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16.2 [2 + 2] 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, (20181472.

The [2 + 2] cycloaddition is one of the most well-known photochemical reactions. Thermally forbidden by Woodward–Hoffman rules, this transformation can instead occur from electronically excited states, providing cyclic structures that are challenging to construct with other strategies. Classical methods for [2 + 2] cycloaddition include either direct excitation of the substrate, or catalysis with a suitable photosensitizer. Alternatively, photoinduced electron transfer (PET) can provide open-shell species that undergo the same net transformation, though by a different mechanism.[‌12‌] High-energy UV irradiation is often required for these reactions, which renders them incompatible with photolytically labile functional groups. The substrates employed are also generally cyclic (to avoid unproductive E/Z isomerization) and react intramolecularly (to ensure that cycloaddition occurs faster than relaxation to the ground state). This section will focus upon catalytic methods that operate under visible-light irradiation, and will highlight recent advances that address some of the synthetic shortcomings of previous protocols.