Polyaniline-Induced C-H Arylation of Arenes with Arenediazonium Salts

Photochemical acridone-mediated direct arylation of (hetero)arenes with aryl diazonium salts

A metal-free photochemical C-H direct arylation using acridone as a photoredox catalyst to facilitate the reaction is described. Diazonium salts as precursors for aryl radicals, demonstrated by a fluorescence quenching experiment and free radical trapping experiment, allow the functionalization of (hetero)arenes under mild conditions. A series of valuable substituted biaryl and aryl-heteroaryl compounds were prepared in moderate to good yields via the coupling. Moreover, this methodology is shown to be applicable to scale-up synthesis.

Recent advances in electrochemical functionalization using diazonium salts

Arenediazonium salts have gained attention in the scientific community due to their numerous synthetic applications. In the traditional method of dediazoniation of arenediazonium salts, the requirements for toxic oxidants and costly catalysts affect their cost-effectiveness and sustainability. However, recent advances in synthetic organic electrochemistry allow for the in situ reduction of arenediazonium salts, affording different functionalizations under mild reaction conditions and with a shorter reaction time. Herein, we report advances up to now of facile organic electrochemical syntheses using arenediazonium salt precursors that avoid the use of hazardous reductants.

Synergism between few-layer black phosphorus and graphitic carbon nitride enhances the photoredox C–H arylation under visible light irradiation

A volcano-shaped relation between the amount of FLBP in the FLBP/g-CN heterojunctions in the photoredox C–H arylation was exhibited. To understand the activity of 35 wt% FLBP/g-CN, band alignments of heterojunction was studied in detailed.

Real Metal-Free C–H Arylation of (Hetero)arenes: The Radical Way

Synthetic methodologies involving the formation of carbon–carbon bonds from carbon–hydrogen bonds are of significant synthetic interest, both for efficiency in terms of atom economy and for their undeniable usefulness in late-stage functionalization approaches. Combining these aspects with being metal-free, the radical C–H intermolecular arylation procedures covered by this review represent both powerful and green methods for the synthesis of (hetero)biaryl systems.

1 Introduction

2 Arylation with Arenediazonium Salts and Related Derivatives

2.1 Ascorbic Acid as the Reductant

2.2 Hydrazines as Reductants

2.3 Gallic Acid as the Reductant

2.4. Polyanilines as Reductants

2.5 Chlorpromazine Hydrochloride as the Reductant

2.6 Phenalenyl-Based Radicals as Reductants

2.7 Electrolytic Reduction of Diazonium Salts

2.8 Visible-Light-Mediated Arylation

3 Arylation with Arylhydrazines: Generation of Aryl Radicals Using an Oxidant

4 Arylation with Diaryliodonium Salts

5 Arylation with Aryl Halides

6 Conclusions

Photochemical (Hetero-)Arylation of Aryl Sulfonium Salts

The construction of (hetero)biaryls, which are ubiquitous scaffolds among medical substances, functional materials, and agrochemicals, constitutes a key application of cross-coupling methods. However, these usually require multiple synthetic steps. Herein, we report a simple photoinduced and catalyst-free C–H/C–H (hetero)arylation cross-coupling through aryl thianthrenium salts, which are formed site-selectively by direct C–H functionalization. The key to this approach is the UV-light, which can disrupt the C–S bond to form thianthrene radical cations and aryl radicals.

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.

Radical Arylation of Anilines and Pyrroles via Aryldiazotates

The radical arylation of anilines and pyrroles can be achieved under transition-metal- and catalyst-free conditions by using aryldiazotates in strongly alkaline aqueous solutions. The aryldiazotates act as protected diazonium ions, which do not undergo azo coupling with electron-rich aromatic substrates, but can still serve as an aryl radical source at slightly elevated temperatures. Based on an improved preparation of aryldiazotates in aqueous solution, homolytic aromatic substitutions of anilines and pyrroles were conducted with good overall yields and high regioselectivity. Moreover, DFT calculations provided further mechanistic insights.

Carbocatalytic reductive coupling reactions via electron transfer from graphene to aryldiazonium salt

A reductive coupling reaction using two-dimensional nanocarbon, i.e., reduced graphene oxide (rGO), as a carbocatalyst and/or a reaction initiator was developed.

Radical C–H arylations of (hetero)arenes catalysed by gallic acid

Gallic acid is able to catalyse radical C–H arylations of (hetero)arenes with diazonium salts at room temperature in water–acetone. As gallic acid is present in multiple bio-wastes, this methodology unleashes an alternative method for waste reutilization.