Reaction Mechanism Underlying Pd(II)-Catalyzed Oxidative Coupling of Ethylene and Benzene to Form Styrene: Identification of a Cyclic Mono-Pdᴵᴵ Bis-Cuᴵᴵ Complex as the Active Catalyst

A recent advance in the synthesis of alkenylated arenes was the demonstration that the Pd(OAc)₂ catalyst precursor gives >95% selectivity toward styrene from ethylene and benzene under optimized conditions using excess Cu(II) carboxylate as the in situ oxidant [ Organometallics 2019, 38(19), 3532−3541]. To understand the mechanism underlying this catalysis, we applied density functional theory (DFT) calculations in combination with experimental studies. From DFT calculations, we determined the lowest-energy multimetallic Pd and Pd–Cu mixed metal species as possible catalyst precursors. From the various structures, we determined the cyclic heterotrinuclear complex PdCu₂(μ-OAc)₆ to be the global minimum in Gibbs free energy under conditions of excess Cu(II). For cyclic PdCu₂(μ-OAc)₆ and the parent [Pd(μ-OAc)₂]₃, we evaluated the barriers for benzene C–H activation through concerted metalation deprotonation (CMD). The PdCu₂(μ-OAc)₆ cyclic trimer leads to a CMD barrier of 33.5 kcal/mol, while the [Pd(μ-OAc)₂]₃ species leads to a larger CMD barrier at >35 kcal/mol. This decrease in the CMD barrier arises from the insertion of Cu(II) into the trimetallic species. Because cyclic PdCu₂(μ-OAc)₆ is likely the predominant species under experimental conditions (the Cu to Pd ratio is 480:1 at the start of catalysis) with a predicted CMD barrier within the range of the experimentally determined activation barrier, we propose that cyclic PdCu₂(μ-OAc)₆ is the Pd species responsible for catalysis and report a full reaction mechanism based on DFT calculations. For catalytic conversion of benzene and ethylene to styrene at 120 °C using Pd(OAc)₂ as the catalyst precursor and Cu(OPiv)2 (OPiv = pivalate) as the oxidant, an induction period of ∼1 h was observed, followed by catalysis with a turnover frequency of ∼2.3 × 10⁻³ s⁻¹. In situ1H NMR spectroscopy experiments indicate that during the induction period, Pd(OAc)₂ is likely converted to cyclic PdCu₂(η²-C₂H₄)₃(μ-OPiv)₆, which is consistent with the calculations and consistent with the proposal that the active catalyst is the ethylene-coordinated heterotrinuclear complex cyclic PdCu₂(η2-C₂H₄)₃(μ-OPiv)₆.