Insight into the Steric and Electronic Effects of Ancillary Ligands: Synthesis and Structure−Reactivity Relationship of Well-Defined, Air- and Moisture-Stable (NHC)Pd(sal)Cl Complexes (sal = Salicylaldimine)

Synthesis of a series of (NHC)Pd(sal)Cl complexes, 1−5, bearing a salicylaldimine (sal) unit was described. The structure−reactivity relationship, especially the steric and electronic effects of the salicylaldimine ligands on their catalytic activities, was investigated intensively in the Buchwald−Hartwig coupling reaction. Our study demonstrated that the sterically encumbered N-aryl groups in the salicylaldimine unit give rise to an obviously negative effect in the aryl amination reaction. Additionally, incorporating a steric substituent into the ortho-position of the phenoxide oxygen atom does not appear to be beneficial to their catalytic activity. Notably, the electronic nature of N-substituted aryl groups in the salicylaldimine ligands plays a significant role in their catalytic activities. An electron-rich N-aryl group causes an obviously decreasing yield, while an electron-deficient one, such as complex 5b, leads to enhanced reactivity. The aryl amination reaction with complex 5b was also found to be remarkably tolerant to both air and moisture. Under optimized reaction conditions, a range of aryl chlorides and amines could be coupled smoothly under aerobic conditions.