Bruns David L, Stahl Shannon S
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue Madison, WI, 53706, United States.
Organometallics. 2022 Nov 28;41(22):3161-3166. doi: 10.1021/acs.organomet.2c00017. Epub 2022 Mar 25.
Palladium(II) catalysts promote oxidative dehydrogenation and dehydrogenative coupling of many organic molecules. Oxidations of alcohols to aldehydes or ketones are prominent examples. Hydroquinone (HQ) oxidation to benzoquinone (BQ) is conceptually related to alcohol oxidation, but it is significantly more challenging thermodynamically. The BQ/HQ redox potential is sufficiently high that BQ is often used as an oxidant in Pd-catalyzed oxidation reactions. A recent report ( , , 19678-19688) showed that certain ancillary ligands can raise the Pd redox potential sufficiently to reverse this reactivity, enabling (L)Pd(OAc) to oxidize hydroquinone to benzoquinone. Here, we investigate the oxidation of -butylhydroquinone ( BuHQ) and 4-fluorobenzyl alcohol (BnOH), mediated by (bc)Pd(OAc) (bc = bathocuproine). Although alcohol oxidation is thermodynamically favored over HQ oxidation by more than 400 mV, the oxidation of BuHQ proceeds several orders of magnitude faster than BnOH oxidation. Kinetic and mechanistic studies reveal that these reactions feature different rate-limiting steps. Alcohol oxidation proceeds via rate-limiting -hydride elimination from a Pd-alkoxide intermediate, while HQ oxidation features rate-limiting isomerization from an O-to-C-bound Pd-hydroquinonate species. The enhanced rate of HQ oxidation reflects the kinetic facility of O─H relative to C─H bond cleavage.
钯（II）催化剂可促进许多有机分子的氧化脱氢和脱氢偶联反应。醇氧化为醛或酮就是典型的例子。对苯二酚（HQ）氧化为苯醌（BQ）在概念上与醇氧化相关，但在热力学上更具挑战性。BQ/HQ的氧化还原电位足够高，以至于BQ经常在钯催化的氧化反应中用作氧化剂。最近的一份报告（，，19678 - 19688）表明，某些辅助配体可以充分提高钯的氧化还原电位，从而逆转这种反应活性，使（L）Pd（OAc）能够将对苯二酚氧化为苯醌。在此，我们研究了由（bc）Pd（OAc）（bc = 红菲绕啉）介导的叔丁基对苯二酚（t - BuHQ）和4 - 氟苄醇（BnOH）的氧化反应。尽管醇氧化在热力学上比HQ氧化更有利，电位差超过400 mV，但t - BuHQ的氧化比BnOH的氧化快几个数量级。动力学和机理研究表明，这些反应具有不同的限速步骤。醇氧化通过从钯醇盐中间体进行限速的β - 氢消除反应进行，而HQ氧化的限速步骤是从O - 键合的钯对苯二酚物种到C - 键合的异构体化。HQ氧化速率的提高反映了O─H相对于C─H键断裂的动力学便利性。
