Samarium(III) triflate as an efficient and reusable catalyst for facile synthesis of benzoxazoles and benzothiazoles in aqueous medium

Article abstract of DOI:10.1055/s-0033-1339758

A simple, green, and efficient method is presented for the synthesis of benzoxazoles and benzothiazoles from reaction of o-aminophenols, o-aminothiophenols, and aliphatic or aromatic aldehydes using samarium triflate as a reusable acid catalyst under mild reaction conditions in aqueous medium. Georg Thieme Verlag Stuttgart, New York.

Full text of DOI:10.1055/s-0033-1339758

LETTER
▌2241
lSetatermarium(III) Triflate as an Efficient and Reusable Catalyst for Facile
Synthesis of Benzoxazoles and Benzothiazoles in Aqueous Medium
Synthesis of Benzoxazoles and Benzothiazoles
Pratapsinha B. Gorepatil, Yogesh D. Mane, Vilas S. Ingle*
Department of Chemistry, S. C. S. College, Omerga, Dist-Osmanabad 413 606, India
Fax +91(2475)252020; E-mail: gorepatilpratap1986@gmail.com; E-mail: Inglevilas71@yahoo.in
Received: 16.06.2013; Accepted after revision: 18.08.2013
NH2
Sm(OTf)3 (10 mol%)
N
R
X
Abstract: A simple, green, and efficient method is presented for the
RCHO
+
synthesis of benzoxazoles and benzothiazoles from reaction of o-
aminophenols, o-aminothiophenols, and aliphatic or aromatic alde-
hydes using samarium triflate as a reusable acid catalyst under mild
reaction conditions in aqueous medium.
EthOH–H2O
2–5 h
XH
72–96%
X = O, S
R = aliphatic, aromatic, heteroaromatic
Scheme 1 Samarium triflate [Sm(OTf)3] catalyzed synthesis of
Key words: benzoxazoles, benzothiazoles, samarium triflate
2-substituted benzoxazoles and benzothiazoles
found to be best suited for reaction in an ethanol–water
(2:2) mixture at 60 °C compared to solvents such MeCN,
dioxane, or toluene (Table 1, entry 7).
Benzoxazoles and benzothiazoles have attracted consider-
able attention due to their biological properties.1–10 The
most commonly used synthetic approach involves con-
densation of o-aminothiophenols or o-aminophenols with
aldehydes,9,11 although carboxylic acids,12 alcohols,13 and
acid chlorides14 have been utilized. However, most of the
traditional synthetic methods suffer from drawbacks such
as the need for strongly acidic conditions and high tem-
peratures,15a–d long reaction times,15e,f and significant
amounts of catalyst or toxic solvents.15g,h Heterogeneous
Lewis acid catalysis has attracted considerable attention16
but, although a wide range of Lewis acids has been devel-
oped, most of them are used only under strictly anhydrous
conditions.17 Recently, with the objective of developing
environmentally benign reaction conditions and media for
organic reactions with excellent efficiency and selectivity,
water has been shown to be a useful solvent for certain
Lewis acids.18 For instance, In(OTf)3, Yb(OTf)3,
Bi(OTf)3, Sc(OTf)3, Sm(OTf)3, and other metal triflates
have been found to be water-tolerant, reusable Lewis ac-
ids catalysts and have received considerable interest for
condensation reactions17,19 and other organic transforma-
tions.20–22
After optimization, we extended the study to various o-
aminophenols and o-aminothiophenols with a range of al-
iphatic, aromatic, and heteroaromatic aldehydes. In gener-
al, most of reactions proceeded very smoothly to give the
corresponding 2-substituted benzoxazoles and benzo-
thizoles in moderate to excellent yields and aldehydes
containing a range of sensitive functional groups were ac-
ceptable under these conditions. As a general trend, elec-
tron-deficient aldehydes gave better yields in shorter
reaction times as compared to electron-rich aldehydes
(Table 2).
Table 1 Optimization of Reaction Conditions for the Synthesis of
2-Phenylbenzoxazolea
NH2
N
catalyst
OHC
+
solvent
O
OH
Entry Catalyst (mol%) Solvent
Time (h) Yield (%)b
1
2
–
EtOH
12
5
5
2
6
2
2
5
5
5
5
25
45
67
62
76
85
92
53
57
60
82
In continuation of our interest in the synthesis of fused
heterocyclic compounds,23 we report herein the use of sa-
marium triflate24 as a water-tolerant Lewis acid catalyst
for the synthesis of 2-substituted benzoxazoles and benzo-
thiazoles by the condensation of o-aminophenols or o-
aminothiophenols with aldehydes in aqueous ethanol as
shown in Scheme 1.
ZnO (5)
EtOH–H2O (2:2)
EtOH–H2O (2:2)
EtOH
3
In2O3 (5)
Sm(OTf)3 (5)
Sm(OTf)3 (2)
Sm(OTf)3 (5)
4
5
EtOH–H2O (2:2)
EtOH–H2O (2:2)
6
Initially, the samarium triflate catalyzed reaction between
equimolar quantities of o-aminophenol and benzaldehyde
was selected as a model reaction for optimization under
different concentrations and using different solvents as
summarized in Table 1. Samarium triflate (10 mol%) was
7
Sm(OTf)3 (10) EtOH–H2O (2:2)
8
Sm(OTf)3 (5)
Sm(OTf)3 (5)
Sm(OTf)3 (5)
MeCN
dioxane
toluene
9
10
11
Sm(OTf)3 (10) toluene
SYNLETT 2013, 24, 2241–2244
Advanced online publication: 13.09.2013
DOI: 10.1055/s-0033-1339758; Art ID: ST-2013-D0555-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
a Stirring at 50–60 °C.
b Isolated yield.

2242
P. B. Gorepatil et al.
LETTER
After completion of the reaction, the catalyst was re- catalyst was then dried at 100 °C for one hour and could
moved by simple filtration and washed with ethanol. The
Table 2 Sm(OTf)3-Catalyzed Synthesis of 2-Substituted Benzoxazoles and Benzothiazolesa
Entry
Aldehyde
Product
Time (h)
2
Yield (%)b
92
N
1
CHO
O
N
O2N
CHO
NO2
2
3
4
3
94
85
96
O
N
CHO
4
O
N
Cl
CHO
Cl
3.5
O
Cl
Cl
Cl
N
O
CHO
5
4
95
Cl
N
HO
CHO
OH
6
7
3.5
4.5
3
82
74
89
89
75
92
94
72
87
82
O
N
CHO
O
N
O
8
CHO
O
O
N
CHO
9
2
S
N
HO
CHO
CHO
CHO
OH
NO2
Br
10
11
12
13
14
15
4
S
N
O2N
Br
3.5
4
S
N
S
N
CHO
n-Bu
5
S
N
N
N
CHO
4.5
3.5
S
N
MeO
CHO
CHO
OMe
S
OMe
OMe
OMe
MeO
MeO
MeO
N
S
16
6
78
a Reaction conditions: aldehyde (1 mmol), o-aminophenol/o-aminothiophenol (1 mmol), Sm(OTf)3 (10 mol%), EtOH–H2O (4:2 mL), stirring
at 50–60 °C.
b Isolated yield.
Synlett 2013, 24, 2241–2244
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Benzoxazoles and Benzothiazoles
2243
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Entry
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Catalyst recovery (%)
1
2
3
4
92
90
88
86
94
92
90
88
a Reaction conditions: benzaldehyde (1 mmol), o-aminophenol
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b Isolated yield.
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be reused. It was found that the catalytic activities of the
recovered catalyst were almost the same as that of fresh
catalyst over several runs (Table 3).
In conclusion, a simple and efficient method has been de-
veloped for the synthesis of 2-substituted benzoxazoles
and benzothiazoles by using samarium triflate as a water-
tolerant, recyclable Lewis acid catalyst in aqueous etha-
nol.
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Acknowledgment
One of the authors, Mr. P.B. Gorepatil, is grateful to the CSIR-New
Delhi for a Junior Research Fellowship (JRF).
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2241–2244

2244
P. B. Gorepatil et al.
LETTER
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7.51 (t, 1 H, J = 8.4 Hz), 7.96 (d, 1 H, J = 8.0 Hz), 8.00–8.06
(m, 3 H). 13C NMR (75 MHz, DMSO-d6): δ = 21.2, 121.6,
122.9, 125.0, 126.2, 127.3, 129.7, 131.0, 135.0, 141.6,
154.2, 168.0. IR (KBr): 760, 1186, 1409, 1590, 1621, 2968
cm–1. MS (EI): m/z = 209.07 [M+].
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2-(4-Chlorophenyl)benzo[d]oxazole (Table 2 Entry 4)
Mp 147–149 °C. 1H NMR (300 MHz, DMSO-d6): δ = 7.44
(t, 1 H, J = 7.6 Hz), 7.50–7.56 (m, 3 H), 7.98 (d, 1 H, J = 8.1
Hz), 8.05–8.09 (m, 3 H). 13C NMR (75 MHz, DMSO-d6):
δ = 121.7, 123.2, 125.4, 126.5, 128.7, 129.2, 132.2, 135.1,
136.8, 154.1, 166.5. IR (KBr): 775, 1175, 1371, 1640, 2980
cm–1. MS (EI): m/z = 229 [M+] and 231 [M + 2].
2-(4-Nitrophenyl)benzo[d]thiazole (Table 2 Entry 11)
Mp 225–227 °C. 1H NMR (300 MHz, DMSO-d6): δ = 7.51
(t, 1 H, J = 7.5 Hz), 7.60 (t, 1 H, J = 7.6 Hz), 8.01 (d, 1 H,
J = 7.8 Hz), 8.16 (d, 1 H, J = 8.1 Hz), 8.36 (q, 4 H, J = 9.4
Hz). 13C NMR (75 MHz, DMSO-d6): δ = 121.9, 123.8,
124.4, 126.1, 126.8, 128.2, 135.6, 139.2, 149.0, 154.1,
165.0. IR (KBr): 807, 1169, 1370, 1556, 1690, 3056 cm–1.
MS (EI): m/z = 256.0 [M+].
2-(Pyridin-4-yl)benzo[d]thiazole (Table 2 Entry 14)
Mp 130–132 °C. 1H NMR (300 MHz, DMSO-d6): δ = 7.44–
7.50 (m, 2 H), 7.57 (t, 1 H, J = 8.1 Hz), 8.01 (d, 1 H, J = 7.9
Hz), 8.13 (d, 1 H, J = 8.2 Hz), 8.40 (dt, 1 H, J = 8.0, 1.7 Hz),
8.73 (dd, 1 H, J = 4.8z, 1.7 Hz). 13C NMR (75 MHz, DMSO-
d6): δ = 121.8, 123.3, 123.8, 125.6, 129.6, 134.4, 135.1,
148.5, 154.0, 164.7. IR (KBr): 690, 1150, 1400, 1653, 2360,
2905 cm–1. MS (EI): m/z = 212.03 [M+].
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(24) General Procedure
To a mixture of the requisite o-aminophenol/o-amino-
thiophenol (1 mmol) and aldehyde (1 mmol) in EtOH–H2O
(2:2 mL), samarium triflate catalyst (10 mol%) was added,
and the resulting mixture was stirred at 60 °C. After
completion of the reaction, as monitored by TLC, the
mixture was diluted with H2O–EtOAc (1:1, 10 mL) and
catalyst recovered by filtration. The filtrate was extracted
with Et2O (2 × 10 mL) and dried with anhydrous Na2SO4.
After filtration and evaporation of solvent, the crude product
was recrystallized from EtOAc or MeOH. All the structures
were confirmed by their analytical data and comparison with
literature data.25–30
2-(4-Methoxyphenyl)benzo[d]thiazole (Table 2 Entry 15)
Mp 122–124 °C. 1H NMR (300 MHz, DMSO-d6): δ = 3.89
(s, 3 H), 7.02 (d, 2 H, J = 8.4 Hz), 7.39 (t, 1 H, J = 7.2 Hz),
7.51 (t, 1 H, J = 7.2 Hz), 7.94 (d, 1 H, J = 7.8 Hz), 8.06 (m,
3 H). 13C NMR (75 MHz, DMSO-d6): δ = 55.4, 114.3, 121.6,
122.7, 124.8, 126.2, 126.4, 129.0, 135.0, 154.3, 162.0,
167.7. IR (KBr): 781, 1170, 1450, 1597, 1650, 3010 cm–1.
MS (EI): m/z = 241 [M+].
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Spectroscopic Data for Selected Compounds
2-Phenylbenzo[d]oxazole (Table 2 Entry 1)
Mp 100–102 °C. 1H NMR (300 MHz, DMSO-d6): δ = 7.39
(m, 2 H), 7.57 (m, 3 H), 7.64 (m, 1 H), 7.80 (m, 1 H), 8.29
(m, 2 H). 13C NMR (75 MHz, DMSO-d6): δ = 110.6, 119.9,
124.5, 125.1, 127.3, 127.5, 128.9, 131.5, 142.2, 150.8,
163.0. IR (KBr): 740, 1171, 1355, 1511, 1640, 3100 cm–1.
MS (EI): m/z = 195.1 [M+].
2-(4-Methylphenyl)benzo[d]oxazole (Table 2 Entry 3)
Mp 114–116 °C. 1H NMR (300 MHz, DMSO-d6): δ = 2.45
(s, 3 H), 7.36 (d, 2 H, J = 8.1 Hz), 7.41 (t, 1 H, J = 8.4 Hz),
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