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production pathway, were known to form the imines from the
amines in the presence of molecular oxygen.[12f] To evaluate the
generation of singlet oxygen, we also measured the changes of
absorption spectra of 1,3-diphenylisobenzofuran (DPBF), which
is known to react with singlet oxygen, in the presence of
Ir(imp)(ppy) 1 upon irradiation. When the acetonitrile solution of
DPBF (25 μmol/L) containing Ir(impy)(ppy) 1 (1.0 μmol/L) was
irradiated with blue LED, the absorption of DPBF decreased with
the blue LED irradiation (Figure S2). This result strongly
suggests that the present visible-light induced oxidative coupling
reaction involves the generation of singlet oxygen.
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Recently, the three component Ugi-type reaction including the
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isocyanide and carboxylic acid has been reported.13 Thus, we also
demonstrated the same reaction using our photocatalytic system.
As a result, the Ugi product 3 was obtained in 98% yield by the
photocatalytic generation of imine 2a followed by the reaction
with tert-butyl isocyanide and cinnamic acid (Scheme 2).
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Scheme 2. Ugi-type reaction including the visible-light-induced
oxidative coupling of benzylamine.
5.
6.
In summary, we have prepared the novel iridium(III) complex
of the pincer type imidazo[1,5-a]pyridine ligand. The complex
has a broad absorption band and exhibited the yellow emission in
solution. When we conducted the visible-light-induced oxidative
coupling reaction of a series of benzylic amines using the
complex as a photocatalyst, the corresponding imines were
obtained in excellent yields. In addition, this photocatalytic
system was successfully applied to the three component Ugi-type
reaction to afford the dipeptide product. Further studies on the
application of the Ir complex to other photocatalytic systems and
optoelectronic devices are in progress.
7.
Acknowledgments
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We thank the Research Cluster program of Tokushima
University (No. 1802001) for partial financial support and
NICHIA corp. for supplying LEDs.
References and notes
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