Organic Process Research & Development
Article
stream, other reagents and solvents were added: I (0.12
6
4
2
2
Information). These contradictory results obtained for ligands
XnPy suggest that pyridine affects other steps. We propose that
XnPy plays a crucial role during the formation of Pd(0) active
species (involving their shape, size, and catalytic activity). This
hypothesis is based on the results of our studies (to be
mmol), Py or X Py (6.2 mmol), nitrobenzene or its liquid
n
derivative (40 mmol), water (0−1.5 mL), and ethanol (20 mL).
The cover was closed, and the autoclave was directly filled with
carbon monoxide (4 MPa), fixed, placed in a hot oil bath, and
maintained at 100−180 °C for 30−120 min, depending on the
reaction. After the reaction was completed, the autoclave was
cooled in a water bath and vented, and a liquid sample of the
reaction mixture was analyzed. The yield of the reaction was
calculated on the basis of GC-FID analysis using n-decyl
alcohol as standard.
22
published) on the thermal decomposition of PdCl (X Py) .
2
n
2
We observed different shapes and sizes of the obtained Pd(0)
depending on the X Py applied, namely, smaller particles were
n
obtained in the presence of chloropyridines. It is possible that
the same phenomena might be responsible for different activity
of Pd(0) generated in the presence of Cl Py and Me Py.
n
n
ASSOCIATED CONTENT
Supporting Information
■
CONCLUSIONS
*
S
■
(
The effects of the structures of the catalyst and substrate
aromatic nitro compound) on the reaction rate were
investigated. The catalytic activity of Pd(II) complexes always
Elemental analysis of Pd(II) complexes; conversion,
yield, and selectivity for the reduction of nitrobenzene by
CO/H O in the presence of PdCl Py complexes as a
function of the amount of water; conversion, yield, and
selectivity for the reduction of nitrobenzene and various
nitro compounds in the presence of Pd(II) catalysts;
Hammett type plots for the reduction of nitro
increased with decreasing basicity of X Py. The best results
n
during the reduction of aromatic nitro compounds
2
2
2
(
YC H NO ) with CO/H O as a reducing agent were obtained
6
4
2
2
for three catalysts with chloropyridine ligands, PdCl (2-ClPy) ,
2
2
PdCl (3-ClPy) , and PdCl (3,5-Cl Py) , regardless the struc-
2
2
2
2
2
ture of the YC H NO substrate. On the basis of these results,
6
4
2
it is impossible to clearly identify the RDS. Nevertheless, our
experiments provide new insights into the structure−activity
relationship for the reduction of aromatic nitro compounds by
AUTHOR INFORMATION
Corresponding Author
■
CO/H O, which might contribute to understanding its
2
mechanism better. Strategies to develop catalysts for the
reduction process should be based on the introduction of
electron-withdrawing substituents to the 3- or 4- position of the
pyridine ring.
Notes
The authors declare no competing financial interest.
EXPERIMENTAL SECTION
■
ACKNOWLEDGMENTS
Materials. Carbon monoxide (99.9%), PdCl , iodine, and
iron powder were used as received. Pyridine (Py), liquid-
substituted pyridines (2-MePy, 3-MePy, 4-MePy, 2,6-Me Py,
■
2
This project was funded by the Ministry of Science and Higher
Education (research project no. IP2011027071). We also thank
2
Dr. Jadwiga Skupin
manuscript.
́
ska for helpful discussions regarding this
2
,4-Me Py, 3,5-Me Py, 2-ClPy, 3-ClPy, 2,4-Cl Py), nitro-
2
2
2
benzene (NB) and its liquid derivatives (3-MeNB), and
ethanol were distilled (or fractionally distilled) over a drying
agent and stored under argon. Solid-substituted pyridines (2,6-
Cl Py and 3,5-Cl Py) and solid-substituted nitrobenzenes (3-
REFERENCES
(1) Ragaini, F. Dalton Trans. 2009, 6251.
(2) Mestroni, G.; Camus, A.; Zassinovich, G. Aspects of Homogeneous
Catalysis; Ugo, R., Ed.; Springer: Dordrecht, 1981; p 71.
3) Tafesh, M.; Weiguny, J. Chem. Rev. 1996, 96, 2035.
4) Mdleleni, M. M.; Rinker, R. G.; Ford, P. C. J. Mol. Catal. A: Chem.
003, 204-205, 125.
5) Nomura, K. J. Mol. Catal. A: Chem. 1995, 95, 203.
6) Cann, K.; Cole, T.; Slegeir, W.; Pettit, R. J. Am. Chem. Soc. 1978,
00, 3969.
7) Bhaduri, S.; Gopalkrishnan, K. S.; Clegg, W.; Jones, P. G.;
Sheldrick, G. M.; Stalke, D. J. Chem. Soc., Dalton Trans. 1984, 1765.
(8) Alessio, E.; Clauti, G.; Mestroni, G. J. Mol. Catal. 1985, 29, 77.
(9) Watanabe, Y.; Tsuji, Y.; Ohsumi, T.; Takeuchi, R. Bull. Chem. Soc.
Jpn. 1984, 57, 2867.
10) Ragaini, F.; Cenini, S. J. Mol. Catal. A: Chem. 1996, 105, 145.
11) Nomura, K. J. Mol. Catal. A: Chem. 1998, 130, 1.
12) Alper, H.; Amaratunga, S. Tetrahedron Lett. 1980, 21, 2603.
́
13) Longo, C.; Alvarez, J.; Fernandez, M.; Pardey, A. J.; Moya, S. A.;
■
2
2
ClNB, 4-MeNB, 4-ClNB) were used as received.
Synthesis of PdCl (X Py) (Compounds I−XII). The
2
n
2
21
(
(
2
(
(
procedure has been described elsewhere. Palladium chloride
complexes with pyridines were prepared under argon. PdCl2
(
1.128 mmol) was placed in a 10 mL flask equipped with a
magnetic stirrer, and 2.26 mmol of Py or substituted X Py in 10
n
mL of acetonitrile was added. The reaction was carried out at
room temperature for 24 h. Elemental analyses of complexes I,
II, V, IX, XI, and XII were performed by a conventional
1
(
2
1
method (see Table S1, Supporting Information). Single
yellow crystals of III, IV, VI, VII, VIII, and X, obtained by slow
evaporation of their acetone solutions, were characterized by X-
2
2,39
(
(
(
(
ray diffraction.
Reduction Procedure. The reaction was carried out in a
00 mL stainless steel autoclave equipped with a magnetic
stirrer. Before starting the experiment, the autoclave was heated
2
Baricelli, P.; Mdleleni, M. M. Polyhedron 2000, 19, 487.
14) Baker, E. C.; Hendriksen, D. E.; Eisenberg, R. J. Am. Chem. Soc.
1980, 102, 1020.
15) Ragaini, F.; Cenini, S.; Gasperini, M. J. Mol. Catal. A: Chem.
at 120 °C for 3 h and then cooled to room temperature.
(
Subsequently, PdCl (X Py) catalyst (0.056 mmol), Fe powder
2
n
2
(
2.68 mmol), and a solid derivative of nitrobenzene (40 mmol)
were placed in the autoclave, the air was evacuated, and the
(
2001, 174, 51.
system was filled with purified argon. Then, under an argon
(16) Pardey, A. J.; Ford, P. C. J. Mol. Catal. 1989, 53, 247.
D
Org. Process Res. Dev. XXXX, XXX, XXX−XXX