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

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.