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16 Photocatalytic Cycloadditions

DOI: 10.1055/sos-SD-229-00260

Amador, A. G.; Scholz, S. O.; Skubi, K. L.; Yoon, T. P.Science of Synthesis: Photocatalysis in Organic Synthesis, (20181467.

General Introduction

Photocycloaddition reactions rank among the most important transformations available through photochemical activation. Like all cycloaddition reactions, they can establish multiple new bonds and new stereocenters in a single operation, which makes them powerful tools for the rapid construction of molecular and stereochemical complexity. However, the unique reactivity profile of electronically excited intermediates can lead to distinctive product distributions, regioselectivities, and stereoselectivities that can differ dramatically from those of thermal cycloadditions. Thus, photochemical activation allows direct access to complex molecular structures that are often not available through other approaches. The success of these processes is often dependent on the method of photochemical activation. For example, excited-state organic intermediates generated via direct UV irradiation often suffer from unproductive singlet-state deactivation pathways such as fluorescence and E/Z isomerization that make these reactions relatively inefficient. In some cases, it is possible to access similar product classes utilizing photocatalytically generated radical ion, radical, and triplet excited-state intermediates. Classically, this approach exploited UV-activated organic photocatalysts such as benzophenone or anthracene-9,10-dicarbonitrile. Recent methodologies, however, have made use of photocatalysts that operate using visible irradiation. This chapter will highlight the most robust methodology in this area that makes use of visible-light-absorbing photocatalysts to promote cycloadditions, organized by the size of the ring formed. For a more comprehensive overview of modern photocatalysis, several general reviews have been recently published,[‌1‌,‌2‌] including applications in total synthesis.[‌3‌‌7‌]