10.1002/anie.201711467
Angewandte Chemie International Edition
COMMUNICATION
favored, even if a fairly large driving force of ~16 kcal/mol can be
estimated for that process based on the relevant bond
dissociation energies (BDEs) (Figure 1b).12 Instead, attack of
nucleophilic-aminoalkyl radicals at the S-H bond of MPA leads
to rapid formation of an electrophilic thiyl radical even though
this process is associated with much less thermodynamic driving
force.12 However, the subsequent reaction between electrophilic
thiyl radicals and nucleophilic ascorbate is much favored and
occurs in irreversible fashion, thereby suppressing undesired
reverse HAT.
electron-rich reactive radical intermediates in an efficient,
irreversible manner, and this concept should be broadly
applicable to other photoredox and radical processes. The
inherent value of reductive amination by photoredox catalysis
compared to traditional (thermal) methods was demonstrated by
labeling an activated cellulose surface with fluorescent
anthracene markers, showing that our method permits temporal
and spatial reaction control under irradiation with visible light.
The inherent value of the photoredox-catalyzed reductive
amination is further demonstrated by a photo-patterning reaction
on activated cellulose support, illustrating the so far unique
possibility of performing reductive amination with temporal and
spatial control. A filter paper was treated with NaIO4 to expose
aldehyde groups (Figure 2), and then it was layered with a
methanol solution of 9-(aminomethyl)-anthracene (1 mM),
[Ru(bpy)3]Cl2 (1 mM), ascorbic acid (20 mM), and MPA (100
mM). A collimated LED lamp providing 455 nm light that passed
through a hand-made mask was employed for photo-irradiation
during 1 hour (SI page S16). The mask ensured illumination of
only selected parts of the activated filter paper. After removing
the filter paper from the solution and washing it with brine and
hydroxylamine hydrochloride solution, the anthracene-labeled
zones of the filter paper can easily be seen under UV irradiation
(Figure 2, bottom left) due to anthracene fluorescence. We
believe this new methodology of substrate immobilization
enabled by visible light with both temporal and spatial control
could be useful for a variety of applications, for example in
biochemical contexts (biochips, biosensors, etc).13
Acknowledgements
Funding from the Swiss National Science Foundation through
the NCCR Molecular Systems Engineering is gratefully
acknowledged.
Keywords: photocatalysis • photochemistry • amination •
hydrogen transfer • immobilization
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Figure 2. Photo-patterning of a cellulose support (filter paper) with visible light,
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