Synthesis of 2-aryl benzothiazoles via K2S2O 8-mediated oxidative condensation of benzothiazoles with aryl aldehydes

Article abstract of DOI:10.1021/jo300740j

Nontransition metal-catalyzed synthesis of 2-aryl benzothiazoles was achieved through K₂S₂O₈-mediated oxidative condensation of benzothiazoles with aryl aldehydes. The same transformation can also be effected when the aryl

Full text of DOI:10.1021/jo300740j

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Note
Synthesis of 2-aryl benzothiazoles via K2S2O8 mediated
oxidative condensation of benzothiazoles with aryl aldehydes
Zhiyong Yang, Xiang Chen, Sizhuo Wang, Jidan Liu, Kai Xie, Anwei Wang, and Ze Tan
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/jo300740j • Publication Date (Web): 25 Jul 2012
Downloaded from http://pubs.acs.org on July 31, 2012
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Synthesis of 2-aryl benzothiazoles via K2S2O8 mediated oxidative
condensation of benzothiazoles with aryl aldehydes
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Zhiyong Yang, Xiang chen, Sizhuo Wang, Jidan Liu, Kai Xie, Anwei Wang, Ze Tan,*
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State Key Laboratory of Chemo/Biosensing and Chemometrics ,College of Chemistry and Chemical Engineering, Hunan University, Changsha
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410082, P. R. China
*E-mail: ztanze@gmail.com
ABSTRACT: Non-transition metal catalyzed synthesis of 2-aryl benzothiazoles was achieved through K2S2O8 mediated oxidative condensation of
benzothiazoles with aryl aldehydes. The same transformation can also be effected when the aryl aldehydes were replaced with phenylglyoxylic
acids.
2-Aryl benzothiazoles are important molecules due to their use not only as medicinal agents but also as organic functional materials such as
fluorescent dyes and liquid crystals.1-2 Traditional methods for their syntheses typically involve the condensation of 2-amino thiophenols with aryl
aldehydes (Eq. 1),3 carboxylic acids,4 nitriles,4a acyl chlorides,4d,5 alcohols,6 or through Jacobson’s potassium ferricyanide mediated cyclization of
thiobenzanilides.1c,7 Alternatively, transition metal catalyzed couplings such as Pd- or Cu-catalyzed cross coupling between benzothiazoles and
aryl halides or 2-halide substituted benzothiazoles with aryl metals have also been developed (Eq. 2).8-9 Among these reports, we have shown that
2-aryl benzothiazoles can also be synthesized via Pd-catalyzed decarboxylative coupling between benzothiazoles and aryl carboxylic acids.9
Herein we report 2-aryl benzothiazoles can be efficiently synthesized via K2S2O8 mediated oxidative condensation of benzothiazoles with aryl
aldehydes (Eq. 3).
NH2
N
oxidant
+
PhCHO
PhX(M)
PhCHO
Ph
Ph
Eq. 1
Eq. 2
S
N
SH
N
cat. Cu or Pd
+
H(X)
S
N
S
S
N
1
K2S2O8
+
Ph
Eq. 3
H
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Inspired by recent reports that K2S2O8 can be used to mediate the Minisci reaction between heteroaryls and phenyl boronic acids,10 we envision
a similar reaction can be effected between aryl aldehydes and benzothiazoles. When benzaldehyde (2) was reacted with benzothiazole (1) in the
presence of 10 mol% of AgNO3 and 1 equiv of K2S2O8 in DMSO and H2O at 130 oC for 12 h, we were delighted to see that our desired product,
2-phenyl benzothiazole (3a) was indeed formed in 57% yield (table 1, entry 1). However, test showed that the reaction still proceeded smoothly
without the Ag catalyst (table 1, entry 2). Better yield (65%) was obtained when the reaction temperature was dropped to 100 oC (table 1, entry 3)
while further decrease of the reaction temperature resulted a much slower reaction, thus leading to incomplete reactions (table 1, entry 4 and 5).
Changing the solvent to DMF/H2O, CH3CN/H2O or Toluene/H2O afforded no product whereas the use of (NH4)2S2O8 as the oxidant or using
DMSO alone as the solvent gave inferior yields (table 1, entries 6-11). If the ratio of DMSO against H2O was changed from 2/1 to 1/2, the yield
also dropped (table 1, entry 13). Control experiment also showed that K2S2O8 was necessary for the reaction to proceed (table 1, entry 12).
Extensive tests also showed that the best yield was achieved when benzothiazoles were used in excess. On the basis of these results, we decided to
set heating the benzothiazoles and aryl aldehydes at 100 oC in the presence of 1 equiv of K2S2O8 in DMSO/H2O (2/1) as our standard condition.
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Table 1 Reaction condition optimizationa
O
N
S
N
S
K2S2O8
+
H
DMSO/H2O
1
3a
2
Entry
Oxidant (equiv)
T (oC)
Yieldb (%)
Solvent
1c
2
AgNO3/K2S2O8
K2S2O8 (1)
130
130
100
57
57
65
DMSO/H2O
DMSO/H2O
K2S2O8 (1)
DMSO/H2O
3
rt
trace
4
K2S2O8 (1)
DMSO/H2O
5
6
K2S2O8 (1)
K2S2O8 (1)
DMSO/H2O
DMSO
60
100
45
38
(NH4)2S2O8 (1)
K2S2O8 (2)
K2S2O8 (1)
K2S2O8 (1)
K2S2O8 (1)
7
8
DMSO/H2O
DMSO/H2O
DMF/H2O
100
100
100
100
100
100
100
55
60
0
9
10
CH3CN/H2O
toluene/H2O
DMSO/H2O
DMSO/H2O
trace
0
11
12d
13e
0
56
K2S2O8 (1)
a Conditions: benzothiazole (1.5 mmol), aldehyde (1.0 mmol), oxidant (1.0
mmol), DMSO/H2O = 2 mL : 1 mL, 100 oC, 3 h. b Yields are determined
by GC. c K2S2O8 (1.0 mmol), AgNO3 (0.1 mmol), 12 h with K2S2O8 in
DMSO/H2O. d no K2S2O8 added. e DMSO/H2O = 1 mL : 2 mL.
With the optimized protocol in hand, we next set out to explore the scope and limitation of the reaction (table 2). We found that the reaction
worked very well for a wide variety of substituted benzaldehydes, affording the desired 2-aryl substituted benzothiazoles in yields ranging from
38-74%. Substituents such as methyl, ethyl, chloro, bromo, fluoro as well as methoxy group were well tolerated. The reaction of 1- or 2-naphthyl
aldehyde also worked well, furnishing the 2-naphthyl substituted benzothiazoles in good yields. 2-Pyridine aldehyde, a heteroaryl aldehyde, was
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also a viable partner, affording the 2-pyridine-substituted benzothiazole in 57% yield. Methoxy, Methy and chloro substituted benzothiazoles gave
no problem, showing this reaction is not very sensitive to the electronic effects of the substituents. In addition, under the standard condition, the
reaction of cinnamon aldehyde with benzothiazole produced the 2-styryl substituted benzothiazole 3p in 68% yield. While the results of aryl
aldehydes were all favorable, the reactions of aliphatic aldehydes were messy, only producing the desired products in low yields. For example, the
reaction of 1-heptanal with benzothiazole only gave the desired product in 24% yield.
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Table 2 Synthesis of 2-aryl benzothiazoles from benzothiazoles and aryl aldehydes a
O
N
S
N
S
K2S2O8(1equiv)
+
H
R1
Br
R1
DMSO : H2O (2 : 1)
100oC, 3h
1
2
3
N2
F
Cl
N
N
N
S
S
S
3c 70%
3b
38%
3d 59%
N
S
N
N
S
S
3e
3f
68%
74%
EtO
3g 65%
OCH3
N
N
S
N
S
OCH3
S
3h
73%
71%
3j 60%
3i
OCH3
N
S
N
S
N
S
3k
3l
63%
67%
N
68%
N
3m
OCH3
N
OCH3
S
S
H3CO
S
N
3o
3r
3n 57%
73%
N
3p
68%
N
N
S
C6H13-n
S
S
Ph
H3CO
3q
3s
24%
N
28%
51%
N
N
Cl
S
S
S
Cl
3u 60%
3t
65%
3v
58%
a Conditions: benzothiazole (1.5 mmol), aldehyde (1.0 mmol), oxidant (1.0 mmol),
DMSO/H2O = 2 mL : 1 mL, 100 oC, 3 h.
Since glyoxylic acids are known to form aldehydes at relative elevated temperature,11 we next tested the possibility of synthesizing 2-aryl
benzothiazoles from glyoxylic acids and benzothiazoles. After a series of investigations, we found that this transformation can be effected in a
two-step/one-pot fashion. Specifically, 1 equiv of glyoxylic acid and 1.5 equiv of benzothiazole were heated at 100 oC in DMSO/H2O for 1 h, then
1 equiv of K2S2O8 was added and the reaction was reacted for another 3 h. After stopping the reaction and regular work up, the desired 2-aryl
benzothiazole can be isolated. Using this method, various 2-aryl substituted benzothiazoles can be synthesized in 35-62% yields starting from
various glyoxylic acids and the results are summarized in Table 3. Surprisingly, directly mixing glyoxylic acid, benzothiazole and K2S2O8 together
in DMSO/H2O did not produce any desired 2-substituted benzothizaole and the reason is still unknown.
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Table 3 Synthesis of 2-aryl benzothiazoles from benzothiazoles and glyoxylic acidsa
1
2
3
4
O
N
S
K2S2O8(1equiv)
N
S
OH
+
DMSO : H2O (1 : 2)
100oC
R2
O
N2
R2
1
3
2a
5
6
7
8
N
N
S
N
S
Br
F
S
3a
56%
3w 54%
3d
3f
55%
62%
F
N
N
S
N
S
9
Cl
S
10
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3b
3x
3y
57%
35%
N
S
N
S
N
S
S
3l
48%
60%
58%
3m
N
S
3z 65%
a Conditions: benzothiazole (1.5 mmol), glyoxylic acid (1.0 mmol), DMSO/H2O = 1 mL : 2 mL,
100 oC, 1 h; then oxidant (1.0 mmol), another 3h.
Though the exact mechanism is still not clear at present, we believe that the reaction is actually not a Minisci type reaction. This is supported by
the control reaction that almost all of the benzaldehyde remained intact when benzaldehyde was heated in DMSO/H2O with 1 equiv of K2S2O8
whereas benzothiazole was completely consumed under the same reaction condition (Eq. 4 and 5). Since 2-amino thiophenol and CO2 were
detected in the reaction of benzothiazole with K2S2O8, a more likely mechanism seems to involve the oxidative opening of the thiazole ring and
oxidative condensation of the resultant 2-amino thiophenol with the aldehyde (Scheme 1).
Scheme 1 Possible reaction mechanism
O
H
N
S
NH2
SH
N
K2S2O8
SH
H
N
N
K2S2O8
S
S
H
To support this, 2-amino thiophenol was reacted with benzaldehyde in the presence of K2S2O8 under the standard reaction condition, and we
found that 2-phenyl benzothiazole was indeed formed in 81% yield (Eq. 6). This proposed mechanism also explains that why 4, 5-dimethyl
thiazole did not afford any desired product, yet the starting materials were completely consumed. Because in this case, once the ring is opened, it
can not be reformed. The reactions of aliphatic aldehydes are messy because they are more easily oxidized by K2S2O8 to produce carboxylic acids,
which are not able to react with 2-amino thiophenol to produce the desired substituted benzothiazoles under the reaction condition. Unfortunately,
benzoxazoles and benzimidazoles can not participate in the reaction and experiments showed that benzimidazoles remained intact under the
reaction condition whereas benzoxazoles were completely consumed in the reaction (Eq. 7 and 8). The failure of the benzoxazole might be due to
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the fact that 2-amino phenol was found not to be able to undergo oxidative condensation with aryl aldehydes to form 2-aryl benzoxazoles under
2
3
4
the standard reaction condition (Eq. 9). In addition, attempts to extend this reaction to carboxylic acid in place of benzaldehyde also met with
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failure.
8
9
O
K2S2O8
DMSO/H2O
H
no reaction
NH2
Eq. 4
Eq. 5
100 o
C
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K2S2O8
N
S
DMSO/H2O
+ CO2
100 o
C
SH
O
K2S2O8
NH2
SH
N
DMSO/H2O
100 o
H
+
Eq. 6
C
S
81%
K2S2O8
DMSO/H2O
100 o
N
N
no reaction
Eq. 7
C
K2S2O8
N
O
DMSO/H2O
SM consumed
Eq. 8
100 o
C
O
K2S2O8
NH2
OH
N
DMSO/H2O
H
+
Eq. 9
100 o
C
O
not formed
In summary, a novel way of synthesizing 2-aryl benzothiazoles from benzothiazoles and aryl aldehydes was developed. By treating
benzothiazoles and aryl aldehydes with K2S2O8 in DMSO and H2O, the benzothiazoles were found to undergo oxidation to produce 2-amino
thiophenols in situ, which subsequently underwent oxidative cyclization with the aryl aldehydes to give a variety of 2-aryl benzothiazoles in yields
ranging from 38-74%. The reaction also worked well when aryl aldehydes were replaced with phenylglyoxylic acids. Since this protocol obviates
the need to spend an extra step to convert the benzothiazoles into 2-amino thiophenols, it is more efficient in terms of step economy than the
classical condensation approach in cases where substituted benzothiazoles are readily available. It also could be complementary to the Cu- or
Pd-catalyzed cross coupling protocols provided that the aryl aldehydes are more readily accessible than the corresponding aryl halides.
EXPERIMENTAL SECTION
General Experimental Methods. Benzothiazole, aldehydes and DMSO were purchased commercially and used without further purification.
Methoxy, Methy, chloro-substituted benzothiazoles were prepared from the corresponding substituted 2-amino benzothiazoles.12 ɑ-oxocarboxylic
acids were prepared from oxidation of corresponding methyl ketones with SeO2 according to the reported procedure.13 1H NMR and 13C NMR
were recorded in CDCl3 at room temperature. The chemical-shifts scale is based on internal TMS.
Procedure for the synthesis of 2-Aryl benzothiazoles via K2S2O8 mediated oxidative condensation of benzothiazoles with aryl aldehydes.
A 25-mL round bottom flask was charged with Benzothiazole (1.5 mmol), aryl aldehydes (1.0 mmol), K2S2O8 (1.0 mmol) and DMSO/H20 (2 mL:
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1 ml). The reaction was stirred at 100 C under N2 for 3 h (monitored by TLC). Upon completion of the reaction, the mixture was diluted with
H2O and extracted with dichloromethane (30 ml×3). The organic layers were combined, washed with brine, dried over Mg2SO4, and filtered. The
solvents were removed via rotary evaporator and the residue was purified with flash chromatography (silica gel, gradient eluent of EtOAc in
n-pentane: 2%, v/v) to yield the product as a colorless (yellow) solid (oil).
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Procedure for the synthesis of 2-Aryl benzothiazoles via K2S2O8 mediated oxidative condensation of benzothiazole with -oxocarboxylic
acids. A 25-mL round bottom flask was charged with Benzothiazole (1.5 mmol), -oxocarboxylic acids (1.0 mmol) and DMSO/H20 (1 ml : 2 ml).
o
The reaction flask was stirred at 100 C under N2 for 1 h, then K2S2O8 (1.0 mmol) was added and resultant mixture was stirred for another 3 h
(monitored by TLC). Upon completion of the reaction, the mixture was diluted with H2O and extracted with dichloromethane (30 ml×3). The
organic layers were combined, washed with brine, dried over Mg2SO4, and filtered. The solvents were removed via rotary evaporator and the
residue was purified with flash chromatography (silica gel, gradient eluent of EtOAc in n-pentane: 2%, v/v) to yield the product as a colorless
(yellow) solid (oil).
2-phenylbenzothiazole(3a)8e,14,18: The product was isolated as a colorless solid in 65% yield (137 mg), m.p. = 98.1-100.7 °C, TLC Rf = 0.55
1
(petroleum ether/ ethyl acetate, 12/1); H NMR (400MHz, CDCl3): δ 7.36 (t, J = 7.6 Hz, 1H), 7.45-7.50 (m, 4H), 7.87 (d, J = 7.6 Hz, 1H),
8.06-8.09 (m, 3H); 13C NMR(100MHz, CDCl3): δ 121.6, 123.1, 125.2, 126.2, 127.5 (2C), 128.9 (2C), 130.9, 133.5, 134.9, 154.0, 168.0; FTIR
(film, cm-1): 1477, 1444, 757, 670; HRMS (EI) m/z calcd for C13H9NS: 211.0456, found 211.0445; Anal. Calcd for C13H9NS: Elemental Analysis:
C, 73.90; H, 4.29; N, 6.63; Found: C, 73.98; H, 4.41; N, 6.50.
2-(2-fluorophenyl)benzothiazole(3b)15: The product was isolated as a colorless solid in 38% yield (87 mg), m.p. = 110.2-112.3 °C, TLC Rf = 0.56
(petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.23 (d, J = 7.6 Hz, 1H), 7.30 (t, J = 7.6 Hz, 1H), 7.42-7.52 (m, 3H), 7.95 (d,
J = 7.6 Hz, 1H), 8.13 (d, J = 8.4 Hz, 1H), 8.42 (t, J = 7.6 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 116.3 (d, JC-F = 21.3 Hz), 121.4, 123.2, 124.6,
124.7, 125.3, 126.3, 129.7, 132.1 (d, JC-F = 8.4 Hz), 135.7 (d, JC-F = 7.7 Hz), 152.5, 160.1 (d, JC-F = 183.7 Hz), 161.8; HRMS (EI) m/z calcd for
C13H8NFS: 229.0361, found 229.0361; Anal. Calcd for C13H8NFS: Elemental Analysis: C, 68.10; H, 3.52; N, 6.11; Found: C, 68.22; H, 3.61; N,
6.00.
2-(3-chlorophenyl)benzothiazole(3c)16: The product was isolated as a colorless solid in 70% yield (170 mg), m.p. = 112.3-114.2 °C, TLC Rf = 0.56
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(petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.38-7.46 (m, 3H), 7.51 (t, J = 7.6 Hz, 1H), 7.90 (d, J = 7.6 Hz, 1H), 7.93 (d,
J = 7.6 Hz, 1H), 8.07 (d, J = 8.0 Hz, 1H), 8.11 (s, 1H); 13C NMR(100MHz, CDCl3): δ 121.7, 123.4, 125.5, 125.6, 126.5, 127.3, 130.2, 130.8,
135.0, 135.1, 135.2, 153.9, 166.2; HRMS (EI) m/z calcd for C13H8ClNS: 245.0066, found 245.0060.
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2-(4-bromophenyl)benzothiazole(3d)17: The product was isolated as a colorless solid in 59% yield (132 mg), m.p. = 105.3-107.1 °C, TLC Rf =
0.56 (petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.40 (t, J = 8.0 Hz, 1H), 7.48 (t, J = 7.6 Hz, 1H), 7.61 (d, J = 8.4 Hz,
2H), 7.89 (d, J = 7.6 Hz, 1H), 7.94 (d, J = 8.0 Hz, 2H), 8.06 (d, J = 8.4 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 121.6, 123.2, 125.4, 126.5, 128.8
(2C), 132.2 (2C), 132.4, 135.0, 154.0, 166.7; MS (m/z) 289 (M+).
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2-(2,4-dimethylphenyl)benzothiazole(3e): The product was isolated as a colorless solid in 68% yield (164 mg), m.p. = 111.1-113.0 °C, TLC Rf =
0.55 (petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 2.36 (s, 3H), 2.64 (s, 3H), 7.09 (d, J = 8.0 Hz, 1H), 7.14 (s, 1H), 7.37 (t,
J = 7.6 Hz, 1H), 7.48 (t, J = 7.6 Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.88 (t, J = 8.0 Hz, 1H), 8.08 (d, J = 8.0 Hz, 1H); 13C NMR(100MHz, CDCl3):
δ 21.2, 21.3, 121.2, 123.1, 124.8, 126.0, 126.8, 130.2, 130.5, 132.3, 135.4, 137.0, 140.1, 153.7, 168.1; HRMS (EI) m/z calcd for C15H13NS:
239.0769, found 239.0764.
2-(2-Methylphenyl) benzothiazole(3f)8e,14 : The product was isolated as a colorless solid in 74% yield (167 mg), m.p. = 56.2-57.7 °C, TLC Rf =
0.55 (petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 2.66 (s, 3H), 7.29-7.35 (m, 3H), 7.38 (t, J = 8.0 Hz, 1H), 7.50 (t, J = 7.6
Hz, 1H), 7.75 (d, J = 7.2 Hz, 1H), 7.90 (d, J = 8.8 Hz, 1H), 8.10 (d, J = 8.4 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 21.3, 121.3, 123.3, 125.0,
126.0, 126.1, 129.9, 130.5, 131.5, 133.0, 135.5, 137.2, 153.7, 167.9; FTIR (film, cm-1): 2926, 1453, 955, 758, 721; HRMS (EI) m/z calcd for
C14H11NS: 225.0612, found 225.0606; Anal. Calcd for C14H11NS: Elemental Analysis: C, 74.63; H, 4.92; N, 6.22; Found: C, 74.75; H, 5.01; N,
6.02.
2-(2-ethylphenyl)benzothiazole(3g)17: The product was isolated as a colorless oil in 65% yield (154 mg), TLC Rf = 0.55 (petroleum ether/ ethyl
acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 1.22 (t, J = 7.6 Hz, 3H), 3.03 (q, J = 7.6 Hz, 2H), 7.30 (t, J = 7.6 Hz, 1H), 7.36-7.44 (m, 3H), 7.51 (t,
J = 7.6 Hz, 1H), 7.66 (d, J = 8.0 Hz, 1H), 7.92 (d, J = 8.4 Hz, 1H), 8.10 (d, J = 8.4 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 15.7, 26.7, 121.4,
123.4, 125.1, 126.0, 126.1, 129.8, 130.2, 130.8, 132.7, 135.7, 143.4, 153.8, 167.9; MS (m/z) 239 (M+).
2-(4-Methylphenyl)benzothiazole(3h)18: The product was isolated as a colorless solid in 73% yield (162 mg), m.p. = 110.3-112.7 °C, TLC Rf =
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0.55 (petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 2.41 (s, 3H), 7.28 (d, J = 8.0 Hz, 2H), 7.36 (t, J = 7.6 Hz, 1H), 7.48 (t, J
= 7.6 Hz, 1H), 7.88 (d, J = 7.6 Hz, 1H), 7.97 (t, J = 8.4 Hz, 2H), 8.05 (d, J = 8.4 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 21.5, 121.5, 123.0, 125.0,
126.2, 127.4 (2C), 129.7 (2C), 130.9, 134.9, 141.4, 154.1, 168.2; FTIR (film, cm-1): 2913, 1477, 1433, 839, 757; HRMS (EI) m/z calcd for
C14H11NS: 225.0612, found 225.0606.
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2-(2-ethoxyphenyl)benzothiazole(3i)19: The product was isolated as a colorless solid in 71% yield (181 mg), m.p. = 123.5-126.1 °C, TLC Rf = 0.36
(petroleum ether/ ethyl acetate, 8/1); 1H NMR (400MHz, CDCl3): δ 1.64 (t, J = 6.8 Hz, 3H), 4.27 (q, J = 6.8 Hz, 2H), 7.02 (d, J = 8.0 Hz, 1H),
7.11 (t, J = 7.6 Hz, 1H), 7.36 (t, J = 7.6 Hz, 1H), 7.42 (t, J = 7.6 Hz, 1H), 7.48 (t, J = 7.6 Hz, 1H), 7.93 (d, J = 8.4 Hz, 1H), 8.08 (d, J = 8.4 Hz,
1H), 8.55 (d, J = 7.6 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 14.9, 64.9, 112.2, 120.9, 121.2, 122.2, 122.7, 124.5, 125.8, 129.4, 131.7, 136.1,
152.0, 156.6, 163.2; MS (m/z) 255 (M+).
2-(2,4-dimethoxyphenyl)benzothiazole(3j)20: The product was isolated as a colorless solid in 60% yield (162 mg), m.p. = 125.6-127.3 °C, TLC Rf
= 0.35 (petroleum ether/ ethyl acetate, 4/1); 1H NMR (400MHz, CDCl3): δ 3.87 (s, 3H), 4.01 (s, 3H), 6.56 (s, 1H), 6.66 (d, J = 8.8 Hz, 1H), 7.33 (t,
J = 7.2 Hz, 1H), 7.46 (t, J = 7.6 Hz, 1H), 7.89 (d, J = 7.6 Hz, 1H), 8.04 (d, J = 8.4 Hz, 1H), 8.46 (d, J = 8.8 Hz, 1H); 13C NMR(100MHz, CDCl3):
δ 55.5, 55.6, 98.4, 105.8, 115.5, 121.1, 122.2, 124.1, 125.7, 130.7, 135.6, 152.1, 158.5, 162.8, 163.3; MS (m/z) 271 (M+).
2-(2-methoxyphenyl)benzothiazole(3k)18: The product was isolated as a colorless solid in 63% yield (152 mg), m.p. = 120.2-121.8 °C, TLC Rf =
0.40 (petroleum ether/ ethyl acetate, 8/1); 1H NMR (400MHz, CDCl3): δ 4.03 (s, 3H), 7.04 (d, J = 8.4 Hz, 1H), 7.13 (t, J = 7.6 Hz, 1H), 7.36 (t, J
= 7.6 Hz, 1H), 7.44-7.48 (m, 2H), 7.92 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 8.4 Hz, 1H), 8.53 (d, J = 8.0 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 55.6,
111.5, 121.1, 121.2, 122.1, 122.7, 124.5, 125.8, 129.4, 131.7, 136.0, 152.0, 157.1, 163.1; FTIR (film, cm-1): 2994, 1603, 1481, 831, 757; HRMS
(EI) m/z calcd for C14H11NOS: 241.0561, found 241.0552.
2-(naphthalen-1-yl)benzothiazole(3l)18: The product was isolated as a yellow oil in 67% yield (174 mg), TLC Rf = 0.55 (petroleum ether/ ethyl
acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.43 (t, J = 7.6 Hz, 1H), 7.52-7.57 (m, 3H), 7.61 (t, J = 7.6 Hz, 1H), 7.91 (d, J = 8.0 Hz, 2H), 7.96 (t,
J = 8.4 Hz, 2H), 8.19 (d, J = 8.0 Hz, 1H), 8.93 (d, J = 8.4 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 121.3, 123.5, 124.9, 125.2, 125.8, 126.2, 126.5,
127.6, 128.4, 129.3, 130.5, 130.7, 131.0, 133.9, 135.4, 154.1, 167.6; HRMS (EI) m/z calcd for C17H11NS: 261.0612, found 261.0604.
2-(naphthalen-2-yl)benzothiazole(3m)15:The product was isolated as a colorless solid in 68% yield (177 mg), m.p. = 99.2-101.7 °C, TLC Rf = 0.56
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(petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.39 (t, J = 7.6 Hz, 1H), 7.48-7.56 (m, 3H), 7.85-7.96 (m, 4H), 8.11 (d, J =
8.0 Hz, 1H), 8.19 (d, J = 8.8 Hz, 1H), 8.55 (s, 1H); 13C NMR(100MHz, CDCl3): δ 120.6, 122.2, 123.4, 124.2, 125.3, 125.8, 126.4, 126.5, 126.8,
127.8 (2C), 129.9, 132.1, 133.5, 134.0, 153.1, 167.1; MS (m/z) 261 (M+).
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2-(pyridin-2-yl)benzothiazole(3n)8e: The product was isolated as a yellow solid in 57% yield (121 mg), m.p. = 132.8-134.1 °C, TLC Rf = 0.41
(petroleum ether/ ethyl acetate, 8/1); 1H NMR (400MHz, CDCl3): δ 7.36 (t, J = 6.0 Hz, 1H), 7.41 (t, J = 7.6 Hz, 1H), 7.50 (t, J = 7.6 Hz, 1H), 7.83
(t, J = 8.0 Hz, 1H), 7.96 (d, J = 8.4 Hz, 1H), 8.09 (d, J = 8.0 Hz, 1H), 8.37 (d, J = 8.0 Hz, 1H), 8.68 (d, J = 4.0 Hz, 1H); 13C NMR(100MHz,
CDCl3): δ 120.7, 122.0, 123.5, 125.3, 125.6, 126.3, 136.1, 137.0, 149.6, 151.3, 154.2, 169.3; FTIR (film, cm-1): 1564, 1433, 979, 758, 728, 720;
HRMS (EI) m/z calcd for C12H8N2S: 212.0408, found 212.0411; Anal. Calcd for C12H8N2S: Elemental Analysis: C, 67.90; H, 3.80; N, 13.20;
Found: C, 68.00; H, 3.93; N, 13.05.
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2-(2,4-dimethoxyphenyl)-5-methoxybenzothiazole(3o)8h: The product was isolated as a yellow solid in 73% yield (220 mg), m.p. = 115.8-117.6 °C,
TLC Rf = 0.26 (petroleum ether/ ethyl acetate, 4/1); 1H NMR (400MHz, CDCl3): δ 3.84 (s, 3H), 3.85 (s, 3H), 3.97 (s, 3H), 6.52 (s, 1H), 6.63 (d, J
= 8.8 Hz, 1H), 7.06 (d, J = 8.8 Hz, 1H), 7.32 (s, 1H), 7.91 (d, J = 8.8 Hz, 1H), 8.83 (d, J = 8.4 Hz, 1H).; 13C NMR(100MHz, CDCl3): δ 55.4, 55.5,
55.6, 98.2, 103.3, 105.7, 115.0, 115.5, 122.7, 130.2, 136.8, 146.6, 156.8, 158.0, 160.9, 162.3; MS (m/z) 301 (M+).
2-styrylbenzothiazole(3p)8e: The product was isolated as a yellow solid in 68% yield (162 mg), m.p. = 98.2-100.1 °C, TLC Rf = 0.45 (petroleum
ether/ ethyl acetate, 8/1); 1H NMR (400MHz, CDCl3): δ 7.37-7.43 (m, 5H), 7.47-7.59 (m, 4H), 7.85 (d, J = 7.6 Hz, 1H), 8.00 (d, J = 7.6 Hz, 1H));
13C NMR(100MHz, CDCl3): δ 121.5, 122.0, 122.9, 125.3, 126.3 (2C), 127.3 (2C), 128.9, 129.4, 134.2, 135.3, 137.6, 153.8, 167.0; HRMS (EI)
m/z calcd for C15H11NS: 237.0612, found 237.0606.
5-methoxy-2-(p-tolyl)benzothiazole(3q)21: The product was isolated as a colorless solid in 51% yield (131 mg), m.p. = 99.2-101.4 °C, TLC Rf =
0.36 (petroleum ether/ ethyl acetate, 8/1); 1H NMR (400MHz, CDCl3): δ 2.41 (s, 3H), 3.87 (s, 3H), 7.07 (d, J = 9.2 Hz, 1H), 7.27 (d, J = 8.0 Hz,
2H), 7.33 (s, 1H), 7.91-7.94 (m, 3H); 13C NMR(100MHz, CDCl3): δ 21.4, 55.7, 104.1, 115.4, 123.4, 127.1 (2C), 129.6 (2C), 131.0, 136.2, 140.8,
148.6, 157.5, 165.7; HRMS (EI) m/z calcd for C15H13NOS: 255.0718, found 255.0712.
2-hexylbenzothiazole(3r)17: The product was isolated as a yellow oil in 24% yield (52 mg), TLC Rf = 0.59 (petroleum ether/ ethyl acetate, 12/1);
1H NMR (400MHz, CDCl3): δ 0.89 (t, J = 8.0 Hz, 3H), 1.31-1.33 (m, 4H), 1.34-1.35 (m, 2H), 1.85-1.87 (m, 2H), 3.12 (t, J = 8.0 Hz, 2H), 7.35 (t,
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J = 7.6 Hz, 1H), 7.45 (t, J = 7.6 Hz, 1H), 7.84 (d, J = 8.0 Hz, 1H), 7.97 (d, J = 8.0 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 14.0, 22.5, 28.8, 29.7,
31.4, 34.3, 121.4, 122.4, 124.6, 125.8, 135.1, 153.1, 172.5; MS (m/z) 219 (M+).
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2-(1-phenylethyl)benzothiazole(3s)22: The product was isolated as a yellow oil in 28% yield (66 mg), TLC Rf = 0.60 (petroleum ether/ ethyl
acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 1.87 (d, J = 7.2 Hz, 3H), 4.58 (q, J = 7.2 Hz, 1H), 7.28 (t, J = 7.2 Hz, 1H), 7.32-7.40 (m, 5H), 7.44 (t,
J = 7.6 Hz, 1H), 7.77 (d, J = 7.6 Hz, 1H), 8.01 (d, J = 8.8 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 21.2, 44.8, 121.4, 122.8, 124.7, 125.8, 127.3,
127.6 (2C), 128.7 (2C), 135.2, 143.0, 153.0, 176.3; FTIR (film, cm-1): 3062, 3002, 2975, 2929, 1513, 1494, 1454, 1437, 1125, 1058, 1026, 1014,
758, 729, 699; MS (m/z) 239 (M+).
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6-methyl-2-phenylbenzothiazole(3t)23: The product was isolated as a white solid in 65% yield (146 mg), TLC Rf = 0.56 (petroleum ether/ ethyl
acetate, 12/1); m.p. = 126-127 °C; 1H NMR (400MHz, CDCl3): δ 2.48 (s, 3H), 7.29 (d, J = 8.0 Hz, 1H), 7.46-7.49 (m, 3H), 7.67 (s, 1H), 7.95 (d, J
= 8.4 Hz, 1H), 8.05-8.08 (m, 2H); 13C NMR(100MHz, CDCl3): δ 21.5, 121.3, 122.7, 127.4 (2C), 127.9, 128.9 (2C), 130.7, 133.7, 135.2, 135.3,
152.2, 167.0; HRMS (EI) m/z calcd for C15H13NOS: 225.0612, found 225.0614.
2-(4-chlorophenyl)-6-methylbenzothiazole(3u)18: The product was isolated as a yellow oil in 60% yield (156 mg), TLC Rf = 0.58 (petroleum ether/
ethyl acetate, 12/1); m.p. = 136-138°C; 1H NMR (400MHz, CDCl3): δ 2.49 (s, 3H), 7.29 (d, J = 8.4 Hz, 1H), 7.41 (d, J = 6.8 Hz, 2H), 7.67 (s, 1H),
7.93 (d, J = 8.4 Hz, 1H), 7.99 (d, J = 6.4 Hz, 2H); 13C NMR(100MHz, CDCl3): δ 21.5, 121.4, 122.7, 128.1, 128.5 (2C), 129.2 (2C), 132.2, 135.2,
135.6, 136.7, 152.2, 165.5; MS (m/z) 259 (M+).
6-chloro-2-phenylbenzothiazole(3v)23: The product was isolated as a yellow oil in 58% yield (141 mg), TLC Rf = 0.58 (petroleum ether/ ethyl
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acetate, 12/1); m.p. = 160-161°C; H NMR (400MHz, CDCl3): δ 7.30 (d, J = 8.8 Hz, 1H), 7.47-7.49 (m, 3H), 7.84 (s, 1H), 7.95 (d, J = 8.8 Hz,
1H), 8.03-8.06 (m, 2H); 13C NMR(100MHz, CDCl3): δ 121.2, 123.9, 127.1, 127.5 (2C), 129.0 (2C), 131.0, 131.2, 133.2, 136.2, 152.6, 165.8; MS
(m/z) 245 (M+).
2-(4-fluorophenyl)benzothiazole(3w)18: The product was isolated as a colorless solid in 54% yield (124 mg), m.p. = 110.1-112.6 °C, TLC Rf = 0.56
(petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.15 (d, J = 7.6 Hz, 1H), 7.17 (d, J = 7.6 Hz, 1H), 7.37 (t, J = 7.6 Hz, 1H),
7.48 (t, J = 7.6 Hz, 1H), 7.87 (d, J = 7.6 Hz, 1H), 8.04-8.08 (m, 3H); 13C NMR(100MHz, CDCl3): δ 116.1 (d, JC-F = 22.1 Hz), 121.5, 123.1, 125.2,
126.3, 129.4 (d, JC-F = 8.4 Hz), 129.8, 135.0, 154.0, 164.4 (d, JC-F = 250.2 Hz), 166.7; HRMS (EI) m/z calcd for C13H8NFS: 229.0361, found
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2-(4-chlorophenyl)benzothiazole(3x)18: The product was isolated as a colorless solid in 57% yield (139 mg), m.p. = 110.3-112.5 °C, TLC Rf = 0.56
(petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.38 (t, J = 7.6 Hz, 1H), 7.45 (d, J = 7.6 Hz, 2H), 7.49 (t, J = 8.0 Hz, 1H),
7.88 (d, J = 8.0 Hz, 1H), 8.00 (d, J = 7.6 Hz, 2H), 8.57 (d, J = 8.0 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 121.6, 123.2, 125.4, 126.5, 128.7 (2C),
129.2 (2C), 132.0, 135.0, 137.0, 154.0, 166.6; HRMS (EI) m/z calcd for C13H8ClNS: 245.0066, found 245.0061.
2-(thiophen-2-yl)benzothiazole(3y)4d,18: The product was isolated as a yellow solid in 48% yield (103 mg), m.p. = 113.2-114.7 °C, TLC Rf = 0.47
(petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 7.13 (t, J = 4.4 Hz, 1H), 7.36 (t, J = 7.6 Hz, 1H), 7.47 (t, J = 7.6 Hz, 1H),
7.50 (d, J = 5.2 Hz, 1H), 7.54 (d, J = 3.2 Hz, 1H), 7.84 (d, J = 8.0 Hz, 1H), 8.03 (d, J = 8.0 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 121.4, 122.9,
125.2, 126.4, 128.0, 128.6, 129.3, 134.6, 137.3, 153.6, 161.4; HRMS (EI) m/z calcd for C11H7NS2: 217.0020, found 217.0030; Anal. Calcd for
C11H7NS2: Elemental Analysis: C, 60.80; H, 3.25; N, 6.45; Found: C, 60.92; H, 3.34; N, 6.32.
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2-mesitylbenzothiazole(3z)8g: The product was isolated as a colorless solid in 65% yield (165 mg), m.p. = 115.3-116.7 °C, TLC Rf = 0.53
(petroleum ether/ ethyl acetate, 12/1); 1H NMR (400MHz, CDCl3): δ 2.17 (s, 6H), 2.34 (s, 3H), 6.96 (s, 2H), 7.43 (t, J = 7.6 Hz, 1H), 7.52 (t, J =
8.0 Hz, 1H), 7.93 (d, J = 8.4 Hz, 1H), 8.11 (d, J = 7.6 Hz, 1H); 13C NMR(100MHz, CDCl3): δ 20.1 (2C), 21.2, 121.5, 123.3, 125.0, 125.9, 128.3
(2C), 130.6, 136.3, 137.1 (2C), 139.4, 153.4, 167.7; MS (m/z) 253 (M+).
ACKNOWLEDGMENTS
This work is supported by grants from the National Science Foundation of China (No. 21072051), NCET program (NCET-09-0334) and the
Fundamental Research Funds for the Central Universities, Hunan university.
Supporting Information: [Including copies of 1H and 13C NMR spectra for the products.] This material is available free of charge via the Internet
at http://pubs.acs.org.
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Siddiqui, N.; Rana, A.; Khan, S. A.; Bhat, M. A.; Haque, S. E. Bioorg. Med. Chem. Lett. 2007, 17, 4178-4182.
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