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

Article abstract of DOI:10.1055/s-0033-1339193

A novel way to synthesize 2-arylbenzothiazoles is described. Reactions of benzothiazoles with diverse benzylamines in the presence of K₂S ₂O₈ and KOt-Bu in DMSO-H₂O afforded the desired 2-arylbenzothiazoles in good yields. It is notable that no transition-metal catalyst is needed in this reaction. Compared with other known methods, this method of synthesizing 2-arylbenzothiazoles can be advantageous in cases where substituted benzothiazoles and benzylamines are readily available. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1339193

LETTER
▌1549
lSetyternthesis of 2-Aryl Benzothiazoles via K2S2O8-Mediated Oxidative Condensa-
tion of Benzothiazoles with Benzylamines
Synthesis of 2-Aryl Benzothiazoles
Zhiyong Yang,a Anwei Wang,a Xiang Chen,a Qingwen Gui,a Jidan Liu,a Ze Tan,*a Hua Wang,a Ji-Cheng Shi*b
a
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, P. R. of China
E-mail: ztanze@gmail.com
b
School of Chemistry and Life Science, Guangdong University of Petrochemical Technology, Maoming, Guangdong 525000, P. R. of China
E-mail: jchshi_mmc@126.com
Received: 17.04.2013; Accepted after revision: 18.05.2013
catalysis at all, instead it is mediated by K2S2O8.15b In-
Abstract: A novel way to synthesize 2-arylbenzothiazoles is de-
spired by these exciting results, we wonder if these two re-
scribed. Reactions of benzothiazoles with diverse benzylamines in
actions can still work if benzaldehyde is replaced with
other molecules. Herein we report that 2-arylbenzothia-
the presence of K2S2O8 and KOt-Bu in DMSO–H2O afforded the
desired 2-arylbenzothiazoles in good yields. It is notable that no
transition-metal catalyst is needed in this reaction. Compared with zoles can also be synthesized via K2S2O8-mediated oxida-
other known methods, this method of synthesizing 2-arylbenzothi-
azoles can be advantageous in cases where substituted benzothia-
zoles and benzylamines are readily available.
tive condensation of benzothiazoles with benzylamines
(Scheme 1, eq. 4).
Key words: 2-aryl benzothiazoles, benzylamines, K2S2O8, KOt-Bu,
oxidative cyclization
NH2
N
oxidant
Ph
+
PhCHO
(1)
(2)
S
SH
2-Arylbenzothiazole is a very important structural motif
in organic chemistry because it appears widely in many
natural products and medicinally active compounds.1,2 It
is also frequently used as a building block in other fields
such as fluorescent dyes and liquid crystals.1,2 While nu-
merous methods to construct this motif have been report-
ed in the literature,3–14 including condensation of 2-amino
thiophenols with aryl aldehydes,3 (Scheme 1, eq. 1) car-
boxylic acids,4 nitriles,5 acyl chlorides,4b,6 alcohols,7 ke-
tones,8 and amines,9 interest in the development of general
methods for providing diverse 2-arylbenzothiazoles have
never ceased. For example, it has been demonstrated that
2-arylbenzothiazoles can be efficiently synthesized via
transition-metal-catalyzed (Ni, Pd, or Cu) cross-couplings
between benzothiazoles and aryl halides,10 aromatic car-
boxylic acids,11 aryl boronic acids,12 or 2-halide-substitut-
ed benzothiazoles with aryl metals or metal equivalents13
(Scheme 1, eq. 2) if the parent benzothiazole is available.
Also starting from the parent benzothiazole, Deng, Li, and
we have independently shown that 2-arylbenzothiazoles
can be efficiently synthesized via oxidative condensation
of benzothiazoles with aryl aldehydes (Scheme 1, eq. 3).15
In both protocols, the starting benzothiazole undergoes
opening to produce 2-amino thiophenol in the reaction
mixture, which subsequently reacts with benzaldehyde to
give 2-arylbenzothiazole.16 Deng and Li’s protocol utiliz-
es inexpensive FeSO4 as the catalyst and O2 as the oxi-
dant.15a Our reaction does not rely on transition-metal
N
S
N
S
cat. Ni, Cu or Pd
H(X)
Ph
PhX(M)
+
N
N
K2S2O8
H
H
Ph
(3)
(4)
PhCHO
+
S
S
or FeSO4, O2
N
S
N
S
K2S2O8
Ph
+
BnNR1R2
this work
Scheme 1
Since benzylamines are known to form benzaldehydes un-
der suitable conditions,17,18 coupled with the knowledge
that 2-arylbenzothiazoles can be synthesized from benzo-
thiazoles and benzaldehydes with K2S2O8, we envision 2-
arylbenzothiazoles can be formed in a similar fashion be-
tween benzothiazoles and benzylamines. We started our
research by reacting one equivalent of benzylamine with
1.5 equivalents of benzothiazole in the presence of one
equivalent of K2S2O8 in DMSO and H2O at 100 °C for
three hours. To our delight, the desired product 2-phenyl-
benzothiazole was indeed formed in 15% yield (Table 1,
entry 1). But we found that a large amount of the starting
benzothiazole remained intact, so we decided to change
the reactant ratio by using one equivalent of benzothiazole
with two equivalents of benzylamine in the next trying.
However, despite numerous attempts, including changing
the solvent or increasing the amount of oxidant to three or
four equivalents, the yield could not be improved signifi-
SYNLETT 2013, 24, 1549–1554
Advanced online publication: 17.06.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339193; Art ID: ST-2013-W0351-L
© Georg Thieme Verlag Stuttgart · New York

1550
Z. Yang et al.
LETTER
cantly (Table 1, entries 2–6). Finally, we found that With the optimized protocol in hand, we next set out to ex-
changing the acidity or basicity of the solvent can affect plore the scope and limitation of the reaction (Table 2). As
the process of the reaction dramatically (Table 1, entries we expected, the reaction worked satisfactorily for a wide
7–14). While the use of acetic acid or trifluoroacetic acid variety of substituted benzylamines with substituents such
as additive in the reaction improved the yield to above as methyl, chloro, fluoro, as well as methoxy groups on
40%, better results were obtained with adding base. In the benzene ring, affording the desired 2-aryl-substituted
particular, the addition of two equivalents of KOt-Bu im- benzothiazoles in yields ranging from 39–71% (Table 2,
proved the yield to 55% (Table 1, entry 10). Since KOt-Bu 3a–l). However, only trace amounts of the desired prod-
will react with water to form KOH and t-BuOH in the re- ucts were obtained when we treated benzothiazole with al-
action mixture, we next tested the effectiveness of using iphatic amines under our standard protocol (not shown).
KOH or KOH and t-BuOH as additives (Table 1, entries This is to be expected, as our original protocol also did not
11 and 12). Though the yields were also higher, they are work well with aliphatic aldehydes. Judging from the re-
slightly inferior to the one obtained with KOt-Bu. Finally, sults obtained, we could see that the reactions of second-
the yield of 2-arylbenzothiazole was improved to 61% ary benzylamines as well as tertiary benzylamines all
when we changed the ratio of the solvent mixture DMSO– worked satisfactorily besides primary benzylamines. Sub-
H2O from 2:1 to 4:1 (Table 1, entry 14). We also discov- stituents such as methyl, ethyl, and benzyl groups on the
ered that the yield was much lower if only a catalytic nitrogen atom of the benzylamine did not cause any prob-
amount of KOt-Bu (0.1 equiv) was used (Table 1, entry lems. In addition, the reactions of methoxy-, methyl-, and
13). On the basis of these results, we decided to set treat- chloro-substituted benzothiazoles proceeded smoothly to
ing benzothiazoles and benzylamines at 100 °C in the afford the desired 2-arylbenzothiazoles in good yields
presence of three equivalents of K2S2O8 and two equiva- (Table 2, 3f–l). In contrast, when we treated 2-amino thio-
lents of KOt-Bu in DMSO–H2O (v/v = 4:1) as our stan- phenol with benzylamine under our standard conditions,
dard conditions.
the desired 2-phenylbenzothiazole was only isolated in
26% yield (Scheme 2). We reasoned that the low yield ob-
Table 1 Optimization of Reaction Conditionsa
N
N
K2S2O8, KOt-Bu
NH2
+
DMSO–H2O
S
S
1
2
3a
Entry
Oxidant (equiv)
Additive (equiv)
Solvent (mL)
Temp (°C)
Yield (%)b
1c
2
K2S2O8 (1)
K2S2O8 (1)
K2S2O8 (3)
K2S2O8 (4)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
K2S2O8 (3)
DMSO–H2O (2:1)
DMSO–H2O (1:2)
DMSO–H2O (2:1)
DMSO–H2O (2:1)
MeCN–H2O (2:1)
MeCN–DMSO–H2O
DMSO–H2O (2:1)
DMSO–H2O (2:1)
DMSO–H2O (2:1)
DMSO–H2O (2:1)
DMSO–H2O (2:1)
DMSO–H2O (2:1)
DMSO–H2O (2:1)
DMSO–H2O (2:0.5)
100
100
100
100
100
100
100
100
100
100
100
100
100
100
15
12
22
18
0
3
4
5
6d
7
21
35
43
40
55
49
50
32
61
AcOH (1)
8
AcOH (1)
9
TFA (1)
10
11
12e
13
14f
KOt-Bu (2)
KOH (2)
KOH, t-BuOH
KOt-Bu (0.1)
KOt-Bu (2)
a Reaction conditions: benzothiazole (0.5 mmol), benzylamine (1.0 mmol), 100 °C, 8 h, unless otherwise mentioned.
b Yields are determined by GC.
c Reaction conditions: benzothiazole (1.5 mmol), benzylamine (1.0 mmol), oxidant (1.0 mmol), 3 h.
d MeCN–DMSO–H2O = 1:1:1 (mL).
e KOH (2 equiv), KOt-Bu (2 equiv).
f Reaction conditions: benzothiazole (0.5 mmol), benzylamine (1.0 mmol), oxidant (1.5 mmol), additive (1.0 mmol), DMSO–H2O = 2:0.5 (mL),
100 °C, 8 h.
Synlett 2013, 24, 1549–1554
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of 2-Aryl Benzothiazoles
1551
tained may be possibly due to the fact that the rate of 2- benzothiazoles 3m and 3n (Scheme 3). For the synthesis
amino thiophenol being oxidized was much faster than the of 3m, 4-methylbiphenyl was brominated at the benzylic
rate of benzaldehyde formation from benzylamine under position, and the benzyl bromide was treated with dimeth-
the standard conditions.
ylamine to give the corresponding benzylamine. The re-
sultant benzylamine was subjected to our standard
conditions with benzothiazole to afford 3m in 50% yield
over three steps. In a similar fashion, 3n was synthesized
in just two operations from the commercially available 4′-
(bromomethyl)-[1,1′-biphenyl]-2-carbonitrile, a common
Sartan intermediate, in 53% overall yield.
N
NH2
NH2
+
K2S2O8, KOt-Bu
DMSO–H2O
100 °C, 8 h
S
SH
26%
Scheme 2 Reaction between 2-amino thiophenol and benzylamines
Some control experiments have been carried out to probe
the reaction mechanism (Scheme 4). When benzylamine
was treated with K2S2O8 and KOt-Bu under our standard
In order to demonstrate the synthetic utility of our method,
we carried out the syntheses of the two 2-aryl-substituted
N
NMe2
iii
i, ii
Ph
Ph
S
N
S
Ph
3m
50% over three steps
CN
CN
N
S
Br
NMe2
iii
ii
N
S
NC
3n
53% over two steps
Scheme 3 Reagents and condition: i. NBS, CCl4, benzoyl peroxide; ii. HNMe2, Et2O–H2O; iii. standard conditions.
K2S2O8 (1 equiv)
CHO
KOt-Bu (1 equiv)
NH2
100 °C, DMSO–H2O
8 h
82%
K2S2O8 (2 equiv)
CHO
+
N
S
N
N
S
NH2
KOt-Bu (2 equiv)
K2S2O8 (1 equiv)
3 h
Ph
+
100 °C, DMSO–H2O
8 h
S
(1 equiv)
(2 equiv)
mostly intact
60%
NH2
K2S2O8 (1 equiv)
SH
N
S
S
KOt-Bu (1 equiv)
S
+
100 °C, DMSO–H2O
5 h
NH2
H2N
5%
77%
Scheme 4 Control experiments
O
K2S2O8
NH2
hydrolysis
H
NH
H
N
N
N
S
NH2
SH
K2S2O8
S
H
SH
N
K2S2O8
S
Scheme 5 Possible reaction mechanism
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1549–1554

1552
Z. Yang et al.
LETTER
conditions, we found that benzaldehyde was formed in amine is faster than the ring opening the thiazole ring. If
82% yield. On the other hand, when we reacted two equiv- another equivalent of K2S2O8 is added into the reaction
alents of benzylamine with one equivalent of benzothia- mixture, the desired 2-phenyl benzothiazole can be
zole, two equivalents of K2S2O8 and two equivalents of formed in 60% yield. We also discovered that the ring
KOt-Bu under our standard conditions, no 2-phenyl ben- opening of benzothiazole under our conditions led mainly
zothiazole was formed. The main product was benzalde- to 2-amino thiophenol instead of 2,2′-diaminodiphenyl-
hyde while most of the starting benzothiazole remained disulfide.
intact. This result showed that the oxidation of benzyl-
Table 2 Synthesis of 2-Arylbenzothiazoles from Benzothiazoles and Benzylaminesa
.
R2
R1
N
S
NR3R4
N
S
K2S2O8, KOt-Bu
R1
+
R2
DMSO–H2O
1 R1 = Me, Cl, OMe, H 2 R2 = H, F, Cl, OMe, etc.
3
R3, R4 = H, Me, Et, Bn
Entry
1
Substrate
Product
Yield (%)b
61
N
R1, R2 = H
R3, R4 = H
S
3a
3a
3a
3a
3a
N
S
R1, R2 = H
2
3
4
5
65
67
63
66
R3 = H, R4 = Me
N
S
R1, R2 = H
R3 = Me,
R4 = Me
N
S
R1, R2 = H
R3 = H, R4 = Et
N
S
R1, R2 = H
R3 = H, R4 = Bn
F
N
S
R1 = H, R2 = F
R3, R4 = H
6
39
3b
Cl
N
S
R1 = H, R2 = Cl
R3, R4 = H
7
8
9
54
62
68
3c
N
S
R1 = H, R2 = Me
R3, R4 = H
3d
MeO
N
S
R1 = H, R2 = OMe
R3, R4 = H
3e
Synlett 2013, 24, 1549–1554
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of 2-Aryl Benzothiazoles
1553
Table 2 Synthesis of 2-Arylbenzothiazoles from Benzothiazoles and Benzylaminesa (continued)
.
R2
R1
N
S
NR3R4
N
S
K2S2O8, KOt-Bu
R1
+
R2
DMSO–H2O
1 R1 = Me, Cl, OMe, H 2 R2 = H, F, Cl, OMe, etc.
3
R3, R4 = H, Me, Et, Bn
Entry
10
Substrate
Product
Yield (%)b
55
N
S
R1 = OMe, R2 = Cl
R3, R4 = H
Cl
MeO
3f
N
S
R1 = OMe, R2 = Me
R3, R4 = H
11
12
13
66
71
68
MeO
3g
MeO
N
S
R1, R2 = OMe
R3, R4 = H
MeO
3h
N
S
R1 = OMe, R2 = H
R3, R4 = H
MeO
3i
N
R1 = Me, R2 = H
R3, R4 = H
14
15
66
57
S
3j
N
S
Cl
R1 = Me, R2 = Cl
R3, R4 = H
3k
N
S
R1 = Cl, R2 = H
R3, R4 = H
16
54
Cl
3l
a Reaction conditions: benzothiazole (0.5 mmol), benzylamine (1.0 mmol), oxidant (1.5 mmol), additive (1.0 mmol), DMSO–H2O = 2:0.5 (mL),
100 °C, 8 h.
b Isolated yields are based on 1.
On the basis of the results reported,15–18 we think a likely with K2S2O8 and KOt-Bu at 100 °C in DMSO and H2O, a
mechanism involves the oxidation of benzylamine by variety of 2-arylbenzothiazoles were synthesized in good
K2S2O8 to the corresponding imine which is subsequently yields. In addition to primary benzylamines, we found the
hydrolyzed in the reaction mixture to form benzaldehyde. reaction also worked well with secondary and tertiary
The addition of KOt-Bu to the reaction will produce KOH benzylamines. Operationally, the reaction is simple to run
in the DMSO–H2O mixture which can help with the and is transition-metal-free. It can be quite useful in cases
hydrolysis process, thus facilitating the formation of benz- when benzylamines are more readily available than the
aldehyde. At the same time, the thiazole ring of benzothi- corresponding benzaldehydes.
azole is opened in the reaction mixture to give 2-amino
thiophenol which undergoes oxidative cyclization with
benzaldehyde to produce the desired 2-arylbenzothiazole
(Scheme 5).
A 25 mL round-bottom flask was charged with benzothiazole (1,
0.5 mmol), benzylamine (2, 1.0 mmol), K2S2O8 (1.5 mmol), KOt-
Bu (1.0 mmol), and DMSO–H2O = 2:0.5 (mL). The reaction was
stirred at 100 °C for 8 h (monitored by TLC). Upon completion of
the reaction, the mixture was diluted with H2O, extracted with
CH2Cl2 (3 × 10 mL). The organic layers were combined and washed
with brine, dried over MgSO4, filtered, and concentrated. The crude
In summary, we have developed a novel way of synthesiz-
ing 2-arylbenzothiazoles from benzothiazoles and benzyl-
amines. By treating benzothiazoles and benzylamines
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1549–1554

1554
Z. Yang et al.
LETTER
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Acknowledgment
Z. Tan would like to thank the National Science Foundation of
China (No. 21072051), NCET program (NCET-09-0334), and the
Fundamental Research Funds for the Central Universities,
Hunan University, for financial support.
Supporting Information Experimental procedures, characte-
rization data, and copies of 1H NMR and 13C NMR spectra for this
article is available online at http://www.thieme-connect.com/ejour-
nals/toc/synlett.SnugIifop
mnoartiuSpngIfpi
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Synlett 2013, 24, 1549–1554
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