Shining Visible Light on Vinyl Halides: Expanding the Horizons of Photocatalysis

ConspectusOver the past decade, photoredox catalysis has blossomed as a powerful methodology because of its wide applicability in sustainable free-radical-mediated processes, in which light is used as a cleaner energy source to alter the redox properties of organic molecules and to drive unique chemical transformations. Numerous examples of highly selective C-C and C-heteroatom bond formation processes have been achieved this way in an efficient and waste-reducing way. Therein, the activation of widely available organic halides via single-electron reduction has been broadly applied for organic synthesis. However, in comparison with alkyl and aryl halides, the analogous utilization of vinyl halides is less developed, most likely as a consequence of the highly unstable vinyl radicals generated as intermediates along with their strong tendency to abstract hydrogen atoms from a suitable source (e.g., the solvent), resulting in a synthetically less useful reduction.Nevertheless, during the last years, a number of photocatalytic processes involving vinyl halides have been developed, featuring the generation of vinyl radicals, diradicals, or radical cations as the key transient species. Moreover, photoredox processes in which a radical reacts with a vinyl halide or with an in situ-generated vinylmetal halide have been developed. Thus, identifying suitable conditions to generate and manipulate these reactive species has resulted in novel synthetic processes in a controllable manner. Moreover, in view of the great versatility of vinyl halides in palladium-catalyzed cross-coupling reactions, their activation by visible light might provide an attractive alternative to such processes, especially when non-noble metals could be used as photoinitiators in the future.In this Account, we discuss the various strategies of photoredox processes involving vinyl halides, classifying the material into four categories: (a) formation of a vinyl radical upon receipt of an electron from the photocatalyst, (b) formation of a radical cation after donation of an electron to the photocatalyst, (c) energy transfer corresponding to diradical formation upon triplet-triplet sensitization, and (d) dual transition metal and photocatalysis employing vinyl halides as precursors. While in the first three approaches the activation of vinyl halides is part of the photochemical step, the fourth one involves the interaction of a photochemically generated radical with a vinylnickel(II) halide obtained in turn by the oxidative addition of nickel(0) to the vinyl halide. Therefore, we highlight these important developments for conceptual comparison to the direct activation of vinyl halides by light, but they are not covered in depth in this Account.