Combining Water and Biorenewable Solvents for Sustainable Anodic Reactions

Abstract

Enabling technologies are perceived as inherently sustainable, due to the replacement of stoichiometric oxidants and reductants with traceless redox mediators such as photons or electrons. However, the reaction solvent plays a crucial role in the environmental impact of the process. To aspire towards ideal sustainable electrochemical transformations, the use of water as solvent would be the most preferred choice, but poor solubility of organic compounds hinders its employment. Another option could be the use of biorenewable solvents, which require wasteful organic supporting electrolytes. In this work, the application of a solvent mixture of water and propylene carbonate is presented. This media combines the best features of the two chemicals, enabling sustainable electrochemical synthesis without solubility issues of organic material or inorganic salts. The adoption of high mass-transfer electrochemical reactor overcomes the limitations caused by heterogeneous mixtures, streamlining potential industrial applications. The generality of this concept was validated across four different representative anodic reactions, comprising a cross-coupling, a rearrangement, a direct and a mediated oxidation. In all the studies, process scalability could be demonstrated, reaching excellent productivities (from 39 g/day to 3.4 kg/day) with exquisite green metrics (PMI up to 27, Green Score > 84). Finally, guidelines for general application of this approach were outlined.