上，这些关键相互作用类似于通过苯磺酸酯/苯氨基磺酸酯和苯酯/苯氨基甲酸酯的五中心过渡态的相互作用。我们的结果表明，通过五中心过渡态涉及与Ni额外供电子相互作用的途径促进了酚类衍生物的芳基C-O键氧化加成。通过这条途径，适当使用有机铝可以提高Ni催化剂用于惰性芳基醚交叉偶联反应的效率。
