Vinyl radicals in transition metal-catalyzed organic transformations

Abstract

The vinyl radical is a destabilizing radical with hybridized orbitals in equilibrium between sp² and sp, exhibiting high reactivity and some electrophilicity. It was mentioned in seminal works published by Stork and Singer/Kong in the 20^th century. Due to its special structure and properties, the vinyl radical frequently acts as a strong single-electron donor, and subsequent diverse transformations have become the focus of research. Transition metal catalysts facilitate the generation of reactive radicals from radical sources or oxidizing radicals to cations via single electron transfer (SET) in the reaction, thereby promoting electron donor transfer and facilitating the transition to electron acceptors. In addition, the combination of the radical with the substrate and the subsequent cyclization process will be facilitated by displacement and coordination–insertion catalytic modes. The combination of radicals and transition metal catalysts yields functionalization products through vinylated metal complexes via oxidative addition and reductive elimination processes, which can be summarized as transmetallation catalytic modes. In recent years, research involving vinyl radicals in transition metal-catalyzed organic transformations has been steadily published. There have been numerous proposals to control the high activity of vinyl radicals through various methods to obtain different kinds of structural compounds (containing alkenyl and alkynyl compounds as well as polycyclic compounds, etc.). Therefore, in this review, we summarize the advancements in transition metal-catalyzed organic transformations involving vinyl radicals and provide a categorical overview based on the reaction types. These transformations are categorized as cyclization, substitution, elimination, oxidation, reduction, rearrangement, and oxidative addition reactions. Within each category, representative studies are presented, and discussed in terms of their general mechanistic perspectives when needed.