亚胺配体协同氮杂环卡宾钯配合物催化碳碳偶联反应的作用机制

Mechanism of Carbon-Carbon Coupling Reactions Catalyzed by Imine-Ligand-Assisted N-Heterocyclic Carbene Palladium Complexes

在氮杂环卡宾-钯(NHC-Pd)配合物催化的碳-碳偶联反应中,配体结构对催化反应起到关键作用.为了实现更好的催化效果,不仅要对NHC配体进行结构优化,还需要第二配体的协同作用.然而,由于第二配体配位能力大大弱于NHC配体,其在催化反应中的协同作用往往被忽视.合成出了3种由相同结构NHC配体以及不同结构亚胺配体组成的NHC-Pd配合物来催化Suzuki-Miyaura交叉偶联反应.实验结果表明,在相同的反应条件下,3种亚胺配体协同的NHC-Pd配合物表现出了明显不同的催化效果.进一步采用理论计算来深入研究亚胺配体协同的NHC-Pd配合物在催化反应中的作用机制.通过完整催化循环过程的计算,发现虽然亚胺配体并没有参与到催化循环过程中,但亚胺配体与氯苯会形成竞争配位,它们与NHC-Pd(0)的配位能力差异直接导致实际参与催化循环的有效活性中心的浓度的差异,计算表明大位阻缺电子的亚胺配体更有利于反应的进行.通过研究,更好地阐明了亚胺配体在催化碳-碳偶联反应中的作用机制,并为NHC-Pd配合物的结构调控提供了新策略.

Structure of ligands would play a crucial role in carbon-carbon coupling reactions catalyzed by N-heterocyclic carbene palladium(NHC-Pd)complexes.Not only the modification of NHC ligands but also the synergistic action of the second ligands were required to achieve better catalytic performance.However,the synergistic effect of the second ligands was often overlooked because of their considerably weaker coordination ability.Three NHC-Pd complexes composed of the same structural NHC ligand and different structural imine ligands were utilized to catalyze the Suzuki-Miyaura cross-coupling reaction.The results showed that the three imine-ligand-assisted NHC-Pd complexes exhibited significantly different catalytic effects under the same reaction conditions.Further theoretical calculations were applied to investigate the catalytic reaction mechanism of these imine-ligand-assisted NHC-Pd complexes in depth.By calculations of the complete catalytic cycle process,it was found that although imine ligands were not involved in the catalytic cycle process,competitive coordination between imine ligands and chlorobenzene should be metioned.The difference in their coordination ability with NHC-Pd(0)directly lead to the difference in the concentration of effective active centers actually involved in the catalytic cycle.The large site-blocking and electron-deficient imine-ligand-assisted NHC-Pd complexes were more favorable for the reaction.The study elucidated the mechanism of imine ligands in catalytic carbon-carbon coupling reactions and provided new strategies for the structural modulation of NHC-Pd complexes.