Electrochemistry & Catalysis
Electrosynthesis is recognized as a promising green methodology for organic chemistry. It fulfills several important criteria that are needed to develop environmentally compatible processes. For instance, toxic or dangerous oxidizing or reducing reagents can be replaced by electric current. Moreover, the total energy consumption can be reduced and unstable and hazardous species can be generated in situ in a controlled manner.
One of the major challenges in electrosynthesis is the kinetic inhibition of the electron transfer, which oftentimes leads to selectivity problems and increased energy consumption (“overpotential”). To address this issue, our work is focused on the development of molecular electrocatalytic systems both for the conversion of organic compounds and for the utilization of carbon dioxide. The use of sub-stoichiometric charge quantities for the catalysis of redox-neutral reactions (electrochemical catalysis) offers a further promising and thus far little investigated approach, which we currently explore in the context of molecular rearrangements. In order to enable a knowledge-based optimization of our reactions, the mechanistic understanding of the catalytic process is of special interest for us. For this purpose, we employ electroanalytical techniques and, if necessary, combine them with spectroscopic methods and control experiments.
The necessity for using large quantities of supporting electrolytes represents a further challenge in electrosynthesis. Once the reaction is complete, the salt must be separated from the reaction mixture. Since the salt additive is both a potential source of waste and a significant cost factor, its recycling is highly desirable. Similarly, molecular catalysts pose a separation and waste problem. In this context, we are placing further emphasis on the topic of multifunctional electrolytes.
Molecular Electrocatalytic Systems
Over the past years, we have established a research program that is focused on the utilization of organometallic catalysts for electrosynthesis.[1-4] In one case we have studied a system based on cyclopentadienone iron complexes,[1,2] which allows for the reduction of carbon dioxide to CO in nearly quantitative Faradaic yield and high current density. Using a combination of cyclic voltammetry, controlled potential electrolysis, FTIR spectroelectrochemistry (in collaboration with the group Ludwig Group, Rostock University) and quantum chemical calculations (in collaboration with the Roemelt Group, Bochum University), we were able to obtain mechanistic insights and to propose a sequence that proceeds via the key-intermediate shown in the left figure (top right). In another project, we have investigated a diimine manganese framework under electro- and photochemical conditions (in collaboration with the Beller Group, LIKAT Rostock).[3]
References:
[1] E. Oberem, A. F. Roesel, A. Rosas-Hérnandez, T. Kull, S. Fischer, A. Spannenberg, H. Junge, M. Beller, R. Ludwig, M. Roemelt, R. Francke, Organometallics 2019, 38, 1236–1247.
[2] A. Rosas-Hernández, H. Junge, M. Beller, M. Roemelt, R. Francke, Catal. Sci. Technol. 2017, 7, 459−465.
[3] C. Steinlechner, A. F. Roesel, E. Oberem, A. Paepcke, N. Rockstroh, F. Gloaguen, S. Lochbrunner, R. Ludwig, A. Spannenberg, H. Junge, R. Francke, M. Beller, ACS Catal. 2019, 9, 2091−2100
[4] R. Francke, M. Roemelt, B. Schille, Chem. Rev. 2018, 118, 4631–4701.
Electrochemical Catalysis
Compared to electrocatalysis, in which a chemical interaction between a substrate and a catalyst facilitates the electrode process, electrochemical catalysis represents exactly the opposite case. Here, the injection (or removal) of an electron into (or from) a substrate triggers a redox-neutral reaction that may otherwise require harsh conditions and/or the use of reagents.[1] Such reactions involve the electrogeneration of an ionic or radical ionic species, which after a coupled chemical step either undergoes a backward electron exchange with the electrode (ECEb mechanism) or triggers a chain process in the bulk solution. Under these circumstances, sub-stoichiometric amounts of charge are sufficient to achieve a full conversion and conceptionally, the electrons and holes can be understood as being catalysts. In this context, we have developed a research program focusing on electrochemically catalyzed redox-neutral reactions. A major accomplishment was the development of the electrochemical Newman-Kwart rearrangement (ENKR) of O-aryl thiocarbamates to the corresponding S-aryl compounds (see right figure). Generally, the NKR represents the key-reaction in the three-step synthesis of thiophenols from their phenol derivatives and proceeds between 200 and 300 °C. Electrochemical catalysis, however, allows for operation at room temperature and provides a complementary reactivity with respect to the arene substitution.[2,3]
References:
[1] R. Francke, R. D. Little, ChemElectroChem. 2019, 6, 4373.
[2] T. Broese, A. F. Roesel, A. Prudlik, R. Francke, Org. Lett. 2018, 20, 7483.
[3] A. F. Roesel, M. Ugandi, N. T. T. Huyen, M. Májek, T. Broese, M. Roemelt, R. Francke, J. Org. Chem. 2020, 85, 8029-8044.
Multifunctional Electrolytes
During the past five years, we have investigated several approaches toward simplifying the workup of indirect electrosyntheses. One of these approaches is based on merging redox mediator and supporting electrolyte into a single molecularly defined species (see left figure). Due to the wide range of applications of hypervalent iodine reagents in organic synthesis,[1] we decided to use the iodine(I)/iodine(III) couple as test case. A series of investigations on ionically tagged iodoarenes showed that under certain conditions, electrolysis in the batch cell is possible without the use of salt additives and that the separation and reuse of the mediator is significantly facilitated.[2-4] Using 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) as solvent, we found that the active dialkoxy-λ3-iodanes are formed in nearly quantitative Faradaic efficiencies. The electrolyzed solutions served as reactive media for several oxidative C-O, C-C, and C-N couplings.
Another approach is based on attaching catalyst molecules and supporting electrolyte units to soluble polymers (resulting in “polymediators and polyelectrolytes”), which allows for separation of both components in a single step using size-exclusive membranes. Using the example of TEMPO-catalyzed alcohol oxidation as a test case (see right figure), we have demonstrated for the first time that indirect electrosynthesis can be efficiently coupled to dialysis or ultrafiltration using polyelectrolyte HP-1 and polymediator HP-2.[5] The polymer solutions are sufficiently conductive, exhibit a high electrocatalytic activity toward oxidation of various alcohols, and can be reused for multiple times.
References:
[1] R. Francke, Curr. Opin. Electrochem. 2019, 15, 83-88.
[2] A. F. Roesel, T. Broese, M. Májek, R. Francke, ChemElectroChem. 2019, 6, 4229–4237.
[3] O. Koleda, T. Broese, J. Noetzel, M. Roemelt, E. Suna, R. Francke, J. Org. Chem. 2017, 82, 11669–11681.
[4] T. Broese, R. Francke, Org. Lett. 2016, 18, 5896–5899.
[5] B. Schille, N. O. Giltzau, R. Francke, Angew. Chem. Int. Ed. 2018, 57, 422.
[6] N. Mohebbati, A. Prudlik, A. Scherkus, A. Gudkova, R. Francke, ChemElectroChem 2021, 8, 3837-3843.