Kinetic Analysis of Electrochemical Lactonization of Ketones Using Water as the Oxygen Atom Source

Lactones serve as key synthetic intermediates for the large-scale production of several important chemicals, such as polymers, pharmaceuticals, and scents. Current thermochemical methods for the formation of some lactones rely on molecular oxidants, which yield stoichiometric side products that result in a poor atom economy and impose safety hazards when in contact with organic substrates and solvents. Electrochemical synthesis can alleviate these concerns by exploiting an applied potential to enable the possibility of a clean and safe route for lactonization. In this study, we investigated the mechanism of electrochemical lactone formation from cyclic ketones. When using a platinum anode and cathode in acetonitrile with 10 M H2O and 400 mM cyclohexanone, we found that non-Baeyer-Villiger products, δ-hexanolactone and γ-caprolactone, are formed with a total Faradaic efficiency of ∼20%. Isotope labeling experiments support that water is the oxygen atom source for this reaction. In addition, electrochemical kinetic data suggest a first-order dependence on water at low water concentrations (<2 M H2O) and a zeroth order dependence on the substrate, cyclohexanone. A Tafel slope of 139 mV/decade was measured at 400 mM cyclohexanone and 10 M H2O, implying an initial electron transfer as the rate-determining step. Literature-proposed mechanisms for similar transformations suggest an outer-sphere pathway. However, on the basis of the collected electrochemical kinetic data, we propose the possibility that Pt reacts with water in an initial electron transfer that forms Pt-OH, which can subsequently react with the ketone substrate. A subsequent electron transfer forms a ring-opened carboxylic acid cation that can reclose to form either of the observed five-or six-membered ring lactone products.