Allylic C-H acetoxylations are among the most widely studied palladium(II)-catalyzed C-H oxidation reactions. While the principal reaction steps are well established, key features of the catalytic mechanisms are poorly characterized, including the identity of the turnover-limiting step and the catalyst resting state. Here, we report a mechanistic study of aerobic allylic acetoxylation of allylbenzene with a catalyst system composed of Pd(OAc) 2 and 4,5-diazafluoren-9-one (DAF). The DAF ligand is unique in its ability to support aerobic catalytic turnover, even in the absence of benzoquinone or other co-catalysts. Herein, we describe operando spectroscopic analysis of the catalytic reaction using X-ray absorption and NMR spectroscopic methods that allow direct observation of the formation and decay of a palladium(I) species during the reaction. Kinetic studies reveal the presence of two distinct kinetic phases: (1) a burst phase, involving rapid formation of the allylic acetoxylation product and formation of the dimeric Pd I complex [Pd I (DAF)(OAc)] 2 , followed by (2) a post-burst phase that coincides with evolution of the catalyst resting state from the Pd I dimer into a π-allyl-Pd II species. The data provide unprecedented insights into the role of ancillary ligands in supporting catalytic turnover with O 2 as the stoichiometric oxidant and establish an important foundation for the development of improved catalysts for allylic oxidation reactions.

Scheme1.a) Generalm ethod for allylic acetoxylation using Pd(OAc) 2 as the catalyst and AcOH as the solvent or cosolvent. b) Prior approaches to allylic acyloxylation using either Pd(OAc) 2 or Pd(OBz) 2 as the catalyst and the carboxylate in high quantitieso ri ncorporated through the oxidant.c )T wo approaches for the use of low-stoichiometry carboxylic acid using palladium sourceswith noncoordinatinganions.[a] C.

Palladium(II)-catalyzed allylic acetoxylation has been the focus of extensive development and investigation. Methods that use molecular oxygen (O 2 ) 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 Pd 0 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 O 2 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) O 2 alone, (2) O 2 with cocatalytic BQ, and (3) O 2 with cocatalytic BQ and Co(salophen). The beneficial effect of BQ in the absence of Co(salophen) is traced to the synergistic roles of O 2 and BQ, both of which are capable of oxidizing Pd 0 to Pd II . The reaction of O 2 generates H 2 O 2 as a byproduct, which can oxidize hydroquinone to quinone in the presence of Pd II . 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 the reaction, resulting in better reaction performance.