Excited State Anions in Organic Transformations

Abstract

Utilizing light is a smart way to fuel chemical transformations as it allows the energy to be selectively focused on certain molecules. Many reactions involving electronically excited species proceed via open‐shell intermediates, which offer novel and unique routes to expand the hitherto used synthetic toolbox in organic chemistry. The direct conversion of non‐prefunctionalized, less activated compounds is a highly desirable goal to pave the way towards more sustainable and atom‐economic chemical processes. Photoexcited closed‐shell anions have been shown to reach extreme potentials in single electron transfer reactions and reveal unusual excited‐state reactivity. It is, therefore, surprising that their use as a reagent or photocatalyst is limited to a few examples. In this Review, we briefly discuss the characteristics of anionic photochemistry, highlight pioneering work, and show recent progress which has been made by utilizing photoexcited anionic species in organic synthesis.

How Radical are ‘Radical’ Photocatalysts? A Closed-Shell Meisenheimer Complex is Identified as a Super-reducing Photoreagent

“…12,17,37 Such closed-shell anthrolate anions are known to be competent photocatalysts for very similar photoreductions of chloroarenes. [38][39][40] In summary, the ultrafast relaxation resulting from internal conversion of doublet excited states to a doublet ground states suggests that synthetically useful photochemically active radical species would be extraordinary, as opposed to their reaction to produce closed shell species in which photoredox processes are driven from the more common singlet/triplet lowest energy excited state manifold.…”

Recent advances in visible light-activated radical coupling reactions triggered by (i) ruthenium, (ii) iridium and (iii) organic photoredox agents

“…There is a vast array of metal 9,31 and organic 8 photoredox catalysts with different excited state redox potentials. The redox potential of metal catalysts can be altered with the selection of ligands around the metal centre, with more electron-poor ligands resulting in more oxidising catalysts and more electron-rich ligands giving better reducing catalysts.…”

“…Our aim here is to provide an overview of some of the areas of rapid development that deploy (i) transition metal photoredox reagents based on ruthenium and iridium complexes or (ii) organic photoredox catalysts. 9…”

Sensitization-initiated electron transfer via upconversion: mechanism and photocatalytic applications

“…recent mechanistic studies of biphotonic excitation in photoredox catalysis, 2,6,8,18,[28][29][30][33][34][35][36][37][38][39][40][41][42] and this seems important for the further rational development of this thriving research area.…”