Communication
Green Chemistry
tides can be easily converted into peptides under aqueous
conditions.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to the Deutsche Forschungsgemein-
schaft for the financial support (DFG-An 156 13-1). Open
Access funding provided by the Max Planck Society.
Scheme 6 Possible mechanism of the photocatalytic thioamide
synthesis.
Notes and references
tionate under the evolution of H2S. The presence of hydrogen
sulfide was confirmed as Ag2S by passing the gases evolved
during the reaction through the AgNO3 solution. The for-
mation of imine 3a from the benzylamine 1a and imine 5
seems to occur according to the previously reported mecha-
nism.37 The addition of H2S to the imine’s 3a CvN bond pro-
duces an intermediate α-aminothiol 7, as was postulated
earlier.30,31 Our results agree with these data. Thus, the inde-
pendently synthesized imine 3a did not give thioamide 2a
under identical photocatalytic conditions. Another observation
taken as evidence supporting this mechanism is that 2-methyl-
benzylamine 1l under the photocatalytic conditions gives only
imine 3l probably because of the steric hindrance of the
methyl groups that effectively shield CvN bond against H2S
addition (Scheme 5). Notably, the yield of thioamide 2a
decreases significantly when the reaction is accomplished in
an open reactor enabling H2S to escape from the reaction
medium. The oxidation of α-aminothiol 7 to thioamide 2a
using chemical oxidants such as molecular iodine or K2S2O8
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organic molecules, such as thioamides and even short chain
thiopeptides, from small molecules under very mild
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77–80.
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lated in good to excellent yields. Limitations of this reaction
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Green Chem.
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