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, KC 8 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.

Complexation‐Triggered Fluctuation of π‐Conjugation on an Antiaromatic Dicyanoanthracene Dianion

“…Bathochromically shifted to that of the borane‐free DCA radical anion ( λ max =710 nm) [33] indicated smaller energy gap between the SOMO and LUMO due to the presence of the borane functionality. A recent report by Scott on structurally defined DCA radical anions also supports these results [34] . A similar bathochromic shift in the absorption maxima was observed for the neutral borane adduct of DCA 1 (490 nm) compared to that of DCA (420 nm).…”

“…Full experimental data including spectra, crystallographic and computational data are given in the Supporting Information. The authors have also cited additional references within the Supporting Information [47–63] …”

A Hierarchy of Ligands Controls Formation and Reaction of Aryl Radicals in Pd-Catalyzed Ground-State Base-Promoted Coupling Reactions

“…In this regard, phosphines are excellent radical traps. Radical addition to a phosphorus atom would give hypervalent phosphoranyl radicals − that are reasonably strong electron donors; indeed, some phosphoranyl radicals (derived from phosphites) have been proposed to undergo electron transfer to iodobenzenes . [Computational calculations show that the transition states for such reactions might be achievable (see the SI). ]…”

Ketyl Radical Coupling Enabled by Polycyclic Aromatic Hydrocarbon Electrophotocatalysts

“…In many of these systems, radical ions are invoked as active photocatalysts. A variety of mechanisms have been proposed to reconcile the photoredox activity of these species with the expected short doublet excited state lifetimes, , including precomplexation, SET to solvent, and further catalyst transformation to closed shell analogs …”