DBU-based ionic-liquid-catalyzed carbonylation of o-phenylenediamines with CO2 to 2-benzimidazolones under solvent-free conditions

Article abstract of DOI:10.1021/cs400256j

Herein, a new route was presented to synthesize 2-benzimidazolones via the carbonylation of o-phenylenediamines with CO₂ catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-based ionic liquids under solvent-free conditions. DBU acetate ([DBUH][OAc]) displayed high efficiency for catalyzing the reactions of CO₂ with o-phenylenediamines, and a series of benzimidazolones were obtained in high yields. It was demonstrated that [DBUH][OAc] could serve as a bifunctional catalyst for these reactions with the cation activating CO₂ and the anion activating o-phenylenediamines. This protocol provides an effective and environmentally friendly alternative route for production of benzimidazolones, and extends the chemical utilization of CO₂ in organic synthesis as well.

Full text of DOI:10.1021/cs400256j

Research Article
pubs.acs.org/acscatalysis
DBU-Based Ionic-Liquid-Catalyzed Carbonylation of
o‑Phenylenediamines with CO2 to 2‑Benzimidazolones under
Solvent-Free Conditions
Bo Yu, Hongye Zhang, Yanfei Zhao, Sha Chen, Jilei Xu, Leiduan Hao, and Zhimin Liu*
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid, Interface and Thermodynamics, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: Herein, a new route was presented to
synthesize 2-benzimidazolones via the carbonylation of o-
phenylenediamines with CO2 catalyzed by 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU)-based ionic liquids under solvent-
free conditions. DBU acetate ([DBUH][OAc]) displayed high
efficiency for catalyzing the reactions of CO2 with o-
phenylenediamines, and a series of benzimidazolones were
obtained in high yields. It was demonstrated that [DBUH][OAc] could serve as a bifunctional catalyst for these reactions with
the cation activating CO2 and the anion activating o-phenylenediamines. This protocol provides an effective and environmentally
friendly alternative route for production of benzimidazolones, and extends the chemical utilization of CO2 in organic synthesis as
well.
KEYWORDS: carbon dioxide, ionic liquid, catalysis, carbonylation, benzimidazolone
paramount importance from a standpoint of green chemistry
and sustainable development. We recently reported a simple
and efficient synthesis of benzimidazole from a reaction of CO2
with o-phenylenediamine in the presence of H2 catalyzed by
homogeneous catalyst Ru(Cl)2(dppe)2 under solvent-free
conditions.19 It was noteworthy that a byproduct, 2-
benzimidazolone was detected with a trace amount accom-
panied with benzimidazole, which may form only from the
reaction of o-phenylenediamine with CO2, excluding the H2
involvement. To test this hypothesis, the reaction of o-
phenylenediamine with CO2 using organic base DBU (1,8-
diazabicyclo[5.4.0]undec-7-ene) as catalyst was performed.
Excitingly, benzimidazolone was proved to be the sole product
in high yield. Compared to the traditional synthetic methods or
the recent new routes, the synthesis of benzimidazolones
utilizing CO2 as a feedstock is more attractive from the green
and sustainable chemistry viewpoint. In preparation of this
work, a bifunctional tungstate catalyst for chemical fixation of
CO2 was reported to produce various useful chemicals,
including benzimidazolones.20 Though significant progress
has been made, developing a highly efficient, environmentally
benign, thermally stable, recyclable catalyst for the synthesis of
benzimidazolones from CO2 and o-phenylenediamines is still
desirable.
1. INTRODUCTION
Benzimidazolones are important derivatives of benzimidazoles
found to exhibit a wide range of biological activities,1−3 and the
heterocyclic compounds incorporating the benzimidazolone
moiety have received considerable attention.4−6 To date, many
synthetic approaches have been developed for the synthesis of
benzimidazolone and its derivatives. The conventional methods
are based on phosgenation of o-phenylenediamine,7 which are
being gradually discarded since phosgene is highly toxic. Several
nonphosgenen approaches have been reported recently,
including reactions of o-phenylenediamines with urea,8
dimethyl carbonate,9 or carbon monoxide,10 and reaction of
2-amino-benzamide with iodosylbenzene.11 Although much
progress has been made for the synthesis of benzimidazolones,
most of the synthetic methods suffer from their inherent
limitations, such as the use of harmful chemicals (e.g., carbon
monoxide), involving volatile organic solvents, low product
yields, requirements for excess of reagents or catalysts, etc.
Consequently, developing environmentally benign approaches
using the cheap and unharmful starting materials is highly
desirable.
In recent years, the conversion of CO2 into valuable
chemicals has received increasing attention, because of its
economical and nontoxic characteristics, potential as a
renewable C1 resource, and a carbonyl reagent alternative to
toxic phosgene and CO.12,13 So far, CO2 has been converted
into some valuable chemicals, such as formic acid,14
methanol,15 ureas,16 esters,17 formamidine derivatives,18
benzimidazoles,19 etc. In particular, the chemical conversion
of CO2 into heterocycles, such as benzimidazoles, is of
Room-temperature ionic liquids (ILs) have currently
attracted significant attention due to their distinctive properties,
Received: April 6, 2013
Revised: July 18, 2013
© XXXX American Chemical Society
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such as negligible vapor pressure, high thermal stability, wide
liquid temperature range, easy recyclability, excellent chemical
stability, and strong solvent power for a wide range of organic
and inorganic molecules. By modification of cations and/or
anions, the properties of ILs can be turned in many ways. To
date, a great number of functional ILs have been designed for
different purposes.21 Especially, they have been widely applied
in organic synthesis as solvents or catalysts,22−25 mainly
including coupling reaction,26 Michael addition,27 Diels−
Alder reaction,28 Knoevenagel condensation,29 Aldol reaction,30
oxidation,31 and reduction.32
DBU is a strong organic base and has been extensively
applied in the base-induced reactions with excellent catalytic
activity. However, the separation of DBU from the product
mixture is generally difficult. The DBU-based ILs (DBU-ILs)
overcome this drawback and exhibit the similar basicity to DBU
accompanied with the general features of ILs. In this work, four
DBU-ILs, including DBU acetate ([DBUH][OAc]), DBU
lactate ([DBUH][Lac]), DBU chloride ([DBUH][Cl]),and n-
butyl DBU acetate ([n-Bu-DBUH][OAc]), were synthesized,
and their activity for catalyzing the carbonylation of o-
phenylenediamine with CO2 under solvent-free conditions
was investigated. In addition, a series of benzimidazole
derivatives were synthesized via the reactions of CO2 with o-
phenylenediamine catalyzed by [DBUH][OAc]. The possible
reaction mechanism was discussed as well.
were prepared by the reactions of DBU with lactic acid and
hydrochloric acid, respectively, based on the reported
procedures.35
2.3. General Procedures for the Carbonylation of o-
Phenylenediamines to Benzimidazolones. All reactions
for the carbonylation of o-phenylenediamines to benzimidazo-
lones were carried out in a Teflon-lined stainless steel reactor of
22 mL coupled with a magnetic stirrer. In a typical experiment
to synthesize benzimidazolone, o-phenylenediamine (2.0
mmol) and IL as the catalyst with the desired amount (e.g.,
[DBUH][OAc], 0.2 mmol) were loaded into the reactor, and
moved subsequently to an oil bath at 120 °C, which was
controlled by a Haake-D3 temperature controller. CO2 was
then charged into the reactor up to the desired pressure (e.g., 9
MPa), and the stirrer was started. After the reaction, the reactor
was cooled in ice−water and the gas inside was slowly vented.
The reaction mixture was extracted with ethyl acetate three
times to separate the IL from the product. The combined
solution of ethyl acetate was dried with Na2SO4 and then
concentrated through vacuum evaporation to give the crude
product, which was further purified by column chromatog-
raphy. The recovered IL (e.g., [DBUH][OAc]) was dried in
vacuo at 60 °C for 8 h and reused in the next reaction as the
catalyst.
Similarly, the other benzimidazolone derivatives were
synthesized via the reactions of CO2 with the corresponding
substituted o-phenylenediamines catalyzed by [DBUH][OAc].
2.4. NMR Spectra Data of the Synthesized Benzimi-
dazolone Derivatives. 2A: 2-Benzimidazolone. This com-
pound was prepared according to the general procedure and
was subjected to column chromatography on silica gel to afford
the product (0.241 g, 90%). The characterization data obtained
for 2-benzimidazolone were identical to those previously
reported in the literature.36 1H NMR (400 MHz, DMSO-d6,
293 K): δ 10.49 (s, 2H), 6.81 (s, 4H). 13C NMR (100 MHz,
DMSO-d6, 293 K): δ 155.7 (C), 130.1 (C), 120.8 (CH), 108.9
(CH).
2B: 5-Methylbenzimidazolone. This compound was pre-
pared according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
(0.281 g, 95%). The characterization data obtained for 5-
methylbenzimidazolone were identical to those previously
reported in the literature.36 1H NMR (400 MHz, DMSO-d6):
δ 10.41 (s, 1H), 10.37 (s, 1H), 6.66 (d, J = 7.7 Hz, 1H), 6.59
(d, J = 10.4 Hz, 2H), 2.16 (s, 3H). 13C NMR (100 MHz,
DMSO-d6): δ 155.92 (CO), 130.34 (C), 129.76 (C), 127.93
(C), 121.30 (CH), 109.45 (CH), 108.60 (CH), 21.47 (CH3).
2C: 4,5-Dimethylbenzimidazolone. This compound was
prepared according to the general procedure and was subjected
to column chromatography on silica gel to afford the product
(0.311 g, 96%). The characterization data obtained for 5,6-
dimethylbenzimidazolone were identical to those previously
reported in the literature.37 1H NMR (400 MHz, DMSO-d6): δ
10.40 (s, 2H), 6.71 (s, 2H), 2.16 (s, 6H). 13C NMR (101 MHz,
DMSO-d6): δ 155.96 (CO), 141.41 (C), 129.63 (C), 128.25
(CH), 20.34 (CH3).
2. EXPERIMENTAL SECTION
2.1. Materials and Methods. CO2 (99.99%) was provided
by Beijing Analytical Instrument Company. DBU (99%), o-
phenylenediamine (1a: 98%), 3,4-diaminotoluene (1b: 97%),
4,5-dimethyl-o-phenylene diamine (1c: 98%), 4-chloro-o-
phenylenediamine (1d: 97%), 4-bromo-o-phenylenediamine
(1e: 97%), 4-fluoro-o-phenylene diamine (1f: 97%), 4-
trifluoromethyl-o-phenylenediamine (1g: 98%), 4-nitro-o-phe-
nylenediamine (1h: 98%), ethyl-3,4-diaminobenzoate (1i:
97%), 3,4-diaminobenzophenone (1j: 97%), n-phenyl-o-phenyl-
enediamine (1k: 98%), N-methyl-1,2-phenylenediamine (1l:
98%), 2′-aminoacetanilide (1m: 98%), and 2-aminothiophenol
(1o: 97%) were purchased from Alfa Aesar and used without
further purification. N,N′-Dimethy-1,2-phenylenediamine (1n)
was prepared from o-phenyldiamine in two steps.33
TLC analysis was performed on silica gel 60 F254, and the
spots were visualized with UV light at 254 nm or under iodine.
1H and 13C NMR spectra were collected in CDCl3 or
(CD3)2SO on a Bruker Avance NMR (400 MHz) at ambient
temperature, and chemical shifts were recorded relative to
1
tetramethylsilane (TMS). H and 13C NMR chemical shifts
were reported in parts per million downfield from tetrame-
thylsilane. The following abbreviations were used in the NMR
follow-up experiments: s, singlet; d, doublet; t, triplet; q,
quartet; m, multiplet.
2.2. General Procedures for the Synthesis of DBU-
Based ILs. [DBUH][OAc] was synthesized according to the
reported procedures.34 In a typical experiment, in a N2
atmosphere, DBU (5 mmol) was loaded into a 50 mL two-
neck flask cooled in an ice−water bath, and acetic acid (5
mmol) was then added dropwise to the flask with stirring. After
the mixture was stirred at 50 °C for 24 h, a light yellow, viscous
oily liquid was obtained. Dried at 80 °C for 24 h under vacuum,
the resultant product was characterized by 1H NMR, which was
in good agreement with the reported spectra data of
[DBUH][OAc]. Similarly, [DBU][Lac] and [DBUH][Cl]
2D: 5-Chlorobenzimidazolone. This compound was pre-
pared according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
(0.258 g, 77%). The characterization data obtained for 5-
chlorobenzimidazolone were identical to those previously
reported in the literature.36 1H NMR (400 MHz, DMSO-d6):
δ 10.65 (s, 2H), 6.87−6.83 (m, 2H), 6.81 (s, 1H). 13C NMR
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a
Table 1. Carbonylation of o-Phenylenediamine to Benzimidazolone with CO2
b
entry
catalyst
amount (equiv)
PCO (MPa)
time (h)
yield (%)
2
c
1
2
none
0
9
9
9
9
9
9
9
6
3
1
9
9
12
24
24
24
30
24
40
24
24
24
24
NR
97
[DBUH][OAc]
[DBUH][Lac]
[n-BuDBU][OAc]
[DBUH][Cl]
1
3
1
91
4
1
25
c
5
1
NR
d
e
f
g
h
6
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
[DBUH][OAc]
0.1
0.1
0.1
0.1
0.1
0.05
0.01
90 ,89 ,89 ,90 ,89
7
96
81
70
51
71
47
8
9
10
11
12
24
a
b
c
d
All reactions were carried out with o-phenylenediamine (2 mmol), 120 °C. Yields of isolated product. No reaction. Yield of product (runs: 1).
e
f
g
h
Runs: 2. Runs: 3. Runs: 4. Runs: 5.
(101 MHz, DMSO-d6): δ 155.69 (CO), 131.32 (C), 129.10
(C), 124.94 (C), 120.56 (CH), 109.98 (CH), 108.84 (CH).
2E: 5-Bromobenzimidazolone. This compound was pre-
pared according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
(0.302 g, 71%). The characterization data obtained for 5-
bromobenzimidazolone were identical to those previously
reported in the literature.38 1H NMR (400 MHz, DMSO-d6):
δ 10.76 (s, 2H), 7.07 (dd, J = 8.2, 1.9 Hz, 1H), 7.05 (s, 1H),
6.87 (d, J = 8.2 Hz, 1H). 13C NMR (101 MHz, DMSO-d6): δ
155.51 (CO), 131.68 (C), 129.46 (C), 123.37 (C), 112.47
(CH), 111.50 (CH), 110.54 (CH). HRMS calcd for C7H5-
BrN2O: 213.0314. Found: 213.0311.
2F: 5-Fluorobenzimidazolone. This compound was pre-
pared according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
(0.258 g, 85%). The characterization data obtained for 5-
fluorobenzimidazolone were identical to those previously
reported in the literature.36 1H NMR (400 MHz, DMSO-d6):
δ 10.74 (s, 1H), 10.62 (s, 1H), 6.87 (dd, J = 8.4, 4.7 Hz, 1H),
6.79−6.69 (m, 2H). 13C NMR (101 MHz, DMSO-d6): δ
156.12 (CO), 130.86 (C), 130.74 (C), 126.51 (C), 109.24
(CH), 109.14 (CH), 107.06 (CH).
2G: 5-Trifluoromethylbenzimidazolone,. This compound
was prepared according to the general procedure and was
subjected to column chromatography on silica gel to afford the
product (0.283 g, 70%). The characterization data obtained for
5-trifluoromethylbenzimidazolone were identical to those
previously reported in the literature.39 1H NMR (400 MHz,
DMSO-d6): δ 11.02 (2H, s), 7.27 (d, J = 8.0 Hz, 1H), 7.16 (1
H, s), 7.09 (d, J = 8.4 Hz, 1H). 13C NMR (101 MHz, DMSO-
d6): δ 157.57 (CO), 132.37 (C), 129.36 (C), 125.71 (C),
123.01 (CF3), 120.75 (CH), 120.43 (CH), 117.37 (CH).
2H: 5-Nitrobenzimidazolone. This compound was prepared
according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
(0.247 g, 69%). The characterization data obtained for 5-
nitrobenzimidazolone were identical to those previously
reported in the literature.36 1H NMR (400 MHz, DMSO-d6):
δ 11.23 (s, 2H), 7.86 (dd, J = 8.7, 2.2 Hz, 1H), 7.65 (d, J = 2.2
Hz, 1H), 7.03 (d, J = 8.7 Hz, 1H). 13C NMR (101 MHz,
DMSO-d6): δ 157.77 (CO), 155.14 (C), 140.87 (C), 135.30
(C), 129.32 (CH), 117.35 (CH), 107.65 (CH).
2I: 5-Ethoxycarbonylbenzimidazolone. This compound
was prepared according to the general procedure and was
subjected to column chromatography on silica gel to afford the
product (0.313 g, 76%). Melting point: 135−136 °C. 1H NMR
(400 MHz, DMSO-d6): δ 11.04 (s, 1H), 10.86 (s, 1H), 7.63
(dd, J = 8.2, 1.3 Hz, 1H), 7.47 (s, 1H), 7.01 (d, J = 8.2 Hz, 1H),
4.26 (q, J = 7.1 Hz, 2H), 1.30 (t, J = 7.1 Hz, 3H). 13C NMR
(101 MHz, DMSO-d6): δ 166.35 (CO), 155.86 (CO), 134.35
(C), 130.03 (C), 123.30 (C), 122.48 (CH), 109.40 (CH),
108.59 (CH), 60.79 (CH2), 14.68 (CH3). FTIR (KBr): 3089,
2983, 2801,1710, 1624, 1582, 1520,1475, 1414, 1367, 1305,
1232, 1123, 956, 892, 887,776,753 cm−1. HRMS calcd for
C10H10N2O3: 206.1980. Found: 206.1982.
2J: 5-Benzoylbenzimidazolone. This compound was
prepared according to the general procedure and was subjected
to column chromatography on silica gel to afford the product
(0.347 g, 73%). The characterization data obtained for 5-
benzoylbenzimidazole were identical to those previously
reported in the literature.36 1H NMR (400 MHz, DMSO-d6):
δ 11.10 (s, 1H), 10.87 (s, 1H), 7.74−7.60 (m, 3H), 7.54 (t, J =
7.5 Hz, 2H), 7.42 (dd, J = 8.2, 1.6 Hz, 1H), 7.32 (d, J = 1.4 Hz,
1H), 7.06 (d, J = 8.1 Hz, 1H). 13C NMR (101 MHz, DMSO-
d6): δ 194.65 (CO), 156.79 (CO), 137.97 (C), 135.90 (C),
131.15 (C), 130.24 (C), 128.63 (CH), 128.04 (CH), 127.79
(CH), 123.63 (CH), 109.07 (CH), 107.83 (CH).
2K: N-Phenylbenzimidazolone. This compound was pre-
pared according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
(0.336 g, 80%). The characterization data obtained for N-
phenylbenzimidazole were identical to those previously
reported in the literature.11 1H NMR (400 MHz, DMSO-d6):
δ 11.15 (s, 1H), 7.62−7.51 (m, 4H), 7.46−7.40 (m, 1H), 7.10−
7.04 (m, 2H), 7.04−6.97 (m, 2H). 13C NMR (101 MHz,
DMSO-d6): δ 153.68 (CO), 135.04 (C), 130.46 (C), 129.86
(C), 128.91 (CH), 127.77 (CH), 126.37 (CH), 122.26 (CH),
121.35 (CH), 109.63 (CH), 108.57 (CH).
2L: N-Methylbenzimidazolone. This compound was pre-
pared according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
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(0.255 g, 86%). 1H NMR (400 MHz, CDCl3): δ 10.62 (s, 1H),
7.10−6.92 (m, 4H), 3.38 (s, 3H). 13C NMR (101 MHz,
CDCl3): δ 155.95 (CO), 130.93 (C), 128.17 (C), 121.51
(CH), 121.14 (CH), 109.63(CH), 107.53 (CH), 26.76 (CH3).
HRMS calcd for C8H8N2O: 148.0637. Found: 148.0643.
2M: 2-Methyl-1H-benzimidazole. This compound was
prepared according to the general procedure and was subjected
to column chromatography on silica gel to afford the product
(0.258 g, 98%). The characterization data obtained for
benzimidazole were identical to those previously reported in
the literature.19 1H NMR (400 MHz, CDCl3): δ 9.78 (s, 1H),
7.55−7.57 (dd, 2H), 7.26−7.21 (dd, 2H), 2.66 (s, 3H). 13C
NMR (101 MHz, CDCl3): δ 150.31 (CO), 137.63 (C), 121.12
(CH), 113.45 (CH), 13.90 (CH3).
2O: 2-Benzothiazolone. This compound was prepared
according to the general procedure and was subjected to
column chromatography on silica gel to afford the product
(0.095 g, 63%). The characterization data obtained for
benzimidazole were identical to those previously reported in
the literature.40 1H NMR (400 MHz, DMSO-d6): δ 11.87 (s,
1H), 7.53 (d, J = 7.7 Hz, 1H), 7.26 (t, J = 7.7 Hz, 1H), 7.10 (t, J
= 7.6 Hz, 2H). 13C NMR (101 MHz, DMSO-d6): δ 170.48
(CO), 136.79 (C), 126.82 (C), 123.77 (CH), 123.10 (CH),
123.02 (CH), 111.93 (CH).
1, entry 7). Moreover, the reaction still proceeded as the IL
amount further decreased to 0.05 equiv, and even to 0.01 equiv
(Table 1, entries 11, 12). This indicates that this IL was very
effective in catalyzing the reaction of CO2 with o-phenylenedi-
amine.
The CO2 pressure was found to considerably influence the
product yields, which reduced from 90% to 51% as the pressure
decreased from 9 to 1 MPa, as shown in Table 1 (Entries 6, 8−
10). This should be related to the phase behavior of the
reaction system. The reaction system included two phases, the
CO2-rich phase and the liquid phase mainly containing o-
phenylenediamine and the IL, and the reaction occurred in the
liquid phase. Decreasing the CO2 pressure rendered less CO2
to dissolve into the liquid phase, which was unfavorable to CO2
contacting with diamine and the catalyst, leading to low
product yield finally. From the above results, the optimal
experimental conditions were chosen as follows: [DBUH]-
[OAc] as the catalyst with an amount of 0.1 equiv of the
substrate and a CO2 pressure of 9 MPa.
The recyclability of the catalyst [DBUH][OAc] for the
reaction was investigated, and the results are shown in Figure 1.
3. RESULTS AND DISCUSSION
In this work, four DBU-ILs, including [DBUH][OAc],
[DBUH][Lac], [DBUH][Cl], and [n-Bu-DBUH][OAc], were
first synthesized, and they were used to catalyze the reaction of
CO2 with o-phenylenediamine. At the reaction temperature of
120 °C, o-phenylenediamine was in the liquid state and
miscible with all the DBU-ILs. The reaction of CO2 with o-
phenylenediamine was performed in a two-phase reaction
system under the experimental conditions as listed in Table 1.
This reaction did not occur in the absence of any IL (Table 1,
entry 1), whereas the presence of an IL, including [DBUH]-
[OAc], [DBUH][Lac], and [n-BuDBU][OAc], in the reaction
system, respectively, resulted in the production of benzimida-
zolone (Table 1, entries 2−4). However, [DBUH][Cl] could
not catalyze the reaction of CO2 with o-phenylenediamine
(Table 1, entry 5). As listed in Table 1, [DBUH][OAc],
[DBUH][Lac], and [DBUH][Cl] showed the activities in the
order: [DBUH][OAc] > [DBUH][Lac] > [DBUH][Cl],
though they had the same cation [DBUH]+. This suggests
that the anions of the ILs had an impact on the activities of the
ILs for catalyzing this reaction. [n-BuDBU][OAc] could
catalyze this reaction; however, it displayed a much lower
activity compared to [DBUH][OAc] (Table 1, entry 4). This
implies that the cations of the ILs may be involved in catalyzing
the reaction. From the above findings, it can be deduced that
both the cation and the anion of the IL played important roles
in catalyzing the reaction of CO2 with o-phenylenediamine,
which will be discussed in the following section.
Figure 1. Yields of benzimidazolone (1A) obtained using the recycled
[DBUH][OAc] as the catalyst.
It can be observed that the product yield almost stayed
unchanged as the IL was reused five times, suggesting that the
IL still retained its original activity. This indicates that the used
IL was recyclable for catalyzing the reaction of CO2 with o-
phenylenediamine.
To broaden the scope and generality of the developed
protocol, a wide range of substituted o-phenylenediamines were
used to react with CO2 under the optimal conditions, and the
results are summarized in Table 2. It was demonstrated that all
the substituted o-phenylenediamines bearing electron-donating
and electron-withdrawing groups could react with CO2
catalyzed by [DBUH][OAc], affording the corresponding
benzimidazolone derivatives in moderate to high yields under
the experimental conditions. Compared to o-phenylenediamine,
the substituted o-phenylenediamines with electron-donating
groups, for example, the Me group, produced the correspond-
ing benzimidazolones with improved yields (Table 2, entries 2,
3), suggesting that the electron-donating groups made the o-
phenylenediamines more active to react with CO2. However,
the o-phenylenediamines with electron-withdrawing groups
from the weak electron-withdrawing groups, for example, F, Cl,
and Br, to the strong electron-withdrawing groups, for example,
CF3, NO2, CO2Et, and PhCO, all generated the corresponding
benzimidazolones in declined yields (Table 2, entries 4−10),
Among the four ILs, [DBUH][OAc] exhibited the best
performance for catalyzing the reaction of CO2 with o-
phenylenediamine. Therefore, it was selected as the catalyst
to investigate the influences of catalyst amount, CO2 pressure,
and reaction time on the reaction. With the equal equivalent of
IL, a product yield of 97% was obtained under the experimental
conditions (Table 1, entry 2). When the IL amount was
decreased to 0.1 equiv, the benzimidazolone yield just reduced
to 90% (Table 1, entry 6), and a product yield of 96% was
achieved when the reaction time was prolonged to 40 h (Table
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Research Article
Instead, 2-methyl benzimidazole (2M) was obtained in a yield
of 98%, which may be attributed to the cyclization of 2′-
aminoacetanilide under the experimental conditions (Scheme
1). It is noteworthy that, when N,N′-dimethyl-o-phenylenedi-
Table 2. Carbonylation of o-Phenylenediamines to
Benzimidazolones Catalyzed by [DBUH][OAc]
ab c
, ,
Scheme 1. Reaction of 2′-Aminoacetanilide with CO2
Catalyzed by [DBUH][OAc]
amine was used as the substrate, no product was formed, even
though the reaction time was prolonged to 40 h (Table 2, entry
13), thus implying that the reaction may proceed via the
isocyanate intermediate, which was consistent with that
reported in the literature.20
2-Aminothiophenol was examined to react with CO2
catalyzed by [DBUH][OAc]. Interestingly, benzothiazolone
was successfully obtained with a yield of 63% (Scheme 2). This
Scheme 2. Synthesis of Benzothiazolone via 2-
a
Aminothiophenol Reacting with CO2
a
Reaction conditions: substrate, 1 mmol; [DBUH][OAc], 1 equiv;
CO2 pressure, 9 MPa; reaction temperature, 100 °C; reaction time, 24
h.
is the first example to produce benzothiazolone via the reaction
of CO2 with 2-aminothiophenol to the best of our knowledge,
which may provide a new route to the production of 2-
benzothiazolones.
To better understand the reaction mechanism, NMR analysis
was employed to identify the possible intermediates during the
reaction. In the spectrum of the mixture of an equivalent of
[DBUH][OAc] with o-phenylenediamine (Figure 2A), the
signal assigned to the NH2 proton of o-phenylenediamine
shifted from δ = 4.34 to 4.52 ppm and the peak became wider,
thus indicating the formation of a hydrogen-bonding
interaction between [DBUH][OAc] and o-phenylenediamine.
Moreover, in the corresponding 13C NMR spectrum, the signal
at δ = 178.6 ppm to the C1 atom of [DBUH][OAc] shifted to
δ = 173.1 ppm upon mixing with o-phenylenediamine (Figure
2B), suggesting the stronger hydrogen-bonding interaction
between [OAc]− and o-phenylenediamine. The hydrogen bond
could weaken the N−H bond and facilitate the nucleophilic
attack of the NH2 group in o-phenylenediamine on the carbon
atom of CO2. In the spectrum of the intermediate of
[DBUH][OAc] exposed to CO2 (9 MPa), a new 13C signal
appeared at δ = 172.3 ppm (Figure 3), probably ascribed to the
[DBUH]+/CO2 adduct, which was a key intermediate for
accomplishing the reaction through activating CO2 by the
a
Reaction conditions: substrate, 2 mmol; [DBUH][OAc], 0.1 equiv;
CO2 pressure, 9 MPa; reaction temperature, 120 °C; reaction time, 24
h. Isolated yield. No reaction.
b
c
suggesting that the electron-withdrawing groups lowered the
activity of the o-phenylenediamines reacting with CO2. The
above findings indicate that the substituents in the phenyl ring
of diamines considerably influenced their activities to react with
CO2, and the o-phenylenediamines with electron-donating
groups are more active than those with electron-withdrawing
groups. The o-phenylenediamine with the N-substituent also
generated the corresponding benzimidazolone in high yields
(Table 2, entries 11 and 12), which indicates that the steric
hindrance of the N-phenyl substituent seemed not to hamper
the reaction. When 2′-aminoacetanilide (1m) was used as the
diamine substrate under the similar conditions, the desired
product (i.e., N-acetyl benzimidazolone) was not obtained.
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Research Article
Scheme 3. The Possible Reaction Pathway
1
Figure 2. NMR spectra: (A) H NMR of o-phenylenediamine (0.1
mmol) without and with [DBUH][OAc] (0.1 mmol), (DMSO-d6, 0.6
mL, 298 K); (B) 13C NMR of [DBUH][OAc] (0.1 mmol), without
and with o-phenylenediamine (0.1 mmol), (DMSO-d6, 0.6 mL, 298
K).
NH group on the carbon atom of isocyanate takes place to give
the corresponding product 2A.
4. CONCLUSION
The carbonylation of o-phenylenediamines with CO2 was
achieved using the DBU-based ILs as catalysts without any
solvents and additives. [DBUH][OAc] displayed the best
efficiency for the reactions of CO2 with o-phenylenediamines,
and a series of 2-benzimidazolones were synthesized in high
yields. In these reactions, [DBUH][OAc] served as a
bifunctional catalyst with the cation activating CO2 and the
anion activating o-phenylenediamines. This protocol to
synthesize 2-benzimidazolones from CO2 and o-phenylenedi-
amine provides an effective and environmentally friendly
alternative for production of benzimidazolones, and it also
extends the chemical utilization of CO2 in the synthesis of
industrially important chemicals.
Figure 3. 13C NMR spectra of pure [DBUH][OAc] (0.1 mmol) and
the intermediate of [DBUH][OAc] exposed to CO2 (9 MPa)
(DMSO-d6, 0.6 mL, 298 K).
tertiary nitrogen atom of the [HDBU]+ cation. From the above
results, it can be deduced that the cation of the IL activated
CO2 and the anion activated o-phenylenediamines, meaning
that the IL acted as a bifunctional catalyst for the reaction of
CO2 with o-phenylenediamine. This can explain why [n-
BuDBU][OAc] showed lower activity than [DBUH][OAc].
Because of the steric hindrance effect of its bulky cation, [n-
BuDBU]+ was incapable of activating CO2, which led to a lower
efficiency of [n-BuDBU][OAc] for catalyzing the reaction of
CO2 with o-phenylenediamine. In the case of [DBUH]Cl as the
catalyst, Cl− could not activate o-phenylenediamine, which may
be responsible for the no activity for this reaction.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of the products. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Phone: + 8610-62562852. Fax: +8610-62559773. E-mail:
liuzm@iccas.ac.cn.
■
Notes
The authors declare no competing financial interest.
On the basis of the NMR experimental results and the
previous report,20,41 the possible reaction mechanism, similar to
that proposed by Mizuno,20 is shown in Scheme 3. We
supposed that the reaction of CO2 with o-phenylenediamine
might undergo as follows. In the presence of [DBUH][OAc],
CO2 was activated by the tertiary N atom of [DBUH]+ to form
intermediate 2; meanwhile, o-phenylenediamine (1a) was
activated by hydrogen bonding with O atoms of the [OAc]−
anion to form intermediate 3. The nucleophilic nitrogen of
intermediate 3 would attack the carbon atom of intermediate 2
to form intermediate carbamate 4, followed by the dehydration
of intermediate isocyanate 5, which was not detected in our
experiment, but could be deduced from the result of entry 12 in
Table 2. Finally, the intramolecular nucleophilic attack of the
ACKNOWLEDGMENTS
■
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 their financial
support.
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