(68e) Kinetic Analysis of Electrochemical Lactonization of Ketones Using Water As the Oxygen Atom Source

The adoption of single use plastic has had devastating effects on the environment, and the need for circular processing is evident. However, there are often difficulties with chemical recycling many of the polymers that make up single use plastics, in part due to their innate chemical stability. Lactones have the potential to serve as a key molecular class in the development of a sustainable circular polymer economy. This is because of their ability to form polymers that can be depolymerized back into their constituent monomers with relative ease when compared to the current materials used for plastic packaging, such as polyolefins, PET, and PVC. Current thermochemical methods for the formation of some lactones rely on molecular oxidants, which yield stoichiometric side products that result in poor atom economy and impose safety hazards when in contact with organic substrates. Electrochemical synthesis can alleviate these concerns by exploiting applied potential to enable a sustainable and safe route for lactonization. In this study, we investigated 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 É£-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 2^nd order dependence on water and a 0^th order dependence on the substrate, cyclohexanone. A Tafel slope of 139 mV/decade was measured at 400 mM cyclohexanone and 10 M H₂O, implying a first electron transfer as the rate determining step. Literature proposed mechanisms for similar transformations suggest an outer sphere pathway. However, based on 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-member ring lactone products.