Syntheses of 2-aryl benzothiazoles via photocatalyzed oxidative condensation of amines with 2-aminothiophenol in the presence of bodipy derivatives

Article abstract of DOI:10.1080/00397911.2014.932811

A simple, convenient, and efficient new method for synthesis of 2-aryl benzothiazoles under mild conditions with nonmetal catalyst has been developed. Boron-dipyrromethene (BODIPY) dyes were used as photocatalysts for aerobic oxidative reactions of amine with 2-aminothiophenol. The approach will be very useful for the synthesis of benzothiazole derivatives and the development of photocatalytic reactions.

Full text of DOI:10.1080/00397911.2014.932811

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Syntheses of 2-Aryl Benzothiazoles via Photocatalyzed
Oxidative Condensation of Amines with 2-
Aminothiophenol in the Presence of BODIPY Derivatives
Zeyin Zhou a & Weijun Yang a
a Department of Chemistry and Chemical Engineering , Hunan University , Changsha , Hunan
Province , P. R. China
Accepted author version posted online: 17 Jul 2014.
To cite this article: Zeyin Zhou & Weijun Yang (2014): Syntheses of 2-Aryl Benzothiazoles via Photocatalyzed Oxidative
Condensation of Amines with 2-Aminothiophenol in the Presence of BODIPY Derivatives, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, DOI: 10.1080/00397911.2014.932811
To link to this article: http://dx.doi.org/10.1080/00397911.2014.932811
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Syntheses of 2-Aryl Benzothiazoles via Photocatalyzed Oxidative Condensation of
Amines with 2-Aminothiophenol in the presence of BODIPY Derivatives
Zeyin Zhou1, Weijun Yang1
1Department of Chemistry and Chemical Engineering, Hunan University, Changsha,
Hunan Province, P. R. China
Abstract
A simple, convenient, efficient and new method for synthesis of 2-Aryl benzothiazoles
under a mild condition with non-metal catalyst has been developed. BODIPY dyes were
used as photocatalyst for aerobic oxidative reactions of amine with 2-aminothiophenol.
The approach will be very useful for the synthesis of benzothiazole derivatives and the
development of photocatalytic reactions.
KEYWORDS: synthesis; benzothiazoles; BODIPY; photocatalysis; oxidation
INTRODUCTION
2-Aryl benzothiazoles play a very important role as organic functional materials in
chemistry and are also widely used as biologically active products,[1,2] as well as
marketed drugs or drug candidates.[3] For example, 2-Aryl benzothiazoles are not only
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important fluorescent dyes that are used in fibre and plastic,[4] but also as liquid crystals
and off-color material.[5,6] In medicine, 2-aryl benzothiazoles serve as fungicide,[7]
acaricide,[8] and anticancer.[9]
Some synthesis methods for 2-aryl benzothiazoles have been reported in the
literature.[10–17] Classic methods for the synthesis involve the condensation of
2-aminothiophenols with aryl aldehydes,[10] acyl chlorides,[11] carboxylic acids,[12]
alcohols,[13] and amines[14] in the presence of oxidants. Another method is intramolecular
cyclization reaction of N-(2-bromophenyl)benzothioamide with the Pd complex as
catalyst. Moreover, it also has been demonstrated that 2-aryl benzothiazoles can be
synthesized efficiently via transition-metal-catalyzed (Ni, Pd) cross-couplings between
benzothiazoles and aryl halides,[15] aryl boronic acids,[16] or aromatic carboxylic acids.[17]
Unfortunately, so many disadvantages, such as rigorous conditions (i.e. high temperature,
long reaction time, and high pressure), hazardous oxidants,[18] or potential toxicity and
high cost of the metal catalysts, presented in the above synthetic reactions, result in these
transformations uneconomical and unfriendly to environment. Therefore, a new method
for its construction is highly desirable.
With the demand for green chemistry, visible-light-responsive photoredox reactions show
much prominence for the use of the visible light which is a clean energy source provided
by the solar irradiance. BODIPY (boron-dipyrromethene) is a class of novel fluorescent
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dyes. It is composed of dipyrromethene complexes with a substituted boron atom,
typically a BF2 unit.[19,20] Currently, BODIPY has been used as visible-light-responsive
photocatalyst applied in oxidation of thioanisole and dihydroxylnaphthalenes,[21,22] which
shows excellent photocatalytic activity. In these reactions, oxygen/air was used as the
terminal oxidant. Mild condition and metal-free photoredox reaction driven by the
BODIPY photosensitizer seems to be highly economical for their excellent properties
such as strong fluorescence and absorption. Meanwhile, the molecular structure of
BODIPY can be so easily tuned by a way of small modifications that we can choose a
better catalyst to photocatalyze oxidative reaction. In this context, one-pot synthesis of
2-aryl benzothiazoles via BODIPY photocatalyzed oxidation of amines with
2-aminothiophenol in visible light was explored.
RESULTS AND DISCUSSION
There are several BODIPY photocatalysts prepared in this paper. As shown in Scheme 1,
BODIPY 1 are made from aldehyde as electrophilic component to form the methane
bridge between two pyrrole units. Then BODIPY 2 and BODIPY 3 are synthesized by
using copper (II) bromide as bromination reagents in mild conditions with excellent
yields and selectivity. Their structures are confirmed by the 1H NMR, 13C NMR and MS
spectra. With BODIPYs in hand, we further studied the photophysical properties of these
compounds. BODIPYs are colorful to our eyes, and most of them are brilliant upon
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irradiation. UV-vis absorption and fluorescence spectra of BODIPYs were studied (see
Figure 1).
BODIPYs’ spectra show the typical narrow absorption and sharp fluorescence emission
bands of classic difluoroboron dipyrrins, and the max fluorescence emission band
(BODIPY 3b, the blue dash line in Figure 1) can approach 580 nm, which displays
near-IR emission. With the increased conjugation on BODIPY b moleculer, the spectra
are red-shifted (10 nm for absorption and 10 nm for fluorescence emission) compared
with those of BODIPY a. Meanwhile, with the increase of the amount of bromine atom in
the BODIPY structure, obvious red-shifts of the absorption and emission was observed in
BODIPY 3a, and the Stokes shift are 10 nm and 20 nm respectively compared with
BODIPY 1a and BODIPY 2a. On the other hand, fluorescence quantum yield was
weakened to a great extent for this reason (see Table 1).
To solve the problems occurred in synthesis of 2-aryl benzothiazoles mentioned above,
one-pot synthesis of 2-aryl benzothiazoles via BODIPY photocatalyzed oxidation of
amines with 2-aminothiophenol in visible light was explored. We are aimed at
accomplishing the transformation effectively under a mild condition with non-metal
catalyst.
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Results of control experiments and optimization of reaction conditions for the oxidation
of benzylamine are shown in Table 2. Initially, the reaction was conducted without visible
light (Table 2, entry 6), no product was detected in GC. Similarly, no transformation
occurred in the absence of oxygen even with longer reaction time to 8h (Table 2, entry 5).
When the reaction was conducted under light without BODIPY catalyst, no product was
observed either (Table 2, entry 7). Meanwhile, we also found the mounts of
2-aminothiophenol (from 2 eq to 3 eq) and the reaction atmosphere (air to O2) just have a
slight effect to the reaction results, for the yields only rise 2 percent and 3 percent
respectively. Therefore, the conclusion was made as follows: the light, photosensitizer
and oxygen are essential to the photocatalytic oxidation. Subsequently, a better yield
(80%) was obtained with temperature rising to 50 °C (Table 2, entry 4). We further raised
the reaction temperature to 80 °C (Table 2, entry 10), GC shows that more side products
were generated (the main side product is benzaldehydes). Changing the photosensitizer,
however, compared with BODIPY 1 and BODIPY 2, it is obvious that BODIPY 3
presents significant influences in the aerobic oxidation of benzylamine with
2-aminothiophenol at the same condition (Table 2, entry 1-3). The more the amount of
bromine in the BODIPY structure, the better yield was obtained, which may suggest that
bromine can promote the generation of 1O2 in accordance with some reported results that
the bromine substituted BODIPY extremely promoted the generation of 1O2.[23,24] Based
on these results, we decided to set heating the benzylamine and 2-aminothiophenol at
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50 °C in the presence of BODIPY 3a irradiated by a 35 W xenon lamp as our optimized
condition.
With the optimized protocol results in hand, a number of experiments were carried out to
explore the scope and limitation of the reaction (Table 3). We found that the reaction
worked very well for a wide variety of substituted benzylamines, obtaining the expected
substituted benzothiazoles in yields ranging from 42 to 81%. With various substituents
such as methyl, chloro, fluoro, bromine, methoxy as well as naphthyl, all groups
proceeded smoothly in good yields. For example, benzylamines substituted with
electron-donating groups (CH3 and OCH3) (Table 3, entry 3, 4, 11), afford the target
products in 78-81% yields; Halogen, such as chloro, bromine (Table 3, entry6, 9, 10)
gives no problem, no matter for para-substituted benzylamines, meta or ortho, except the
fluoro (Table 3, entry 2, 7), which are strong electron-withdrawing groups and only
undergo the oxidative condensation with 42% yields. So the oxidation of the
electron-donating groups proceeded more efficiently than the electron-withdrawing
groups. We also found that no reaction occurred when amine lacking of α-H, such as
aniline (Table 3, entry 15). Heterocyclic amine, such as thiophen-2-ylmethanamine, also
proceeded smoothly with better yield.
Based on foregoing results, we proposed a mechanism that it is highly likely that the
presence of singlet oxygen (1O2)[25] is responsible for the photooxidation of benzylamine
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with 2-aminophenol. We further studied the influence of DABCO (1,
4-diazabicyclo[2.2.2]octane, a singlet oxygen scanvenger) to the oxidation. We found that
the photocatalytic reaction can be significantly quenched by the DABCO. Therefore, we
think 1O2 is involved in the photooxidative process. The proposed mechanism was shown
in Scheme 2. First, BODIPY accepted a photon from the visible light to form BODIPY*
under the irradiation by visible light. Then, benzylamine was oxidized to form
phenylmethanimine by singlet oxygen generated by energy transfer from BODIPY*.
Finally, 2-aryl benzothiazole was formed by the condensation of intermediate. Some
details about the singlet oxygen involved in the photosensitized oxidation reaction had
been reported and discussed.[26,27] In terms of the energy requirements, BODIPY dyes
generating singlet oxygen from the triplet excited state have been used in photodynamic
therapy.[28–30]
In summary, several BODIPY dyes were synthesized, and their structures were confirmed
by 1H NMR, 13C NMR, and MS spectra, and they were used as photocatalysts for aerobic
oxidative reactions of amine with 2-aminothiophenol. A new method to synthesize 2-aryl
benzothiazoles under a mild condition with a metal-free procedure using BODIPY as
photocatalysts was developed. Our approach will be very useful for the synthesis of
benzothiazoles derivatives and the development of photocatalytic reactions.
EXPERIMENTAL
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General Procedure For BODIPY 1
Aryl-aldehyde (2.0 mmol), 2,4-dimethylpyrrole (423 mg, 4.5 mmol) were dissolved in
(100 mL) absolute CH2Cl2. Trifluoroacetic acid (one drop) was added in the solution
under argon. After the reaction mixture was stirred about 4 hours at room temperature
until TLC-control showed the complete consumption of the aldehyde. Then DDQ
(2,3-Dichloro-5,6-dicyano-1,4-benzoquinone) (454 mg, 2.0 mmol) in CH2Cl2 (50 mL)
was added. The mixture was stirred for 30 min followed by the addition of Et3N (6 mL)
and BF3·Et2O (4 mL) at ice-cold condition and further stirred at room temperature for 3h.
The reaction mixture was washed with water (3×100 mL), then the organic layer were
combined and dried over with anhydrous MgSO4 and evaporated to dryness. The crude
product was further purified using column chromatography.
General Procedure For BODIPY 2
1,3,5,7-tetramethyl-BODIPY 1 (0.2 mmol) and K2CO3 (0.6 mmol) were dissolved in
MeCN (20 mL). CuBr2 (0.3 mmol) in MeCN (25 mL) was slowly added in the solution
under an O2 atmosphere (balloon). The mixture was stirred at r.t. for 24 h. The reaction
mixture was washed with EtOAc (3×40 mL), and then washed with H2O (3x30 mL). The
organic layer were combined and dried over with anhydrous MgSO4 and evaporated to
dryness. The crude product was further purified using column chromatography.
General Procedure For BODIPY 3
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1,3,5,7-tetramethyl-BODIPY 1 (0.2 mmol) and CuBr2 (0.5 mmol) were dissolved in
MeCN (20 mL) under an O2 atmosphere (balloon). The mixture was stirred at r.t. for 12 h.
The reaction mixture was washed with EtOAc (3×40 mL), and then washed with H2O
(3×30 mL). The organic layer were combined and dried over with anhydrous MgSO4 and
evaporated to dryness. The crude product was further purified using column
chromatography.
Typical Procedures For Photocatalytic Oxidation Of Amine With
2-Aminothiophenol
To a dry 10 mL flask were added amine (1 mmol), 2-aminothiophenol (2mmol), BODIPY
photosensitizer (0.01mmol, 1.0 mol %,) and acetonitrile(5 mL). The flask was
pressurized with air (2 bar), and then heated to 50°C. The solution was then irradiated
using a 35 W xenon lamp through a cut off filter (0.72 M NaNO2 aqueous solution, which
is transparent for light > 385 nm, because lamps could emit a small amount of ultraviolet
light). After the reaction is completed, the solvent was evaporated under reduced pressure.
The crude product was further purified using column chromatography.
SUPPORTING INFORMATION
Supplemental data for this article can be accessed on the publisher’s website.
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Table 1. Photophysical Properties of BODIPYs in Dichloromethane*
BODIPY
λmax/nm
ᇞλ/nm
λf/nm
ε/105M-1cm-1
Фf
521
533
551
530
540
570
1a
2a
3a
1b
2b
3b
502
510
528
510
518
538
19
23
23
20
22
32
0.64
0.58
0.57
0.70
0.62
0.59
0.61
0.12
0.08
0.76
0.15
0.11
*With fluorescein as the standard (Фf= 0.85 in NaOH).
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Table 2. Reaction Condition Optimization
NH2
N
S
NH2
+
SH
photosensitizer
condition
T/°C
Yield/%b
Entry
T/h
1
2
BODIPY 1a/1b
BODIPY 2a/2b
BODIPY 3a/3b
BODIPY 3a
BODIPY 3a
BODIPY 3a
No catalyst
In air
In air
In air
In air
In N2
In air
In air
In air
In O2
In air
23
23
23
50
50
50
50
50
50
80
5
5
24/22
31/28
40/36
80
3
5
4
5
5
10
5
--e
6c
--
7
5
--
8d
9
BODIPY 3a
BODIPY 3a
BODIPY 3a
5
82
5
83
10
5
73
aReaction conditions:benzylamine (1mmol), 2-aminothiophenol (2eq, 2 mmol), photosensitizers
catalyst (1 mol%), acetonitrile as solvent (5 mL), λ > 380 nm, P=0.20MPa.
bYield are determined by GC.
cNo photo-irradiation.
d2-aminothiophenol (3 eq, 3 mmol).
eNo reaction.
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Table 3. Oxidation of various substituted amines using BODIPY 3a
R
NH2
N
S
BODIPY 3a
Visible light
NH2
R
+
SH
Entry
Substrate
Product
T/h
Yield/%b
NH2
1
5
80
N
S
F
2
5
5
5
5
5
5
5
5
5
5
42
81
78
76
72
49
69
75
74
79
NH2
N
S
F
3
NH2
NH2
N
S
MeO
EtO
4
N
S
OMe
5
NH2
OEt
NH2
N
S
Cl
6
N
S
Cl
N
7
NH2
F
S
F
N
S
8
NH2
NH2
N
S
9
Br
Br
N
S
10
11
NH2
NH2
Cl
Cl
N
S
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O
12
5
5
78
76
NH2
N
S
O
O
O
13
N
S
NH2
N
S
14
15
NH2
5
5
81
0
S
S
NH2
N
N
HS
aReaction conditions:amine (1mmol), 2-aminothiophenol (2 eq, 2 mmol), BODIPY 3a (1 mol%),
acetonitrile as solvent (5 mL), λ > 380 nm, T=50°C, in air P=0.20 MPa.
bIsolated yield based on amine.
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Table 4. Mechanism study of the oxidative experiments
NH
2
N
S
BODIPY 3a
Visible light
NH
2
+
SH
Entry
Add DABCO%
Yield %b
1
2
3
0
2
5
80
35
0
aReaction conditions: benzylamine (1 mmol), 2-aminopheno (2 mmol), photosensitizers catalyst (1
mol%), acetonitrile (5 mL), in air P=0.2 MPa, λ> 380 nm, 50°C.
bYield was determined with GC.
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Scheme 1. Synthesis of BODIPYs.
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Scheme 2. Proposed reaction mechanism for the photocatalytic aerobatic oxidation with
the photosensitizer BODIPY 3a
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Figure 1. Steady state UV−vis absorption (solid line) and fluorescence (dash line) spectra
measured in dichloromethane.
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