Mild-Base-Promoted Arylation of (Hetero)Arenes with Anilines

In situ Diazonium Salt Formation and Photochemical Aryl-Aryl Coupling in Continuous Flow Monitored by Inline NMR Spectroscopy

A novel class of diazonium salts is introduced for the photochemical aryl-aryl coupling to produce (substituted) biphenyls. As common diazonium tetrafluoroborate salts fail, soluble and safe aryl diazonium trifluoroacetates are applied. In this mild synthesis route no catalysts are required to generate an aryl-radical by irradiation with UV-A light (365 nm). This reactive species undergoes direct C-H arylation at an arene, forming the product in reasonable reaction times. With the implementation of a continuous flow setup in a capillary photoreactor 13 different biphenyl derivatives are successfully synthesized. By integrating an inline 19F-NMR benchtop spectrometer, samples are reliably quantified as the fluorine-substituents act as a probe. Here, real-time NMR spectroscopy is a perfect tool to monitor the continuously operated system, which produces fine chemicals of industrial relevance even in a multigram scale.

Recent Advances in the Transition-Metal-Free Arylation of Hetero­arenes

Transition-metal-free arylation reactions have attracted considerable attention for economic and environmental reasons over the past 40 years. In recent years, much effort has been made to develop efficient transition-metal-free approaches for the arylation of heteroarenes. Covering the literature from 2015 to early 2021, this review aims to provide a thorough overview of the synthetic and mechanistic aspects of these atom-economical and environmentally benign reactions.

1 Introduction

2 Arylation of Pre-functionalized Heteroarenes

2.1 Arylation of Heteroaryl Halides

2.2 Decarboxylative Arylation of Heteroarenes

3 Direct C–H Arylation of Heteroarenes

3.1 C(sp2)–H Arylation

3.2 C(sp3)–H Arylation

4 N-Arylation of Heteroarenes

5 Summary and Outlook

Generation of aryl radicals by redox processes. Recent progress in the arylation methodology

Arylation methods based on the generation and use of aryl radicals have been a rapidly growing field of research in recent years and currently represent a powerful strategy for carbon – carbon and carbon – heteroatom bond formation. The progress in this field is related to advances in the methods for generation of aryl radicals. The currently used aryl radical precursors include aryl halides, aryldiazonium and diaryliodonium salts, arylcarboxylic acids and their derivatives, arylboronic acids, arylhydrazines, organosulfur(II, VI) compounds and some other compounds. Aryl radicals are generated under mild conditions by single electron reduction or oxidation of precursors induced by conventional reagents, visible light or electric current. A crucial role in the development of the radical arylation methodology belongs to photoredox processes either catalyzed by transition metal complexes or organic dyes or proceeding without catalysts. Unlike the conventional transition metal-catalyzed arylation methods, radical arylation reactions proceed very often at room temperature and have high functional group tolerance. Without claiming to be exhaustive, this review covers the most important advances of the current decade in the generation and synthetic applications of (het)aryl radicals. Examples of reactions are given and mechanistic insights are highlighted.

The bibliography includes 341 references.

Biaryl synthesis with arenediazonium salts: cross-coupling, CH-arylation and annulation reactions

The rich legacy of arenediazonium salts in the synthesis of unsymmetrical biaryls, built around the seminal works of Pschorr, Gomberg and Bachmann more than a century ago, continues to make important contributions at various evolutionary stages of modern biaryl synthesis. Based on in-depth mechanistic analysis and design of novel pathways and reaction conditions, the scope of biaryl synthesis with arenediazonium salts has enormously expanded in recent years through applications of transition metal/photoredox-catalysed cross-coupling, thermal/photosensitized radical chain CH-arylation of (hetero)arenes and arylative radical annulation reactions with alkynes. These recent developments have provided facile synthetic access to a wide variety of unsymmetrical biaryls of pharmaceutical, agrochemical and optoelectronic importance with green scale-up options and created opportunities for late-stage modification of peptides, nucleosides, carbon nanotubes and electrodes, the details of which are captured in this review.

Aromatic substitutions of arenediazonium salts via metal catalysis, single electron transfer, and weak base mediation

Radical aromatic substitutions of arenediazonium salts can be initiated by various methods. The recent developments of weak base-mediated protocols provide great advantages over conventional metal-mediated or photoredox reactions by their operational simplicity, price, hazard potential and scalability.