Operando Spectroscopic and Kinetic Characterization of Aerobic Allylic C–H Acetoxylation Catalyzed by Pd(OAc)₂/4,5-Diazafluoren-9-one

Abstract

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.

Base‐Assisted C−H Bond Cleavage in Cross‐Coupling: Recent Insights into Mechanism, Speciation, and Cooperativity

“…Stahl recently reported a unique capacity of the 4,5‐diazafluorenone (DAF) ligand to enable efficient aerobic turnover versus other typical ligands or “ligandless” conditions during oxidative Pd‐catalyzed reactions. Recent mechanistic studies by this group correlated the special stabilizing effect of DAF, at least in part, to a funneling of Pd(0) to a stable DAF‐bridged Pd(I) dimer 91 by comproportionation with an equivalent of Pd(II) (Figure b) . Independently prepared [Pd(μ 2 ‐DAF)(κ 1 ‐OAc)] 2 ( 91 ) was found to be a kinetically competent catalyst in both allylic C−H acetoxylation and aza‐Wacker reactions; mechanistic experiments confirmed that this species slowly evolves to active catalyst during the steady state phase of catalysis.…”

“… (a) General representation of potential catalyst deactivation through speciation (deligation) at the Pd(0) oxidation state and (b) DAF‐facilitated shunting of Pd(0) to a stable DAF‐bridged Pd(I) dimer during allylic C−H acetoxylation reported by Stahl (reversible substrate binding omitted for clarity) …”

Operandomonitoring of mechanisms and deactivation of molecular catalysts

“…60,61 It is widely used in heterogeneous operando studies, however, only few cases have been presented on molecular catalysis in situ analysis. [62][63][64][65][66] Among one of the first in situ XAS studies is a study on the bromobenzene homocoupling with a [Ni(cod) (bpy)] by Tanaka's group. 67 Using time-resolved XANES and EXAFS analysis they were able to monitor the reactant [Ni(cod) (bpy)], the intermediate [Ni(bpy)(Ph)Br(DMF) 2 ] and the subproduct [Ni(bpy)Br 2 (DMF)] 2 .…”

Mechanistically guided survey of enantioselective palladium-catalyzed alkene functionalization

“…The ideal solution would be the development of robust systems that allow reduced Pd loadings, which may be generally enabled by the use of greener oxidants such as molecular oxygen in the absence of a cocatalyst (see Outstanding questions). One of the most prominent contributors to this line of research, albeit largely in the context of Pd-mediated oxidation reactions, is the Stahl group [46,[92][93][94].…”