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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

Musgrave, Charles B., III and Bennett, Marc T. and Ellena, Jeffrey F. and Dickie, Diane A. and Gunnoe, T. Brent and Goddard, William A., III (2022) 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. Organometallics, 41 (15). pp. 1988-2000. ISSN 0276-7333. doi:10.1021/acs.organomet.2c00183.

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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.

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URLURL TypeDescription
Musgrave, Charles B., III0000-0002-3432-0817
Bennett, Marc T.0000-0002-1449-0923
Dickie, Diane A.0000-0003-0939-3309
Gunnoe, T. Brent0000-0001-5714-3887
Goddard, William A., III0000-0003-0097-5716
Additional Information:© 2022 American Chemical Society. Published online 18 July 2022. The authors gratefully acknowledge Xiaofan Jia, Weihao Zhu, and Robert Nielsen for useful discussions. C.B.M. and W.A.G. were supported by ONR (N00014-19-1-2081). M.T.B. and T.B.G. were supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division (DE-SC0000776). Single-crystal X-ray diffraction experiments were performed on a diffractometer at the University of Virginia funded by the NSF-MRI program (CHE-2018870). The authors declare no competing financial interest. Accession Codes. CCDC 2114500 contains the supplementary crystallographic data for this paper.
Funding AgencyGrant Number
Office of Naval Research (ONR)N00014-19-1-2081
Department of Energy (DOE)DE-SC0000776
Subject Keywords:Aromatic compounds, Catalysts, Hydrocarbons, Palladium, Styrenes
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Issue or Number:15
Record Number:CaltechAUTHORS:20220720-918301000
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Official Citation: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 Charles B. Musgrave III, Marc T. Bennett, Jeffrey F. Ellena, Diane A. Dickie, T. Brent Gunnoe, and William A. Goddard III Organometallics Article ASAP DOI: 10.1021/acs.organomet.2c00183
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:115703
Deposited By: George Porter
Deposited On:22 Jul 2022 22:11
Last Modified:12 Oct 2022 04:04

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