Selective and Scalable Dehydrogenative Electrochemical Synthesis of 3,3′,5,5′-Tetramethyl-2,2′-biphenol

Abstract

3,3′,5,5′-Tetramethyl-2,2′-biphenol is a compound of high technical significance, as it exhibits superior properties as building block for ligands in the transition-metal catalysis. However, side reactions and overoxidation are challenging issues in the conventional synthesis of this particular biphenol. Here, an electrochemical method is presented as powerful and sustainable alternative to conventional chemical strategies, which gives good yields up to 51%. Despite using inexpensive and well-available bromide-containing supporting electrolytes, the issue of bromination and general byproduct formation is effectively suppressed by adding water to the electrolyte. Additionally, the scalability of this method was demonstrated by conducting the electrolysis on a 122 g scale.

Electrosynthesis of 3,3′,5,5’-Tetramethyl-2,2′-biphenol in Flow

“…In this section the development of the electrosynthesis of 5 in a flow electrolysis cell scaled-up to the technical scale is reported. On basis of the scaled beaker-type cell electrolysis (Scheme 3) [82], a successive approach was chosen to convert the synthesis into a flow process: For the transfer of the reaction from a batch to a flow process the 2 cm× 6 cm flow electrolysis cell developed for laboratory scale was used [21]. The electrolysis was than scaled-up.…”

“…For the transfer of the electrosynthesis of 5 in the 2 cm × 6 cm flow electrolysis cell, the already optimized conditions of the beaker-type cell electrolysis were chosen (Scheme 3): [82] The electrolysis of 4 was carried out in HFIP with 15 vol.% of water and the use of tetraethylammonium bromide (Et 4 NBr) as supporting electrolyte at a temperature of 50°C. Et 4 NBr was chosen as supporting electrolytes because tetraalylammonium bromides generally give the best yield of 5 in this system and is the most inexpensive compound of the tested supporting electrolytes.…”

“…Based on the results from the investigated batch electrolysis [82], initial experiments were carried out in the 2 cm × 6 cm flow electrolysis cell (Fig. 1).…”

“…The first steps in this direction have already been successfully accomplished. We could already publish first results in the beaker-type cell using glassy carbon electrodes and very common and inexpensive bromide-containing supporting electrolytes [61,82]. In addition, a supporting electrolyte-free method was developed as flow electrolysis and scaled-up to the semi-technical scale using the 4 cm × 12 cm electrolysis cell equipped with a glassy carbon anode and a stainless steel cathode.…”

Electrosynthesis 2.0 in 1,1,1,3,3,3‐Hexafluoroisopropanol/Amine Mixtures

“…[54] The synthesis of this particular structural motif either requires economically and ecologically unfavorable transition metal catalysis or can be performed in an electro-organic transformation which requires supporting electrolytes. [55] The use of HFIP is vital to avoid undesired CÀ O coupling reactions and formation of polycyclic products. [56] The electrochemical synthesis of 44 was major research topic within the Waldvogel group [57] because of the technical relevance.…”