Synfacts 2020; 16(06): 0701
DOI: 10.1055/s-0040-1706943
Organo- and Biocatalysis
© Georg Thieme Verlag Stuttgart · New York

Photoexcitation of an Acridine Radical Generates a Strong Organic Single-Electron Reductant

Benjamin List
Manuel J. Scharf
MacKenzie IA, Wang L, Onuska NP. R, Williams OF, Begam K, Moran AM, Dunietz BD, Nicewicz DA. * University of North Carolina at Chapel Hill, USA
Discovery and Characterization of an Acridine Radical Photoreductant.

Nature 2020;
580: 76-80
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15. Mai 2020 (online)



Nicewicz and co-workers report the characterization and utilization of an acridine radical derived from the common photoredox-catalyst Mes-Acr-BF4 . The stable radical was investigated experimentally and computationally in terms of its photochemical and redox properties. Upon irradiation, two distinct excited states are populated: a doublet state (D1) and a twisted intramolecular charge-transfer state (TICT) in which the N-phenyl ring is twisted to allow charge transfer to the central acridine unit. The reduction potential of this compound was found to be as high as –3.36 V (vs. saturated calomel electrode), which is comparable to that of main-group alkali metals.



The described Mes-Acr radical could be generated in situ from Mes-Acr-BF4 through the use of N,N-diisopropylethylamine as a single-electron reductant. By applying this strategy, the photocatalytic cleavage of aromatic C–Cl and N–Ts bonds could be accomplished, the latter of which is usually performed by using strongly reducing alkali metals. The presented photocatalytic method is thus advantageous in terms of experimental convenience, functional-group tolerance, and atom economy. This work makes one question the inherent advantages associated with metals in organic chemistry, as well as the future limitations in reactivity for small-molecule organic catalysts.