2021 Annual Meeting
(67a) Design Criteria for Solvent and Additive Choices to Facilitate Selective and Sustainable Electrocarboxylation.
Authors
Nathan Corbin - Presenter, Massachusetts Institute of Technology
Karthish Manthiram, Massachusetts Institute of Technology
Synthesizing carboxylic acids from carbon dioxide (CO2) represents an attractive way to develop new pathways to value-added chemicals using a sustainable and abundant C1 synthon. Compared to the more commonly used oxidative routes, carboxylation with CO2 is reductive, increasing tolerance to oxidizable functional groups, and extends the carbon chain length. Additionally, performing carboxylation electrochemically has advantages over established synthetic routes such as Kolbe-Schmitt or Grignard reagents including ambient reaction conditions, heterogeneous catalysts, renewable energy source in the form of electricity, and avoidance of stoichiometric reducing agents. While electrocarboxylation has been studied for several decades now, a limitation in its sustainability and practicality results from the usage of sacrificial anodes of metals such as magnesium and aluminum. We show in this work that replacing the sacrificial anode is not a trivial task for carboxylating many types of carbon-halide bonds, as the metal cations produced from the sacrificial anode play a key role in the selectivity of the cathodic reaction. Carboxylation without the presence of metal cations such as Mg2+ leads to the formation of esters from an SN2 nucleophilic reaction between an unprotected carboxylate and substrate; for more susceptible substrates, nucleophilic reactions with CO32- can lead to organic carbonates and alcohols. The addition of an Mg-containing salt such as MgBr2 can effectively suppress esterification via coordination with the carboxylate group. The effectiveness of this additive is demonstrated by carboxylating a wide range of organic halide substrates without sacrificial anodes and in several representative cases, by showing comparable if not better yields compared to using a magnesium sacrificial anode. The ability of the carboxylate product to protect the cathode against passivation by insoluble inorganic carbonates is also discussed. The electrolyte solvent is also shown to have a large impact on selectivity, controlling the amount of unwanted substrate hydrogenation that occurs.
