Intramolecular cascade C-S bond formation: Regioselective synthesis of substituted benzothiazoles

Article abstract of DOI:10.1055/s-0029-1218798

Carbon-fluorine bond fission has been coupled with C-S bond formation under metal-free conditions, which is seldom reported in the literature. The intramolecular C-S bond formation reaction takes place by fission of the carbon-fluorine bond in situ, and is believed to proceed through an S _NAr mechanism. The approach represents a practical and atom-economical approach for regioselective and metal-free cascade synthesis of substituted benzothiazoles. This one-pot, environmentally benign protocol involves 2-fluoroanilines, benzoyl chlorides and Lawessons reagent under microwave irradiation (5 min) or conventional thermal conditions (3h). Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1218798

SHORT PAPER
1983
Intramolecular Cascade C–S Bond Formation: Regioselective Synthesis of
Substituted Benzothiazoles
S
ynthesis of Subst
.
itute
d
Benzo
S
thiazole
s
ubhas Bose,* Mohd. Idrees
Organic Chemistry Division III, Fine Chemicals Laboratory, Indian Institute of Chemical Technology, Hyderabad 500607, India
Fax +91(40)27160387; E-mail: dsb@iict.res.in; E-mail: bose_iict@yahoo.co.in
Received 23 February 2010; revised 17 March 2010
tious methodologies for the synthesis of privileged struc-
tures for use in drug discovery and development
processes.
Abstract: Carbon–fluorine bond fission has been coupled with C–
S bond formation under metal-free conditions, which is seldom re-
ported in the literature. The intramolecular C–S bond formation re-
action takes place by fission of the carbon–fluorine bond in situ, and
is believed to proceed through an SNAr mechanism. The approach
represents a practical and atom-economical approach for regio-
selective and metal-free cascade synthesis of substituted benzothia-
zoles. This one-pot, environmentally benign protocol involves 2-
fluoroanilines, benzoyl chlorides and Lawesson’s reagent under mi-
crowave irradiation (5 min) or conventional thermal conditions
(3 h).
Benzothiazoles are an important class of privileged struc-
tures of medicinal significance due to their recognized
biological and therapeutic activities.3 As such, these het-
erocycles constitute key structural motifs that exhibit a
wide range of biological properties, such as imaging
agents for b-amyloid plaques,4 photosensitizers,5 inhibi-
tors of stearoyl-coenzyme A d-9 desaturase,6 antitumor
(1),7 and antimicrobial8 agents, LTD4 receptor antago-
nists,9 orexin receptor antagonist (2),10 and Gram-positive
selective antibacterials11 (3; Figure 1).
Key words: benzothiazole, metal-free, cascade reactions, S-aryla-
tion, regioselective
O
Large dissociation energies of C–F bonds in fluorocar-
bons are widely used to explain their low reactivity, and
are caused mainly due to the small size and high elec-
tronegativity of the fluorine atom. The activation of C–F
bonds in fluoroaromatic compounds has attracted the at-
tention of many organometallic chemists and new routes
that enable functionalization of these bonds are under in-
tensive investigation. In this regard, a number of reports
describing stoichiometric activation and functionalization
of C–F bonds by transition-metal complexes have been re-
ported.1 Despite the recent advances in this field, catalytic
transformations of C–F bonds using transition-metal cen-
ters have been only poorly developed and, moreover, only
a few methods that enable fluorine to be replaced are cur-
rently available. However, among fluoroaromatic com-
pounds, nickel-catalyzed activation of the C–F bond of 2-
fluoropyridines and pyrimidines have been reported.2
S
NH2•2HCl
N
H
H
(CH2)4NH2
N
F
1
S
N
O
O
N
N
N
H
2
H2N
N
N
Cl
S
N
H
N
3
Figure 1 Structures of pharmaceutically important benzothiazoles
Although 2-substituted benzothiazoles play an important
role in pharmaceutical science, the number of available
synthetic strategies that lead to these compounds are lim-
ited compared with those that lead to the structurally relat-
ed benzothiophene. Traditional methods for the
preparation of the benzothiazole framework include con-
densation reactions of 2-aminothiophenols with substitut-
ed nitriles, aldehydes, carboxylic acids, acyl chlorides, or
esters.12 These methods, however, suffer from limitations
such as difficulties encountered in the preparation of
readily oxidizable 2-aminothiophenols bearing substitu-
ents. Some of these drawbacks have recently been over-
come by the development of a novel and more sustainable
process13 that allows the efficient assembly of the target
heterocycles under comparatively mild reaction condi-
tions. Among those protocols, copper- or palladium-cata-
lyzed Buchwald–Hartwig-type cross-coupling reactions
In this context, we were interested in studying the anoma-
lous reactivity of the fluorine atom towards aromatic nu-
cleophilic substitution reactions (SNAr). Halogen atoms
generally show the reactivity trend F > Cl > Br > I towards
SNAr replacement reactions, which can be explained by
considering the high electronegativity of the fluorine atom
that facilitates the initial attack of nucleophile and stabi-
lizes the transition state. This interesting property of the
fluorine atom could be effectively investigated and dem-
onstrated for the synthesis of biologically significant,
privileged structures. From our part, we have focused on
the development of environmentally benign and expedi-
SYNTHESIS 2010, No. 12, pp 1983–1988
x
x.
x
x
.
2
0
1
0
Advanced online publication: 27.05.2010
DOI: 10.1055/s-0029-1218798; Art ID: Z04410SS
© Georg Thieme Verlag Stuttgart · New York

1984
D. S. Bose, M. Idrees
SHORT PAPER
represent an elegant and straightforward synthetic ap- products.18 In this regard, we envisaged the application of
proach to 2-substituted benzothiazoles, although some a metal-free cascade intramolecular C–S bond formation
limitations still remain. For example, the palladium-cata- protocol to obtain functionalized benzothiazoles from
lyzed preparation of aryl thiols from aryl halides via thiol readily available o-fluoroanilines.
surrogates is difficult and can only be applied to a limited
As part of our ongoing efforts to develop environmentally
range of available starting materials.14 Moreover, the high
benign processes,19 we examined the effect of microwave
cost and air-sensitivity of palladium catalyst systems
irradiation20 on the formation of intramolecular C–S
commonly limit their application to large- and industrial-
bonds. Thus, a mixture of 2,4-difluoroaniline (4b), p-
scale formation of these bonds. The process is also inher-
anisoyl chloride, and Lawesson’s reagent (LR)21 was irra-
ently unfavorable from the point of view of atom econo-
diated for five minutes in a domestic microwave oven
my. The replacement of one (or both) of these
(300 W, 115 °C) in toluene, to give the 2-arylbenzothia-
zole 5d, albeit in trace amounts (Table 1). We then opti-
mized the reaction conditions with respect to yield by
functionalized reagents by a metal-free protocol is thus a
key goal in the development of green methods and is of
particular appeal with respect to the organometallic re-
varying the reaction time and temperature, and by using a
agents, which are often the least stable component and
range of solvents with suitable boiling points, with two
commonly used ionic liquids, and with no added solvent
most difficult to prepare.
Over the past decade, extensive efforts have been made to (Table 1). These investigations revealed that compound
develop methodologies that directly functionalize aromat- 5d could be obtained in 97% yield at 155 °C in N,N-dieth-
ic C–H bonds to construct C–C, C–N, C–O, and C–S ylaniline under microwave irradiation for five minutes,
bonds by using transition-metal catalysis.15 Among the whereas at 100 °C, intermediate thiobenzanilide 6b was
various intramolecular reactions, S-arylation is less stud- isolated (Scheme 1). The temperature was one of the key
ied, mainly due to the tendency of thiols to undergo oxi- factors that improved the yields, but longer times and/or
dative self coupling (S–S coupling) and to undergo higher temperatures did not increase the yield further
chelation to metals, which in turn leads to catalytic deac- (Table 1). To check for the possible involvement of mi-
tivation.16,17 Despite the importance of sulfur-containing crowave effects, the reaction was also conducted using a
aromatic and heteroaromatic biologically active com- pre-heated oil bath for the same duration and at the same
pounds, it is desirable to find novel procedures that pro- final temperature as that measured at the end of the micro-
vide efficient access to such highly useful organic wave-assisted synthesis. It was found that the reaction
Table 1 Synthesis of Benzothiazoles via Cascade C–S Bond Formationa
i) 4-MeOC6H4COCl
F
F
F
S
N
ii) Lawesson's reagent (LR)
OMe
MW
NH2
5d
4b
Entry
Solventb
Temp (°C)c
Time (min)d
Yield (%)e
Microwave
Thermalf
1
2
3
4
5
6
7
8
9
toluene
115
5
8
5
7
5
5
5
7
8
traceg
80h
84h
88
–
o-xylene
145
–
anisole
154
–
o-dichlorobenzene
N,N-diethylaniline
diphenyl ether
165
–
155, 188i
155, 188i
150
97
90
88
75
–
95
1-hexyl-3-methylimidazolium chloride
82
1-butyl-3-methylimidazolium tetrafluoroborate
no solvent
145
80
165
60
–
a Reaction conditions: 4b (1.1 equiv), benzoyl chloride (1.0 equiv), LR (0.55 equiv), all reactions were carried out in a sealed tube.
b DMF and DMSO were avoided due to their reactivity with LR.
c Temperature of reaction mixture was recorded after the microwave irradiation at the given reaction time.
d The reactions were carried out in a domestic microwave oven with IR sensor.
e Yield of isolated product.
f Oil bath temperature 145–180 °C, 3 h, reflux.
g Benzene, THF and 1,4-dioxane gave intermediate thiobenzanilide 6b (see Scheme 1).
h When the NO2 group was present in p-anisoyl chloride, formation of 6b was observed (see Scheme 1).
i When the NO2 group is present.
Synthesis 2010, No. 12, 1983–1988 © Thieme Stuttgart · New York

SHORT PAPER
Synthesis of Substituted Benzothiazoles
1985
5
(trace)
organic bases: heating up to
NMM, Py, Et3N their boiling
points
F
F
S
R1 or/and R2 = EDG
MW heating 150–155 °C
O
R2
S
Lawesson's reagent
R1
R1
N
N
R1 or/and R2 = NO2
MW heating >180 °C
conventional heating
110 °C for 1 h
R2
R2
N
H
H
R1
6b
6a
benzothiazoles
5
heating up to 280 °C
R2
MW heating <145 °C
O
N
5
(trace)
R1
benzoxazoles
7
Scheme 1
proceeded slowly, with 5–10% yield in five minutes, The effects of some of the most common ionic liquids,
whereas a 90% yield of 5d was obtained after three hours which are known to have the ability to act as hydrogen
under conventional heating at 155 °C (Table 1, entry 5). bond acceptors, were also studied. The results were in ac-
Interestingly, in derivatives of compound 4 containing cord with those obtained with the molecular solvents. Fur-
other halogen atoms (R1 = Cl, Br or I) in lieu of the fluo- thermore, thiobenzanilide (6b), when exposed to identical
rine atom, the same reaction did not produce the cyclized MW reaction conditions in DMF, resulted in the forma-
product 5d. Instead, the reaction efficiently led to the for- tion of 5 in a quantitative yield. However, in DMSO and
mation of intermediate 6b (Scheme 1), presumably due to polyethylene glycol (PEG) the reaction gave exclusively
relative differences in the size and electronegativity of the starting material. Because an additive, such as a base, was
halogen atoms.
thought to be necessary for efficient S-arylation and high
conversion into 5, the use of several additives, including
triethylamine, pyridinium tosylate, and N-methylmorpho-
line were explored. Ultimately, these additives were
found to be unnecessary because changes to other param-
eters resulted in much higher yields of products
(Scheme 1). Moreover, we conducted further investiga-
tions to see whether the benzoxazole ring system (7) was
formed from the intermediate amide (6a) under the opti-
mized conditions. Interestingly, we observed that the 6a
was found to be very stable even at temperatures up to
280 °C, and did not yield the expected benzoxazole.
Experimental and theoretical investigations have indicat-
ed that more polar organic compounds absorb more mi-
crowave energy, which usually causes heating of the
material. The different microwave energy absorption
properties of the solvent and/or the reactants and/or inter-
mediates can thus be advantageously used to alter the
course of the reaction when it is carried out under micro-
wave irradiation. On the basis of these ideas, we have
studied the influence of the solvent on the formation of
benzothiazoles from the corresponding anilines. Clearly,
the nature of the solvent has a dramatic effect on the rate
of reaction (Table 1), e.g., xylene (nonpolar, aprotic sol- The mechanism by which these reactions proceed is par-
vent) gave good yields and diphenylether or N,N¢-diethyl- ticularly intriguing, and we could find no literature prece-
aniline (polar) afforded high yields, hence, these solvents dence for benzothiazole formation from 2-fluoro-
are considered to be suitable solvents for this reaction.
thioformanilide under cascade metal-free microwave irra-
diation. A proposed reaction mechanism is depicted in
slow
rate-limiting
F
F
S
SH
R1
R1
N
N
H
R2
6c
R2
6b
H
S
δ+
H
S
F
F
S
N
R2
fast
R1
R1
δ–
R1
N
N
R2
R2
5
HF
6d
Meisenheimer complex
Scheme 2 SNAr mechanism as the key step in the cascade sequence
Synthesis 2010, No. 12, 1983–1988 © Thieme Stuttgart · New York

1986
D. S. Bose, M. Idrees
SHORT PAPER
Scheme 2. Nucleophilic aromatic substitution reactions actions between the transition sate and the solvent, it is
involving activated substrates and good leaving groups important to consider the hydrogen bonding interactions
are known to proceed through an addition–elimination22 that can occur between the sulfur/solvent, transition-state/
mechanism involving the formation of an intermediate solvent, and Meisenheimer complex/solvent.
Meisenheimer complex.23 During the first step of this re-
Solvents with hydrogen-bond accepting properties can in-
action, the nucleophilic sulfur attacks the fluorobenzene
teract with the protons on the sulfur, increasing the elec-
and bonds to the carbon that bears the halogen. This step,
tron density on the sulfur atom and therefore its
which involves the loss of aromaticity, generally proceeds
nucleophilic character. This increase has been shown
when ionic liquids, N,N-diethylaniline or diphenyl ether is
used as the solvent for substituted anilines. Furthermore,
slowly. In the second step, the complex formed loses the
fluorine in a fast step, where the aromaticity of the ring is
restored. The solvent can affect the rates of these reactions
the similarity between the transition state in these SNAr re-
by interaction with starting materials, transition states,
actions and the Meisenheimer complex intermediate pre-
and/or the intermediate. In addition to simple dipole inter-
Table 2 Synthesis of 2-Subtituted Benzothiazoles
Entry
2-Fluoroanilines 4
Acid chloride
Product 5a
Yield (%)b
MW
Thermal
F
COCl
S
1
2
3
4
4a
4a
5a
5b
5c
5d
92
9024a
8924b
90
N
S
N
NH2
F
COCl
93
95
95
OMe
NH2
MeO
F
F
COCl
F
F
S
N
S
N
4b
NH2
F
F
COCl
COCl
4b
92
OMe
NH2
MeO
O2N
NO2
F
S
N
5
6
4a
5e
5f
90
95
86
88
NH2
COCl
NO2
F
F
F
S
N
4b
NH2
NO2
NO2
COCl
NO2
MeO
F
MeO
MeO
S
N
7
4c
5g
92
90
NH2
F
COCl
MeO
Me
Me
S
N
8
9
4c
5h
5i
85
92
85
Me
N
N
NH2
F
Me
O
MeO
COCl
O
O
MeO
S
N
4c
8513d
O
NH2
COCl
S
N
F
10
4a
5j
95
9013d
O
NH2
O
a All reactions were carried out in diphenyl ether as solvent.
b Yield refers to the pure isolated product.
Synthesis 2010, No. 12, 1983–1988 © Thieme Stuttgart · New York

SHORT PAPER
Synthesis of Substituted Benzothiazoles
1987
slightly more concentrated solution and slightly extended overall
heating time in the microwave oven.
dicts that factors that contribute to the stabilization of the
transition state will also tend to stabilize the complex.
To explore the generality and scope of this method, a
range of 2-fluoroanilines and acid chlorides were studied
for the synthesis of arylbenzothiazoles (5a–j) and the re-
sults are illustrated in Table 2. It is noteworthy that both
acid-sensitive and alkaline-sensitive groups were com-
pletely unaffected under the reaction conditions. Further-
more, it appears that electron-donating or electron-
withdrawing groups do not significantly affect the rate of
reaction. It is pertinent to mention here that when a nitro
group was present in the benzoyl chloride, an optimum re-
action temperature of 180 °C was required.
6-Methoxy-2-(3,4-methylenedioxyphenyl)-1,3-benzothiazole
(5i)
White solid; mp 164–166 °C (Lit.13d 165–166 °C).
1H NMR (300 MHz, CDCl3, TMS): d = 3.88 (s, 3 H), 6.04 (s, 2 H),
6.84–6.87 (d, J = 8.31 Hz, 1 H), 6.99–7.04 (dd, J1 = 9.06, J2 = 2.26
Hz, 1 H), 7.27–7.28 (d, J = 3.02 Hz, 1 H), 7.48–7.58 (m, 2 H), 7.85
(d, J = 9.06 Hz, 1 H).
13C NMR (CDCl3, 75 MHz): d = 55.8, 101.6, 104.4, 107.3, 108.6,
115.4, 122.1, 123.5, 128.3, 136.3, 148.4, 148.7, 149.7, 157.7, 165.1.
MS (EI): m/z (%) = 285 (100) [M]+, 270 (80), 242 (15), 143 (13), 95
(20).
HRMS (ESI): m/z [M + H]+ calcd for C15H11NO3S: 286.0537;
found: 286.0537.
In summary, we have developed a novel and regioselec-
tive, metal-free cascade intramolecular S-arylation reac-
Anal. Calcd for C15H11NO3S: C, 63.14; H, 3.88; N, 4.90; S, 11.24.
Found: C, 63.05; H, 3.82; N, 4.96; S, 11.15.
tion, which represents
a
simple, practical and
straightforward approach towards 2-aryl-substituted ben-
zothiazole derivatives from readily available starting
materials. Further investigations into the detailed mecha-
nistic aspects and expanded scope of this protocol for the
construction of novel heterocyclic ring systems are under-
way in our laboratory.
Acknowledgment
One of the authors (M.I.) thanks CSIR, New Delhi, for financial
support.
References
All chemicals were obtained from Sigma-Aldrich and used without
further purification. Column chromatography was performed using
Acme silica gel (60–120 mesh). Common solvents for chromatog-
raphy (petroleum ether, EtOAc) were distilled prior to use. Routine
monitoring of the reaction was made using thin layer chromatogra-
phy (TLC) on glass plates precoated with silica gel (Merck 60 F-
254) of 0.25 mm thickness and visualized with iodine or UV light
(254 nm) or by coloration with cerium molybdenum solution [phos-
phomolybdic acid (25 g), Ce(SO4)2·H2O (10 g), concd H2SO4 (60
mL), H2O (940 mL)]. Optical rotations were measured on a Jasco P-
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Nexus 670 FT-IR spectrophotometer. 1H and 13C NMR spectra were
recorded on either a Bruker Avance 300 spectrometer (300.132
MHz for 1H, 75.473 MHz for 13C), or a Varian FT-200 MHz (Gem-
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ppm) as an internal standard. Melting points were obtained using a
precision digital melting point Veego VMPDS apparatus and are
uncorrected. Elemental analyses were performed on a Elementar’s
Vario EL microanalyzer. Low-resolution mass spectra (ESI-MS)
and HRMS were recorded on Quattro LC (Micromass) and Q STAR
XL (Applied Biosystems), respectively.
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dure
A heterogeneous mixture of 2-fluoroaniline (11.0 mmol), benzoyl
chloride (10.0 mmol), Lawesson’s reagent (5.0 mmol, correspond-
ing to the intermediate amide formed in situ) in N,N-diethylaniline
(or diphenyl ether; 5 mL), was irradiated in a sealed tube with mi-
crowaves in a domestic microwave oven fitted with an IR sensor to
measure temperature, for 5–8 min at 145–180 °C (Method A) or us-
ing conventional heating at 145–180 °C for 3 h under N2 (Method
B). After cooling to r.t., the reaction mixture was transferred to a
stirred solution of dilute HCl (2 M, 5 mL) followed by 1 N NaOH
(10 mL). The precipitated product was filtered, washed with H2O
(2 × 5 mL) and dried in vacuo to give crude 5, which was further pu-
rification by column chromatography (EtOAc–hexane, 1:5 v/v) to
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SHORT PAPER
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