Transition metal ion-assisted photochemical generation of alkyl halides and hydrocarbons from carboxylic acids

Abstract

Near-UV photolysis of aqueous solutions of propionic acid and aqueous Fe³⁺ in the absence of oxygen generates a mixture of hydrocarbons (ethane, ethylene and butane), carbon dioxide, and Fe²⁺. The reaction becomes mildly catalytic (about five turnovers) in the presence of oxygen which converts a portion of alkyl radicals to oxidizing intermediates that reoxidize Fe²⁺. The photochemistry in the presence of halide ions (X⁻ = Cl⁻, Br⁻) generates ethyl halides via halogen atom abstraction from FeX_n³⁻ⁿ by ethyl radicals. Near-quantitative yields of C₂H₅X are obtained at ≥0.05 M X⁻. Competition experiments with Co(NH₃)₅Br²⁺ provided kinetic data for the reaction of ethyl radicals with FeCl²⁺ (k = (4.0 ± 0.5) × 10⁶ M⁻¹ s⁻¹) and with FeBr²⁺ (k = (3.0 ± 0.5) × 10⁷ M⁻¹ s⁻¹). Photochemical decarboxylation of propionic acid in the presence of Cu²⁺ generates ethylene and Cu⁺. Longer-chain acids also yield alpha olefins as exclusive products. These reactions become catalytic under constant purge with oxygen which plays a dual role. It reoxidizes Cu⁺ to Cu²⁺, and removes gaseous olefins to prevent accumulation of Cu⁺(olefin) complexes and depletion of Cu²⁺. The results underscore the profound effect that the choice of metal ions, the medium, and reaction conditions exert on the photochemistry of carboxylic acids.