Outstandingly robust anodic dehydrogenative aniline coupling reaction
Graphical abstract
Introduction
In recent years, electrochemistry has again gained significant importance, in research as well as in industrial processes. Especially, electrosynthetic transformations became more and more popular in developing green and sustainable processes for synthesizing a broad variety of substance classes [1,2]. Because of the mild reaction conditions, electrochemistry benefits from its high functional group tolerance and chemoselective transformations. Furthermore, electrochemistry exhibits a better atom economy than conventional organic synthesis and is therefore remarkably progressive in terms of avoiding reagent waste. In particular, in times of advancing climate change and dwindling resources, it is becoming increasingly important to strive for alternatives to existing, often polluting and outdated processes for a more sustainable use of existing resources. However, renewable energy production from wind power or photovoltaics often generate large surplus electricity, which requires storage in order to compensate a higher demand at a later point (Fig. 1).
This requires more efficient energy storage in order to compensate the differing supply and demand for electricity. In addition to large mechanical storage devices, such as pumped hydro power plants, the storage of electrical energy in chemical bonds is also feasible. The energy is therefore stored in substances that can later serve as energy sources or chemicals. In this context, the electrolysis of water to hydrogen and oxygen is often mentioned. However, the direct use of surplus electricity for the production of basic or fine chemicals is also an attractive method for energy valorization [3,4]. In particular, the electrochemical C,C bond formation between aromatic subunits is an attractive method for the preparation of valuable fine chemicals [5].
Though constant-potential experiments exhibit a high selectivity by specifically addressing one substrate’s potential, the major drawback is the continuously decreasing current because of depletion of the starting material in the course of conversion. This leads to a prolonged electrolysis, if a complete conversion is desired. If the electrosynthesis is conducted in a constant-current mode, only a simple two-electrode setup is needed. In this case, no expensive reference electrode and sophisticated electronic periphery is required. In addition, large quantities of starting material can be converted with an excellent space-time-yield. Especially for technical applications, the basic cell design with an affordable current source and the use of high current densities is highly attractive. Usually, the majority of electrosynthetic processes are rather limited to low current densities of 0.5–5 mA/cm2 or less and are restricted to a rather small current density window [[6], [7], [8]]. Exceeding this window leads to emerging side reactions or no desired conversion at all. High current density is only used for conversions where electrochemically resilient products are formed like in the Kolbe electrolysis or Monsanto’s Baizer process [9,10]. Future applications of electrosynthesis for energy storage will need robust transformations that can easily digest fluctuations in the electric power used. In our group it was found that the electrosyntheses of (partially protected) biphenols and m-terphenyl-2,2″-diols is unexpectedly robust and can be conducted in a broad range of current density [11]. Other attempts to use high electric currents are e.g. the use of macroporous electrodes to maximize the available electrode surface [[12], [13], [14], [15], [16]], electrolysis in biphasic systems [17] or utilization of mediating systems in equimolar quantity [18]. Here we report the extraordinary electrochemical synthesis of 2,2′-diaminobiaryls and meta-terphenyl amines by anodic coupling of aniline and benzidine derivatives. Such amino-substituted bi- or teraryls are highly valuable building blocks for organocatalysts, ligands, functionalized materials or biologically active molecules [[19], [20], [21]]. In conventional chemistry these substance classes are synthesized via multi-step or transition metal-catalyzed processes that have a poor atom economy and generate large amounts of reagent waste [[22], [23], [24], [25], [26]]. The electrochemical protocol provides the desired products in good yields and selectivity without the need of pre-functionalized substrates and is therefore remarkably advanced in sustainability.
Section snippets
Experimental procedure for electroorganic homo-coupling reaction
If not stated otherwise, the following standard reaction parameters were employed in the electrolyses:
For each reaction, the respective substrate A (0.75 mmol) and methyltributylammonium methyl sulfate (MTBS, 0,5 mmol, 154 mg) were dissolved in HFIP (5.0 mL). Undivided Teflon electrolysis cells (self-made [27] or purchased from IKA-Werke GmbH & Co KG, Staufen, Germany) equipped with glassy carbon (Sigradur® G, obtained from HTW Hochtemperaturwerkstoffe GmbH, Thierhaupten, Germany), boron-doped
Optimization of the homo-coupling of N-(3,4-Dimethylphenyl)formamide
The anodic synthesis of symmetric and non-symmetric 2,2′-diaminobiaryls by direct electrolysis of protected anilines as simple starting materials was already established [28,29]. No leaving groups or expensive catalysts are required and only a simple two-electrode arrangement in a beaker-type cell is needed for this exceptional conversion. The best yield and selectivity are obtained in 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). Studies in our group revealed that HFIP has the outstanding ability
Conclusions
Using the exceptional solvent properties of HFIP-based electrolytes allows both, the selective cross-coupling of protected benzidine derivatives with anilines and the application of very high electron densities up to 100 mA/cm2 for the homo-coupling of N-(3,4-dimethylphenyl)formamide. Thanks to HFIP’s outstanding performance, the conversion of aniline derivatives is feasible within a broad range of current density and therefore easily adaptable to fluctuating renewable energy sources because
CRediT authorship contribution statement
Lara Schulz: Investigation, Writing - original draft. Jan-Åke Husmann: Visualization, Investigation. Siegfried R. Waldvogel: Supervision, Conceptualization, Writing - review & editing.
Acknowledgements
S.R.W. thanks the DFG (WA1276/17–1) for funding.
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