Supporting-Electrolyte-Free and Scalable Flow Process for the Electrochemical Synthesis of 3,3′,5,5′-Tetramethyl-2,2′-biphenol

An Electrochemical Oxo-amination of 2H-Indazoles: Synthesis of Symmetrical and Unsymmetrical Indazolylindazolones

We have established a supporting-electrolyte free electrochemical method for the synthesis of indazolylindazolones through oxygen reduction reaction (eORR) induced 1,3-oxo-amination of 2H-indazoles where 2H-indazole is used as both aminating agent as well as the precursor of indazolone. Moreover, we have merged indazolone and indazole to get unsymmetrical indazolylindazolones through direct electrochemical cross-dehydrogenative coupling (CDC). This exogenous metal-, oxidant- and catalyst-free protocol delivered a number of multi-functionalized products with high tolerance of diverse functional groups.

Electrochemical C-H Sulfonylation of Hydrazones

We have developed an electrochemical method for the direct C-H sulfonylation of aldehyde hydrazones using sodium sufinates as the sulfonylating agent under supporting electrolyte-free conditions. This straightforward sulfonylation strategy afforded a library of (E)-sufonylated hydrazones with high tolerance of various functional groups. The radical pathway of this reaction has been revealed by the mechanistic studies.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Flow Electrosynthesis of Sulfoxides, Sulfones, and Sulfoximines without Supporting Electrolytes

An efficient electrochemical flow process for the selective oxidation of sulfides to sulfoxides and sulfones and of sulfoxides to N-cyanosulfoximines has been developed. In total, 69 examples of sulfoxides, sulfones, and N-cyanosulfoximines have been synthesized in good to excellent yields and with high current efficiencies. The synthesis was assisted and facilitated through a supporting electrolyte-free, fully automated electrochemical protocol that highlights the advantages of flow electrolysis.

Evaluating the Green Credentials of Flow Chemistry towards Industrial Applications

Continuous flow chemistry is becoming an established technology platform that finds frequent application in industrial chemical manufacture with support and endorsements by the FDA for pharmaceuticals. Amongst the various advantages that are commonly cited for flow chemistry over batch processing, sustainability continues to require further advances and joint efforts by chemists and chemical engineers in both academia and industry. This short review highlights developments between 2015 and early 2021 that positively impact on the green credentials associated with flow chemistry, specifically when applied to the preparation of pharmaceuticals. An industrial perspective on current challenges is provided to whet discussion and stimulate further investment towards achieving greener modern synthetic technologies.

1 Introduction

2 Subject Areas and Relevant Case Studies

3 Industrial Outlook on Future Sustainability Driven through Continuous Manufacturing Approaches

4 Conclusions and Outlook

Reproducibility in Electroorganic Synthesis-Myths and Misunderstandings

The use of electric current as a traceless activator and reagent is experiencing a renaissance. This sustainable synthetic method is evolving into a hot topic in contemporary organic chemistry. Since researchers with various scientific backgrounds are entering this interdisciplinary field, different parameters and methods are reported to describe the experiments. The variation in the reported parameters can lead to problems with the reproducibility of the reported electroorganic syntheses. As an example, parameters such as current density or electrode distance are in some cases more significant than often anticipated. This Minireview provides guidelines on reporting electrosynthetic data and dispels myths about this technique, thereby streamlining the experimental parameters to facilitate reproducibility.

Developments in the Dehydrogenative Electrochemical Synthesis of 3,3',5,5'-Tetramethyl-2,2'-biphenol

The symmetric biphenol 3,3′,5,5′‐tetramethyl‐2,2′‐biphenol is a well‐known ligand building block and is used in transition‐metal catalysis. In the literature, there are several synthetic routes for the preparation of this exceptional molecule. Herein, the focus is on the sustainable electrochemical synthesis of 3,3′,5,5′‐tetramethyl‐2,2′‐biphenol. A brief overview of the developmental history of this inconspicuous molecule, which is of great interest for technical applications, but has many challenges for its synthesis, is provided. The electro‐organic method is a powerful, sustainable, and efficient alternative to conventional synthesis to obtain this symmetric biphenol up to the kilogram scale. Another section of this article is devoted to different process management strategies in batch‐type and flow electrolysis and their respective advantages.

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

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.