In:
ECS Meeting Abstracts, The Electrochemical Society, Vol. MA2020-02, No. 43 ( 2020-11-23), p. 2753-2753
Abstract:
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 H 2 O and 400 mM cyclohexanone, we found that non-Baeyer-Villiger products, δ-hexanolactone and ɣ-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 1 st order dependence on water at low water concentrations ( 〈 2 M H 2 O) 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 H 2 O, implying an initial 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.
Type of Medium:
Online Resource
ISSN:
2151-2043
DOI:
10.1149/MA2020-02432753mtgabs
Language:
Unknown
Publisher:
The Electrochemical Society
Publication Date:
2020
detail.hit.zdb_id:
2438749-6
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