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

Palladium(II) catalysts promote oxidative dehydrogenation and dehydrogenative coupling of many organic molecules. Oxidations of alcohols to aldehydes or ketones are prominent examples. Hydroquinone (H 2 Q) 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 investigate the oxidation of tert -butylhydroquinone ( t BuH 2 Q) and 4-fluorobenzyl alcohol ( 4F BnOH), 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 4F BnOH 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 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.