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tion-metal reductions are employed in detosylation reactions36–38
.
A variety of electronically diverse tosylated aniline derivatives were
smoothly converted to the desired free anilines in moderate-to-excel-
lent yield. Interestingly, substrates containing aryl halides were toler-
ated under the reaction conditions. As this reaction is conducted at
a much lower concentration of substrate compared to the reductive
dehalogenation method (0.1 M versus 0.5 M), the observed lack of aryl
halide reduction may be a function of concentration. Esters (43, 73),
free carboxylic acids (44), ketones (48) and free alcohols (58) were
tolerated under the reaction conditions, showing the high functional-
group tolerance of this method relative to methods relying on harsh
dissolving-metal conditions. Benzylic (52) and secondary alkyl amines
(45, 53, 65–68) were efficient substrates for this transformation as
well. Medicinally relevant heterocycles—including pyridines (59),
indoles (58), pyrroles (62), pyrrolidines (67), indazoles (63), benzi-
midazoles (64) and morpholines (65)—were deprotected in good-to-
excellent yields, with no reduction of the aromatic system observed
in all cases. Of note is the ability of this method to chemoselectively
and efficiently deprotect tosyl amines over mesyl-protected amines,
as shown by the reaction of substrate 51, yielding the desired deto-
sylation product in 61% yield with no observed cleavage of the mesyl
amine. Additionally, the reaction performed well with 1.28 g of starting
tosylamine, with substrate 64 giving 92% yield when the desired deto-
sylation was conducted in a standard round-bottom flask irradiated
with light-emitting diode lamps (see Supplementary Information for
experimental details).
In conclusion, an acridine radical generated in situ from single-
electron reduction of an acridinium derivative may act as a potent
single-electron reductant upon excitation with 390-nm light. Spectro-
scopic and computational investigations indicate the formation of at
least two distinct excited states, one of which may be characterized as
a TICT state. The development of chemoselective dehalogenation and
desulfonylation reactions using Mes-Acr• complement the well known
oxidative chemistry associated with acridinium salts and highlight
the potential for the development of other types of reaction based on
excitation of organic radicals.
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