Diazafluorenone-Promoted Oxidation Catalysis: Insights into the Role of Bidentate Ligands in Pd-Catalyzed Aerobic Aza-Wacker Reactions

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• 作者：Paul B. White ; Jonathan N. Jaworski ; Geyunjian Harry Zhu ; Shannon S. Stahl
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：May 6, 2016
• 年：2016
• 卷：6
• 期：5
• 页码：3340-3348
• 全文大小：736K
• 年卷期：0
• ISSN：2155-5435

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 Pd^II center. The insights from these studies introduce new ligand concepts that could promote numerous other classes of Pd-catalyzed aerobic oxidation reactions.