Opening a Pandora’s Flask on a Prototype Catalytic Direct Arylation Reaction of Pentafluorobenzene: The Ag2CO3/Pd(OAc)2/PPh3 System

Direct C–H functionalization reactions have opened new avenues in catalysis, removing the need for prefunctionalization of at least one of the substrates. Although C–H functionalization catalyzed by palladium complexes in the presence of a base is generally considered to proceed by the CMD/AMLA-6 mechanism, recent research has shown that silver­(I) salts, frequently used as bases, can function as C–H bond activators instead of (or in addition to) palladium­(II). In this study, we examine the coupling of pentafluorobenzene 1 to 4-iodotoluene 2a (and its analogues) to form 4-(pentafluorophenyl)­toluene 3a catalyzed by palladium­(II) acetate with the commonplace PPh3 ligand, silver carbonate as base, and DMF as solvent. By studying the reaction of 1 with Ag2CO3/PPh3 and with isolated silver (triphenylphosphine) carbonate complexes, we show the formation of C–H activation products containing the Ag­(C6F5)­(PPh3)n unit. However, analysis is complicated by the lability of the Ag–PPh3 bond and the presence of multiple species in the solution. The speciation of palladium­(II) is investigated by high-resolution-MAS NMR (chosen for its suitability for suspensions) with a substoichiometric catalyst, demonstrating the formation of an equilibrium mixture of Pd­(Ar)­(κ1-OAc)­(PPh3)2 and [Pd­(Ar)­(μ-OAc)­(PPh3)]2 as resting states (Ar = Ph, 4-tolyl). These two complexes react stoichiometrically with 1 to form coupling products. The catalytic reaction kinetics is investigated by in situ IR spectroscopy revealing a two-term rate law and dependence on [Pdtot/nPPh3]0.5 consistent with the dissociation of an off-cycle palladium dimer. The first term is independent of [1], whereas the second term is first order in [1]. The observed rates are very similar with Pd­(PPh3)4, Pd­(Ph)­(κ1-OAc)­(PPh3)2, and [Pd­(Ph)­(μ-OAc)­(PPh3)]2 catalysts. The kinetic isotope effect varied significantly according to conditions. The multiple speciation of both AgI and PdII acts as a warning against specifying the catalytic cycles in detail. Moreover, the rapid dynamic interconversion of AgI species creates a level of complexity that has not been appreciated previously.