文摘
2,2′-Bipyridine (bpy), 1,10-phenanthroline (phen), and related bidentate ligands often inhibit homogeneous Pd-catalyzed aerobic oxidation reactions; however, certain derivatives, such as 4,5-diazafluoren-9-one (DAF), can promote catalysis. In order to gain insight into this divergent ligand behavior, eight different bpy- and phen-derived chelating ligands have been evaluated in the Pd(OAc)2-catalyzed oxidative cyclization of (E)-4-hexenyltosylamide. Two of the ligands, DAF and 6,6′-dimethyl-2,2′-bipyridine (6,6′-Me2bpy), support efficient catalytic turnover, while the others strongly inhibit the reaction. DAF is especially effective and is the only ligand that exhibits “ligand-accelerated catalysis”. Evidence suggests that the utility of DAF and 6,6′-Me2bpy originates from the ability of these ligands to access κ1-coordination modes via dissociation of one of the pyridyl rings. This hemilabile character is directly observed by NMR spectroscopy upon adding 1 equiv of pyridine to solutions of 1/1 L/Pd(OAc)2 (L = DAF, 6,6′-Me2bpy) and is further supported by the X-ray crystal structure of Pd(py)(κ1-DAF)OAc2. DFT computational studies illuminate the influence of three different chelating ligands (DAF, 6,6′-Me2bpy, and 2,9-dimethyl-1,10-phenanthroline (2,9-Me2phen)) on the energetics of the aza-Wacker reaction pathway. The results show that DAF and 6,6′-Me2bpy destabilize the corresponding ground-state Pd(N∼N)(OAc)2 complexes, while stabilizing the rate-limiting transition state for alkene insertion into a Pd–N bond. Interconversion between κ2 and κ1 coordination modes facilitates access to open coordination sites at the PdII center. The insights from these studies introduce new ligand concepts that could promote numerous other classes of Pd-catalyzed aerobic oxidation reactions.