RESUMO
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.
RESUMO
The tricyclic aromatic ketone 9-anthrone and its derivatives are under basic conditions in equilibrium with their corresponding anionic forms. Unlike the neutral species, the 9-anthrolate anions can be excited by blue LED light and thus, are able to initiate a photoinduced electron transfer (PET) reaction. To demonstrate the synthetic applicability of the catalytic system, various (hetero)aryl chlorides were converted in C-C and C-Het bond-forming reactions affording the corresponding arylation products in moderate to excellent yields. The reactions proceed under very mild conditions without the need for a sacrificial electron donor. Besides 9-anthrone, other closely related derivatives were synthesised and investigated concerning their ability to catalyse demanding reductive transformations. Based on spectroscopic findings and radical trapping experiments a conceivable mechanism is proposed.