6146
T. Amaya et al. / Tetrahedron Letters 53 (2012) 6144–6147
EtO2C
CO2Et
EtO2C
CO2Et
PMAS/Au NPs (5 mol%)
aqueous solution,
O2,
90 °C, 1 h
N
H
N
98%
Scheme 1. Dehydrogenative oxidation of Hantzsch ester.
system will allow to provide an environmental friendly synthetic
method.
(a)
Acknowledgments
This work was partially supported by a Grant-in-Aid for Scien-
tific Research on Innovative Areas ‘Advanced Molecular Transfor-
mations by Organocatalysts’ from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
References and notes
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Figure 4. UV–vis spectra of (a) PMAS/Au NPs (blue line) and after the reaction of
PMAS/Au NPs with 1 under Ar (vermillion line), and (b) PMAS(red)/Au NPs under Ar
(vermillion line) and after O2 bubbling (green line) (1.0 Â 10À4 M based on the
aniline monomer unit, at 25 °C).
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Asian J. 2009, 4, 1551–1561; (d) Aschwanden, L.; Mallat, T.; Maciejewski, M.;
Krumeich, F.; Baiker, A. ChemCatChem 2010, 2, 666–673; (e) Miyamura, H.;
Morita, M.; Inasaki, T.; Kobayashi, S. Bull. Chem. Soc. Jpn. 2011, 84, 588–599; (f)
Preedasuriyachai, P.; Chavasiri, W.; Sakurai, H. Synlett 2011, 1121–1124.
In order to investigate the redox behavior of PMAS in the oxida-
tion reaction, the reaction of 1 under argon atmosphere was fol-
lowed by using UV–vis spectroscopy. Significant spectral change
was observed before and after the reaction (Fig. 4a, blue and
vermillion line, respectively), whereas the yield of the product
was 4%. The characteristic polaron band (470 nm)12 disappeared
after the reaction, in the meantime the peak appeared at 400 nm,
which is assigned to the reduced form of PMAS13 (Fig. 4a). These
results suggest the hydrogen transfer from 1 to PMAS. Oxygen bub-
bling to the reaction mixture restored the polaron band with de-
crease of the peak for PMAS (red), where PMAS (red) was
reoxidized. In this way, redox mediating function of PMAS was
indicated in these processes.
8.
A procedure for the preparation of PMAS/Au NPs solution: PMAS (kindly
provided by Mitsubishi Rayon Co.) was deionised through cation-exchange
resins before use. Other reagents were used as recieved. The water used in the
present study is of
a milliQ grade. An aqueous solution (1 mL) of PMAS
(0.06 mmol based on the aniline monomer unit, 12 mg) was mixed with 3 mL
of 0.5 M B(OH)3–NaOH (pH 9.0) aqueous solution. An aqueous solution (1 mL)
of N2H4ÁH2O (0.095 mmol, 4.7
lL) was added to the PMAS solution, which was
stirred at room temperature under air for 3.5 h. Then, an aqueous solution
(1 mL) of NaAuCl4Á2H2O (0.06 mmol, 24 mg) was added to the stirring solution
at 0 °C. The mixture was stirred at room temperature under air for 24 h.
9. A general procedure for the catalytic aerobic dehydrogenative oxidation: A
20 mL two-necked flask was evacuated and backfilled with molecular oxygen.
Then, substrate (1 mmol) and 5 mL of PMAS/Au NPs solution (Au: 0.05 mmol,
5 mol %) were added at room temperature. The mixture was stirred at 80 °C
under molecular oxygen for 4 h. The reaction mixture was extracted with ethyl
acetate. The organic layer was evaporated and examined by 1H NMR analysis
(JEOL ECS-400, 400 MHz) with 1,3,5-trimethoxybenzene as an internal
standard.
In conclusion, the PMAS/Au NPs catalyst was demonstrated for
the dehydrogenative oxidation reaction of 2-substituted indoline
and dihydropyridine under molecular oxygen in aqueous solution.
The recycle use was also allowed. The redox mediating function of
PMAS was revealed by following the UV–vis spectra. The reaction