Titanium Tetrabutoxide (TTBO) as Efficient Catalyst for Rapid One Pot Synthesis of 2-Arylbenzothiazoles under Mild Conditions

Article abstract of DOI:10.1002/jccs.201400105

An effective strategy has been developed for rapid and efficient one pot synthesis of 2-arylbenzothiazoles from readily available 2-aminothiophenol and aromatic aldehydes catalyzed by TTBO in high yields and short reaction times. This strategy allows acce

Full text of DOI:10.1002/jccs.201400105

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Titanium Tetrabutoxide (TTBO) as Efficient Catalyst for Rapid One Pot Synthesis of
2-Arylbenzothiazoles under Mild Conditions
Hossein Naeimi* and Arash Heidarnezhad
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan, 87317, I.R. Iran
(Received: Feb. 26, 2014; Accepted: Mar. 17, 2014; Published Online: ??; DOI: 10.1002/jccs.201400105)
An effective strategy has been developed for rapid and efficient one pot synthesis of 2-arylbenzothiazoles
from readily available 2-aminothiophenol and aromatic aldehydes catalyzed by TTBO in high yields and
short reaction times. This strategy allows access to a structurally diverse array of products for further ma-
nipulation. The reactions were preceded under mild conditions to afford 2-arylbenzothiazole derivatives.
The pure products were identified and characterized by physical and spectroscopic data such as; IR, 1H
NMR and 13C NMR.
Keywords: Titanium tetrabutoxide; 2-Arylbenzothiazoles; Lewis acid; 2-Aminothiophenol; Synthe-
sis; Catalyst.
INTRODUCTION
Benzothiazole is a heterocyclic compound, weak
plex work-up and purification, strongly acidic conditions,
high temperatures, use of toxic metal catalysts and long re-
action times. Therefore, in the development of facile and
mild general method to overcome these short comings re-
mains a challenge for organic chemists in the synthesis of
2-arylbenzothiazoles.18-27
base, having varied biological activities and still of great
scientific interest now a days. They are widely found in
bioorganic and medicinal chemistry with application in
drug discovery. Benzothiazoles are known to inhibit sev-
eral enzymes such as acetyl cholinesterase,1 mon-amine
oxidase,2,3 lipoxygenase, lysophosphatidic acid acyltrans-
ferase-b,4 aldose reductase, cyclooxygenase,5 carbonic
anhydrase, HCV helicase, and protease.6 Other recognized
pharmacological activities of benzothiazoles include anti-
tumor,7 antimicrobial, antioxidant and antiglutamate.8 Var-
ious benzothiazoles such as 2-arylbenzothiazole received
much attention due to unique structure and its uses as radio-
active amyloid imagining agents9 and anticancer agents.10
The most useful synthesis of benzothiazoles include con-
densation of 2-aminobenzenethiol with carboxyl deriva-
tives,11 cyclization of ortho-haloanilides,12 the radical
cyclization of thioacylbenzanilides,13 DMP mediated intra-
molecular cyclization of thioformanilides.14 The direct
condensation of 2-aminothiophenol with aromatic alde-
hyde15,16 under microwave irradiation17 in the presence of
various catalysts such as MnSiO2, P-TsOH or graphite sup-
ported on solid minerals I2/DMF, 1-phenyl-3-methylimida-
zolium bromide[PmIm]Br, O2 or H2O2 in the presence of
Sc(OTf)3, cerium(IV) ammonium nitrate(CAN). Despite
their potential utility, some of these methods are not envi-
ronmentally friendly and suffer from one or more disad-
vantages for example hazardous reaction conditions, com-
In this study, in continuation of our interest on the
synthesis of 2-arylbenzothiazoles, we decided to explore
ability of Lewis acids as suitable catalysts for synthesis of
significant and benefit 2-arylbenzothiazoles. Therefore, a
number of different Lewis acids was used for synthesis of
2-arylbenzothiazoles through condensation reaction of
2-aminothiophenol with aromatic aldehydes under mild
conditions.
RESULTS AND DISCUSSION
Anumber of methods are reported for the synthesis of
heterocyclic compounds by using different catalysts. In
this research, in order to the development of novel syn-
thetic methodologies using Lewis acids as catalyst, we
have found that TTBO can be efficient catalyst for synthe-
sis of 2-arylbenzothiazoles from 2-aminothiophenol and
aldehydes in excellent yields under mild conditions
(Scheme 1).
To achieve the best results in terms of yields and reac-
tion times we examined the efficiency of different reaction
media and catalyst amounts in condensation reaction of
4-nitrobenzaldehyde with 2-aminothiophenol as a reaction
model in ethanol solution. In order to find the optimum re-
* Corresponding author. Tel: +983615912388; Fax: +983615912397; Email: naeimi@kashanu.ac.ir
Supporting information for this article is available on the www under http://dx.doi.org/10.1002/jccs.201400105
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1

Article
Naeimi and Heidarnezhad
Table 1. Effect of various Lewis acids on the reaction of 4-nitro-
benzaldehyde with 2-aminothiophenol
Synthesis of benzothiazole derivatives from
2-aminothiophenol and aldehydes
Scheme 1
Entry
Catalyst
Time (min)
Yielda (%)
action conditions for the condensation reaction, prelimi-
nary efforts were mainly focused on the evaluation of dif-
ferent Lewis acids. So a number of different Lewis acids
was examined on the model reaction, the results are shown
in Table 1. As can be seen in this Table, when the reaction
was carried out in the presence of TTBO as catalyst, the
products were obtained in excellent yields and very short
reaction times, while, the reaction using the other catalysts
were occurred in lower yields and longer reaction times
(Table 1, entry 12 vs entries 1-11).
1
AlCl3
FeCl3
15
20
48
7
2
3
Cu(OAc)2.2H2O
Zn(OAc)2
Al2O3
20
25
30
30
80
10
70
40
15
65
90
4
15
5
6b
25
SSA
10
7
Nano TiO2
BF3(Et2O)
Nano SiO2
SiO2
20
8
15
9
25
10
11
12c
25
P2O5/SiO2
TTBO
15
Immed.
In according to the Table 1, the results related to dif-
ferent Lewis acids were indicated that TTBO as catalyst
plays a critical role in this reaction and was highly effective
catalyst for condensation reaction of different aldehydes
with 2-aminothiophenol. Thus, TTBO was used as the pre-
ferred catalyst for further studies. The catalytic role of
TTBO can be chelating the oxygen of carbonyl group by Ti,
and also acting as a base to generate the thiolate anion that
let to release the nucleophilisity of sulfur atom. So the ef-
fect of catalyst amount on this reaction was studied in Table
2. Also, it was found that with increase the amount of
TTBO, yield of the desired 2-arylbenzothiazole increased.
Hence 6.6 mol% of TTBO catalyst was chosen as the opti-
mum concentration of catalyst in this reaction. The results
are shown in Table 2.
[a] Isolated yield.
[b] Silica sulfuric acid.
[c] Titanium tetrabutoxide.
Table 2. Effect of TTBOa as Lewis acid catalyst on the reaction
of 4-nitrobenzaldehyde with 2-aminothiophenol
Entry
TTBO loading (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
1.1
2.2
3.3
4.4
5.5
6.6
7.7
8.8
50
62
75
80
90
95
95
92
After the optimization of TTBO amount as catalyst,
to demonstrate the generality and scope of this method, it
was examined the condensation reaction with a number of
different aldehydes and 2-aminothiophenol at room tem-
perature. The results of these reactions are summarized in
Table 3.
[a] Titanium tetrabutoxide.
[b] Isolated yield.
As it is shown in Table 3, the yields of aldehydes with
EWG groups like p-NO2 (entry 9) is more than the alde-
hydes with EDG groups like o-Me (entry 4). Certain atoms
or groups of atoms can add or withdrawal electron density
to a system. Electron withdrawing groups (EWG) remove
electron density from a system and tend to stabilize anions
or electron rich structures and electron donating groups
(EDG) add electron density to a system and tend to stabilize
cations or electron poor systems. In other hand, the steric
hindrance, or crowdedness around the electrophile, is an
important factor that influences reactivity.
The different aldehydes were reacted with 2-amino-
thiophenol without any significant difference in the reac-
tion time to give the corresponding 2-arylbenzothiazole de-
rivatives in good yields (Table 3). All the products were
synthesized in good to excellent yields and characterized
2
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Synthesis of 2-Arylbenzothiazoles Catalyzed by Ti(OBu)4
1
Scheme 2 The proposed reaction mechanism
with physical and spectroscopic data such as, FT-IR, H
NMR and 13C NMR.
The proposed reaction mechanism
The proposed reaction mechanism for synthesis of
2-arylbenzothiazoles is shown in Scheme 2. In this reac-
tion, TTBO as a strong Lewis acid can be activated the car-
bonyl group of aldehyde I for initial nucleophilic attack of
NH2 group to aldehyde to form a Schiff base intermediate
II. On the other hand, the catalyst can be absorbed hydro-
gen atom from intermediate II followed by nucleophilic at-
tack of thiolate anion to C=N resulted the intermediate IV.
Then, the subsequent oxidation of IV in the presence of air
oxygen to furnish 2-arylbenzothiazoles as product V
(Scheme 2).
CONCLUSIONS
the materials were of commercial grade. The FT-IR spectra were
obtained with potassium bromide pellets in the range 400-4000
cm-1 with a Perkin Elmer 550 spectrometer. Melting points were
determined in open capillaries using an Electrothermal MK3 ap-
paratus and are uncorrected. All 1H NMR and 13C NMR spectra
were recorded with a Bruker DRX-400 spectrometer at 400 MHz.
The NMR spectra were obtained in CDCl3 solutions and are re-
ported as parts per million (ppm) downfield from tetramethyl-
silane (TMS) as internal standard. The abbreviations used are:
singlet (s), doublet (d), triplet (t) and multiplet (m). The purity de-
termination of the substrates and reaction monitoring were ac-
complished by TLC on silica-gel polygram SILG/UV 254 plates
In conclusion, we have explored a useful and practi-
cal approach to 2-arylbenzothiazoles with a facile and mild
procedure by condensation reaction of 2-aminothiophenol
and aromatic aldehydes using TTBO as an efficient cata-
lyst. The main advantages of the present synthetic protocol
are mild, high yields, short reaction times and easy reaction
work up procedure.
EXPERIMENTAL
The chemicals and TTBO as catalyst were purchased from
the Fluka and Merck Chemical Companies in high purity. All of
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Article
Naeimi and Heidarnezhad
1
(from Merck Company).
830; H NMR (400 MHz, CDCl3)/ d ppm: 3.89 (s, 3H, OCH3),
7.015 (d, 2H, J = 8.8 Hz, Ar-H), 7.36 (t, 1H, J = 8.0 Hz, Ar-H),
7.48 (t, 1H, J = 8.0 Hz, Ar-H), 7.89 (d, 1H, J = 8.0 Hz, Ar-H),
8.04-8.06 (d, 3H, J = 8.8 Hz, Ar-H); 13C NMR/ (400 MHz,
CDCl3)/ d ppm: 55.47, 114.38, 121.53, 122.83, 124.81, 126.23,
126.42, 129.13, 134.86, 154.21, 161.94, 167.89. 2-(4-Nitro-
phenyl)-benzothiazole: Yellow solid; m.p. = 227-229 oC (m.p. =
226-228 oC);29 IR (KBr)/ u(cm-1): 3042, 2937, 1520, 1461, 1342,
1106, 851, 762; 1H NMR (400 MHz, CDCl3)/ d ppm: 7.45-7.49 (t,
1H, J = 7.2 Hz, Ar-H), 7.54-7.58 (t, 1H, J = 7.2 Hz, Ar-H), 7.97 (d,
1H, J = 7.6 Hz, Ar-H), 8.14 (d, 1H, J = 7.6 Hz, Ar-H), 8.27 (d, 2H,
J = 8.0 Hz, Ar-H), 8.36 (d, 2H, J = 8.0 Hz, Ar-H); 13C NMR/ (400
MHz, DMSO) / d ppm: 121.85, 123.94, 124.32, 126.24, 126.94,
128.23, 133.1, 135.40, 148.8, 154.0, 164.90. 2-(2,3-Dichloro-
phenyl)-benzothiazole: White solid; m.p. = 118-120 oC (m.p. =
119-121 oC);28 IR (KBr)/ u(cm-1): 3020, 2900, 1635, 1512,
General procedure for synthesis of 2-arylbenzothiazoles
catalyzed by TTBO: TTBO (0.18 gr) was added to a mixture of
2-aminothiophenol (8 mmol, 0.995 gr) and aldehyde (9 mmol) in
ethanol (10 mL) as solvent in a beaker and the reaction mixture
was properly mixed with the help of glass rod and stirred in ambi-
ent temperature and the progress of the reaction was monitored by
TLC (ethyl acetate: n-hexane, 6:4), after completion of the reac-
tion the mixture was cooled and dichloromethane (2 × 15 mL) was
added to the mixture for extracting the product. Then, the filtrate
was evaporated under reduced pressure to isolate a solid residue,
and recrystallized from ethanol (10 mL) to afford the correspond-
ing products.
General procedure for synthesis of 2-arylbenzothiazoles
catalyzed by solid Lewis acids: A mixure of 2-aminothiophenol
(8 mmol, 0.995 gr), aldehyde (9 mmol) and different Lewis acids
(0.15 gr) that are shown in Table 1, were added in ethanol (10 mL)
as solvent in a beaker and the reaction mixture was properly
mixed with the help of glass rod and stirred in ambient tempera-
ture for the time indicated in Table 1. The progress of the reaction
was monitored by TLC (ethyl acetate:hexane, 6:4), after comple-
tion of the reaction the solvent was removed under reduced pres-
sure, then the mixture was cooled and dichloromethane (15 mL)
was added to the mixture and filtered to remove the catalyst. Then
the filtrate was evaporated under reduced pressure to isolate a
solid residue, and recrystallized from ethanol (10 mL) to afford
the corresponding products.
1
1470,1432, 1348, 780; H NMR (400 MHz, CDCl3)/ d ppm:
7.35-7.39 (t, 1H, J = 8.0 Hz, Ar-H), 7.45-7.49 (t, 1H, J = 8.0 Hz,
Ar-H), 7.54-7.58 (t, 1H, J = 8.0 Hz, Ar-H), 7.62 (d, 1H, J = 8.0 Hz,
Ar-H), 7.98 (d, 1H, J = 8.0 Hz, Ar-H), 8.07 (d, 1H, J = 8.0 Hz,
Ar-H), 8.15 (d, 1H, J = 8.0 Hz, Ar-H); 13C NMR/ (400 MHz,
CDCl3) / d ppm: 117.77, 118.13, 123.30, 124.54, 127.10, 128.11,
128.73, 129.27, 129.95, 130.91, 131.67, 146.01, 149.00. 2-(3-
Nitrophenyl)-benzothiazole: Colourless solid; m.p. =181-183
oC (m.p. =181-182 oC);29 IR (KBr)/ u(cm-1): 3039, 2936, 1522,
1460, 1345, 1103, 1041, 851, 750; 1H NMR (400 MHz, CDCl3)/ d
ppm: 7.47 (m, 1H, Ar-H), 7.56 (m, 1H, Ar-H), 7.71-7.79 (m, 1H,
Ar-H), 7.97 (d, 1H, J = 7.6 Hz, Ar-H), 8.14 (m, 1H, Ar-H), 8.36
(m, 1H, Ar-H), 8.44 (d, 1H, J = 7.6 Hz, Ar-H), 8.95 (s, 1H, Ar-H);
13C NMR/ (400 MHz, DMSO) / d ppm: 121.90, 122.4, 123.32,
125.24, 126.11, 126.80, 128.75, 131.1, 133.63, 134.40, 147.8,
153.0, 162.23. 2-(2-Nitrophenyl)-benzothiazole: Brown solid;
Spectral Data of the Selected Compounds: 2-(2-Hy-
droxyphenyl)-benzothiazole: Yellow solid; m.p. = 127-128 oC
o
(m.p. = 126-128 C);28 IR (KBr)/ u(cm-1): 3285, 3090, 2900,
1619, 1590, 1490, 1423, 874, 751; 1H NMR (400 MHz, CDCl3)/ d
ppm: 6.95-699 (t, 1H, J = 8.0 Hz, Ar-H), 7.12 (d, 1H, J = 8.0 Hz,
Ar-H), 7.38-7.44 (m, 2H, Ar-H), 7.50-7.54 (t, 1H, J = 8.4 Hz,
Ar-H), 7.71 (d, 1H, J = 8.0 Hz, Ar-H), 7.92 (d, 1H, J = 8.4 Hz,
Ar-H), 8.01 (d, 1H, J = 8.0 Hz, Ar-H), 12.54 (s, 1H, OH); 13C
NMR/ (400 MHz, CDCl3)/ d ppm: 116.80, 117.88, 119.54,
121.52, 122.19, 125.56, 126.70, 128.43, 132.60, 132.77, 151.84,
157,96, 169.39. 2-Phenyl-benzothiazole: White solid; m.p. =
110-112 oC (m.p. = 111-112 oC);28 IR (KBr)/ u(cm-1): 3066, 3017,
o
o
m.p. = 94-96 C (m.p. = 95-97 C);28 IR (KBr)/ u(cm-1): 3033,
2930, 1545, 1466, 1349, 1111, 880, 851, 762; 1H NMR (400 MHz,
CDCl3)/ d ppm: 7.44-7.56 (m, 2H, Ar-H), 7.64-7.73 (m, 2H,
Ar-H), 7.81 (d, 1H, J = 7.6 Hz, Ar-H), 7.93-7.96 (m, 2H, Ar-H),
8.09 (d, 1H, J = 7.6 Hz, Ar-H); 13C NMR/ (400 MHz, DMSO) / d
ppm: 120.85, 122.18, 124.30, 124.98, 126.03, 126.94, 128.88,
133.12, 136.14, 138.90, 143.8, 157.0, 164.09. 2-(4-Methyl-
1
2835, 1608, 1587, 1476,1430, 830, 763; H NMR (400 MHz,
o
phenyl)-benzothiazole: White solid; m.p. = 82-84 C (m.p. =
o
83-85 C);28 IR (KBr)/ u(cm-1): 3024, 2905, 1610, 1519, 1480,
CDCl3)/ d ppm: 7.02-7.06 (t, 1H, J = 7.6 Hz, Ar-H), 7.25 (t, 1H, J
= 8.4 Hz, Ar-H), 7.56-7.70 (m, 5H, Ar-H), 7.93(d, 1H, J = 7.6 Hz,
Ar-H), 8.41(d, 1H, J = 8.4 Hz, Ar-H); 13C NMR/ (400 MHz,
CDCl3)/ d ppm: 121.50, 123.38, 125.07, 126.36, 127.58, 128.91,
130.77, 133.89, 135.11, 155.05, 167.98. 2-(4-Metoxyphenyl)-
benzothiazole: White solid; m.p. = 120-122 oC (m.p. = 120-122
oC);29 IR (KBr)/ u(cm-1): 3021, 3048, 2837, 1609, 1590, 1483,
1435, 1383, 1312, 821, 759; 1H NMR (400 MHz, CDCl3)/ d ppm:
2.45 (s, 3H, CH3), 7.34-736 (m, 4H, Ar-H), 7.54-759 (m, 1H,
Ar-H), 7.75-7.78 (m, 1H, Ar-H), 8.14-8.16 (m, 2H, Ar-H); 13C
NMR/ (400 MHz, CDCl3) / d ppm: 23.6, 110.42, 120.33, 121.76,
123.34, 128.56, 131.65, 132.67, 143.15, 155.63, 160.83, 165.11.
2-(4-Chlorophenyl)-benzothiazole: Yellow solid; m.p. = 114-
4
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JOURNAL OF THE CHINESE
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Synthesis of 2-Arylbenzothiazoles Catalyzed by Ti(OBu)4
o
o
116 C (m.p. = 116-118 C);29 IR (KBr)/ u(cm-1): 3055, 2358,
1560, 1455, 1430, 1317, 1275, 1060, 965, 750, 725; 1H NMR (400
MHz, CDCl3)/ d ppm: 7.37-739 (m, 2H, Ar-H), 7.51-753 (d, 2H, J
= 7.6 Hz, Ar-H), 7.58-7.61 (m, 1H, Ar-H), 7.77-7.79 (m, 1H,
Ar-H), 8.19-8.22 (d, 2H, J = 7.6 Hz, Ar-H); 13C NMR/ (400 MHz,
CDCl3) / d ppm: 121.8, 123.13, 125.17, 126.80, 129.10, 129.78,
132.67, 135.2, 137.1, 154.4, 166.21. 2-(4-N,N-dimethylphen-
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o
yl)-benzothiazole: Brown solid; m.p. = 160-162 C (m.p. =
160-162 oC);28 IR (KBr)/ u(cm-1): 3355, 2358, 1598, 1478, 1210,
1017, 965, 743; 1H NMR (400 MHz, CDCl3)/ d ppm: 3.08 (s, 6H,
-N(CH3)2), 6.77-6.79 (d, 2H, J = 8 Hz, Ar-H), 7.27-7.31 (m, 2H,
Ar-H), 7.52-7.54 (d, 1H, J = 8.2 Hz, Ar-H), 7.70 (d, 1H, J = 8.2
Hz, Ar-H), 8.11-8.13 (d, 2H, J = 8 Hz, Ar-H); 13C NMR/ (400
MHz, CDCl3) / d ppm: 39.8, 111.13, 121.1, 122.0, 124.1, 126.78,
128.67, 132.25, 135.2, 152.1, 154.4, 168.2. 2-(2-Chlorophen-
yl)-benzothiazole: Colorless solid; m.p. = 72-74 oC (m.p. = 72-74
oC);28 IR (KBr)/ u(cm-1): 2955, 1538, 1502, 1421, 1242, 1117,
965, 750, 733; 1H NMR (400 MHz, CDCl3)/ d ppm: 7.38-7.49 (m,
4H, Ar-H), 7.58-760 (m, 1H, Ar-H), 7.63-7.65 (m, 1H, Ar-H),
7.66-7.68 (m, 1H, Ar-H), 8.15-8.18 (m, 1H, Ar-H); 13C NMR/
(400 MHz, CDCl3) / d ppm: 111.8, 123.13, 124.17, 125.80,
128.19, 129.88, 131.67,133.14, 134.21, 137.1, 143.51, 154.4,
162.21. 2-(2-Methylphenyl)-benzothiazole: Yellow solid; m.p.
= 53-55 oC (m.p. = 54-56 oC);28 IR (KBr)/ u(cm-1): 3040, 2980,
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1
1598, 1550, 1451, 1244, 1115, 862, 754; H NMR (400 MHz,
CDCl3)/ d ppm: 2.47 (s, 3H, CH3), 7.35-7.37 (m, 3H, Ar-H),
7.41-7.45 (t, 1H, J = 8 Hz, Ar-H), 7.58-7.60 (m, 1H, Ar-H),
7.77-7.79 (m, 1H, Ar-H), 8.05-8.07 (d, 1H, J = 8 Hz, Ar-H), 8.08
(d, 1H, J = 8 Hz, Ar-H); 13C NMR/ (400 MHz, CDCl3) / d ppm:
22.6, 112.13, 119.38, 120.53, 122.34, 126.17, 128.34, 129.12,
130.67,132.47, 144.15, 152.68, 159.93, 162.17.
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ACKNOWLEDGEMENTS
The authors are grateful to University of Kashan for
supporting this work by Grant No. 159148/18.
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