Abstract
3,3′,5,5’-Tetramethyl-2,2′-biphenol is well known as an outstanding building block for ligands in transition-metal catalysis and is therefore of particular industrial interest. The electro-organic method is a powerful, sustainable, and efficient alternative to conventional synthetic approaches to obtain symmetric and non-symmetric biphenols. Here, we report the successive scale-up of the dehydrogenative anodic homocoupling of 2,4-dimethylphenol (4) from laboratory scale to the technically relevant scale in highly modular narrow gap flow electrolysis cells. The electrosynthesis was optimized in a manner that allows it to be easily adopted to different scales such as laboratory, semitechnical and technical scale. This includes not only the synthesis itself and its optimization but also a work-up strategy of the desired biphenols for larger scale. Furthermore, the challenges such as side reactions, heat development and gas evolution that arose during optimization are also discussed in detail. We have succeeded in obtaining yields of up to 62% of the desired biphenol.
Highlights
Biphenols are an important structural motif in organic chemistry as they are part of many natural products and pharmaceuticals [1,2,3,4,5,6,7,8]
The yield was successfully increased up to 60% by using graphite as anode material and 1,1,1,3,3,3hexafluoropropan-2-ol (HFIP) as solvent [71]. This example shows the specific influence of HFIP, which is known for its Scheme 2 Electrochemical synthesis of 3,3′,5,5’-tetramethyl2,2′-biphenol (5) by direct dehydrogenative oxidative homocoupling of 2,4dimethylphenol (4)
The continuous process of flow chemistry offers many advantages for scale-up, whereas batch processes are limited to a few liters’ reaction volume [84,85,86,87,88,89,90,91,92,93,94]. It simplifies the heat management and the surfaceto-volume ratio of the electrode is superior compared to batch electrolysis. In this manuscript we show the scale-up of the electrochemical synthesis of 5 in the flow cell up to technical scale
Summary
Biphenols are an important structural motif in organic chemistry as they are part of many natural products and pharmaceuticals [1,2,3,4,5,6,7,8]. The yield was successfully increased up to 60% by using graphite as anode material and 1,1,1,3,3,3hexafluoropropan-2-ol (HFIP) as solvent [71] This example shows the specific influence of HFIP, which is known for its Scheme 2 Electrochemical synthesis of 3,3′,5,5’-tetramethyl2,2′-biphenol (5) by direct dehydrogenative oxidative homocoupling of 2,4dimethylphenol (4). The continuous process of flow chemistry offers many advantages for scale-up, whereas batch processes are limited to a few liters’ reaction volume [84,85,86,87,88,89,90,91,92,93,94] It simplifies the heat management and the surfaceto-volume ratio of the electrode is superior compared to batch electrolysis. In this manuscript we show the scale-up of the electrochemical synthesis of 5 in the flow cell up to technical scale
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