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H. Min et al. / Journal of Organometallic Chemistry 755 (2014) 7e11
Scheme 2. The proposed mechanism of the reduction of nitro arene.
toluene in 98% yield (entry 1). Iodoanisole derivatives all gave the
anisole in good yields (entries 2e4). 1-Iodonaphthalene and 2-
iodopyridine afforded the corresponding hydrodehalogenated
products in good yields (entries 5 and 6). All aryl bromides such as
4-tert-butylbromobenzene, 4-bromophenol, 5-bromobenzodioxole
and 4-bromobiphenyl produced the desired hydrodehalogenated
products in good yields (entries 7e10). However, aryl chlorides
showed low yields (entries 11 and 12).
conditions, nitro arenes (or aryl halides) are reacted with PdO
(10 mol%) and K3PO4 (1.5 equiv.) in DMF/cyclohexanol at 110 ꢀC for
12 h. We found that cyclohexanol worked as hydrogen source, and
this catalyst showed good activities after 6 cycles of reuse.
4. Experimental section
4.1. Synthesis of catalyst
To investigate the mechanism of the reduction of nitro arenes,
diphenyldiazene was reacted under these optimized conditions. As
shown in Scheme 1, diphenyldiazene was converted to aniline with
60% yield.
The reaction vessel charged with water (200 mL), styrene (10 g,
96 mmol), divinylbenzene (0.11 g, 8.4 mmol) and potassium per-
sulfate (14 mg) was flushed and bubbled with nitrogen gas for
10 min. The vessel was heated up to 85 ꢀC. After stirring for 18 h,
10 mL portions of the resulting polystyrene emulsion were moved
to the 100 mL flask and NaClO (2.5 mL solution as available chlorine
10e15%) was added. The 2.0 mL aqueous solution dispersed with
PdCl2 (0.15 g, 0.85 mmol) was slowly dropped into the solution for
5 min and heated at 50 ꢀC for 1 h. The resulting emulsion was
centrifuged and washed with water and ethanol to yield a dark
black powder (485 mg).
At this time, the reaction mechanism for the reduction of nitro
arenes is not clear. However, based on a previous report, we suggest
a plausible reaction pathway as shown in Scheme 2. Both paths A
and B were proposed. Considering that diphenyldiazene was not
detected in the reaction mixture under these optimized conditions,
path A might be the major pathway. However, we can suggest that
path B is also a possible way based on the detection of diphe-
nyldiazene in the reaction conditions of Cs2CO3. In addition, the
hydrogen source was cyclohexanol due to the fact that cyclohexa-
none, which was formed from the oxidation of cyclohexanol, was
found in the reaction mixture of both the reduction and the
hydrodehalogenation.
To evaluate the reusability of this catalyst, the catalyst was
recovered and reused in the reduction of nitrobenzene. After
complete the reaction, the catalyst was filtered out and washed
with Et2O and water, and then dried. As shown in Scheme 3, when
the catalyst was reused 6 times, the desired product was formed.
4.2. The reduction of nitro arenes
The reaction flask was charged with appropriate amounts of
nitro arene (4.0 mmol), K3PO4 (6.0 mmol) and PdO. Then, DMF
(6.0 mL) and cyclohexanol (6.0 mL) were added. The flask was
sealed and heated at 110 ꢀC for 12 h. The reaction mixture was
cooled and then purified by column chromatography. In order to
recycle the catalyst, the filtered catalyst was washed with Et2O and
water, and dried in vacuum. The products were confirmed by GCe
MS and the data of an authentic sample.
3. Conclusions
4.3. The hydrodehalogenation of aryl halides
In summary, we easily prepared a reusable palladium catalyst of
PdO nanoparticles bound on the surface of cross-linked polystyrene
(PS) beads. This preparation method is very simple and economical.
This catalyst showed good activities in the reduction of nitro arenes
and the hydrodehalogenation of aryl halides. In the optimized
The reaction flask was charged with appropriate amounts of aryl
halides (4.0 mmol), K3PO4 (6.0 mmol) and PdO. Then, DMF (6.0 mL)
and cyclohexanol (6.0 mL) were added. The flask was sealed and
heated at 110 ꢀC for 12 h. The reaction mixture was cooled, and an
aliquot was taken and dissolved in Et2O for GCeMS analysis. The
products were confirmed by GCeMS and the data of an authentic
sample.
Acknowledgments
This research was supported by Nano Material Technology
Development Program and the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education, Science and Technology (grant
number 2012M3A7B4049655, NRF-2011-0013095). Analytical data
Scheme 3. The reusability of PdO.