Hexa-Fe(III) Carboxylate Complexes Facilitate Aerobic Hydrocarbon Oxidative Functionalization: Rh Catalyzed Oxidative Coupling of Benzene and Ethylene to Form Styrene

• Published in: ACS catalysis Vol. 14; no. 13
• Main Authors: Bennett, Marc T., Park, Kwanwoo A., Musgrave, Charles B., Brubaker, Jack W., Dickie, Diane A., Goddard, William A., Gunnoe, T. Brent
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society (ACS) 24-06-2024
• Subjects: C−H activation; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; iron; oxidative arene alkenylation; rhodium; styrene
• ISSN: 2155-5435; 2155-5435

Fe(II) carboxylates react with dioxygen and carboxylic acid to form Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 (X = acetate or pivalate), which is an active oxidant for Rh catalyzed arene alkenylation. Heating (150-200 °C) the catalyst precursor [(η2-C2H4)2Rh(µ-OAc)]2 with ethylene, benzene, Fe(II) carboxylate and dioxygen yields styrene production > 30-fold faster than reaction with dioxygen in the absence of Fe(II) carboxylate additive. It is also demonstrated that Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 is an active oxidant under anaerobic conditions, and the reduced material can be re-oxidized to Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 by dioxygen. At optimized conditions, a turnover frequency of ~0.2 s-1 is achieved. Unlike analogous reactions with Cu(II) carboxylate oxidants, which undergo stoichiometric Cu(II)-mediated production of phenyl esters (e.g., phenyl acetate) as side products at temperatures ≥ 150 °C, no phenyl ester side product is observed when Fe carboxylate additives are used. Kinetic isotope effect experiments using C6H6 and C6D6 give kH/kD=3.5(3), while use of protio or mono-deutero pivalic acid reveals a small KIE with kH/kD = 1.19(2). First-order dependencies on Fe(II) carboxylate and dioxygen concentration are observed in addition to complicated kinetic dependencies on the concentration of carboxylic acid and ethylene, which both inhibit the reaction rate at high concentration. Mechanistic studies are consistent with irreversible benzene C–H activation, ethylene insertion into the formed Rh–Ph bond, β–hydride elimination and reaction of Rh–H with Fe6(µ-OH)2(µ3-O)2(µ-X)12(HX)2 to regenerate a Rh-carboxylate complex.