Palladium(II) catalysts promote oxidative dehydrogenation and dehydrogenative coupling of many organic molecules. Oxidations of alcohols to aldehydes or ketones are prominent examples. Hydroquinone (H2Q) oxidation to benzoquinone (BQ) is conceptually related to alcohol oxidation, but it is significantly more challenging thermodynamically. The BQ/H2Q redox potential is sufficiently high that BQ is often used as an oxidant in Pd-catalyzed oxidation reactions. A recent report (J. Am. Chem. Soc.2020, 142, 19678–19688) showed that certain ancillary ligands can raise the PdII/0 redox potential sufficiently to reverse this reactivity, enabling (L)PdII(OAc)2 to oxidize hydroquinone to benzoquinone. Here, we investigate the oxidation of tert-butylhydroquinone (tBuH2Q) and 4-fluorobenzyl alcohol (4FBnOH), mediated by (bc)Pd(OAc)2 (bc = bathocuproine). Although alcohol oxidation is thermodynamically favored over H2Q oxidation by more than 400 mV, the oxidation of tBuH2Q proceeds several orders of magnitude faster than 4FBnOH oxidation. Kinetic and mechanistic studies reveal that these reactions feature different rate-limiting steps. Alcohol oxidation proceeds via rate-limiting β-hydride elimination from a PdII-alkoxide intermediate, while H2Q oxidation features rate-limiting isomerization from an O-to-C-bound PdII-hydroquinonate species. The enhanced rate of H2Q oxidation reflects the kinetic facility of O–H relative to C–H bond cleavage.
Benzoquinone Cocatalyst Contributions to DAF/Pd(OAc) 2 -Catalyzed Aerobic Allylic Acetoxylation in the Absence and Presence of a Co(salophen) Cocatalyst
Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O2) as the terminal oxidant typically benefit from the use of benzoquinone (BQ) and a transition-metal (TM) cocatalyst, such as Co(salophen), to support oxidation of Pd0 during catalytic turnover. We previously showed that Pd(OAc)2 and 4,5-diazafluoren-9-one (DAF) as an ancillary ligand catalyze allylic oxidation with O2 in the absence of cocatalysts. Herein, we show that BQ enhances DAF/Pd(OAc)2 catalytic activity, nearly matching the performance of reactions that include both BQ and Co(salophen). These observations are complemented by mechanistic studies of DAF/Pd(OAc)2 catalyst systems under three different oxidation conditions: (1) O2 alone, (2) O2 with cocatalytic BQ, and (3) O2 with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to the synergistic roles of O2 and BQ, both of which are capable of oxidizing Pd0 to PdII. The reaction of O2 generates H2O2 as a byproduct, which can oxidize hydroquinone to quinone in the presence of PdII. NMR spectroscopic studies, however, show that hydroquinone is the predominant redox state of the quinone cocatalyst in the absence of Co(salophen), while inclusion of Co(salophen) maintains oxidized quinone throughout more »
- Award ID(s):
- 1953926
- Publication Date:
- NSF-PAR ID:
- 10228074
- Journal Name:
- ACS Catalysis
- Volume:
- 11
- Page Range or eLocation-ID:
- 6363 to 6370
- ISSN:
- 2155-5435
- Sponsoring Org:
- National Science Foundation
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- https://doi.org/10.1021/acscatal.1c01074
