- Can Donor Ligands Make Pd(OAc)2a Stronger Oxidant? Access to Elusive Palladium(II) Reduction Potentials and Effects of Ancillary Ligands via Palladium(II)/Hydroquinone Redox Equilibria
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Palladium(II)-catalyzed oxidation reactions represent an important class of methods for selective modification and functionalization of organic molecules. This field has benefitted greatly from the discovery of ancillary ligands that expand the scope, reactivity, and selectivity in these reactions; however, ancillary ligands also commonly poison these reactions. The different influences of ligands in these reactions remain poorly understood. For example, over the 60-year history of this field, the PdII/0 redox potentials for catalytically relevant Pd complexes have never been determined. Here, we report the unexpected discovery of (L)PdII(OAc)2-mediated oxidation of hydroquinones, the microscopic reverse of quinone-mediated oxidation of Pd0 commonly employed in PdII-catalyzed oxidation reactions. Analysis of redox equilibria arising from the reaction of (L)Pd(OAc)2 and hydroquinones (L = bathocuproine, 4,5-diazafluoren-9-one), generating reduced (L)Pd species and benzoquinones, provides the basis for determination of (L)PdII(OAc)2 reduction potentials. Experimental results are complemented by density functional theory calculations to show how a series of nitrogen-based ligands modulate the (L)PdII(OAc)2 reduction potential, thereby tuning the ability of PdII to serve as an effective oxidant of organic molecules in catalytic reactions.
- Bruns, David L.,Musaev, Djamaladdin G.,Stahl, Shannon S.
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supporting information
p. 19678 - 19688
(2020/12/18)
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- Detection of Palladium(I) in Aerobic Oxidation Catalysis
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Palladium(II)-catalyzed oxidation reactions exhibit broad utility in organic synthesis; however, they often feature high catalyst loading and low turnover numbers relative to non-oxidative cross-coupling reactions. Insights into the fate of the Pd catalyst during turnover could help to address this limitation. Herein, we report the identification and characterization of a dimeric PdI species in two prototypical Pd-catalyzed aerobic oxidation reactions: allylic C?H acetoxylation of terminal alkenes and intramolecular aza-Wacker cyclization. Both reactions employ 4,5-diazafluoren-9-one (DAF) as an ancillary ligand. The dimeric PdI complex, [PdI(μ-DAF)(OAc)]2, which features two bridging DAF ligands and two terminal acetate ligands, has been characterized by several spectroscopic methods, as well as single-crystal X-ray crystallography. The origin of this PdI complex and its implications for catalytic reactivity are discussed.
- Jaworski, Jonathan N.,McCann, Scott D.,Guzei, Ilia A.,Stahl, Shannon S.
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supporting information
p. 3605 - 3610
(2017/03/21)
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- Oxidation of 1,10-Phenanthroline by Tetraoxomanganate (VI) and (VII). Preparation, Structure and Properties of 1H-Cyclopentadipyridine-2,5-dione
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Oxidation of 1,10-phenanthroline with tetraoxomanganate (VI) gave good yields of ketone 3 and the previously unknown dione 5, formed by the unusual further oxidation of 3 at the 2 position of a pyridine ring.In contrast, use of the tetraoxomanganate (VII) gave the bipyridine diacid 2 (69percent), ketone 3 (20percent) and only a trace of the dione 5.The X-ray crystal structure of the anion of 5 indicates that the negative charge is located mainly on the 2-O rather than 5-O atom, with some delocalisation into the pyridine ring.
- Baxter, Paul N. W.,Connor, Joseph A.,Wallis, John D.,Povey, David C.,Powell, Anne K.
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p. 1601 - 1606
(2007/10/02)
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