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.