Electrosynthesis Using Carboxylic Acid Derivatives: New Tricks for Old Reactions.

Abstract

Electrosynthetic organic chemistry is an old discipline that takes its root in Faraday's seminal works. The field has a rich history and is in the midst of a renaissance, due to the growing impetus of the chemical community to develop greener, more economical, and more efficient synthetic methodologies. Indeed, electrosynthesis relies on one of the greenest and cheapest reagents in the world: the electron itself. In this Account, the recent developments in the use of carboxylic acid derivatives in electrosynthesis are summarized. Until lately, the fate of the monoelectronic reduction of aromatic esters in nonprotic solvents remained unclear. Recent investigations have shown that aromatic esters are reduced and form surprisingly long-lived radical anions. Under the right conditions, these radical anions decompose into the corresponding carboxylates and alkyl radicals. These principles have been used to develop a novel electrochemical alcohol deoxygenation reaction using aromatic esters as stable and versatile radical precursors. In contrast to esters, the electrochemistry of carboxylic acids has been intensively studied. Pioneering works by Faraday and Kolbe in the late 1800s have revealed that the anodic oxidation of carboxylic acids leads to a radical decarboxylation. Interestingly, radical recombination is observed due to the very high concentration of radicals in the vicinity of the electrode. Such radical recombination is rarely observed during classical homogeneous radical reactions. The "Kolbe" reaction and its carbocationic variation have been intensively used across the fields due to their versatility. As we will develop in this Account, almost two hundred years after its discovery, the anodic decarboxylation of carboxylic acids is still relevant to modern organic chemists. For instance, we will examine how the non-decarboxylate Kolbe reaction of aromatic acids forms aroyloxy radicals and how oxycarbonyl radicals could be generated from hemioxalates. Finally, the carbocationic variant of the Kolbe reaction, known as the Hofer-Moest reaction, will be examined in the context of two newly developed reactions: a green MOM-type ether formation and the use of malonic acid derivatives as carbonyl synthons. Electrosynthesis is a powerful synthetic tool. Even if it might still be underutilized at the moment, there is little doubt that it will become one of the "classic" methods to activate small organic molecules in a very near future.