(759d) Electrochemical Epoxidation of Olefin Substrates Using Water As the Oxygen Atom Source

The direct transfer of an oxygen-atom into a target substrate is a fundamental reaction in organic chemistry. Over several decades, these types of reactions have been investigated in order to make various key chemical feedstock. These key chemical feedstock include epoxides, which are ubiquitous as versatile intermediates for the manufacture of many chemical products including plastics, adhesives, surfactants, epoxy resins, and pharmaceuticals. Current routes of olefin epoxidation involve either hazardous reagents or generate stoichiometric side products, leading to challenges in separation and significant waste streams.

Here, we demonstrate a sustainable and safe route to epoxidize olefin substrates using water as the oxygen atom source at room temperature and ambient pressure. Manganese oxide nanoparticles are utilized as electrocatalysts and shown to catalyze cyclooctene epoxidation. Taken together with the data of the product analysis, isotopic labeling experiments, and electrokinetic studies, the overall electrochemical epoxidation reaction is proposed to involve the reaction of cyclooctene and water to generate cyclooctene oxide and hydrogen gas. Greater than 30% Faradaic efficiency towards epoxidation was achieved at the anode, while hydrogen was coproduced at the cathode with over 94 ± 4.5% Faradaic efficiency. In addition, we demonstrate that this new route can also achieve a cyclooctene conversion of ~ 50% over 4 hours. Hence, compared to a conventional water electrolyzer which produces valuable hydrogen and vents oxygen gas, we are able to make use of the anodically produced oxygen atoms to conduct an epoxidation reaction.

Electrokinetic studies provide insights into the mechanism of olefin epoxidation, including an approximate first order dependence on substrate and water and a rate determining step which involves the first electron transfer. Mn(IV)=O species are proposed as the resting state, which was reported in previous studies on manganese oxide catalyst for water oxidation. We consider a reversible oxygen atom transfer step that exists as a pre-equilibrium step, followed by rate limiting one-electron transfer step. In addition, we have examined a series of linear, cyclic, and other aliphatic substrates to show the broader applicability of our concept and obtained more than 50% selectivity toward epoxides from most of the substrates. We believe that our results will provide a new pathway to catalyze oxygen atom transfer reactions electrochemically.