Efficient synthesis of benzothiophenes by an unusual palladiumcatalyzed vinylic C-S coupling

Full text of DOI:10.1002/anie.200902843

Communications
DOI: 10.1002/anie.200902843
Tandem Catalysis
Efficient Synthesis of Benzothiophenes by an Unusual Palladium-
À
Catalyzed Vinylic C S Coupling**
Christopher S. Bryan, Julia A. Braunger, and Mark Lautens*
Transition-metal-catalyzed cross-coupling reactions have
become indispensable tools for the synthetic chemist.[1,2]
The construction of carbon–heteroatom bonds is of particular
interest. Consequently, a large number of general and
efficient systems for the catalytic amination and etherification
of aryl and vinyl halides have been described.[3,4] In contrast,
research in the field of carbon–sulfur coupling reactions has
lagged behind, largely because of sulfur’s long-standing
reputation as a catalyst poison.[5–7] Despite this difficulty,
coupling reactions of thiols with aryl halides under the
catalysis of Pd,[8] Cu,[9] Ni,[10] Co,[11] Fe,[12] and In[13] have been
reported. Reactions of vinyl halides are comparatively rare:
only a handful of publications describe the vinylation of
thiols,[14,15] and the palladium-catalyzed intramolecular cou-
pling is unknown.[16]
We anticipated that the use of Pd could facilitate access to
a number of tandem coupling reactions in which an S-
vinylation reaction could be combined with orthogonal
carbon–carbon bond formation in a single synthetic oper-
ation. Tandem/domino reactions are an area of intense
research, as they enable streamlined synthetic sequences
and a consequent reduction of waste.[17] Herein, we disclose a
tandem catalytic reaction of a gem-dihalovinyl thiophenol
system in which an intramolecular S-vinylation is paired with
routes to these targets have only recently been developed.[20]
To the best of our knowledge, the reaction described herein is
the first example of a tandem catalytic process that incorpo-
À
rates a C S coupling, as well as the first example of the
palladium-catalyzed vinylation of a thiol.
We and others have previously reported the synthesis of
indoles substituted at the 2-position by tandem coupling
reactions of gem-dihalovinyl anilines.[21–27] In these systems, an
intramolecular amination or amidation was combined with an
inter- or intramolecular Suzuki–Miyaura, Heck, or Sonoga-
shira coupling, amidation, direct arylation, or carbonylation.
À
Similar transformations involving C S coupling would pro-
vide rapid access to benzothiophenes and expand the range of
available tandem coupling reactions.
À
an intermolecular C C bond-forming reaction (a Suzuki–
Miyaura, Heck, or Sonogashira reaction) to yield 2-substi-
tuted benzothiophenes (Scheme 1). These heterocycles form
the core of a number of medicinally important molecules,
such as raloxifene[18] and zileuton.[19] However, catalytic
Our initial attempts were aimed at combining the carbon–
sulfur bond-forming process with a Suzuki–Miyaura coupling
(Table 1). We found that the conditions developed previously
for the aniline system[22b] were less effective for the thiophenol
substrate 1a: the desired product was isolated in only 48%
yield (Table 1, entry 1).[28] The screening of various bases
(Table 1, entries 1–4, 8), precatalysts (entries 5–7), and ligand/
catalyst ratios (entries 9 and 10) revealed that the best results
were obtained when the reaction was carried out with PdCl2,
SPhos,[29] and anhydrous K3PO4/Et3N in dioxane at 1108C.
Under these conditions, 2a was formed in 89% yield (Table 1,
entry 10). Lowering of the catalyst loading to 1 mol% led to
enhanced turnover with minimal impact on the yield (Table 1,
entry 12).
With these optimized conditions in hand, we investigated
the effect of varying the boronic acid on the tandem process
(Table 2). The reaction was found to tolerate changes in the
electronic nature of the boronic acid. Electron-poor boronic
acids displayed similar reactivity to that of electron-rich 3,4-
dimethoxyphenylboronic acid: the boronic acid used for
optimization of the reaction (Table 2, entries 1–4). The
coupling proceeded equally well when a sterically congested
boronic acid was used (Table 2, entry 5). The use of hetero-
aromatic boronic acids gave the best results: the correspond-
ing heterocyclic products were obtained in up to 99% yield
Scheme 1. Retrosynthetic strategy.
[*] C. S. Bryan, J. A. Braunger, Prof. Dr. M. Lautens
Davenport Laboratories, Department of Chemistry
University of Toronto
80 St. George Street, Toronto, Ontario (Canada)
Fax: (+1)416-946-8185
E-mail: mlautens@chem.utoronto.ca
Homepage: http://www.chem.utoronto.ca/staff/ML/
[**] This research was supported by the Natural Sciences and Engi-
neering Research Council of Canada (NSERC), the Merck Frosst
Centre for Therapeutic Research, and the University of Toronto. We
thank Dr. Valentina Aureggi for her intellectual contributions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200902843.
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Table 1: Tandem coupling under different reaction conditions.[a]
(Table 2, entries 6–8). Finally, a styryl boronic acid was as
effective as a coupling partner as aryl boronic acids (Table 2,
entry 9).
We then probed the compatibility of other boron nucle-
ophiles with the tandem process (Table 3). When a boronic
ester was used, it was necessary to add a small amount of
water to obtain for full conversion (Table 3, entry 1). Both
Entry Pd source
Pd
L/Pd ratio Base
Yield
[%][b]
loading [%]
Table 3: Alternative boron reagents.[a]
1
2
3
4
5
6
7
8
9
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2
[Pd2(dba)3]
Pd/C
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
6
6
6
6
6
6
6
6
6
6
3
1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
4:1
1:1
1:1
1:1
K3PO4·H2O
48
n.d.
71
72
78
65
39
Cs2CO3
K3PO4
Et3N
K3PO4
K3PO4
K3PO4
K3PO4/Et3N 80
K3PO4/Et3N 74
K3PO4/Et3N 89
K3PO4/Et3N 87
K3PO4/Et3N 81
Entry
1[c]
Organoboron
reagent
Product
Yield
[%][b]
10
11
12
2k
80
[a] Reactions were conducted with 1a (0.17 mmol), the boronic acid
(0.29 mmol), and the base (3 equiv) in dioxane at 1108C in a sealed tube
for 16 h; see the Supporting Information for more details. [b] Yield of the
isolated product. dba=dibenzylideneacetone, n.d.=not determined,
SPhos=2-dicyclohexylphosphanyl-2’,6’-dimethoxybiphenyl.
2[d]
3[d]
4
2k
2l
84
85
80
BEt3
2m
[a] Reactions were conducted with 1a (0.17 mmol), the boron reagent
(1.7 equiv), and K3PO4/Et3N (3.0 equiv) in dioxane at 1108C in a sealed
tube for 16 h. [b] Yield of the isolated product. [c] The reaction was
carried out in the presence of water (10 mL). [d] The reaction was carried
out with 5.0 equivalents of the base K3PO4/Et3N.
Table 2: Scope of the reaction with respect to the boronic acid.[a]
aryl and vinyl trifluoroborate salts were coupled effectively in
the tandem reaction (Table 3, entries 2 and 3).[30] With these
substrates, the amount of base used needed to be increased
from 3 to 5 equivalents. Finally, the use of a trialkyl borane
provided access to 2-ethylbenzothiophene in good yield
(Table 3, entry 4).
Our next goal was to increase the utility of the method-
ology by examining substituent effects in the thiophenol
substrate (Table 4). In contrast to the boronic acids, the
electronic nature of the thiophenol fragment had a significant
effect on the outcome of the reaction. Substrates with mildly
electron withdrawing fluoride and chloride substituents
underwent the desired coupling as expected (Table 4,
entries 1 and 2). However, the presence of a more electron-
deficient CF3 substituent led to lower conversion and yield
(Table 4, entry 7), and the reaction of a substrate with a
powerfully electron withdrawing nitro substituent gave none
of the tandem coupling product but instead produced the
2-bromobenzothiophene derivative 4h in modest yield
(entry 8).
Entry
Boronic acid
Product
Yield
[%][b]
1
2
3
4
2b
2c
2d
2e
91
83
84
76
5
2 f
82
6[c]
7
2g
2h
99
96
8
9
2i
2j
83
87
A thiophenol derivative with a mildly electron donating
methyl substituent showed similar reactivity to that of the
unfunctionalized substrate (Table 4, entry 5), whereas the
high efficiency of the reaction of the more electron rich
substrate 3 f was masked by difficulty in isolating the product:
100% conversion was observed (Table 4, entry 6). Bromi-
nated derivatives were found to react unselectively at all
[a] Reactions were conducted with 1a (0.17 mmol), the boronic acid
(1.7 equiv), and K3PO4/Et3N (3.0 equiv) in dioxane at 1108C in a sealed
tube for 16 h. [b] Yield of the isolated product. [c] Reaction time: 40 h.
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Table 4: Variation of the thiophenol derivative.[a]
of the thiol to an intermediate alkyne formed by elimination
of HBr.[31]
Finally, the method could be extended from Suzuki–
Miyaura coupling reactions to other palladium-catalyzed
À
cross-coupling processes. The tandem C S coupling/Heck
reaction of thiophenol 1a with N-acryloylmorpholine or 1-
decen-3-ol proceeded under Jeffrey-type conditions to give
the corresponding product in 75 and 69% yield, respectively
[Eqs. (1) and (2)].[32] The substrate is also amenable to a
Entry Thiophenol
Product
Yield
[%][b]
À
tandem C S coupling/Sonogashira reaction. The treatment of
1
2
3a
3b
4a 78
thiophenol 1a with a terminal alkyne under palladium-
catalyzed conditions in the presence of CuI gave the product
of the tandem reaction in moderate yield [Eqs. (3) and (4);
THP = tetrahydropyranyl]. Notably, this reaction can be
4b 76
3[c]
4[c]
5
3c
3d
3e
3 f
4c 84
4d 75
4e 84
6
4 f
64[d]
7
3g
4g 45
8
9
3h
3i
4h 46
2a 77
10
3j
4j
81
catalyzed by Pd/C, which is a vastly preferable precatalyst
to more expensive and air-sensitive Pd0 or PdII complexes.
In conclusion, we have developed a general, efficient
method for the catalytic synthesis of diversely functionalized
benzothiophenes from gem-dihalovinyl thiophenols. This
reaction is both the first example of a tandem catalytic
[a] Reactions were conducted with the thiophenol 3 (0.17 mmol), the
boronic acid (1.7 equiv), and K3PO4/Et3N (3.0 equiv) in dioxane at 1108C
in a sealed tube for 16 h. [b] Yield of the isolated product. [c] The reaction
was carried out with 2.4 equivalents of the boronic acid. [d] The reaction
proceeded to 100% conversion.
À
process that incorporates C S coupling, and the first example
bromine-substituted positions; thus, a highly efficient process
was observed in which three bonds were formed in a single
operation (Table 4, entries 3 and 4). Interestingly, when fewer
than two equivalents of the boronic acid were used in the
reaction, no monoarylated product was isolated; instead, a
mixture of the dibromide starting material and the corre-
sponding diarylated product was obtained. We found that the
reactivity of dichlorovinyl substrates was similar to that of the
dibromo compounds (Table 4, entries 9 and 10). Importantly,
tetrasubstituted olefins can also be used in the reaction
(Table 4, entry 10). This result provides evidence of a
mechanism that proceeds through reductive elimination
from a thiopalladium species, rather than through addition
of the palladium-catalyzed vinylation of a thiol. Further
studies to elucidate the mechanism of this process and to
apply the transformation to the synthesis of natural products
are in progress in our laboratories.
Experimental Section
Representative procedure: Thiophenol 1a (50 mg, 0.17 mmol),
4-fluorophenylboronic acid (53 mg, 0.29 mmol, 1.7 equiv), K3PO4
(108 mg, 0.51 mmol, 3.0 equiv), Et3N (0.07 mL, 0.51 mmol,
3.0 equiv), and dioxane (2.5 mL) were placed in an oven-dried 2–
5 mL Biotage microwave vial, and the vial was capped with a septum
and degassed with argon for 5 min. The septum was removed, PdCl2
(0.8 mg, 0.0051 mmol, 3 mol%) and SPhos (1.7 mg, 0.0051 mmol,
3 mol%) were added quickly, the septum was replaced, and the
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solution was degassed with argon for an additional 5 min. The septum
was then replaced with a Teflon microwave cap, the vial was sealed,
and the reaction mixture was stirred for 10 min and then heated in an
oil bath for 16 h at 1108C. The flask was then cooled to room
temperature, and the solids were removed by vacuum filtration
through a plug of silica gel and washed with EtOAc. The solution was
concentrated, and the crude product was purified by column
chromatography over silica gel (eluent: Et2O/pentane 0–2%).
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Keywords: cross-coupling · palladium · sulfur heterocycles ·
synthetic methods · tandem catalysis
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