Science of Synthesis

Amador, A. G.; Scholz, S. O.; Skubi, K. L.; Yoon, T. P., Science of Synthesis: Photocatalysis in Organic Synthesis, (2018) 1, 472.

Alkenes are known to undergo a variety of excited-state reactions, including [2 + 2] cycloadditions, which can occur from either the singlet or triplet state. In the latter case, inefficient rates of intersystem crossing and competitive singlet reactions can lead to poor selectivity and yield. Instead, one can employ a suitable triplet sensitizer, which can activate the alkene substrate through Dexter energy transfer. Classical triplet sensitizers include ketones such as acetophenone, benzophenone, or acetone. However, these catalysts require UV irradiation, limiting their practicality. Recent renewed interest in synthetic photochemistry has encouraged the study and development of visible-light-absorbing alternatives. Importantly, this approach generates the excited triplet alkene even though the substrate does not absorb visible light.