Electrifying palladium-catalyzed reactions broaden electrochemical methods in synthetic chemistry

A study from a team of chemists working at McGill University in Montreal, Canada has proposed a new method for synthesising palladium catalysts using electrochemical potential, supporting both oxidative addition and reductive elimination with two-electron exchange in mild temperature and pressure conditions.

Palladium has enjoyed a major role in modern organic synthesis, and is a popular catalyst in bond-forming reactions such as carbonylations with feedstock reagents, whereby a carbon monoxide molecule is incorporated into a substrate. A one of the obstacles to scalability in these reactions, however, is the need for catalyst regeneration and keeping a stability. These balancing operations, in which steps on the path to product formation must be reversed, often require high temperature conditions and can limit the scope of application.

Electrochemistry is enjoying renewed popularity at present, thanks to its ability to create sustainable transformations, particularly in oxidative and reductive reactions. Redox neutral reactions – those which do not require an external agent for reduction or oxidation – have been less well researched, though there is significant potential for the deployment of electrochemistry here. The researchers proposed driving catalysis using electrochemical potential, using two-electron cycling during the metal oxidation state. This is possible with the use of metal complexes that favour two-electron changes, and the electrochemical potential serves as the energy source as opposed to oxidising and reducing substrates or a thermal catalyst.

The abovementioned redox steps support carbonylation reactions at very mild ambient temperature and pressure conditions. One such reaction is the catalytic synthesis of high-energy aroyl halide electrophiles. It was also shown that the catalyst functions according to a multi-electron exchange pathway, where two-electron reduction drives oxidative addition, and two-electron oxidation leads to reductive product elimination. The combination of these two processes is described by the authors as unique. In addition to using electrochemical potential energy as the only additional energy source, this process eliminates the need for the classic ligand balanced thermal operations.

In the experiment, the researchers examined the carbonylative synthesis of acyl chlorides, in environmental conditions of up to 130 °C and 50 atm CO. Palladium is well suited for this type of reaction, as it favours two-electron oxidation state changes more strongly relative to nickel. The presence of carbon monoxide can also stabilise palladium through various states of oxidation.

According to the authors, this form of electrocatalysis facilitated by multi-electron exchange has wide-ranging potential applications in the chemical and pharmaceutical industries. Furthermore, the ability to carry out oxidative addition and reductive elimination with low environmental barriers opens the door to more complex bond-forming reactions. The findings could prove to be an important milestone in the drive to electrify catalytic synthesis.

The authors of the study are all based at McGill University’s Department of Chemistry in Montreal, Canada. The work was carried out with a Natural Sciences and Engineering Research Council of Canada Discovery Grant, a McGill Sustainable Systems Initiative Ideas Grant and a Molson & Hilton Hart Fellowship, in addition to support from the Centre in Green Chemistry and Catalysis.

Source: https://pubs.acs.org/doi/10.1021/jacs.5c03354