Catalysis of Redox-Neutral Reactions By Electron Exchange between Electrodes and Organic Compounds

Abstract

To most chemists the term ‘electrocatalysis’ is known as the facilitation of a heterogeneous electron transfer via a chemical interaction between the electrode and a substrate [1]. The opposite case, the utilization of a heterogeneous electron exchange to catalyze a chemical reaction (‘electrochemical catalysis’), is a much less known but yet a very powerful approach in electrosynthesis [2]. Here, the injection (or removal) of an electron into (or from) a substrate triggers a redox-neutral reaction (e.g. a molecular rearrangement) that may otherwise require harsh conditions and/or the use of reagents. Such processes involve the electrogeneration of an ionic or radical ionic species, which after a coupled chemical step either undergoes a backward electron exchange with the electrode (ECEb mechanism) or triggers a chain process in the bulk solution. Under these circumstances, sub-stoichiometric amounts of charge are sufficient to achieve a full conversion and conceptionally, the electrons and holes can be understood as being catalysts.In this contribution, the concept of electrochemical catalysis will be illustrated using the Newman-Kwart rearrangement (NKR) of O-aryl thiocarbamates 2 to the corresponding S-aryl compounds 3 as an example (see scheme below) [3]. Generally, the NKR represents the key-reaction in the three-step synthesis of thiophenols 4 from phenols 1 and proceeds between 200 and 300 °C [3]. Electrochemical catalysis, however, allows for operation at room temperature and provides a complementary reactivity with respect to the arene substitution [3,4]. The results of batch and flow electrolysis will be presented, along with a detailed mechanistic analysis based on cyclic voltammetry studies, control experiments, and quantum chemical calculations.