DBSA mediated chemoselective synthesis of 2-substituted benzimidazoles in aqueous media

Article abstract of DOI:10.1016/j.tetlet.2013.07.147

An efficient synthetic method has been developed for the facile synthesis of 2-substituted benzimidazoles in organized aqueous media in the presence of a surfactant (viz. DBSA) as catalyst and I₂ as co-catalyst. The method described has the advantages of operational simplicity, excellent yields, high chemoselectivity, and clean and green reaction profile.

Full text of DOI:10.1016/j.tetlet.2013.07.147

Tetrahedron Letters 54 (2013) 5505–5509
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
DBSA mediated chemoselective synthesis of 2-substituted
benzimidazoles in aqueous media
⇑
⇑
Vikash Kumar, Dipratn G. Khandare, Amrita Chatterjee , Mainak Banerjee
Department of Chemistry, Birla Institute of Technology and Science, Pilani, K.K. Birla Goa Campus, Goa 403 726, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient synthetic method has been developed for the facile synthesis of 2-substituted benzimidazoles
in organized aqueous media in the presence of a surfactant (viz. DBSA) as catalyst and I2 as co-catalyst.
The method described has the advantages of operational simplicity, excellent yields, high chemoselectiv-
ity, and clean and green reaction profile.
Received 3 June 2013
Revised 25 July 2013
Accepted 28 July 2013
Available online 3 August 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Surfactant catalyzed reaction
Dehydration reaction in water
Dodecylbenzenesulfonic acid (DBSA)
Chemoselective
2-Substituted benzimidazoles
Benzimidazole is a ubiquitous heterocyclic moiety in natural
and synthetic compounds. Benzimidazole derivatives with substit-
uents particularly at N-1 and/or C-2 positions have received para-
mount interest in recent times because of their broad range of
biological functions1 and pharmacological applications.2 They are
an integral part of various clinical medicines as well, for example
2-substituted benzimidazole, Esomeprazole3 is an anti-ulcerative
drug and Albendazole4 is used to treat parasitic diseases, whereas,
1,2-substituted benzimidazole, Astemizole is an antihistamine
drug.5 In addition, they are important intermediates in many or-
ganic reactions6 and structural subunits (or major components)
of many functional materials.7 This has led to the development of
several methods for the synthesis of benzmidazole derivatives in
recent times. The traditional methods for the synthesis of benzim-
idazoles involve the condensation of an o-diaminoarene with a car-
boxylic acid or its derivatives under harsh dehydrating conditions.8
Several alternative approaches such as reductive cyclization reac-
tion of o-nitroaniline with aldehydes,9 palladium catalyzed tandem
carbonylation–cyclization reaction of o-diaminoarene,10 iron(II)
bromide catalyzed coupling of 2-azidoarylimine,11 rhodium cata-
lyzed hydroformylation reaction of N-alkenyl phenylenediam-
ines,12 solid-phase supported synthesis,13 Cu2O catalyzed cascade
reaction between o-haloaniline and amidine hydrochlorides14
etc., have also been developed for the synthesis of benzimidazole
derivatives. However, dehydrative Schiff’s base formation followed
by oxidative cyclization in the same pot involving o-diaminoarene
and aldehydes turned out to be the most popular method for the
synthesis of 2-substituted benzimidazoles, 315 and, 1,2-disubsti-
tuted benzimidazoles, 4.16 The reported procedures for this proto-
col involved a wide spectrum of reagents such as In(OTf)3,15a
Sm(OTf)3,15b WOx/ZrO2,15d H2O2/CAN,15e p-TsOH/DMF15j for 2-
substituted benzimidazoles and HClO4–SiO2,16a proline,16c Zn-pro-
line,16d and oxalic acid16e for 1,2-disubstituted benzimidazoles.
Interestingly, acid catalyzed conditions favor 2-substituted benz-
imidazoles over 1,2-disubstituted benzimidazoles in most of the
cases.15a,b,j However, most of the methods use considerable
amounts of hazardous organic solvents for reaction and extraction
processes, which are not environmentally benign. Moreover, sev-
eral of these reactions were carried out at higher temperatures
and using costly reagents. In the last few years, several eco-friendly
methods have been reported for 1,2-disubstituted benzimidaz-
oles17 but similar methods for the selective synthesis of 2-substi-
tuted benzimidazoles are rare.17f,18
As the use of large volumes of volatile hazardous organic
solvents in industrial processes poses a serious threat to the
environment, development of novel, milder, and sustainable syn-
thetic processes involving environmentally-friendly solvents and
nontoxic reagents has become imperative. One of the most sustain-
able alternatives to organic solvents is water, which has gained
increasing popularity due to being inexpensive, non-toxic, non-
flammable, widely abundant in nature, and environmentally be-
nign. The problem of insolubility and hydrolytic decomposition
of many organic compounds in water may be solved by the use
⇑
Corresponding authors. Tel.: +91 832 2580 320; fax: +91 832 2557 033 (A.C.);
tel.: +91 832 2580 347; fax: +91 832 2557 033 (M.B.).
E-mail addresses: amrita@goa.bits-pilani.ac.in (A. Chatterjee), mainak@goa.bits-
pilani.ac.in (M. Banerjee).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.147

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V. Kumar et al. / Tetrahedron Letters 54 (2013) 5505–5509
of surface-active compounds. They form a colloidal, micellar, or
other organized phase and thereby, solubilize the organic reagents
inside the hydrophobic interior of the organized aqueous media
and allow the reaction to occur. The use of surfactants as catalysts
is widespread, and has been investigated in detail for various reac-
tions in aqueous media.19,20 In particular, dehydration reactions,
which otherwise need anhydrous conditions and thus one of the
most challenging tasks to accomplish in water, have been success-
fully carried out by our group21 and others22 in aqueous media in
the presence of surfactants. Synthesis of benzimidazoles involves
a dehydration step and micellar condition has been successfully
utilized, separately, by Bahrami et al.17f and Ghosh and co-work-
ers17g for the construction of this heterocyclic system in the pres-
ence of mildly basic SDS as catalyst. It is interesting to note that
both the methods predominantly yield 1,2-disubstituted benzimi-
dazoles. As the literature survey reveals that acidic catalysts favor
2-substituted benzimidazoles over 1,2-disubstituted benzimidaz-
oles,15a,b,j we assumed, acidic surfactant, DBSA (dodecylbenzene-
sulfonic acid) could be effective in achieving selectivity toward
2-substituted benzimidazole derivatives. As part of our continued
efforts on the development of safe and ‘green’ protocols for organic
reactions in aqueous media,21 we report herein, DBSA catalyzed
chemoselective synthesis of 2-substituted benzimidazoles in aque-
ous media in which iodine acts as co-catalyst to enhance selectivity
of the desired product (Scheme 1). The primary roles of DBSA are
(a) to assist in solubilizing the organic substrates in aqueous media
by forming micelles or other organized phase and (b) to act as a
catalyst to promote condensation of o-diaminoarene with the
aldehyde.
We started our work with a focus on optimizing the reaction
conditions. In this direction, the formation of the emulsion droplets
was confirmed by taking optical micrograph of different surfac-
tants containing aqueous solutions of reactants before the reaction
would actually proceed (Fig. 1a). Dynamic light scattering (DLS)
experiments of those solutions revealed that the corresponding
size of emulsion droplets is in the nanometer range (Fig. 1b). We
screened catalytic activity of six surfactants on a model reaction
between equimolar mixture of o-phenylenediamine (1a) and benz-
aldehyde (2a) to find out the best catalyst that induces higher
selectivity to 2-phenylbenzimidazoles. To our delight, all six sur-
factants (DBSA, SDS, CTAB, Triton X100, Tween 20, and Tween
80) could catalyze the above reaction at a variable rate to produce
the desired products (3a and 4a) in different proportions indicating
a micellar condition is useful to carry out this condensation reac-
tion (Table 1). As expected, DBSA showed highest selectivity (Ta-
ble 1, entry 1) among all toward 2-phenylbenzimidazole (3a)
with about 10% of undesired 1-benzyl-2-phenyl-1H-benzo[d]imid-
azole (4a). At elevated temperatures, the rate of the reaction was
increased with slightdrop in the selectivity (Table 1, entry 2). SDS
was found to be the most suitable catalyst in terms of time re-
quired for the completion of the reaction with much reduced selec-
tivity toward 2-phenylbenzimidazole (3a) (Table 1, entry 4). The
reaction was slower and selectivity was poor for other cases (Ta-
ble 1, entries 6–9). We also examined the chemoselectivity of
SDS and DBSA on the same reaction upon the addition of 2 equiv
of benzaldehyde at one portion. In case of SDS as catalyst, the re-
sult was close to what is reported by others17f showing pro-
nounced selectivity toward 1,2-disubstituted benzimidazole (4a)
but about 10% of 2-phenylbenzimidazole (3a) was also obtained
(Table 1, entry 5). On the other hand, the reaction was relatively
slow in the presence of DBSA producing nearly equal proportion
of both 3a and 4a even after 6 h (Table 1, entry 3). From the above
results we inferred that acidic nature of DBSA and slow reaction
rate are helpful for the formation of 2-substituted benzimidazoles.
The excellent chemoselectivity induced by DBSA inspired us to
investigate this transformation in detail. Formation of some
amount of undesired 1,2-disubstituted benzimidazole was still
our concern. Literature survey revealed that use of oxidizing agent
under micellar condition17f increases selectivity toward 2-substi-
tuted benzimidazoles by quick conversion of monoimine into the
aromatic system before diimine would form, which is the interme-
diate of 1,2-disubstituted benzimidazole.16a In this regard, various
nonhazardous, easily available, cheap oxidizing agents are chosen
to accelerate oxidative aromatization process from monoimine
and thereby, minimize formation of 4a. Initially, stoichiometric
amounts of oxidizing agents such as I2, H2O2, p-benzoquinone,
ammonium persulfate, and oxone were used separately for con-
densation of equimolar mixture of o-phenylenediamine (1a) and
benzaldehyde (2a) in the presence of 10 mol % of DBSA in water
to find out their influence in the final outcome on the product ratio
(Table 2). Selectivity was improved at various extents toward the
formation of 3a in each case. However, both hydrogen peroxide
and iodine were found equally suitable to bring about higher selec-
tivity as well as to reduce reaction time (Table 2, entries 1 and 5). A
few more reactions were carried out with different aromatic alde-
hydes under same condition in the presence of both I2 and H2O2,23
and based on the observed selectivity and considering the fact that
iodine is milder and easier to handle, it was selected as an additive
for further study. We presumed that the role of iodine could be
twofold: (a) to act as Lewis acid to increase electrophilicity of the
imine bond and thus, facilitate cyclization process and (b) to oxi-
dize dihydroimidazole to corresponding aromatic system. In order
to optimize the amount of iodine required for this condensation,
we carried out reactions with various proportions of iodine at sub-
stoichiometric level keeping other conditions same. We were de-
lighted to observe that use of 10 mol % of iodine is equally
effective as that of stoichiometric amount to impose similar selec-
tivity toward 2-phenylbenzimidazole (Table 2, entry 7). This indi-
cates that the primary role of iodine is to act as Lewis acid24 to
expedite cyclization process and presumably, oxidation is mostly
done by the dissolved oxygen15h,18a in the system. We also exam-
ined that use of more than 10% of DBSA does not enhance che-
moselectivity (or yield) (Table 2, entry 8). Neither the use of less
than 10% of DBSA is suitable for this transformation (Table 2, entry
9). Therefore, we decided to use 10 mol % of iodine as co-catalyst
along with DBSA (10 mol %) for this condensation reaction to
achieve
highest
chemoselectivity
toward
2-substituted
benzimidazoles.
To test the generality of this method, a series of aromatic alde-
hydes was treated with various o-diaminoarenes under optimal
reaction conditions.25 The developed process was found to be
excellent in terms of yield and selectivity resulting in a variety of
2-substituted benzimidazoles in very high yield (Table 3). The
aldehydes with electron donating (Table 3, entries 6, 7, 24, 31,
etc.) as well as with electron withdrawing groups (Table 2, entries
2, 15, 22, 30, etc.) participated in the reaction uniformly with no
significant distinction with regard to the yields of the target prod-
ucts. Similarly, no distinct substituent effect was observed on the
yields of 2-substituted benzimidazoles by varying substituents in
o-diaminoarenes. Even sensitive substrates like furfuraldehyde
(Table 3, entries 9, 19 and 25) produced the desired product
without any difficulty. The present method was fairly applicable
to aliphatic aldehydes as well (Table 3, entry 11, 28, etc.). How-
ever, reactions were sluggish for water soluble aldehydes viz.
NH2
NH2
N
N
DBSA (10 mol%)
N
R'
R
+
R'
+ R' CHO
I2 (10 mol%)
H2O, rt, stir
N
H
R
R
3
2
4
1
R'
80-94%
0-5%
Scheme 1. DBSA catalyzed synthesis of 2-substituted benzimidazoles.

V. Kumar et al. / Tetrahedron Letters 54 (2013) 5505–5509
5507
a
b
Size Distribution by Intensity
4
3
2
1
0
0.1
1
10
100
1000
10000
Diameter (nm)
Figure 1. (a) A typical optical micrograph of vesicles formed in an aqueous solution of DBSA, o-diaminoarene and benzaldehyde. (b) DLS data of DBSA showing formation of
aggregates.
Table 1
Catalytic activity of different surfactants on benzimidazole formation
NH2
N
N
N
Surfactant
H2O
Ph
+
Ph
PhCHO
+
N
H
3a
NH2
2a
1a
4a
Ph
Entry
Surfactanta (10 mol %)
No. of equiv of PhCHO
T (°C)
Time (h)
Yield of 3a (%)
Yield of 4a (%)
1
2
3
4
5
6
7
8
9
DBSA
DBSA
DBSA
SDS
SDS
CTAB
Triton X100
Tween 20
Tween 80
1.0
1.0
2.0
1.0
2.0
1.0
1.0
1.0
1.0
rt
55
rt
rt
rt
55
rt
rt
6.0
2.0
6.0
1.0
0.5
6.0
3.0
6.0
6.0
72
68
52
44
10
32
48
42
39
10
13
44
26
78
24b
25
28
rt
26b
a
The reaction was carried out between 0.5 mmol of o-phenylenediamine (1a) and 0.5 mmol of benzaldehyde (2a) in the presence of 0.05 mmol of surfactants in 2 mL of water.
12–15% of o-phenylenediamine was isolated along with the products.
b
Table 2
Effect of co-catalysts and optimization of the reaction condition
DBSA (A) /
NH2
N
N
N
co-catalyst (B)
Ph
+
Ph
PhCHO
+
H2O, rt, stir
N
H
3a
NH2
2a
1a
4a
Ph
Entry
mol % of A
Co-catalyst (B)
mol % of B
Time (h)
Yield of 3a (%)
Yield of 4a (%)
1
2
3
4
5
6
7
8
9
10
10
10
10
10
10
10
20
5
H2O2
p-Benzoquinone
Oxone
(NH4)2S2O8
I2
I2
I2
I2
I2
100
100
100
100
100
20
10
20
05
2
5
4
4
1.5
2
2
2
6
90
82
78
75
89
88
92
88
74
3
5
6
8
0a
0a
0a
0a
6
a
No trace of 4a was found in the TLC.
butyraldehyde (Table 3, entry 12), which is quite expected as they
will hardly stay inside the micelles. Apparently, the nature and po-
sition of the substitutions in the aryl rings did not have great influ-
ence on the reactivity. Most of the reactions were completed
within 3 h. Only aldehyde with strong electron withdrawing group
reacted relatively faster (Table 2, entries 2, 15, 30, etc.) and reac-
tion was little slow for an aldehyde with strong electron donating
group (Table 2, entries 6, 7, 24, 31, etc.), as expected.
Based on the above results a plausible mechanism has been pro-
posed, which is believed to be the major pathway for this dehydra-
tive condensation followed by cyclization reaction (Scheme 2). The
reaction is initiated by the formation of a Schiff’s base when an
aldehyde molecule comes in contact with an o-diaminoarenes in-
side the hydrophobic core of the micelle. This step is always in-
clined toward product as the water molecule is ejected out of the
hydrophobic core as soon as it is formed. Next, iodine acts as a Le-
wis acid to make a partial bond with imine to increase its electro-
philicity and facilitates attack of the other amino group to the
imine carbon resulting in the formation of dihydrobenzimidazole.
In the final step the dihydrobenzimidazole gets oxidized to the
product by the dissolved oxygen in water (Scheme 3).
In order to expand the scope of this method we treated o-diami-
noarene with 2-formylchromone in 1:1 ratio under similar condi-
tions, which resulted in the formation of a macro-heterocyclic

5508
V. Kumar et al. / Tetrahedron Letters 54 (2013) 5505–5509
Table 3
system, 2,3:9,10-dibenzo-6,13-disalicyloyl-1,8-dihydro-1,4,8,11-
DBSA–I2 catalyzed synthesis of 2-substituted benzimidazoles
tetraaza[14]annulene (6) in a very high yield. Tetraaza-[14]-annu-
lenes like analogous systems, porphyrins, and phthalocyanines,
have a vast range of applications.27 However, their syntheses are
generally carried out in organic media at elevated temperatures.28
The present method appeared to be a potential route to construct
this important macrocyclic framework in an environmentally
friendly and sustainable manner. A detailed study on the synthesis
of these systems will be published elsewhere.
In conclusion, a practical and ‘green’ method has been demon-
strated for the chemoselective synthesis of 2-substituted benzimi-
dazoles in organized aqueous media in the presence of DBSA as
catalyst and iodine as co-catalyst. A broad range of 2-substituted
benzimidazoles have been synthesized fromo-diaminoarenes and
a variety of aldehydes using this method. The operational simplic-
ity, excellent yields of the products, and high chemoselectivity are
some of the merits of this method, and furthermore, this method is
cheap, safe, and environmentally benign.
NH2
N
DBSA (10 mol%)
R
R'
R
+ R' CHO
I2 (10 mol%)
H2O, rt, stir
N
H
NH2
3
2
1
Entry
R
R0
Time (h)
Yield of 3a (%)
Ref.
15d
15d
11
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
2.0
1.0
3.0
2.5
2.0
3.0
5.0
2.0
4.0
2.0
2.5
12.0
6.0
2.0
1.0
3.0
2.0
3.0
4.0
2.5
2.5
2.0
4.0
6.0
4.0
2.5
3.0
6.0
2.0
1.5
5.0
92b
94b
80c
89c
87c
84c
86c
84d
80d
90b
88c
12e,f
82c,f
91b
93b
79c
82c
84c
81d
86c
94b
91b
90b
92b
83d
88d
86c
81c,f
89c
90b
93b
4-NO2C6H4
3-ClC6H4
4-ClC6H4
15d
11
4-BrC6H4
2-OHC6H4
4-OCH3C6H4
C6H5–CH@CH–
Furan-2-yl
Pyrrole-2-yl
Thiophene-2-yl
n-Bu
15d
15d
15a
15d
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
15i
17f
18a
17f
14
Cyclohexyl
Ph
4-NO2C6H4
3-ClC6H4
4-Methyl
4-Methyl
4-Methyl
4-Methyl
4-Methyl
4-Methyl
4-Methyl
4-Nitro
Acknowledgements
26a
26a
15h
4-ClC6H4
M.B. thanks DST (India) (project No. SR/FT/CS-023/2010) for
financial support. A.C. is also thankful to DST (India) (project No.
SR/FT/CS-092/2009) for research fund. V.K. and D.G.K. are indebted
to DST (India) for research fellowships. The authors thank to Mr.
Hari, K. of Goa University for some spectral data.
2-OHC6H4
Furan-2-yl
Thiophene-2-yl
Ph
4-NO2C6H4
2-OHC6H4
4-OCH3C6H4
Furan-2-yl
Pyrrole-2-yl
Thiophene-2-yl
Cyclohexyl
Ph
29
16f
4-Nitro
4-Nitro
4-Nitro
4-Nitro
4-Nitro
4-Nitro
4-Nitro
26b
26b
29
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
07.147.
14
4-Chloro
4-Chloro
4-Chloro
4-NO2C6H4
4-OCH3C6H4
a
All yields refer to isolated product, characterized by melting point, 1H NMR and
References and notes
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NH2
NH2
DBSA (10 mol%)
H2O, rt, stir, 4h
HO
OH
+
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NH
CHO
89%
5
1a
6
Scheme 3. DBSA catalyzed synthesis of dibenzotetraaza-[14]-annulene derivative
(6).

V. Kumar et al. / Tetrahedron Letters 54 (2013) 5505–5509
5509
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DBSA and 1 equiv. of either H2O2 or I2. The yields of corresponding 2-
arylbenzimidazoles are as follows. For I2, p-OMe: 86%, p-NO2: 94%. For H2O2, p-
OMe: 84%, p-NO2: 88%.
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25. General procedure for the syntheses of 2-substituted benzimidazoles: To a solution
of DBSA (0.05 mmol) in H2O (2 mL) were added an amine (0.5 mmol) and
iodine (0.05 mmol). An aldehyde (0.5 mmol) was added portionwise at room
temperature. The reaction was stirred at room temperature for several hours
(see Table 3). The progress of the reaction was monitored by TLC. The
completion of the reaction was indicated by separation of the organic phase
from aqueous media. The aqueous layer was decanted. The organic part was
taken in ethyl acetate, washed with saturated NaHCO3, water, brine and then
dried over anhydrous Na2SO4. The organic layer was concentrated and purified
by silica gel chromatography.
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29. The physical data of some of the compounds are provided below. 5-Methyl-2-
(thiophen-2-yl)-1H-benzo[d]imidazole (Table 3, entry 20): Yellow color solid,
mp 226–227 °C; 1H NMR (500 MHz, DMSO-d6): d (ppm) 2.38 (s, 3H), 6.97 (d,
J = 7.6 Hz, 1H), 7.18 (d, J = 8.5 Hz, 1H), 7.19–7.52 (m, 2H), 7.66 (d, J = 4.9 Hz,
1H), 7.76 (d, J = 2.8 Hz, 1H), 12.74 (s, 1H, NH); 13C NMR (125 MHz, DMSO-d6): d
(ppm) 21.9, 111.2, 118.6, 123.8, 126.9 (2C), 128. 7 (2C), 129.0 (2C), 134.4,
147.1; HRMS (ESI): m/z calcd for C12H11N2S [M+H]+ 215.0643, found 215.0642.
2-(Furan-2-yl)-5-nitro-1H-benzo[d]imidazole (Table 3, entry 25): Almond color
20. For some recent examples see: (a) Arenas, D. R. M.; Bonilla, C. A. M.;
Kouznetsov, V. V. Org. Biomol. Chem. 2013, 11, 3655–3663; (b) Ghosh, P. P.;
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Bhaskaran, S.; Samant, S. D. Green Chem. 2011, 13, 1852–1859; (d) Gao, Q.; Liu,
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solid, mp 222–223 °C; 1H NMR (500 MHz, DMSO-d6):
d (ppm) 6.75 (dd,
J1 = 1.5 Hz, J2 = 3.4 Hz, 1H), 7.31 (d, J = 3.4 Hz, 1H), 7.67 (d, J = 8.9 Hz, 1H), 8.00
(s, 1H), 8.07 (dd, J1 = 2.1 Hz, J2 = 8.9 Hz, 1H), 8.38 (s, 1H); 13C NMR (125 MHz,
DMSO-d6): d (ppm) 113.3 (3C), 118.6, 143.3, 145.0 (2C), 146.4 (2C), 148.1,
148.3; HRMS (ESI): m/z calcd for C11H8N3O3 [M+H]+ 230.0566, found 230.0564.
2,3:9,10-Dibenzo-6,13-disalicyloyl-1,8-dihydro-1,4,8,11-tetraaza[14]annulene
(6):28cRed crystalline solid, mp 225–226 °C, 1H NMR (300 MHz, Py-d5): d (ppm)
7.04 (dd, J1 = 3.6 Hz, J2 = 6.0 Hz, 4H), 7.09 (dt, J1 = 1.2, J2 = 7.5, 2H), 7.28–7.32
(m, 6H), 7.48 (dt, J1 = 1.2, J2 = 7.5, 2H), 7.86 (dd, J1 = 1.0 Hz, J2 = 7.5 Hz, 2H), 8.92
(d, J = 6.3 Hz, 4H), 12.30 (br s, 2H, NH), 14.51 (t, J1 = 6.3 Hz, exchangeable, 2H,
OH); 13C NMR (75 MHz, Py-d5): d (ppm) 111.0, 116.2, 118.1, 120.02, 123.2,
127.1, 131.6, 133.5, 137.6, 153.6, 158.7, 194.8; Mass spectrum (ESI–MS): m/z
529.00 [M+H]+.
23. Reactions are separately carried out with o-phenylenediamine and p-
methoxybenzaldehyde/p-nitrobenzaldehyde in the presence of 10 mol % of