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
Key words
photocatalysis - photoreduction - visible light - acridine radical - single-electron
reduction
Significance
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.
Comment
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.
