Synthesis of Substituted Benzo[ b]thiophenes via Base-Promoted Domino Condensation-Intramolecular C-S Bond Formation

Article abstract of DOI:10.1021/acs.orglett.1c00085

A novel synthesis of 2,3-substituted benzothiophenes is reported, involving a tandem base-mediated condensation of o-iodoarylacetonitriles/acetates/ketones with (hetero)aryldithioesters and an intramolecular C-S bond formation. The reaction affords divers

Full text of DOI:10.1021/acs.orglett.1c00085

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Letter
Synthesis of Substituted Benzo[b]thiophenes via Base-Promoted
Domino Condensation−Intramolecular C−S Bond Formation
Yogendra Kumar and Hiriyakkanavar Ila*
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ABSTRACT: A novel synthesis of 2,3-substituted benzothiophenes is reported,
involving a tandem base-mediated condensation of o-iodoarylacetonitriles/
acetates/ketones with (hetero)aryldithioesters and an intramolecular C−S bond
formation. The reaction affords diversely substituted benzothiophenes and
heterofused thiophenes in excellent yields.
Scheme 1. Synthesis of Benzothiophenes via Transition-
Metal-Catalyzed C−S Bond Formation
ubstituted benzo[b]thiophenes are heterocycles of pivotal
Simportance, which were proven for their activity which
include their use as HIV-1 reverse transcriptase inhibitors,
antidepressants, and tubulin polymerization inhibitors.1 These
heterocycles were also the basic structural unit present in
marketed drugs, such as raloxifene and arzoxifene,2 and were
also found to be useful in the development of optoelectronic
materials.3 Owing to their importance, several methods for
their synthesis were reported in literature.4−11 The general
methods of syntheses of these heterocycles include the
intramolecular electrophilic 5-endocyclization of o-alkynylar-
ylthioethers1b,6 and transition-metal (Pd, Cu or Au) catalyzed
intramolecular C−S bond formation.7−11 We recently reported
the synthesis of 2,3-substituted benzo[b]thiophenes and their
analogues via Cu- and Pd-catalyzed intramolecular C−S bond
formation of in situ generated enethiolates, from o-bromoar-
ylacetonitriles, arylacetonitriles, arylacetates and deoxyben-
zoins and het(aryl)dithioesters or aryl/alkylisothiocyanates
(Scheme 1, eqs 1−3).1a,b,12a We have also described the
synthesis of 2-het(aryl)-3-acyl/benzoylbenzothiophenes from
1,3-bishet(aryl)-1,3-monothiodiketones and o-bromoiodoar-
enes, involving successive Cu/Pd-catalyzed intermolecular
C−S bond formation and intramolecular Heck reaction
(Scheme 1, eq 4).4 The advent of transition metal (TM)
free C−C13−15 and C−heteroatom16−18 bond-forming reac-
tions resulted in the synthesis of several five- and six-
membered benzo heterocycles in efficient manner. In
continuation of our efforts toward TM-free synthesis of
heterocycles employing organosulfur reagents,13,19 herein we
report the synthesis of 2,3-substituted benzo[b]thiophenes
from in situ generated thioenolates (Scheme 1, eq 5).
presence of ligands like L-proline or 1,10-phenanthroline, at
rt, 3a was obtained in low yields (Table 1, entries 1 and 2);
however, increasing the temperature (90 °C) furnished 3a in
higher yields (Table 1, entries 3 and 4). On the other hand, use
of DMSO as solvent instead of DMF, in the presence or
absence of ligand, resulted in a dramatic increase in the yield of
3a even at rt (Table 1, entries 5 and 6). With these results in
At the outset, we selected o-iodophenylacetonitrile 1a and
(4-methoxyphenyl)dithioester 2a as model substrates for the
optimization of reaction conditions leading to the synthesis of
benzothiophene 3a under both Cu-catalyzed and base-
mediated conditions (Table 1). Thus, when 1a was reacted
with 2a in the presence of NaH/DMF, followed by CuI-
catalyzed cyclization of resulting thioenolate 4a, in the
Received: January 13, 2021
Published: February 12, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00085
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1698−1702
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Letter
for base-promoted synthesis of 3a (Table 2, entry 1). We
therefore reacted various o-iodo(het)arylacetonitriles and other
active methylene compounds with broad range of (het)-
aryldithioesters in the presence of NaH/DMSO at rt for the
synthesis of various benzothiophenes 3 and their heterofused
analogues in the subsequent studies (Schemes 2 and 3) with
two exceptions (Scheme 2, 3l,m).
Table 1. Optimization of Reaction Conditions for the
Synthesis Benzothiophene 3a
a
catalyst
entry (10 mol %)
ligand
(20 mol %)
temp
(°C)
time % yield,
(h)
b
solvent
3a
Scheme 2. Substrate Scope for the Synthesis of Substituted
Benzothiophene 3
1
2
3
4
5
6
7
8
9
CuI
CuI
CuI
CuI
CuI
CuI
DMF
DMF
DMF
DMF
DMSO
DMSO
DMSO
DMF
L-proline
1,10-phen
L-proline
1,10-phen
L-proline
rt
rt
90
90
rt
rt
rt
rt
12
12
12
12
12
6
3
12
12
41
a
45
72
80
89
90
92
toluene
rt
a
Reaction conditions: 1a (1 mmol), 2a (1 mmol), and NaH (2 equiv)
in 8 mL of solvent under N2, stirred at rt for 30 min, then CuI (10
b
mol %), ligand (20 mol %). Isolated yield.
hand, we next performed the reaction under identical
conditions at rt in the absence of CuI catalyst in DMSO,
and to our delight, the reaction was complete within 3 h,
yielding 3a in 92% yield (Table 1, entry 7). However, no trace
of 3a was obtained when the reaction was conducted in
solvents such as DMF or toluene under identical conditions
(entries 8 and 9).
For the purpose of comparison, we further examined the
reactivity and efficacy of other o-halophenylacetonitriles 5a−7a
bearing bromine, chlorine, and fluorine substitutions, respec-
tively, toward 2a under base-promoted conditions, for the
synthesis of 3a, and the results are summarized in the Table 2.
Table 2. Base-Mediated Cyclocondensation of Various o-
a
Halophenylacetonitrile 1a, 5a−7a with 2a
1a, 5a-
7a
temp
(°C)
time
(h)
% yield
% yield
b
b
entry
X
3a
8
8
a
Reaction conditions: 1, 5b−c (1 mmol), 2, 9−11 (1 mmol), NaH (2
1
2
3
4
5
6
7
8
1a
5a
5a
5a
6a
6a
7a
7a
I
rt
rt
60
90
rt
90
rt
90
3
24
18
12
24
12
24
12
92
8a
8b
8b
8b
8c
8c
8d
8d
0
93
43
42
85
50
83
42
equiv) in 8 mL DMSO under N2, stirred at rt for 3−4 h, isolated
Br
Br
Br
CI
CI
F
b
yields. 3c (0.5 mmol) in 4 mL of TFA, stirred at 50 °C (oil bath) for
40
45
c
d
e
5 h. Stirred at rt for 12 h. Stirred at 90 °C (oil bath) for 6 h. Stirred
at rt for 5−6 h.
43
5
49
We next explored the scope of this protocol for the synthesis
of other substituted benzothiophenes by reacting several o-
iodo(het)arylacetonitriles 1 and various dithioesters 2, and the
results are displayed in Scheme 2. Thus, o-iodophenylacetoni-
triles 1a−c bearing electron-donating methoxy groups reacted
efficiently with aryldithioesters 2a−c with 4-(dimethylamino)
and 2-(4-methoxybenzyloxy) substituents, respectively, under
optimized reaction conditions, yielding the corresponding
benzothiophenes 3b−d in excellent yields (Scheme 2). The
benzothiophene 3c was subsequently transformed into 2-(2-
hydroxyphenyl) derivative 3c′ in 78% yield on treatment with
TFA. Further diversity at the 2-position of benzothiophenes 3
was introduced by reacting various (het)aryldithioesters with
(5-dimethylamino)-2-thienyl- (2d), 2-(N-methyl)pyrrolyl-
(2e), 2-(N-methylimidazolyl)- (2f), 3-(N-methylindolyl)-
F
a
Reaction conditions: 1a, 5a−7a (1 mmol), 2a (1 mmol), NaH (2
equiv) in 8 mL of DMSO under N2. Isolated yield.
b
Thus, no trace or a very low yield of 3a was obtained from the
reaction when 5a−7a were reacted with 2a at rt in the presence
of NaH/DMSO under optimal conditions. Even after
prolonged reaction time, the only products isolated in good
yields were the thioenol adducts 8b−d (Table 2, entries 2, 5,
and 7). At higher temperature, however, 3a was obtained in
moderate yields along with reduced yields of 8b−d (Table 2,
entries 3−4, 6, 8). Thus, similar to our previous studies,13 it is
apparent that the o-iodophenylacetonitrile 1a is most efficient
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reaction of 2-iodo-3-indolylacetonitrile 12 with dithioesters 2g
and 2p afforded the 2,3-substituted thieno[2,3-b]indole 13a,b
in excellent yields (Scheme 3). Similarly, the corresponding
2,5-bis(iodo)thiophene-3-acetonitriles 14 also reacted with
dithioesters 2n and 2q in highly chemoselective manner,
yielding the 5-iodo-substituted thieno[2,3-b]thiophene-3-car-
bonitriles 15a,b in 89−90% yields (Scheme 3). The
corresponding thieno-fused pyrazoles 17a,b could also be
accessed smoothly in high yields by annulation of 4-
iodopyrazole-5-acetonitrile precursors 16a,b with dithioesters
2n and 2m, respectively (Scheme 3).
Scheme 3. Substrate Scope for the Synthesis of Heterofused
Benzothiophene
a
The mechanism of formation of benzothiophene from
thioenolate anion 4 appears to be via a radical pathway through
S
RN1 mechanism.13 However, we conducted few experiments
to substantiate the viability of this mechanism. As shown
previously in Table 2, the other o-haloarylacetonitriles 5a−7a
(X = Br, Cl, F) were found to be much less reactive in
comparison to o-iodophenylacetonitrile 1a, yielding either no
trace of 3a at rt (Table 2, entries 2, 5, and 7) or in moderate
yields at higher temperature, under identical conditions (Table
2, entries 3−4, 6, and 8). These observations rule out SNAr
mechanism, as the reaction does not follow the expected order
of reactivity (F > Cl> Br > I) for nucleophilic aromatic
substitutions. Also, as we had observed previously,13 the
cyclization of thioenolate 4a was nearly completely inhibited
when the reaction was conducted in the presence of TEMPO,
a common trapping agent for free radicals (Scheme 4, eq 1).
a
Reaction conditions: 12, 14, 16 (1 mmol), 2 (1 mmol), NaH (2
equiv) in 8 mL of DMSO under N2, stirred at rt for 3−4 h. Isolated
yields.
(2g), and 3-(pyridyl)- (2h) substituents, with o-iodophenyla-
cetonitriles 1a−c under identical conditions, furnishing the
corresponding 2-(het)aryl-3-cyanobenzothiophenes 3e−j in
excellent yields (Scheme 2). Similarly, the 2-(n-butyl)-3-
cyanobenzothiophene 3k, with an alkyl side chain, could be
obtained in 81% yield, employing an alkyldithioester 2i.
However, our attempted syntheses of o-iodoarylacetonitriles
bearing electron-withdrawing substituents were not success-
ful.20 We therefore reacted the corresponding 2-bromopheny-
lacetonitriles bearing 4-fluoro- (5b) and 5-chloro- (5c)
substituents with dithioesters 2a and 2j, respectively, under
standard reaction conditions, affording the corresponding 6-
fluoro- and 5-chlorobenzothiophenes 3l−m in moderate
yields; however, excellent yields of 3l,m were obtained when
the reactions were conducted at higher temperature (Scheme
2).
Scheme 4. Reaction of 2-Iodophenylacetonitrile 1a with 2a
in the Presence of TEMPO
a
The study was next extended to other 2-iodo-substituted
active methylene compounds such as 2-iodophenyacetates
1d,e, which also reacted efficiently with various dithioesters
2k−m under optimized reaction conditions, furnishing the
corresponding 2-(het)arylbenzothiophene-3-carboxylates 3n−
p in high yields (Scheme 2). Similarly, the corresponding 3-
aroyl-2-substituted benzothiophenes 3q−s could also be
synthesized in high yields when o-iododeoxybenzoins 1f−h
were reacted with various substituted dithioesters 2n,o and 2a,
respectively, under identical conditions. The benzothiophene
3s is methoxy analog of the bioactive tubulin inhibitor.21 The
other thiocarbonyl components like aryl isothiocyanate 9, di(n-
butyl)trithiocarbonate 10 and O,S-dimethylcarbonodithioate
11 also participated smoothly, reacting with o-iodophenylace-
tonitrile 1a, under similar conditions, yielding the 3-
cyanobenzothiophenes 3t,u with 2-(arylamino) and 2-alkylthio
side chains, respectively, in good to excellent yields (Scheme
2). However, with 11, the corresponding 2-methoxybenzo-
thiophene 3v was obtained in 68% yield along with the
formation of 2-(methylthio) analogue 3w in moderate yield.
Some of these newly synthesized benzothiophenes display
yellow-green to yellow-orange fluorescence (see the Support-
ing Information).
a
Reaction conditions: 1a (1 mmol), 2a (1 mmol), NaH (2 equiv),
b
TEMPO (1 mmol) in 8 mL of DMSO. Isolated yield.
Similarly, when 1a and 2a were reacted in air (the absence of
N2 atmosphere), 3a was obtained in decreased yield of only 8%
(Scheme 4, eq 2). These experiments suggest the involvement
of radical intermediates and/or a SET process in this
transformation.
Based on our previous studies,14,15 and the present
experimental observations, we suggest a possible mechanistic
pathway for the formation of 3a from 1a and 2a, involving
S
RN1 mechanism, as shown in the Scheme 5. Since no trace of
benzothiophene 3a was obtained when the reaction was
conducted in solvents like DMF or toluene (Table 1, entries 8
and 9), we believe that the radical process is initiated by single
electron transfer (SET) from dimsyl anion22 to thioenolate
intermediate 4a to afford the radical anion species 18A, which
undergoes fragmentation to deliver the corresponding aryl
radical intermediate 18B along with iodide ion. Intramolecular
trapping of aryl radical by thioenolate anion in the
intermediate 18B, furnishes the benzothiophene radical
anion intermediate 19, which acts as electron donor to the
starting thioenolate 4a, furnishing the final product 3a and
radical anion 18A, thus completing the radical chain cycle.
With successful implementation of the present base-
mediated TM-free methodology for the synthesis of various
substituted benzothiophenes, we next extended this novel
protocol for the synthesis of heterofused thiophenes, by
reacting few 2-iodo(het)arylacetonitriles with various dithioest-
ers under optimized reaction conditions (Scheme 3). Thus, the
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Scheme 5. Proposed Mechanism for Base-Mediated
Synthesis of Benzothiophene 3a from 1a and 2a
ACKNOWLEDGMENTS
■
We thank the Council of Scientific and Industrial Research
(CSIR, New Delhi) for CSIR-SRF (Senior research fellow-
ship), CSIR award no. 09/733(0228)/2017/-EMR-I (to Y.K.),
JNCASR, Bangalore for financial asisstance, and Hindustan
Lever Research Professorship (to H.I.).
DEDICATION
■
This paper is dedicated to Prof. H. U. Reissig on his 72th
birthday.
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ASSOCIATED CONTENT
* Supporting Information
■
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AUTHOR INFORMATION
Corresponding Author
■
Hiriyakkanavar Ila − New Chemistry Unit, Jawaharlal Nehru
Centre for Advanced Scientific Research, Bangalore 560064,
India; orcid.org/0000-0002-6971-8374; Email: hila@
jncasr.ac.in
Author
Yogendra Kumar − New Chemistry Unit, Jawaharlal Nehru
Centre for Advanced Scientific Research, Bangalore 560064,
India
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Complete contact information is available at:
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