The new strategy of AC electrolysis realizes the direct phosphonylation reaction of a variety of hydrocarbons

On January 5, 2023, the team of Professor Lei Aiwen from the School of Chemistry and Molecular Sciences and the Institute for Advanced Study of Wuhan University published a research paper entitled “Asymmetric-waveform alternating current-promoted silver catalysis for C–H phosphorylation” in the journal Nature Synthesis.

This achievement reports a new electrolysis strategy for asymmetric waveform alternating current electrolysis, which realizes the direct phosphonylation reaction of a variety of hydrocarbons. This type of reaction uses silver salt, which is not commonly used in electrosynthesis reactions, as a catalyst, and realizes the recycling of silver catalyst in electrolytic environment through asymmetric waveform alternating current, which solves the limitation of silver catalyst easy reduction deposition inactivation. At the same time, asymmetric waveform AC electrolysis can also be applied to other transition metals such as palladium, copper and other catalytic oxidative coupling reactions, which will have a wide range of application scenarios in the field of organic synthesis.

The corresponding author of the paper is Professor Avenues Rays. The co-first authors are Zeng Li, a postdoctoral fellow in the School of Chemistry and Molecular Sciences, Ying Jiao, a doctoral candidate at the Institute for Advanced Study, and Weishun Yan, a master in the School of Chemistry and Molecular Sciences.

Transition metal catalytic coupling reactions are widely used in organic synthesis, chemical and pharmaceutical, material polymers and other fields. However, in the field of organic electrosynthesis, transition metal catalysts are often difficult to compatible with simple and general electrolysis conditions. The main factor is that transition metals such as silver, palladium, copper and other catalysts have high reduction potential, and it is often easy to produce metal elements by reducing reaction at the cathode, while attaching to the electrode surface to contaminate the electrode. There are two general solutions, one is to use a separate electrolytic cell, using a sand core or ion exchange membrane to isolate the metal catalyst in the anode to avoid metal ion reduction, such methods require more complex electrolysis devices and conditions; The other is to introduce a guide group with strong coordination in the substrate, and the guide group and the metal catalyst coordinate to reduce the reduction potential of the metal species, making it more difficult to be reduced by the cathode, which will lead to a narrow range of application of the reaction. The development of a transition metal catalytic electrolysis strategy with simple electrolytic conditions, mild reaction conditions and good compatibility is of great research value, which will promote the further development of electrosynthesis reactions.

Figure 1: Limitations of transition metal-catalyzed reactions under DC electrolysis and asymmetric waveform AC electrolysis strategies

By controlling the shape of the alternating current waveform, the traditional symmetrical waveform (sine wave and square wave) is transformed into an asymmetric waveform, and an asymmetric electrode combination (carbon and platinum electrode combination) is introduced to adjust the reaction process. Using the difference in redox potential of different electrodes on metal catalysts, changing the duty cycle (the proportion of forward period to the whole cycle) to control the time for different electrodes to participate in redox in each cycle to promote the recycling of metal catalysts. Through the fine-tuning of alternating current parameters, a series of conditions suitable for direct phosphonylation reactions of alkynes, olefins, aromatics and phosphite compounds were obtained, which greatly expanded the scope of application of a variety of C-H/P-H CROSS-COUPLING REACTIONS. At the same time, the method has the advantages of mild conditions, simple device structure, scale-up, and compatibility with a variety of transition metal catalysts. When the reaction is carried out under DC electrolysis conditions, a large number of metal catalysts are precipitated on the cathode, resulting in electrode contamination and catalyst inactivation, and the target product cannot be obtained efficiently, which means that AC electrolysis is not only a supplement to DC electrolysis, but a unique and efficient new electrolysis method.

Figure 2: Schematic diagram of asymmetric AC electrolysis principle and reaction mechanism

This study provides a general, simple and widely compatible new electrolysis strategy to solve the difficult pain points in transition metal catalytic electrosynthesis reactions. The efficient recycling of metal catalysts under AC electrolysis will greatly expand the application scenarios of transition metal catalytic reactions, and will help researchers and industry to develop more efficient, green and compatible electrosynthesis reactions. This work was supported by the National Key R&D Program of China (2021YFA1500104), the Key Project of the Natural Science Foundation of China (22031008), and the Postdoctoral Innovation Talent Support Program (BX2021225). (Source: Science Network)

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