Camphor sulfonic acid catalyzed a simple, facile, and general method for the synthesis of 2-arylbenzothiazoles, 2-arylbenzimidazoles, and 3H-spiro[benzo[d]thiazole-2,3′-indolin]-2′-ones at room temperature

Article abstract of DOI:10.1080/00397911.2020.1870043

A simple, mild, eco-friendly, general, and convenient approach has been developed for the synthesis of structurally diverse 2-arylbenzothiazole derivatives from the reactions of 2-aminothiophenol and various aromatic aldehydes using camphor sulfonic acid

Full text of DOI:10.1080/00397911.2020.1870043

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Camphor sulfonic acid catalyzed a simple, facile,
and general method for the synthesis of 2-
arylbenzothiazoles, 2-arylbenzimidazoles, and
3H-spiro[benzo[d]thiazole-2,3′-indolin]-2′-ones at
room temperature
Gurpreet Kaur, Radha Moudgil, Mussarat Shamim, Vivek Kumar Gupta &
Bubun Banerjee
To cite this article: Gurpreet Kaur, Radha Moudgil, Mussarat Shamim, Vivek Kumar Gupta &
Bubun Banerjee (2021) Camphor sulfonic acid catalyzed a simple, facile, and general method for
the synthesis of 2-arylbenzothiazoles, 2-arylbenzimidazoles, and 3H-spiro[benzo[d]thiazole-2,3′-
indolin]-2′-ones at room temperature, Synthetic Communications, 51:7, 1100-1120, DOI:
10.1080/00397911.2020.1870043
To link to this article: https://doi.org/10.1080/00397911.2020.1870043
Published online: 08 Jan 2021.
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2021, VOL. 51, NO. 7, 1100–1120
https://doi.org/10.1080/00397911.2020.1870043
Camphor sulfonic acid catalyzed a simple, facile, and
general method for the synthesis of 2-arylbenzothiazoles,
2-arylbenzimidazoles, and 3H-spiro[benzo[d]thiazole-2,30-
indolin]-20-ones at room temperature
Gurpreet Kaura, Radha Moudgila, Mussarat Shamima, Vivek Kumar Guptab, and
Bubun Banerjeea
b
aDepartment of Chemistry, Indus International University, Una, India; Post-Graduate Department of
Physics, University of Jammu, Tawi, India
ABSTRACT
ARTICLE HISTORY
Received 20 November 2020
A simple, mild, eco-friendly, general, and convenient approach has
been developed for the synthesis of structurally diverse 2-arylbenzo-
thiazole derivatives from the reactions of 2-aminothiophenol and
various aromatic aldehydes using camphor sulfonic acid a low cost,
commercially available, efficient organo-catalyst in aqueous ethanol
at room temperature. Under the same optimized conditions, a series
of 2-arylbenzimidazoles was also synthesized starting from o-phenyl-
enediamines and various aldehydes whereas synthesis of 3H-spiro[-
benzo[d]thiazole-2,30-indolin]-20-ones was accomplished from the
reactions of 2-aminothiophenol and substituted isatins.
KEYWORDS
2-arylbenzimidazoles; 2-
arylbenzothiazoles; camphor
sulfonic acid; 3H-
spiro[benzo[d]thiazole-2,30-
indolin]-20-ones;
organocatalysis
GRAPHICAL ABSTRACT
Introduction
Benzothiazole and benzimidazole skeletons are very common in naturally occurring bio-
active compounds (Figure 1).[1–8] Many commercially available drug molecules bearing
CONTACT Bubun Banerjee
banerjeebubun@gmail.com; bubun.banerjee@iiuedu.in
Department of Chemistry,
Indus International University, V.P.O. Bathu, Una 174301, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2021 Taylor & Francis Group, LLC

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Figure 1. Naturally occurring bioactive compounds containing benzimidazole or benzothia-
zole skeleton.
Figure 2. Important drug molecules containing benzimidazole or benzothiazole moieties.
benzothiazole or benzimidazole as the main structural unit (Figure 2).[9–41]
Recent studies revealed that various synthetic benzothiazole or benzimidazole derivatives
possess significant biological efficacies such as anti-oxidant, anti-tumor, anti-cancer,
anti-proliferative, anti-anxiety, etc. activities (Figure 3).[42–71]
After noticing the huge biological importance of benzothiazole as well as benzimida-
zole derivatives, scientists were motivated to synthesize a variety of such derivatives
under various reaction conditions. Among many others, during the last two decades,

1102
G. KAUR ET AL.
Figure 3. Glimpse of bioactive synthetic benzimidazole and benzothiazole derivatives.
both 2-arylbenzothiazoles and 2-arylbenzimidazoles gained huge attention. As a result, a
large number of methods were reported for the synthesis of 2-arylbenzothiazoles from
the reactions of various aromatic aldehydes and 2-aminothiophenol in the presence of a
variety of homogeneous or heterogeneous catalysts under various reaction conditions
(Table 1).[72–91] On the other hand, 2-arylbenzimidazoles were synthesized from the
reactions of various aromatic aldehydes and o-phenylenediamine by using various cata-
lytic systems under diverse reaction conditions (Table 2).[92–131] Though these reported
protocols have some merits still many of them are suffering in terms of green chemistry
perspectives due to the use of metal-containing catalysts and toxic organic solvents.
Moreover, in some cases, high catalyst loading, heating, microwave-irradiation, ball mill-
ing, visible light, etc. were required additionally. Besides, all these reported methods are
focused only on a particular scaffold, that is, either synthesis of 2-arylbenzothiazoles or
2-arylbenzimidazoles.
Therefore, search for an efficient and general route for the synthesis of both 2-
arylbenzothiazoles as well as 2-arylbenzimidazoles by using a metal-free organocata-
lyst under greener conditions still remains a valid exercise.
Under these environmentally conscious days, metal-free organocatalysts have gained
tremendous attention due to their environmental friendliness.[131–136] Among many
others, camphor sulfonic acid (CSA) has also come out as an efficient, low cost,

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Table 1. Reported protocols for the synthesis of 2-arylbenzothiazoles. .
Entry
Catalyst
Solvent
H2O
EtOH
CH3OH:H2O
EtOAc
EtOH
Neat
EtOH
Neat
Neat
CH3CN
EtOH
Neat
H2O
EtOH:H2O
DMF
CH2Cl2
Glycerol
MeOH
Toluene
Neat
Temp.
60–65 ꢀC
RT
Time
Yield (%)[REF]
1
2
3
4
5
6
7
8
b-CD
4–10 h
45 min
30 min
6 h
10 min
4 min
15 min
3 min
13 min
2.5 h
2 h
30 min
5 h
2 h
25 min
24 h
81[72]
90[73]
84[74]
91[75]
95[76]
93[77]
87[78]
92[79]
97[80]
88[81]
85[82]
91[83]
97[84]
89[85]
88[86]
80[87]
92[88]
75[89]
69[90]
99[91]
93[this work]
30%H2O2/37%HCl
NH4Cl
12 W blue LED
SnP2O7
RT
RT
Reflux
Acacia concinna
MW
PIFA
I2
MW, 80 ꢀC
MW
9
H2O2 (30 %)/ CAN
tert-butyl hypochlorite
HClO4/PANI
Nano-ZnO
PTSA
50 ꢀC
Reflux
Reflux
Ball-mill
70 ꢀC
55 ꢀC
100 ꢀC
RT
10
11
12
13
14
15
16
17
18
19
20
21
Sm(OTf)3
I2
Baker’s Yeast
RT
RT
30 min
10 h
75 min
1 h
1 h
CAN
ZnBr2-ABM
L-Proline
CSA
110 ꢀC
MW
EtOH:H2O
28–32 ꢀC
TBHP: tert-Butyl hydroperoxide; CAN: ceric ammonium nitrate; HClO4/PANI: perchloric acid/doped polyaniline; PTSA:
para-toluene sulfonic acid; ABM: animal bone meal.
commercially available organocatalyst.[137] In continuation of our strong interest in
organo-catalyzed protocols,[138–143] in this communication we wish to report camphor
sulfonic acid-catalyzed a simple, efficient, and general method for the synthesis of
2-arylbenzothiazoles, 2-arylbenzimidazoles, and 3H-spiro[benzo[d]thiazole-2,30-indolin]-
20-one derivatives in aqueous ethanol at room temperature (Schemes 1 and 2).
Results and discussion
To optimize the reaction condition, we carried out a series of trial reactions between
freshly distilled benzaldehyde (1a; 1 mmol) and 2-aminothiophenol (2; 1 mmol) under
various reaction conditions at ambient temperature. At first, we performed the reaction
under catalyst as well as solvent-free conditions at room temperature which produced
only a trace amount of the desired product after 4 h (Table 3, entry 1). Less than 10%
yield was observed under catalyst-free conditions in water after 4 h (Table 3, entry 2).
After obtaining poor yields under catalyst-free conditions we realized the necessity of a
suitable catalyst to promote this reaction. Initially, in continuation of our strong interest
with mandelic acid as catalyst, for this reaction also we employed 20 mol% mandelic
acids as catalyst in water which afforded 53% of 2-phenylbenzo[d]thiazole (3a) after 2 h
(Table 3, entry 3). Using the same amount of catalyst in ethanol, we obtained 3a with a
62% yield after 2 h (Table 3, entry 4). The same reaction in methanol (Table 3, entry 5)

1104
G. KAUR ET AL.
Table 2. Reported protocols for the synthesis of 2-arylbenzimidazoles.
Entry
Catalyst
Solvent
H2O
DMF
DMAc
dry DCM
DMF
Temp.
Time
Yield (%)[REF]
1
2
3
4
5
6
7
8
SBSA
RT
25 min
10 min
10 min
45 min
2 h
97[92]
p-TsOH
NaHSO3
CAN
FeCl3/Al2O3
ZnCl2-SiO2
CAN
Fe/MgO
La(OTf)3
L-Proline, pH 4.2
80 ꢀC
MW
Reflux
25 ꢀC
MW
50 ꢀC
RT
85[93]
88[94]
75[95]
86[96]
Neat
5 min
94[97]
PEG-400
MeOH
CH3CN-H2O
H2O
CH3CN
EtOH
2 h
98[98]
25 min
3 h
92[99]
9
RT
86[100]
97[101]
88[102]
94[103]
95[104]
96[105]
94[43]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
Reflux
5 h
3 h
8 h
LaCl3
RT
Zn(OTf)2
Reflux
MW
RT
Oxalic acid
Blue LEDs (7 W)
NH4Cl
Neat
4 min
7 h
MeOH
CHCl3
Neat
THF
EtOH
EtOH
EtOH
Neat
Neat
H2O
H2O
EtOH
EtOH
EtOH
Mesitylene
EtOH
RT
4 h
5 h
2 h
2 h
Er(OTf)3
80 ꢀC
RT
78[106]
75[107]
95[108]
63[109]
80[110]
97[111]
94[112]
92[113]
92[114]
86[115]
93[116]
95[117]
78[118]
90[119]
93[120]
81[121]
88[122]
93[123]
93[124]
88[125]
90[126]
92[127]
94[128]
92[129]
94[130]
87[this work]
tert-butyl nitrite
[Cu(BHPPDAH)H2O]
NH4Cl
RT
80–90 ꢀC
RT
2.5 h
11 h
ꢁ
Na3AlF6
K4[Fe(CN)6] grinding
PbO2
RT
RT
RT
10 min
30 min
2 h
H3BO3
[bmim]PF6
Fe3O4 NPs
Co(OH)2 /CoO
MnZrO2-450
Pt/TiO2
RuO2-MoO3
Ce(NO3)3.6H2O
CoCl2.6H2O
LaCl3
Reflux
RT
4 h
6 h
RT
80 ꢀC
Reflux
Reflux
80 ꢀC
RT
60 min
24 h
55 min
1.5 h
4 h
DMF
CH3CN
CH3CN
EtOAc
DMF
CH3CN
Neat
Neat
CH3CN
Neat
Toluene
EtOH:H2O
RT
3 h
6 h
MgI2
RT, hꢀ
60 ꢀC
RT
MW
MW
RT
MW
110 ꢀC
28–32 ꢀC
MgCl2.6H2O
100 min
YCl3
3 h
[Epty]BF4
20 min
4 min
20 min
25 min
24 h
1 h
Amberlite IR-120
H5IO6-SiO2
Glacial acetic acid
CuFe2O4 NPs
CSAa
aInstead of benzaldehyde we have used p-chlorobenzaldehyde.
BHPPDAH: N,N-bis(2-hydroxylphenyl)pyridine-2,6-diarboxamide; CAN: ceric ammonium nitrate; SBSA: silica boron sul-
fonic acid.
or acetonitrile (Table 3, entry 6) as solvent afforded 59% or 38% yields respectively.
Slight better yield (72%) was obtained aqueous-ethanol (1:1 v/v) as solvent through the
time required still 2 h (Table 3, entry 7). From these preliminary studies, it was estab-
lished that aqueous-ethanol may be the best suitable solvent to carry out this reaction.
But we were unsatisfied with the performance of mandelic acid for this reaction and
that is why we reinvestigated the catalytic efficiency of some other metal-free organo-

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Scheme 1. CSA-catalyzed synthesis of 2-arylbenzothiazoles and 2-arylbenzimidazoles at room
temperature.
Scheme 2. CSA-catalyzed synthesis of 3H-spiro[benzo[d]thiazole-2,30-indolin]-20-ones at room
temperature.
Table 3. Optimization of reaction conditions for the synthesis of 2-phenylbenzo[d]thiazole.
Entry
Catalyst (mol%)
Solvent
Time (h)
Yield (%)a,b
1.
2
3
4
5
6
7
8
9
Catalyst-free
Catalyst-free
Neat
H2O
H2O
EtOH
MeOH
4
4
2
2
2
2
2
2
2
1
1
1
trace
10
53
62
59
38
72
66
54
93
82
93
Mandelic acid (20)
Mandelic acid (20)
Mandelic acid (20)
Mandelic acid (20)
Mandelic acid (20)
Itaconic acid (20)
Palmitic acid (20)
CSA (20)
MeCN
H2O:EtOH
H2O:EtOH
H2O:EtOH
H2O:EtOH
H2O:EtOH
H2O:EtOH
10
11
12
CSA (15)
CSA (25)
aReaction conditions: benzaldehyde (1; 1 mmol) and 2-aminothiophenol (2; 1 mmol) in the absence or presence of a
catalytic amount of camphor sulfonic acid in neat/4 mL of water/ethanol/methanol/aqueous ethanol at 28–32 ꢀC; biso-
lated yields.
catalysts. When we employed 20 mol% of itaconic acid (Table 3, entry 8) or palmitic
acid (Table 3, entry 9) as a catalyst in aqueous-ethanol it produced 66% or 54% of 3a
respectively after 2 h. When we employed 20 mol% camphor sulfonic acid (CSA) as a
catalyst in aqueous ethanol, surprisingly, the reaction completed within just 1 h and

1106
G. KAUR ET AL.
yielded 93% of the desired product (Table 3, entry 10). We then standardized the
amount of required catalyst by maintaining other parameters remain fixed. More than
10% less product (82%) was isolated using 15 mol% of CSA (Table 3, entry 11) whereas
no significant improvement was observed in the product formation even after increasing
the catalyst amount to 25 mol% (Table 3, entry 12). Therefore, it was optimized that
20 mol% of camphor sulfonic acid (CSA) is sufficient for the efficient synthesis of 2-
phenylbenzo[d]thiazole (3a) from the unimolar reaction of benzaldehyde (1a) and 2-
aminothiophenol (2) in aqueous ethanol at room temperature (Table 3, entry 10). To
check the generality of our developed protocol we were interested to synthesize a series
of other derivatives of 2-arylbenzothiazoles under the same optimized reaction condi-
tions. It is our delight to mention that we were successful to synthesize ten other deriva-
tives of 2-arylbenzothiazoles (3 b-3k) with excellent yields (74-94%) from the reactions
of various aromatic aldehydes (1b–1k) and 2-aminothiophenol (2) (Table 4, entries 2-
11). Aldehydes with both electron-withdrawing as well as donating substituent under-
went smoothly and afforded the desired products with excellent yields. It may be due to
steric reason o-nitrobenzaldehyde (Table 4, entry 2) yielded a lesser product than
p-nitrobenzaldehyde (Table 4, entry 3). Heteroaryl aldehydes such as indole-3-carboxal-
dehyde (Table 4, entry 10) and chromone-3-carboxaldehyde (Table 4, entry 11) also
produced the desired products with excellent yields. To extend the scope of our devel-
oped protocol, we then planned to carry out the reactions between aldehydes (1,
1 mmol) and o-phenylenediamine (4, 1 mmol) instead of 2-aminothiophenol (2) under
the same reaction conditions which afforded the corresponding 2-arylbenzimidazoles
(5a-5c) with excellent yields (79–91%) within 90 min (Table 5, entries 1–3). Moreover,
under the same optimized reaction conditions synthesis of 3H-spiro[benzo[d]thiazole-
2,30-indolin]-20-ones (7a–7c) was also achieved from the reaction of 2-aminothiophenol
(2; 1 mmol) and isatins (6; 1 mmol) with excellent yields (76–82%) (Scheme 2).
All the synthesized compounds were obtained pure just by simple filtration and wash-
ing the crude products subsequently with aqueous ethanol. No column chromatographic
purification was required. We were successful to synthesize 2-phenylbenzo[d]thiazole (3a;
1.77g, 84%) in gram scale within 3 h from the reactions of freshly distilled 10mmol ben-
zaldehyde (1a; 1.06g) and 10mmol 2-aminothiophenol (2; 1.25g) using 20mol% CSA
(0.46 g) in aqueous ethanol (20ml) at room temperature. All the synthesized compounds
1
were well characterized by the detailed physical as well as spectroscopic analyses of H
NMR, 13C NMR, and HRMS. It is also noted worthy to mention that we were able to
form a single crystal of one derivative, that is, compound 7a. ORTEP view of the com-
pound 7a is shown in Figure 4. In this ORTEP small spheres of arbitrary radii represent
H atoms. Figure 5 shows the packing view of molecules down a-axis within the unit cell
of compound 7a. From the X-ray structure, it is confirmed that the molecules in the unit
cell are bound by the intermolecular N–H … O hydrogen bonds (Figure 6). A plausible
mechanism for the formation of 2-arylbenzothiazoles is shown in Figure 7.

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Table 4. Synthesis of 2-arylbenzothiazoles (3a–3k) using camphor sulfonic acid as catalyst at room
temperature.
Entry Ar-CHO
1
Product
Time (h)
1
Yield (%)a,b
93
2
1
89
3
4
1
1
94
88
5
6
1
91
83
1.5
7
8
1
1
88
86
9
1.5
1.5
82
79
10
11
1.5
74
aReaction conditions: aldehydes (1; 1 mmol) and 2-aminothiophenol (2; 1 mmol) in the presence of 20 mol% camphor
sulfonic acid as catalyst in aqueous ethanol at 28–32 ꢀC. bIsolated yields.

1108
G. KAUR ET AL.
Table 5. Synthesis of 2-arylbenzimidazoles (5a–5c) using camphor sulfonic acid as catalyst at room
temperature.
Entry
1
Ar-CHO
Product
Time (h)
1
Yield (%)a,b
91
2
3
1
87
79
1.5
aReaction conditions: aldehydes (1; 1 mmol) and o-phenylenediamine (4; 1 mmol) in the presence of 20 mol% camphor
sulfonic acid as catalyst in aqueous ethanol at 28–32 ꢀC. bIsolated yields.
Figure 4. ORTEP view of the molecule 7a with displacement ellipsoids drawn at 50% probability level
(CCDC 2025015).

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1109
Figure 5. Packing view of the molecules in the unit cell viewed down the a-axis.
Figure 6. A plot of molecules of the compound 7a showing the intermolecular N–H … O hydro-
gen bonds.

1110
G. KAUR ET AL.
Figure 7. Plausible mechanism for the synthesis of 2-arylbenzothiazoles using CSA as catalyst.
Experimental section
General
Melting points were recorded on a Digital Melting Point Apparatus (Model No. MT-
934) and are uncorrected. TLC was performed on silica gel 60 F254 (Merck) plates.
Infrared spectra were recorded on Agilent (Cary 660) FT-IR spectrophotometer on KBr
1
disks. H and 13C NMR spectra were obtained at 500 MHz Jeol (JNM ECX-500) NMR
machines with CDCl3 as the solvent. Mass spectra (TOF-MS ESþ) were measured on a
Bruker Impact HD QTOF Micro mass spectrometer.
General procedure for the synthesis of 2-phenylbenzo[d]thiazole (3a)
In a dry screw-cap test tube a magnetic stir bar, benzaldehyde (1a; 1 mmol), 2-amino-
thiophenol (2; 1 mmol), 4 mL aqueous ethanol and a catalytic amount of camphor sul-
fonic acid (20 mol%) were taken sequentially. On a magnetic stirrer, the reaction
mixture was then stirred vigorously at room temperature and it was monitored by TLC.
After completion of the reaction, the yellow product was isolated pure just by simple fil-
tration and washing the crude products with aqueous ethanol (H2O:EtOH ¼ 1:2) subse-
1
quently. The structure of the synthesized compound was confirmed by H NMR, 13C
NMR, and HRMS analysis.
1
2-Phenylbenzo[d]thiazole (3a): Yellow solid; H NMR (500 MHz, CDCl3) d/ppm: 8.10-
8.08 (1H, m, aromatic H), 8.07 (1H, s, aromatic H), 7.91 (1H, d, J ¼ 7.5 Hz, aromatic
H), 7.55–7.53 (1H, m, aromatic H), 7.51-7.48 (2H, m, aromatic H), 7.39 (1H, t,
J ¼ 7.5 Hz, aromatic H), 7.35-7.32 (2H, m, arpmatic H); 13C NMR (125 MHz, CDCl3)
d/ppm: 146.35, 131.09, 129.14, 128.88 (2 C), 127.66, 126.66, 126.432, 125.30, 123.32,
121.73, 120.81, 109.83; HRMS (ESI-TOF) m/z: For C13H9NS Calcd. [M þ H]þ 212.0534;
Found [M þ H]þ 212.0535.

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General procedure for the synthesis of 2-(4-nitrophenyl)-1H-
benzo[d]imidazole (5a)
By repeating the before mentioned procedure, 2-(4-nitrophenyl)-1H-benzo[d]imidazole
(5a) was also synthesized from the reaction of p-nitrobenzaldehyde (1c; 1 mmol) and
o-phenylenediamine (4; 1 mmol) in the presence of a catalytic amount of camphor sul-
fonic acid as catalyst under the same optimized conditions. The structure of the synthe-
1
sized compound was confirmed by H NMR, 13C NMR, and HRMS analysis.
2-(4-Nitrophenyl)-1H-benzo[d]imidazole (5a): Reddish solid; 1H NMR (500 MHz,
CDCl3) d/ppm: 8.63 (1H, s), 8.31 (2H, d, J ¼ 8.25 Hz, aromatic H), 8.06 (2H, dd,
J ¼ 8.95, 6.90, & 2.05 Hz, aromatic H), 7.14–7.12 (2H, m, aromatic H), 6.81–6.75 (2H,
m, aromatic H), 4.25 (1H, s); 13C NMR (125 MHz, CDCl3) d/ppm: 153.73, 148.96,
143.09, 141.89, 135.58, 129.14, 129.08 (2 C), 123.99 (2 C), 118.43, 116.87, 115.80. HRMS
(ESI-TOF) m/z: For C13H9N3O2 Calcd. [M þ H]þ 240.0773; Found [M þ H]þ 240.0546.
General procedure for the synthesis of 3H-spiro[benzo[d]thiazole-2,30-indolin]-20-
one (7a)
Synthesis of 3H-spiro[benzo[d]thiazole-2,30-indolin]-20-one (7a) was achieved from the
reaction of 2-aminothiophenol (2; 1 mmol) and isatin (6; 1 mmol) using the same
20 mol% of camphor sulfonic acid as catalyst in aqueous ethanol at room temperature.
3H-Spiro[benzo[d]thiazole-2,30-indolin]-20-one (7a): Off white solid; 1H NMR
(500 MHz, CDCl3) d/ppm: 11.01 (1H, s, NH), 7.63 (1H, d, J ¼ 7.5 Hz, aromatic H), 7.42
(1H, s, NH), 7.29 (1H, t, J ¼ 7.5 & 4.0 Hz, aromatic H), 7.10 (2H, t, J ¼ 10.5 & 7.5 Hz,
aromatic H), 7.00 (1H, t, J ¼ 7.5 Hz, aromatic H), 6.85–6.81 (2H, m, aromatic H), 6.73
(1H, d, J ¼ 7.5 Hz, aromatic H); 13C NMR (125 MHz, CDCl3) d/ppm: 168.49, 146.32,
141.12, 139.49, 137.69, 130.89, 126.11, 125.79, 123.86, 121.79, 121.56, 110.84, 110.42,
73.35; HRMS (ESI-TOF) m/z: For C14H10N2OS Calcd. [M þ Na]þ 277.0412; Found
[M þ Na]þ 277.0262.
Conclusions
In conclusion, we have developed a simple, facile, efficient, and general method for the
synthesis of a series of structurally diverse 2-arylbenzothiazole derivatives via the con-
densation reactions of various aromatic aldehydes and 2-aminothiophenol using a cata-
lytic amount of camphor sulfonic acid as catalyst in aqueous ethanol at room
temperature. Reactions of aromatic aldehydes and o-phenylenediamine afforded the cor-
responding 2-arylbenzimidazole derivatives. Under the same optimized conditions, syn-
thesis of 3H-spiro[benzo[d]thiazole-2,30-indolin]-20-ones was also achieved from the
reactions of 2-aminothiophenol and isatins. Mild reaction conditions, use of metal-free
commercially available low-cost organocatalyst, high yields, short reaction times, column
chromatography-free purification procedure, and gram-scale production are some of the
major advantages of this developed protocol.
1
Full experimental detail, scanned spectra including H-NMR, 13C-NMR, and HRMS
of all the synthesized scaffolds are supplemented in supporting information. This mater-
ial can be found via the “Supplementary Content” section of this article’s webpage.’

1112
G. KAUR ET AL.
Acknowledgments
Dr. B. Banerjee is thankful to the Indus International University, Una, Himachal Pradesh, India
and the Kartha Education Society, Mumbai, India for the support. The authors are grateful to
1
AMRC, IIT Mandi, Himachal Pradesh, India for the spectral measurements such as H and 13C
NMR and HRMS data.
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