Organic transformations using electro-generated polyoxometalate redox mediators

Abstract

The use of electrochemistry in organic synthesis (sometimes termed electro-organic synthesis, or just “electrosynthesis”) is a field that is attracting ever-increasing attention on account of its ability to deliver reaction outcomes that cannot be achieved in any other way and on account of its potential to replace toxic and/or sacrificial reagents with cleanly-generated electricity. In the simplest incarnation of electro-organic synthesis, an organic substrate undergoes direct electron transfer with the electrode in a type of heterogeneous catalysis process. However, such direct electrochemical processes may suffer from slow kinetics, poor product selectivities and/or lack of control over the extent of electron transfer to the substrate, all of which can lead to sub-optimal reaction outcomes. A distinct, but extremely important, sub-section of electro-organic synthesis (so-called “indirect electrolysis”) circumvents these limitations through the use of soluble redox mediator molecules, which act as electron shuttles between the heterogeneous electrode and the organic substrate in bulk solution. The advantages of indirect electrolysis for a number of organic transformations is now becoming increasingly clear, typically using either very simple metal salts or small organic molecules as the mediators. However, one particular class of mediators, the polyoxometalates (well-defined, soluble metal oxide clusters), offers an especially wide diversity of structures and properties that could revolutionise indirect electrolysis for electro-organic synthesis. In this review, we provide a collation of reports that describe use of electro-generated polyoxometalate redox mediators for organic transformations, with a particular focus on the most recent and most exciting results.