Synthesis of Substituted Benzo[b]thiophenes via Sequential One-Pot, Copper-Catalyzed Intermolecular C-S Bond Formation and Palladium-Catalyzed Intramolecular Arene-Alkene Coupling of Bis(het)aryl/alkyl-1,3-monothiodiketones and o-Bromoiodoarenes

Article abstract of DOI:10.1021/acs.orglett.7b00273

A new, convergent, one-pot synthesis of 2,3-substituted benzo[b]thiophenes from readily available 1,3-bis(het)aryl-1,3-monothiodiketones and o-bromoiodoarenes involving a sequential copper-catalyzed intermolecular C-S coupling followed by palladium-catalyzed intramolecular arene-alkene coupling of in situ generated β-(o-bromoaryl)thiovinylketones has been described. Synthesis of a few thieno- and furano-fused 2-(het)aroylethylidenethiochromenes via intramolcular direct C-H (het)arylation of β-(o-bromoaryl)thioenones carrying 2-thienyl/furyl groups under different palladium-catalyzed conditions has also been reported.

Full text of DOI:10.1021/acs.orglett.7b00273

Letter
pubs.acs.org/OrgLett
Synthesis of Substituted Benzo[b]thiophenes via Sequential One-Pot,
Copper-Catalyzed Intermolecular C−S Bond Formation and
Palladium-Catalyzed Intramolecular Arene−Alkene Coupling of
Bis(het)aryl/alkyl-1,3-monothiodiketones and o‑Bromoiodoarenes
Somaraju Yugandar, Saidulu Konda, and Hiriyakkanavar Ila*
New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India
S
* Supporting Information
ABSTRACT: A new, convergent, one-pot synthesis of 2,3-
substituted benzo[b]thiophenes from readily available 1,3-
bis(het)aryl-1,3-monothiodiketones and o-bromoiodoarenes
involving a sequential copper-catalyzed intermolecular C−S
coupling followed by palladium-catalyzed intramolecular
arene−alkene coupling of in situ generated β-(o-bromoaryl)-
thiovinylketones has been described. Synthesis of a few thieno- and furano-fused 2-(het)aroylethylidenethiochromenes via
intramolcular direct C−H (het)arylation of β-(o-bromoaryl)thioenones carrying 2-thienyl/furyl groups under different
palladium-catalyzed conditions has also been reported.
reagents or transition-metal catalysts (pathway A, disconnections
d and a)1−3 and tandem intramolecular palladium- or copper-
catalyzed S-vinylation/interamolecular cross coupling of o-(gem-
dibromovinyl)thiophenols (pathway B).4 The crucial bond-
forming event in these reactions is intramolecular attack of
nucleophilic sulfur atom on the activated C−C multiple bond,
leading to the formation of S(1)−C(2) bond (disconnection a).
Recently, Li and co-workers have reported a direct one step
synthesis of benzo[b]thiophenes through intramolecular oxida-
tive cyclization between simple thiophenols and activated
alkynes (pathway C, disconnections a and d).5 These reactions,
although efficient, require prior synthesis of prefunctionalized
thiophenol precursors. 2-Substituted benzo[b]thiophenes have
also been obtained through copper (or palladium)-catalyzed
double thiolation of o-(2-halovinyl)halobenzene6a or 2-bromoal-
kynylbenzenes6b with metal sulfides or their surrogates (pathway
D, disconnections a and b).6c Our research group and others7
have recently reported syntheses of substituted benzo[b]-
thiophenes and their heteroanalogues involving S(1)−C(7a)
bond formation via copper (or palladium)-catalyzed intra-
molecular C−S cross-coupling/cyclization1b or palladium-
catalyzed intramolecular oxidative C−H functionalization/
thiolation of in situ generated enethiolates of α-aryl-β-(het)-
aryl/alkyl-β-mercaptoacrylonitriles/acrylates/acrylophenones1a
or through radical-mediated intramolecular cyclization of the
corresponding thioethers (pathway E, disconnections b and c).1c
Knochel and co-workers have developed synthesis of substituted
benzo[b]thiophenes via copper-catalyzed carbomagnesiation of
o-(alkynylthio) Grignard reagents (pathway F, disconnection
d).8a Recently, 3-arylbenzothiophenes have also been synthe-
ubstituted benzothiophenes represent an important class of
Sheterocycles displaying a broad range of biological activity
and serving as useful heterocyclic cores to a host of marketed
drugs.1 In addition, benzo[b]thiophene and its condensed
analogues are important structural components in the develop-
ment of optoelectronic materials.1 Consequently, the synthesis
of this privileged structure has been actively pursued in recent
years.1
Scheme 1 shows some of the reported procedures with various
disconnections for benzothiophene synthesis. Among recent
syntheses, the most common approaches for benzo[b]-
thiophenes involve intramolecular 5-endo-dig cyclization of o-
alkynyl arylthioethers or their surrogates employing electrophilic
Scheme 1. Various Disconnection Approaches for the
Synthesis of Benzothiophene Ring System
Received: January 25, 2017
© XXXX American Chemical Society
A
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Letter
sized by intramolecular Heck reaction of 2-(phenylthio)acrylates
(pathway G, disconnection d).8b
which was assigned Z-stereochemistry on the basis of its X-ray
crystallographic data (Figure S1, see the Supporting Informa-
tion).
Having established the synthesis of β-arylthioenone 4a under
copper catalysis, we next explored the optimized conditions for
its conversion to benzothiophene 3a with various palladium
catalysts/ligand systems under varying conditions (Table 1). The
During the course of our studies toward design and
development of new synthetic routes for five- and six-membered
heterocycles from various organosulfur intermediates,9 we now
report a novel, convergent approach to 2,3-substituted benzo-
[b]thiophenes 3 via sequential one-pot, copper-catalyzed
intermolecular Ullmann type C−S coupling of 1,3-monothiodi-
ketones 1 and 2-bromoiodoarenes 2 and intramolecular Heck
reaction of the resulting β-arylthioenones 4 (Scheme 1, pathway
H, disconnections b and d).
Table 1. Optimization of the Reaction Conditions for the
Palladium-Catalyzed Intramolecular Cyclization of 4a to
a
Benzothiophene 3a
We have recently reported synthesis and applications of
unsymmetrically substituted 1,3-bis(het)aryl/alkylmonothiodi-
ketones of the general structure 1 as a new class of easily
accessible versatile 3-carbon organosulfur synthons.9a−d By
employing these monothiodiketones, we have developed
efficient regiocontrolled syntheses of unsymmetrically substi-
tuted 1-aryl-3,5-bis(het)arylpyrazoles,9a 3,5-bishet(aryl)-
oxazoles,9b substituted thiophenes/fluorescent push−pull thio-
phene acrylates,9c and triisubstituted imidazoles.9d In continu-
ation of these studies, we further envisaged development of a new
convergent synthesis of 2,3-disubstituted benzo[b]thiophenes 3
by utilizing these 1,3-monothiodiketones as CC−S coupling
partners in transition-metal-catalyzed cyclization with o-
bromoiodoarenes 2 (Scheme 1).
temp
(°C),
time
(h)
yield
(%)
entry
1
catalyst
ligand
base
solvent
DMF
Pd(OAc)2
Pd(OAc)2
Cs2CO3
100, 8
82
77
b
2
Cs2CO3
Bu4NBr
100, 15 DMF
3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
PPh3
Cs2CO3
100, 8 DMF
81
68
64
62
59
73
52
55
4
P(o-Tol)3 K2CO3
100, 12 DMSO
100, 10 DMF
100, 20 DMF
100, 15 DMF
100, 10 dioxane
100, 18 DMSO
100, 20 DMF
5
BINAP
XPhos
PPh3
K2CO3
6
Cs2CO3
Cs2CO3
NaHCO3
The desired 1,3-monothiodiketones 1a−u were obtained by
base-mediated condensation of the respective active methylene
ketones with various (het)aryl dithioesters (Scheme 2).9a The
7
8
PdCl2
BINAP
9
Pd2(dba)3
Pd2(dba)3
XantPhos Cs2CO3
dppf Cs2CO3
10
Scheme 2. Synthesis of 1,3-Monothiodiketones 1a−u
a
Reaction conditions: 4a (0.3 mmol), catalyst (10 mol %), ligand (20
mol %), base (0.3 mmol) in 4 mL of solvent under N2. Bu4NBr (0.3
mmol) is used.
b
best yields of 3a were obtained with palladium acetate (10 mol
%) under ligand-free conditions or in the presence of PPh3 in
DMF as solvent with Cs2CO3 as base (Table 1, entries 1 and 3).
We therefore conducted all of the subsequent cyclization
reactions of β-arylthioenones 4 using the former reaction
conditions (Table 1, entry 1).
With the realization of a two-step process, we next focused to
develop sequential one-pot protocol for benzothiophene 3a by
first reacting monothiodiketone 1a with 2-bromoiodoarene 2a
under copper catalysis and subsequent in situ palladium-
catalyzed intramolecular cyclization of the resulting β-arylth-
ioenone 4a under optimized conditions, which to our delight,
afforded the benzothiophene 3a in 80% yield comparable to the
stepwise process (Scheme 3 vs Table 1, entry 1).
The optimized one-pot reaction conditions work effectively
with an array of substituted o-iodobromoarenes 2a−h bearing
both electron-donating and -withdrawing groups, affording the
corresponding benzothiophenes 3a−o in good to excellent yields
(Scheme 3). Similarly, it was possible to employ a range of 1,3-
monothiodiketones (1a−m) bearing an aryl/(het)aryl substitu-
ents such as 2-furyl, 2-[(5-dimethylamino)-2-thienyl], 2-(N-
methylpyrrolyl), 2- or 3-pyridyl, 3-(N-methylindolyl), or alkyl
groups adjacent to either carbonyl or thiocarbonyl moieties, thus
affording the corresponding 2-(het)aryl/alkyl-3-(het)-
aroylbenzo[b]thiophenes 3a−m in excellent yields (Scheme
3). Similarly, the corresponding 2-(N-morpholino)-/(N-piper-
idino)-substituted benzothiophenes 3n and 3o could also be
prepared in moderate yields when the thioamides 1n,o were used
as coupling partners (Scheme 3). Finally, by employing 1,3-
a
Prepared by nucleophilic displacement on corresponding β-keto
dithioesters with cyclic amines
1,3-monothiodiketone 1a and 2-bromoiodobenzene 2a were
selected as model substrates for screening of various catalysts and
optimization of reaction conditions. We first examined the
possibility of developing a one-pot process for benzothiophene
3a with a single palladium catalyst/ligand system, catalyzing both
initial C−S bond formation and subsequent intramolecular
arene−alkene coupling of the resulting β-arylthioenone 4a
(Table S1, see the Supporting Information). However, all of our
all attempts to employ various Pd-based catalysts under different
conditions failed to yield the desired benzothiophene 3a.
We then switched our attention to performing the reaction in a
sequential manner with the initial copper-catalyzed C−S bond
formation between 1a and 2a to furnish β-(2-bromoaryl)-
thioenone 4a (Table S2, see the Supporting Information). The
reaction proceeded efficiently in the presence of various copper
catalysts, and the best yield (87%) of 4a was obtained using CuI
(10 mol %) as catalyst in the presence of Cs2CO3 (1.5 equiv) in
1
DMF at 100 °C (3 h) (see Table S2, entry 2). The H NMR
spectra of 4a showed the presence of only one stereoisomer,
B
DOI: 10.1021/acs.orglett.7b00273
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Scheme 3. Synthesis of Benzo[b]thiophenes 3 via Sequential
Scheme 4. One-Pot Synthesis of Thieno- and Furano-Fused 2-
(Het)aroylethylidenethiochromenes 5a−e
One-Pot, Copper-Catalyzed Intermolecular and Palladium-
a
Catalyzed Intramolecular Aryl−Alkene Coupling
a
Yield under two-step conditions from 4q.
monothiodiketones 1r,s bearing a (2-thienyl)thiocarbonyl
moiety, furnishing the corresponding 2-[(het)aroylethylidene]-
thieno[2,3-c]thiochromenes 5b−c in overall high yields on
treatment with o-iodobromoarenes 2g and 2b, respectively,
under the modified sequential one-pot conditions (Scheme 4).
The corresponding monothiodiketones 1t−u bearing a (2-
furyl)thiocarbonyl moiety also followed a similar trend, yielding
the corresponding furano-fused 2-[(het)aroylethylidene]-
thiochromenes 5d,e exclusively in good yields on coupling
with the respective o-bromoiodoarenes (2d,g) under identical
conditions, and no trace of the corresponding (2-thienyl/furyl)-
substituted benzothiophenes 3r−u could be detected in the
reaction mixture (Scheme 4). On the other hand, attempted
intramolecular cyclization of monothioketones 1i and 1j carrying
a [N-methyl-(2-pyrrolyl)]- and [5-dimethylamino(2-thienyl)]-
thiocarbonyl moiety, respectively, with the appropriate o-
bromoiodobenzenes (2e−2f) under modified palladium-cata-
lyzed one-pot reaction conditions did not afford even traces of
the corresponding fused thiochromenes 5f,g, yielding only the
benzothiophenes 3i and 3j exclusively in decreased yields
(Schemes 3 and 4).
a
Reaction conditions: 2 (0.5 mmol), 1 (0.5 mmol), Cul (0.05 mmol),
Cs2CO3 (0.75 mmol) in DMF (4 mL) heated under N2 atm at 100 °C
for 3 h and then Pd(OAc)2 (0.05 mmol), DMF, 100 °C for 8−10 h.
b
Reaction conditions: Pd(OAc)2 (0.05 mmol), Bu4NBr (0.5 mmol),
c
d
DMF, 130 °C for 8−10 h. Yield under two-step process. 4q obtained
in 83% yield.
The probable mechanism for the palladium-catalyzed intra-
molecular arene−alkene coupling of β-(o-bromoaryl)thiovinyl
ketones 4 leading to benzothiophenes 3 is depicted in Scheme 4.
Thus, the insertion intermediate A obtained by oxidative
addition of Pd(0) into bromoarene 4 could be transformed
into the resonance-stabilized cationic intermediate B, formed by
the intramolecular attack of the electron-rich thiovinyl group on
the electrophilic metal center. Subsequent deprotonation of the
acidic proton in intermediate B would lead to palladacycle C,
which on reductive elimination affords the benzothiophene 3
(Scheme 5, route a). A mechanism involving direct Heck-type
insertion into the enone double bond in the intermediate A to
monothiodiketone 1p and 1,4-dibromo-2,5-diiodobenzene 2i,
the linear benzodithiophene 3q could also be synthesized in
moderate yield under identical one-pot or two-step conditions
(Scheme 3).
Interestingly, when the o-bromoiodoarene 2j and the
monothiodiketone 1q (carrying a 2-thienyl moiety attached to
thiocarbonyl group) were subjected to sequential copper- and
palladium-catalyzed coupling under optimized conditions, the
expected 2-(2-thienyl)benzo[b]thiophene 3q was not obtained,
and only β-arylthioenone 4q was isolated in 83% yield (Schemes
3 and 4). We therefore attempted to cyclize the thiovinylketone
4q to 3q under modified reaction conditions by treatment with
palladium acetate (10 mol %) in the presence of tetrabuty-
lammonium iodide in DMF at 130 °C (8 h) (Scheme 4).
However, workup of the reaction mixture afforded an unexpected
product (78%), instead of the desired benzothiophene 3q, which
was characterized as 2-(aroylethylidene)thieno[2,3-c]-
thiochromene derivative 5a on the basis of its spectral and
analytical data (Scheme 4).The thiochromene 5a was obtained in
72% yield, when the reaction was conducted under sequential
one-pot reaction conditions (Scheme 4). This novel trans-
formation, which apparently involves palladium catalyzed
intramolecular direct C−H arylation on thiophene ring10 in
thioenone 4q, was found to be general with the other 1,3-
Scheme 5. Proposed Mechanisms for Palladium-Catalyzed
Intramolecular Arene−Alkene Coupling of β-
Arylthiovinylketones 4 to Benzothiophenes 3
C
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Letter
give intermediates D/D′ (Scheme 5, route b) appears to be
disfavored energetically owing to the strain associated with the 5-
endo-cyclization. Alternatively, the intermediates D/D′ could be
formed by a two-step alkene insertion process from A via
palladacycle B, followed by subsequent β-hydride elimination to
give benzothiophene 3 (Scheme 5, route c). The proposed
mechamisms are similar to those suggested by Kurth and co-
workers11 for the palladium-catalyzed intramolecular cyclization
of N-(2-bromoaryl) enamines to 2,3-substituted/annulated
indoles on the basis of quantum chemical calculations.
ACKNOWLEDGMENTS
■
We thank the Council of Scientific and Industrial Research
(CSIR, New Delhi) for CSIR-SRF (to S.Y.), JNCASR,
Bangalore, for research associateship (to S.K.), and Hindustan
Lever Research Professorship (to H.I.).
DEDICATION
Dedicated to Prof. Lutz F. Tietze on his 75th birthday.
■
REFERENCES
■
The probable mechanism for the formation of thieno- and
furano-fused 2-(aroylethylidene)thiochromenes 5a−e from the
corresponding thiovinyl ketones 4q−u under the modified
palladium-catalyzed conditions apparently involves intramolec-
ular direct C−H (het)arylation10 on the C3 position of the
favorably located 2-thienyl (or 2-furyl) ring (Scheme 4).
However, failure of the corresponding monothiodiketones 1i
and 1j with 2-(N-methylpyrrolyl)- and [(5-dimethylamino)-2-
thienylthiocarbonyl groups to furnish the corresponding fused
thiochromenes 5f−g under identical conditions (Scheme 4)
suggests that the structural features along with various electronic
and steric constraints present in thiovinyl ketones 4 play
important role in the formation of either benzothiophenes or
fused thiochromene derivatives, and further work is needed to
examine various factors responsible for these observations.
In summary, we have developed an efficient, convergent, one-
pot synthesis of 2-(het)aryl/alkyl-3-acylbenzo[b]thiophenes
from readily available 1,3-bis(het)aryl-1,3-monothiodiketones
and o-bromoiodoarenes involving a sequential copper-catalyzed
Ullmann-type intermolecular C−S coupling followed by an in
situ palladium-catalyzed intramolecular Heck reaction of the
resulting β-(arylthio)vinyl ketones.12 It should be noted that
such kind of disconnection approach for the construction of a
benzo[b]thiophene ring has not been reported in the literature.
This new methodology provides access to a broad range of
substituted benzothiophenes displaying functional group diver-
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f e w t h i e n o - a n d f u r a n o - f u s e d 2 - ( h e t ) -
aroylethylidenethiochromenes via intramolcular direct C−H
arylation of β-(2-bromoarylthio)enone precursors under differ-
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establish generality and scope of this novel benzothiophene and
fused thiochromene synthesis, along with detailed mechanistic
studies regarding various factors responsible for the formation of
both kinds of products, is in progress in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00273.
Experimental details; tables for optimization of reaction
conditions; spectral and analytical data (PDF)
X-ray crystallog raphy of 4a (CIF)
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Int. Ed. 2012, 51, 10588.
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Yang, F.; Lv, X.; Bao, W. J. Org. Chem. 2011, 76, 967. (b) Song, R.-J.; Liu,
Y.; Li, R.-J.; Li, J.-H. Tetrahedron Lett. 2009, 50, 3912.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hila@jncasr.ac.in.
ORCID
Hiriyakkanavar Ila: 0000-0002-6971-8374
Notes
The authors declare no competing financial interest.
D
DOI: 10.1021/acs.orglett.7b00273
Org. Lett. XXXX, XXX, XXX−XXX