Getting excited about cycloadditions

Abstract

Photoactivation to long-lived triplet excited states enables cycloadditions with heteroarenes Synthetic chemists use photochemistry to achieve challenging or unusual chemical transformations, but not all compounds are photoactive. Photosensitization is a process by which a molecule that is incapable of efficiently absorbing a particular wavelength of light directly is promoted to its triplet excited state (T1) by an intermolecular triplet energy transfer from a photosensitizer, which is a compound that ideally has a large extinction coefficient, rapid rate of intersystem crossing, and a long-lived T1. A particular advantage of photosensitization is that distinctive reactivity profiles not accessible through ground states become facile from corresponding excited states (1). The use of photosensitizers in chemical synthesis has paralleled the rise in popularity and use of various photoredox catalysts (2). On page 1338 of this issue, Ma et al. (3) report a photosensitized dearomative [4 + 2] cycloaddition that converts simple, unsaturated building blocks into products of increased molecular complexity using visible light.