Electrochemistry: Technology, Synthesis, Energy, and Materials

Editorial Cite This: Chem. Rev. 2018, 118, 4483−4484

pubs.acs.org/CR

Introduction: Electrochemistry: Technology, Synthesis, Energy, and Materials he word “electrochemistry” means different things to different people. For many, the term is undoubtedly colored by experiences in a classroom setting that almost exclusively focus upon physical and analytical chemistry. This is not a surprise. Organic chemistry textbooks and most organic chemistry courses, even advanced ones, spend little time discussing electron-transfer reactions and almost never discuss synthetic organic electrochemistry. The result is that most practicing organic chemists have little to no experience with electrochemistry. It is time for a change. In many instances, a lack of familiarity with a potential area of research would be viewed as a new opportunity for exploration. However, rather than being attracted to new reaction landscapes offered by electrochemistry, many chemists have been intimidated by the unfamiliar territory and by the perception that the equipment needed for organic electrochemistry is complicated and expensive. This is not true. Electrochemical reactions can be conducted with a variety of simple setups including a three-necked flask as the reaction vessel, mechanical pencil leads as electrodes, and a phone charger as a power supply. Nevertheless, the unfamiliarity of these features has caused researchers to turn to the use of reagents and familiar methods for which they have prior experience, even when an electrochemical method might hold a distinct advantage over that more comfortable approach. Those willing to step out of that comfort zone are privileged to experience an ever-evolving landscape of new reactivity that is rich in opportunities involving the generation of new reactive intermediates for synthesis, bioremediation, the transformation and utilization of biomass, the development of sensors and energy storage devices, and the exploration of more sustainable ways to accomplish more traditional approaches to synthesis. In the later case, the electron transfer from or to an electrode can lead directly and efficiently to oxidized or reduced products without the need for traditional, frequently toxic reagents. In this manner, the cost and effort associated with the collection and disposal of waste is greatly diminished. Alternatively, the metal-based reagents used to achieve a wide range of important redox processes can be turned over at the electrode and reused without needing an external, stoichiometric coreagent to convert the metal to its original oxidation state. The use of flow processes for these transformations allows recycling and upscaling, permits use of less solvent, and minimizes or completely eliminates the need for a supporting electrolyte. At the same time, the use of redox mediators as homogeneous electron transfer agents frequently allows reactions to occur at potentials that are substantially less positive (oxidations) or less negative (reductions) than those required for direct electron transfer at the electrode, thereby reducing energy consumption and improving the “energy economy” of a reaction along with the gains in “atom economy”. In short, the opportunities for new, fundamentally important reaction chemistry are great, and as the reviews that follow indicate, the synthetic community has begun to exploit this potential.

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There are still barriers that appear to make the chemistry difficult for some. In the past, many researchers envisaged that reactions carried out electrochemically were difficult to monitor. Perhaps the thought of using a coulometer, yet another unfamiliar piece of equipment, scared some away. In reality, a coulometer is useful but not necessary. A simple TLC analysis will dojust as is the case for the routine monitoring of any organic reaction. Of course, a rich array of additional tools can be used like in situ IR, UV, and ESR as well as a multitude of powerful voltammetric techniques. Of these, cyclic voltammetry again leads to some consternation because of its unfamiliarity to many chemists. This is another area in need of change because cyclic voltammetry can be used to explore a great number of electron-transfer reactions. Cyclic voltammetry allows one to quickly and easily determine the potential at which a substrate is reduced or oxidized, affords one an idea of the stability and reactivity of intermediates, provides a rapid screening tool to assess catalyst turnover in redox mediated processes, and offers an opportunity not only to monitor the progress of a chemical reaction but also to gauge reaction rates for highly reactive radical ion intermediates. There are other technological advances in electrochemistry that currently offer unique synthetic opportunities. Use of the so-called cation pool technology permits the formation, reaction, and spectroscopic visualization of heretofore uncharacterized intermediates by NMR. The ability to form reactive intermediates and carry out reactions at specific locations on microelectrode arrays allows one to probe molecular interactions involving three-dimensional binding motifs of biological receptors. In addition, microflow chemistry and the ability to do rapid parallel synthesis has generated new modes of reactivity based on micromixing. One of the ironies associated with recent developments is the realization that it is frequently not the electron transfer event that is responsible for the failure of an electrochemical transformation. Rather, it is an incomplete knowledge of the nature of the intermediates, and mechanistic insights into what influences their reactivity that leads to trouble. In other words, an electroorganic reaction is very much still an organic reaction. This is good news for an organic chemist because the optimization of an electroorganic reaction they want to use will very likely depend on the physical organic chemistry principles that govern the reactions with which they are more comfortable. The landscape is rich and fertile. We hope you find it inviting and encourage you to engage.

R. Daniel Little*

University of California Santa Barbara (UCSB)

Kevin D. Moeller

Special Issue: Electrochemistry: Technology, Synthesis, Energy, and Materials Published: May 9, 2018 4483

DOI: 10.1021/acs.chemrev.8b00197 Chem. Rev. 2018, 118, 4483−4484

Chemical Reviews

Editorial

Washington University in St. Louis

University of CaliforniaSanta Barbara in 1980 and then his Ph.D. degree in Organic Chemistry (Professor R. Daniel Little) from the same institution in 1985. He was an NIH Postdoctoral Fellow at the University of WisconsinMadison (Professor Barry M. Trost) from 1985 to 1987. His long-standing research interests center on the interplay between electrochemistry and organic synthesis, efforts that have ranged from using electrochemical reactions to construct complex molecules to using electrochemically directed synthetic methods to construct the complex molecular surfaces used on bioanalytical devices. In 2016, he received the Manuel M. Baizer Award in Organic Electrochemistry from the Electrochemical Society. He can be reached at [email protected].

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Kevin D. Moeller: 0000-0002-3893-5923 Notes

Views expressed in this editorial are those of the authors and not necessarily the views of the ACS. Biographies

R. Daniel Little (Dan) was born and educated in Superior, Wisconsin. He majored in math and chemistry, graduating with highest honors from the University of Wisconsin in Superior. During his college years, Dan participated in NSF-URP programs at the University of South Dakota, and spent a semester at Argonne National Laboratory studying with Kaplan and Wilzbach. Graduate studies at Wisconsin with Howard Zimmerman were followed by delightful postdoctoral experiences at Yale with Jerome Berson. Major research interests have focused upon diradical chemistry related to trimethylenemethane and the development and uses of organic electrochemistry. Save for sabbaticals in Canada, China, and Germany, Dan has spent his independent career at UCSB, where he has served as Vice-Chair and Chair of the Department. He is an elected member of the American Association for the Advancement of Science, and in 2015 received the Jaroslav Heyrovsky Prize for Molecular Electrochemistry awarded by the International Society of Electrochemistry.

Kevin D. University Chemistry November

Moeller joined the chemistry faculty at Washington in St. Louis in 1987, and he has been Professor of since 1999. He was born in Scranton, Pennsylvania on 25, 1958, and earned a BA degree in Chemistry from the 4484

DOI: 10.1021/acs.chemrev.8b00197 Chem. Rev. 2018, 118, 4483−4484