Communication
Green Chemistry
ring takes place in several steps wherein each product 22 mL coupled with a magnetic stirrer. In a typical experiment
becomes the substrate for the next reaction. According to this to synthesize benzimidazole, o-phenylenediamine (5.0 mmol)
possible reaction pathway, o-phenylenediamines should be and RuCl (dppe) were loaded into the reactor, and moved
2 2
completely converted into corresponding benzimidazoles. subsequently to an oil bath of 120 °C, which was controlled by
However, due to the presence of the competing reaction of a Haake-D3 temperature controller. Then the mixed gas of CO2
o-phenylenediamines with CO
formed in small amounts.
2
, byproducts are inevitably and H
into the reactor up to the desired pressure (e.g., 5, 10, 15,
0 MPa), and the stirrer started. After the reaction, the reactor
2 2 2
(the molar ratio of CO to H was 3 : 2) was charged
2
was cooled in ice water and the gas inside was slowly vented.
The reaction mixture was dissolved in methanol and trans-
ferred into a volumetric flask. The quantitative analysis was
Conclusions
The cyclization of o-phenylenediamines by CO was achieved conducted by HPLC using a Shimadzu LC-20AT pump, a
2
in the presence of H , producing benzimidazoles. The catalyst Hypersil ODS2 5 μm column, and a Soma UV-Vis LC-830 detec-
2
2 2
RuCl (dppe) is highly active and selective for the production tor at 282 nm. A methanol–water (50 : 50 V/V) solution was
−
1
of a series of benzimidazoles in excellent yields. In addition, used as the mobile phase with a flow rate of 0.8 mL min
the reactions can be performed under solvent-free conditions, The pure products were obtained via column chromatography
.
1
13
rendering the strategy to synthesize benzimidazoles from CO
2
separation. The pure products were identified by H and
C
highly valuable from both environmental and economical NMR. Similarly, substituted benzimidazoles were obtained
points of view. The present synthetic route of benzimidazoles using the corresponding o-phenylenediamines as the
starting from CO
2
represents a novel alternative to the substrates.
traditional routes, and opens a new way for the CO utilization
2
as well.
Acknowledgements
Experimental section
We are grateful to the National Natural Science Foundation of
China (Nos. 21125314, 21021003, 21073202) and the Chinese
Academy of Sciences (KJXC2-YW-H30) for financial support.
Hydrogen (99.99%) and CO
Analytical Instrument Company. o-Phenylenediamine (1a:
8%), 3,4-diaminotoluene(1b: 97%), 4,5-dimethyl-o-phenylene-
diamine (1c: 98%), 4-chloro-o-phenylenediamine (1d: 97%),
-bromo-o-phenylenediamine (1e: 97%), 4-fluoro-o-phenylene-
2
(99.99%) were provided by Beijing
9
Notes and references
4
diamine (1f: 97%), ethyl-3,4-diaminobenzoate (1g: 97%), 3,4-
diaminobenzophenone (1h: 97%), 4-nitro-o-phenylenediamine
1
(a) T. Sakakura, J. C. Choi and H. Yasuda, Chem. Rev., 2007,
07, 2365; (b) P. Munshi, A. D. Main, J. C. Linehan,
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1
(
1i: 98%), 4-trifluoromethyl-o-phenylenediamine (1j: 98%),
N-phenyl-o-phenylenediamine (1k: 98%), 2′-aminoacetanilide
1l: 98%) were purchased from Alfa Aesar and used without
further purification. RuCl (dppe) (dppe = 1,2-bis(diphenyl-
7
(
2
2
phosphino)-ethane) was prepared according to a reported
(
e) O. Krocher, R. A. Koppel and A. Baiker, J. Mol. Catal. A:
3
procedure.
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1
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H (400 MHz) and C (100 MHz) NMR spectra were col-
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3 2
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(
400 MHz) at ambient temperature, and chemical shifts were
1
13
recorded relative to tetramethylsilane (TMS). H and C NMR
chemical shifts are reported in ppm downfield from tetra-
methylsilane. Abbreviations used in the NMR follow-up exper-
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multiplet. Melting points of new products were recorded using
an XT4 microscopy melting point determinator. The quantity
analysis of the products was conducted on an HPLC
3
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(Shimadzu, LC-15C) with a UV detector set at a wavelength of
2
54 nm.
4
5
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Procedure for the synthesis of benzimidazoles from
o-phenylenediamines and CO2
2
The cyclization of o-phenylenediamines by CO in the presence
of H was carried out in a Teflon-lined stainless steel reactor of
2
Green Chem.
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