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Thermodynamic-Kinetic Comparison of Palladium(II)-Mediated Alcohol and Hydroquinone Oxidation

机译：Thermodynamic-Kinetic Comparison of Palladium(II)-Mediated Alcohol and Hydroquinone Oxidation

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摘要

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/H(2)Q 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 Pd-II/0 redox potential sufficiently to reverse this reactivity, enabling (L)Pd-II(OAc)(2) to oxidize hydroquinone to benzoquinone. Here, we the oxidation of tertbutylhydroquinone ((t)BuH(2)Q) and 4-fluorobenzyl alcohol ((BnOH)-Bn-4F), mediated by (bc)Pd(OAc)(2) (bc = bathocuproine). Although alcohol oxidation is thermodynamically favored over H(2)Q oxidation by more than 400 mV, the oxidation of (t)BuH(2)Q proceeds several orders of magnitude faster than (BnOH)-Bn-4F oxidation. Kinetic and mechanistic studies reveal that these reactions feature different rate-limiting steps. Alcohol oxidation proceeds via rate-limiting beta-hydride elimination from a Pd-II-alkoxide intermediate, while H(2)Q oxidation features rate-limiting isomerization from an O-to-C-bound Pd-II-hydroquinonate species. The enhanced rate of H(2)Q oxidation reflects the kinetic facility of O-H relative to C-H bond cleavage.

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