Shining Fresh Light on Complex Photoredox Mechanisms through Isolation of Intermediate Radical Anions.

Abstract

Photoredox catalysis (PRC) has gained enormous and wide-ranging interest in recent years but has also been subject to significant mechanistic uncertainty, even controversy. To provide a method by which the missing understanding can begin to be filled in, we demonstrate herein that it is possible to isolate as authentic materials the one-electron reduction products of representative PRC catalysts (PCs). Specifically, KC8 reduction of both 9,10-dicyanoanthracene and a naphthalene monoamide derivative in the presence of a cryptand provides convenient access to the corresponding [K(crypt)+][PC·-] salts as clean materials that can be fully characterized by techniques including EPR and XRD. Because PC·- states are key intermediates in PRC reactions, such isolation allows for highly controlled study of these anions' specific reactivity and hence their mechanistic roles. As a demonstration of this principle, we show that these salts can be used to conveniently interrogate the mechanisms of recent, high-profile "conPET" and "e-PRC" reactions, which are currently the subject of both significant interest and acute controversy. Using very simple experiments, we are able to provide striking insights into these reactions' underlying mechanisms and to observe surprising levels of hidden complexity that would otherwise have been very challenging to identify and that emphasize the care and control that are needed when interrogating and interpreting PRC mechanisms. These studies provide a foundation for the study of a far broader range of questions around conPET, e-PRC, and other PRC reaction mechanisms in the future, using the same strategy of PC·- isolation.