CH-Functionalization of Heterocycles with the Formation of C-O, C-N, C-S/Se, and C-P Bonds by Intermolecular Addition of Heteroatom-Centered Radicals

In the last decade, free radicals have found a wide application in functionalization of unsaturated compounds, such as alkenes, alkynes, and arenes, via the free-radical addition to carbon-carbon pi-bonds. In these processes, intermolecular free-radical attack on the aromatic substrates represents a challenge due to relatively high resistance of aromatic pi-system to addition reactions in comparison to alkene C=C bonds. The free-radical functionalization of heterocycles is especially interesting due to the diversity of their structures and chemical properties as well as their importance for medicinal chemistry, agrochemistry, and materials science. Addition of C-centered radicals to heterocycles is widely known as the Minisci-type reactions and well-reviewed. In this paper, we have summarized the main achievements in less explored group of processes: functionalization of heterocycles by intermolecular addition of heteroatom-centered radicals (O-, N-, S-/Se-, and P-radicals) with the emphasis on the papers published after 2010. Literature analysis revealed the strong trend towards the usage of electrochemistry and photoredox-catalysis for the generation of free radicals in recent years. The remaining fundamental problem in this field is the lack of strong experimental support for the proposed mechanisms and frequent existence of several plausible reaction pathways. The progress in mechanistic studies can significantly improve the prediction of optimal reaction conditions depending on the substrates structure.

Automated summary

In the last decade, free radicals have been widely used for the functionalization of unsaturated compounds, particularly heterocycles, through intermolecular addition of heteroatom-centered radicals. Recent literature analysis shows a trend towards the use of electrochemistry and photoredox-catalysis for free radical generation. However, there is a need for stronger experimental support and mechanistic studies to predict optimal reaction conditions.