Electrochemical Screening for Electroorganic ... - ACS Publications

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Electrochemical Screening for Electroorganic Synthesis Christoph Gütz,† Bernhard Klöckner,‡ and Siegfried R. Waldvogel*,† †

Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany Electronics Workshop of the Chemical Department, Rheinische-Friedrich-Wilhelm-University Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany

‡

S Supporting Information *

ABSTRACT: Electrochemical screening is usually strongly focused on electroanalytical data, while the parameters of organic synthesis are mostly not used as selection criteria. Typical parameters would be indication of the formation of the product and the efficiency of the electroorganic conversion. The latter data indicate the stability of the product under electrolysis conditions and represent the key for the accumulation of the desired product. We survey the current methods for electroorganic screening. In particular, parallel electrolysis under more defined conditions is discussed in detail since it represents a powerful tool for the development of electroorganic syntheses and processes.

1. INTRODUCTION Since electricity from renewable sources has become increasingly important, electroorganic synthesis is advancing more and more to a future key technology in chemical industry. On the one hand, it represents a cost-efficient and sustainable “green chemistry” method.1−8 On the other hand, electroorganic synthesis can provide outstanding selectivity and reactivity and sometimes the formation of products that are not accessible via conventional methods.8−11 Therefore, a large number of interesting transformations such as fluorinations,12,13 coupling of alkenes,14−16 dehalogenations17−22 oxidative C−C couplings,23,24 and many more are impressive examples of this powerful methodology. In the electrochemical conversion of organic compounds, the substrate has to come close to the electrode surface or sometimes be adsorbed onto it (Figure 1). The electron transfer can occur directly at the electrode (heterogeneous) or indirectly by another redox intermediate that acts as a mediator (homogeneous). Subsequently, the electron transfer occurs, generating an open-shell intermediate. This can desorb into the

electrolyte, since radicals are sufficiently stabilized in organic molecules. The initial electrochemical step is mostly determined by the nature of the electrode material. If so-called innocent electrodes are used, the electrochemical potential for the electron transfer must be reached. However, other chemicals in the solution with different redox potentials may interfere and influence the desired selectivity. A sequence of chemical steps in the electrolyte follows. Then a further electron transfer via other intermediates or by contact with the electrode can occur to generate a neutral or charged nonradical derivative. The electrolyte with its ionic strength stabilizes neutral or charged intermediates differently. Further parameters that affect mass transport and conversion are temperature, solvent, convection, etc. Since many parameters are involved in such an electrolytic conversion of organic compounds, screening of the electrolysis conditions provides the most efficient way to find suitable protocols for electroorganic synthesis of the desired products. In general, there are two different operational modes in which to conduct electrolytic conversions: controlled-potential or constant-current conditions. Electrolysis at controlled potential leads to selective conversion as a result of the applied potential at the working electrode since it is chosen to match that of the substrate. Unfortunately, this requires a threeelectrode arrangement and therefore a much more costly electronic periphery. The galvanostatic mode of conducting the electrolysis provides a technically simple two-electrode setup. Data from this approach can be easily used for scale-up. The important parameter is the current density, which describes the concentration of reactive intermediates formed and also determines the reaction pathway. The potential generated this way is coupled to the current density, and by observation of the applied voltage during the electrolysis, the progress of the electrolysis can be monitored. The optimal current density can be determined by a set of electrolyses. The second set of data obtained by this mode of operation is the applied charge, which

Figure 1. Schematic overview of significant parameters for electroorganic synthesis. © XXXX American Chemical Society

Received: November 16, 2015

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DOI: 10.1021/acs.oprd.5b00377 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

Review

already provides information about the efficiency of the electrolytic conversion. In addition, attention has to be given to the electrochemical and chemical reversibility of the conversion. If the following sequence of reactions involves an irreversible step and the desired product or starting material is stable toward the counter electrode, an undivided cell can be employed. When the conditions described are not fulfilled, an electrolytic cell having separate compartments must be used. In some cases, the desired product might be not sufficiently stable toward the working electrode because it is easier to oxidize or reduce compared with the starting material. In such cases, flow techniques or partial conversions might help.25−27 However, if the desired product is prone to side reactions with the intermediates formed, usually electrolysis is not a useful alternative to obtain this product. Since the choice of the specific electrolysis system is only partially intuitive, several parameters have to be screened, which results in a time-consuming large number of small-scale electrolyses. A common strategy to tackle such an issue is high-throughput screening (HTS).28−33 Whereas HTS is wellknown and established, i.e., in catalysis, no comparable system has been established for electrosynthesis. Therefore, here we survey the different approaches for electrochemical screening, dealing with different issues from electroanalytic to novel electrosynthetic screening concepts.

Figure 3. Setup for the collection of electroanalytical data via an x, y, z positioning unit. Reproduced with permission from ref 36. Copyright 2001 Wiley-VCH GmbH & Co KGaA.

determination of redox potentials of π-conjugated polymers,36 electroanalytical data of biosensors,37 and electrocatalyst screening for methanol oxidation.38 By this approach, the suitability of various substrates with different electrolyte systems based on the cyclovoltammogram data can be analyzed, whereas the variation of electrode material is possible only after the electrochemical analysis of all vials. Another setup is the use of separate controllable electrodes via a combination of a potentiostat and a multiplexer that is controlled by a computer.39 Since the whole array is quite small (64 electrodes with an area of 1 mm2), only a single electrolyte solution with counter and reference electrodes is used. Indeed different electrode materials can be investigated, but alternation of substrates or the supporting electrolyte is not easy. The approaches mentioned above provide only electroanalytical data. Only small microelectrodes and a common onechannel potentiostat are utilized. Thus, only one electrolysis can be analyzed at the same time. Depending on the issues associated with CV measurements (e.g., low scan rates, requiring up to 2 h for several cycles), a complete run through a 96-well microtiter plate might require up to 192 h. Furthermore, the use of only one bundle of electrodes can cause contamination from one vial to the next, especially when a film is formed on the working electrode. To circumvent these challenges, most of the microtiter vials are used for rinsing or calibration electrolytes. Moreover, only electroanalytical data (i.e., peaks in the cyclic voltammogram) are collected, which reveal only that an electron transfer was observed; no detailed structural information about the intermediate and the final product is provided. Isolation and characterization of potential products fail or are difficult since no significant amounts of products are formed. Therefore, a determination of the yield or applied electricity is not possible. Nevertheless, this setup was also used for the potentiostatic electrosynthesis of iminoquinol ether and the cathodic coupling of α,β-unsaturated esters and allyl bromides in quite small amounts of 0.06 mmol.40,41 Because of the small electrode surface, the electrolysis time required for each vial to reach complete conversion is long, and the whole method is very time-consuming. Thus, Speiser et al. ran the electrolysis only for a short time of about 30 min and then analyzed the electrolyte via HPLC−MS or GC−MS. In the case of positive results, the electrolysis was repeated on a larger preparative scale for determination of the yield and purity via various analytical methods. However, the authors note that there were some discrepancies between the results obtained by screening and those obtained on a preparative scale. A further advanced development of potentiostatic screening of electrolytic conversion was achieved by the use of multichannel potentiostats for parallel conversions. Holtmann

2. ELECTROANALYTICAL/POTENTIOSTATIC SCREENING Most of the concepts and developments in the field of electrochemical screening equipment belong to electroanalysis or potentiostatic electrolysis. The most prominent approach is based on the combinatorial measurement of cyclic voltammetry (CV) to identify promising electrocatalysts.34 The first report deals with the combinatorial screening of different metal alloys for methanol oxidation.35 However, only a library of different electrocatalysts for the working electrode was generated in a combinatorial fashion by printing these alloys via a modified inkjet printer on conducting Toray carbon paper (Figure 2).

Figure 2. Combinatorial printing of a quaternary-phase library of electrocatalysts on conducting Toray carbon paper via an inkjet printer. The numbers label the three nested shells of the pyramid (1 = outer shell; 2 = middle shell; 3 = inner shell). Reprinted with permission from ref 35. Copyright 1998 AAAS.

Then each electrolysis was conducted manually by dipping a combination of each with reference and counter electrodes into the corresponding electrolyte system. Hence, taking the parameters of Figure 1 into account, only the electrode material is altered to screen the activity of the alloy for electrolytic methanol oxidation. A significant advancement for electroanalytical measurements is the combination of microtiter plates with a bundle of electrodes (counter, reference, and working electrodes) that can be automatically shifted from vial to vial by a step-motorcontrolled x, y, z positioning unit (Figure 3).34 Thereby, a large number of electroanalytical data can be collected in a highly automated manner. This approach was used for the B



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Figure 4. Parallel potentiostatic screening via a multichannel approach. Reprinted with permission from ref 42. Copyright 2013 Elsevier.

Figure 5. Approach for parallel constant-potential electrolysis by de Campos Rodrigues and Rosenbaum via a six-channel potentiostat. Reproduced with permission from ref 44. Copyright 2014 Wiley-VCH GmbH & Co KGaA.

amount of product, this method is also insufficient since the surface of the working electrode is quite small, which leads to very long reaction times for full conversion. An interesting and straightforward setup was described by de Campos Rodrigues and Rosenbaum.44 They combined a sixchannel potentiostat with electrochemical cells having volumes of up to 9 mL and large electrode surfaces of 12 cm2. Therefore, a time-efficient electrolysis with quantification of product was possible (Figure 5). Unfortunately, they only used this system

and co-workers collected electrochemical data for eight samples in a 24-well microtiter plate at the same time (Figure 4).42 Unfortunately, the change to the next eight samples had to be done manually, so no real automatization via an x, y, z positioning unit was realized. The same arrangement was further adopted for the screening of electroenzymatic processes using cytochrome P450 monooxygenases and variations (muteins) thereof in combination with a UV/vis detector for online analysis.43 For the electrosynthesis of a quantitative C



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for screenings of bioelectrochemical systems, which required unusually low current densities of 2.5 μA/cm2, so each electrolysis run lasted 11 days. However, this system is a promising approach for a potentiostatic screening system, except for the following challenges: First, no stirring of the solution was realized. In principle, convection with a stirring bar is possible, but since the working electrode is located at the bottom of the cell, mechanical abrasion can take place. Second, screening of different electrode materials is difficult. The electrodes represent the bottom and the side wall of the cell. Therefore, alteration of the electrode materials is not trivial. Third, the electrode geometry leads to an inhomogeneous electric field because the electrodes are not arranged in a parallel fashion. Fourth, the combination of large electrode surfaces with adequate current densities requires high-performance potentiostats, which are rather expensive. With respect to the key parameter of Figure 1, the latest presented potentiostatic methods take several aspects into account. By parallel potentiostatic screening, the effects of various electrode materials and electrolytes can be analyzed in a time-efficient manner. Problematic aspects of such a potentiostatic approach are the expensive and with respect to current and voltage output often less powerful potentiostats and the relatively complex three-electrode setup. Additionally, in the case of large working electrodes it is almost impossible to achieve a controlled potential across the whole electrode surface. Finally, potentiostatic methods are time-consuming since the current is asymptotically reduced, and under controlled-potential conditions complete conversion cannot be achieved on a realistic time scale.

suitable only for electrocatalyst screening for volatile reaction products. For typical electroorganic conversions, quantitative analysis of the reaction products has not been yet realized. The system can be considered as one of the cutting-edge approaches, but it requires a significant electronic and analytical periphery.

4. ELECTROSYNTHETIC SCREENING Galvanostatic electrosynthetic screening is the most efficient approach since it combines several advantages: First, the setup is much easier and less expensive. Since a two-electrode arrangement is utilized and no reference electrode is required, the electronic periphery is dramatically simplified. Most of the parts can be purchased in hardware stores. Second, the data found allow scale-up to a preparative batch cell by employing the same galvanostatic protocol.20,46,47 The transfer of the electrolysis parameter cannot be done to a flow cell, but the elaborated data facilitate it.48 The first galvanostatic screening method was reported by Yudin and co-workers, who used a rather simple setup.49 They took a Teflon block with 16 wells or 16 glass vials in a 4 × 4 arrangement. In each cell they immersed a stainless steel cathode and a graphite rod anode. The individual cells had volumes of a few milliliters. The 16 cathodes and 16 anodes were contacted to each other and served by the same power supply (Figure 7). Thereby, only one direct current (DC) power source was necessary. The Canadian group postulated

3. COMBINATION WITH MASS SPECTROMETRY Another highly efficient potentiostatic setup for fast screening of alloy systems with varying composition of the working electrode was elaborated by Mayrhofer and co-workers.45 They used a movable electrochemical flow cell consisting of an inlet and an outlet for the electrolyte, the latter equipped with a counter electrode. Additionally, an outlet to a mass spectrometer for the detection of volatile products was installed. This electrolytic cell was then shifted over the surface of the corresponding alloy composition that represents the working electrode. Using a surface exhibiting a concentration gradient of the corresponding components of the alloy makes it possible to analyze the efficiency of different alloy compositions online within a short time (Figure 6). Of course, this method is

Figure 6. Setup for the screening of different alloy compositions via a movable electrochemical cell in combination with a mass spectrometer (MS outlet = central flow to the top). Reprinted with permission from ref 45. Copyright 2014 AIP Publishing LLC.

Figure 7. Galvanostatic screening approach of Yudin and co-workers in which 16 electrodes are connected to a single DC power source. Reprinted from ref 49. Copyright 2000 American Chemical Society. D



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that under equal conditions the applied electricity and current density for each cell can be calculated by dividing the entire current by a factor of 16. For simple screening of different substrates under the same conditions, this approach is quite suitable. However, for screening of supporting electrolytes or different concentrations, the Ohmic cell resistances may differ. In that case, the applied electricity and current density can be different for each cell, so the reproducibility of the experiment might be problematic. Furthermore, the electrodes are welded into a stainless steel plate for the connection to the power supply, so changing the electrode material is not trivial. However, this setup was successfully used for a few screenings of anodic oxidation to α-alkoxycarbamates and α-alkoxyamides or 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated oxidations.49−51 An alternative setup for galvanostatic electrosynthetic screening was developed by our group and is presented here in detail for the first time. This arrangement is designed not only to verify a screening hit but provide the necessary data for a subsequent scale-up at the same time. The electroorganic protocol can be directly used for the synthesis of preparative quantities. This approach combines high flexibility with the synthesis of a defined amount of product. Therefore, detailed information about the formed products, yield, and purity of the product can be achieved by various analytical methods such as NMR, MS, GC, and HPLC. These standard analytical tools are not directly implemented into the setup, and therefore, this approach can be easily established in an organic synthesis lab. The setup consists of a self-made eight-channel galvanostat (detailed information is provided in the Supporting Information) with an integrated coulomb counter (Figure 8). The electrolysis cells can be divided or undivided (Figure 9) and are mounted on a common magnetic stirrer by a stainless steel block (Figure 10). The stainless steel block can be fixed onto the magnetic stirrer like a standard oil bath. This provides comparable

Figure 9. Cross sections of (left) undivided and (right) divided cells for electrosynthesis.

Figure 10. Cross sections of the stainless steel carousels for undivided and divided screening cells.

intensive stirring in each cell. In the stainless steel block, all of the cells can be heated or kept at a defined temperature by a contact thermometer connected to the magnetic stirrer. Additionally, the stainless steel block can be modified with inner cooling fins and a connector to a thermostat to chill the electrolysis cells. The stainless steel piece is equipped to hold eight undivided electrolysis cells or six divided cells. The undivided cells with a volume of 6 mL are made of Teflon with a Teflon cap to suppress solvent evaporation. Because of the Teflon, they are suitable for fluorination reactions as well. The two electrodes (1 cm × 7 cm × 0.3 cm) are fixed in the cap by polyurethane screws. Therefore, on one hand the electrodes can be exchanged rapidly to screen different electrode materials, and on the other hand, the immersion depth can be varied, which is very useful for biphasic systems such as TEMPO-based oxidations.52,53 A stirring bar at the bottom of the cell guarantees efficient convection. The divided cells are also formed out of Teflon with a half-cell volume of up to 6 mL. Each half-cell can accommodate a stirring bar. The two half-cells are pressed together with four stainless steel screws. This facilitates exchange of the separator material between the half-cells. Common separators are Nafion, sintered glass frits (P4), and different anion exchange membranes. The electrodes are connected to the galvanostat by simple alligator clips. The galvanostat provides eight individual channels, each with a programmable coulomb counter. The system can operate at voltages and currents ranging up to 50 V and 50 mA, respectively, for each channel.

Figure 8. Picture of the whole setup. Left side: stainless steel block with eight undivided screening cells and a magnetic stirrer. Right side: programmable eight-channel galvanostat with an integrated coulomb counter. E



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If necessary, it is also possible to run a potentiostatic screening by adding a reference electrode to each cell and replacing the galvanostat by a more expensive multichannel potentiostat. However, in the case of a later scale-up this would be counterproductive, as mentioned above. The wide applicability and outstanding performance of this setup was demonstrated by the successful optimization of various electrosyntheses, including anodic cross-coupling reactions,23,24 dehalogenations,22 deoxygenations,54−56 and domino oxidation reduction sequences.57 A later scale-up into large batch or flow cells for multigram syntheses can easily be realized, as shown by the process development of the crosscoupling reactions and the dehalogenation reactions.48,58

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.oprd.5b00377. These data should enable an electrical or mechanical engineer to construct such devices. For more detailed information about the galvanostat described in the text, contact the electronic workshop of the University of Bonn (contact: b.kloeckner@ uni-bonn.de), and for information about the electrosynthetic screening cells, contact the machine shop of Johannes Gutenberg University Mainz (via [email protected]). Technical drawings, connection diagrams of the DC power supply, and more detailed pictures of the screening cells (PDF)

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REFERENCES

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5. SUMMARY We have surveyed different approaches for electrochemical screening. Most screening methods deal with potentiostatic electrochemical testing, which allows high throughput but mostly gathers electroanalytical data. These data have limited significance for most electroorganic syntheses. When such approaches are combined with chemical analysis (e.g., mass spectrometry), valuable information about the products formed can be obtained. Screening concepts closer to electroorganic synthesis on a preparative scale require galvanostatic methods. Conducting several electrolyses in such a parallel fashion reduces the number of preparative electrolysis runs performed. The data obtained by this approach allow clear product characterization and the development of suitable protocols for preparative electroorganic conversions. In the future, this method might even be combined with simple mass spectrometric techniques to enhance the power of this approach further.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Malte Brutschy for the preparation of the technical drawings via Solidworks and the Federal Ministry of Education and Research Project MANGAN (FKZ 03S0506) for support. F



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