Synthesis of functionalized 2-arylthiophenes with triarylbismuths as atom-efficient multicoupling organometallic nucleophiles under palladium catalysis

Article abstract of DOI:10.1055/s-0030-1260554

Atom-efficient cross-coupling reactions of functionalized 2-bromo- and 2-iodothiophenes have been demonstrated using triarylbismuths as atom-efficient multicoupling organometallic nucleophiles under palladium-catalyzed conditions. These couplings with various functionalized triarylbismuths proceeded smoothly to afford the corresponding functionalized 2-arylthiophenes in high yields. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0030-1260554

1324
LETTER
Synthesis of Functionalized 2-Arylthiophenes with Triarylbismuths as
Atom-Efficient Multicoupling Organometallic Nucleophiles under Palladium
Catalysis
S
ynthesis of Functiona
a
lized 2-Ary
d
lthiophenes
d
w
ith Triarylb
a
ismuths li L. N. Rao,* Debasis Banerjee, Ritesh J. Dhanorkar
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
Fax +91(512)2597532; E-mail: maddali@iitk.ac.in
Received 19 January 2011
Dedicated to Professor Yashwant D. Vankar on the occasion of his 60th birthday
Table 1 Screening Conditionsa–c
Abstract: Atom-efficient cross-coupling reactions of functional-
ized 2-bromo- and 2-iodothiophenes have been demonstrated using
triarylbismuths as atom-efficient multicoupling organometallic nu-
cleophiles under palladium-catalyzed conditions. These couplings
with various functionalized triarylbismuths proceeded smoothly to
afford the corresponding functionalized 2-arylthiophenes in high
yields.
S
[Pd]
BiPh3
Ph-Ph
+
+
+
Br
Ph
S
S
S
(3.5 equiv) (1 equiv)
1B
1C
1a
1A
Entry Catalyst/ligand
1A
7
1B
23
18
28
27
22
17
12
23
21
32
28
36
24
–
1C
1a
Key words: 2-halothiophenes, cross-coupling, palladium catalysis,
triarylbismuths, atom-efficient, multicoupling
1
2
PdCl2(PPh3)2
Pd2(dba)3
20
–
50 (40)
80
2
2
Utilization of organometallic reagents for C–C bond for-
mation is in high demand for applications in synthetic or-
ganic chemistry.1 Synthesis of various multifunctional
heteroaryl2 systems is of a special interest due to their po-
tential applications in materials,3 and medicinal chemis-
try.4 Various functionalized aryl-substituted thiophenes
have been known to have a variety of applications in phar-
macology,5 and in display and sensor technologies.3 A
popular method of synthesis of these compounds is
through cross-coupling reactions as these methods pro-
vide the desired flexibility6,7 and site selectivity.8
3
Pd2(dba)3–PPh3
Pd(PPh3)4
20
13
–
48 (42)
49 (40)
2
4
10
75
80
2
5
PdCl2(MeCN)2
PdCl2(PhCN)2
Pd(OAc)2–PPh3
Pd(OAc)2–PPh3
Pd(OAc)2–PPh3
Pd(OAc)2–PPh3
Pd(OAc)2–PPh3
Pd(OAc)2–PPh3
Pd(OAc)2
6
–
2
7
5
78 (81)
15d
8
56
49
13
13
32
60
90
4
9
2
19e
10
11
12
13
14
12
8
42 (40)f
50 (48)g
27h
Recently, we have demonstrated the novel coupling reac-
tivity of triarylbismuths9 as atom-efficient multicoupling
organometallic nucleophiles with different organic elec-
trophiles.10 In these reactions, triarylbismuths have been
cross-coupled with three equivalents of organic electro-
philes.9 The nontoxic nature of bismuth compounds,11
makes these reactions highly desirable as green reagents
in organic synthesis.12 The coupling study of triarylbis-
muths with heteroaryl substrates as electrophilic coupling
partners has not been greatly explored hitherto. Langer et
al. reported efficient coupling of tri- and tetrabro-
mothiophenes with organometallic reagents involving
site-selective couplings.8 In this context, we wish to dis-
close an atom-efficient cross-coupling of triarylbismuths
with functionalized 2-bromo- and 2-iodothiophenes for
the novel synthesis of functionalized 2-arylthiophenes un-
der palladium-catalyzed conditions.
2
–
2
PPh3
–
–
a Reaction conditions: BiPh3 (1 equiv), 2-bromothiophene (3.5 equiv),
Pd catalyst (0.1 equiv), ligand (0.4 equiv), K3PO4 (6 equiv), DMF (3
mL), 90 °C, 1 h.
b Conversions are based on GC analysis.
c Isolated yields are given in parenthesis.
d At 40 °C.
e At 60 °C.
f With K3PO4 (2 equiv).
g With K3PO4 (4 equiv).
h Without base.
To understand the coupling efficacy of halothiophenes
with triarylbismuth reagents, a systematic screening was
carried out (Table 1). The coupling of 2-bromothiophene,
1A (3.5 equiv) with triphenylbismuth (1 equiv) provided
varied reactivity using different palladium precursors in
N,N-dimethylformamide with K3PO4 base (Table 1, en-
SYNLETT 2011, No. 9, pp 1324–1330
Advanced online publication: 05.05.2011
DOI: 10.1055/s-0030-1260554; Art ID: D01911ST
© Georg Thieme Verlag Stuttgart · New York
x
x.
x
x.
2
0
1
1

LETTER
Synthesis of Functionalized 2-Arylthiophenes with Triarylbismuths
1325
Table 2 Cross-Couplings with Different Triarylbismuthsa–d
(continued)
tries 1–7). In general, three equivalents of 2-bro-
mothiophene are sufficient to couple with three phenyl
groups from one equivalent of triphenylbismuth reagent.
However, we have employed 3.5 equivalents of bromide
with the excess to facilitate cross-coupling over the homo-
coupling from triphenylbismuth in the presence of palla-
dium catalyst.13
[Pd]
+
Bi
Br
S
S
R
3
R
(3.5 equiv)
(1 equiv)
(3 equiv)
Entry Triarylbismuth
2-Arylthiophene
Yield (%)
72
Under the conditions studied, the best cross-coupling was
obtained with Pd(OAc)2 catalyst. Other catalytic combi-
nations, such as PdCl2(PPh3)2, Pd2(dba)3–2 PPh3 and
Pd(PPh3)4 provided moderate conversions, while
Pd2(dba)3, PdCl2(MeCN)2 and PdCl2(PhCN)2 gave poor
cross-coupling efficiency. Additional screening was car-
ried out by varying the temperature, when coupling reac-
tions at 40 °C and 60 °C proved to be less efficient with
lower yields (Table 1, entries 8 and 9). Further trials with
two and four equivalents of base proved to be less effec-
tive (Table 1, entries 10 and 11). Other reactions carried
out in the absence of base, ligand and catalyst each pro-
vided poor or no cross-coupling product (Table 1, entries
12–14). Minor amounts of both homocoupled biphenyl
(1B)13 from triphenylbismuth, and 2,2′-bisthiophene
(1C)6c from 2-bromothiophene were seen, along with un-
reacted 2-bromothiophene in these screenings. In the pres-
ence of high cross-coupling, formation of side products
was minimized. In summary, the efficient cross-coupling
of 2-bromothiophene with triphenylbismuth was estab-
lished to afford 2-phenylthiophene in high yield (Table 1,
entry 7). From this, the coupling reactivity of triarylbis-
muth was found to be notable in comparison with the
known literature methods with other organometallic re-
agents in similar couplings.6
4
S
Bi
F
3
F
4a
S
5
83
Bi
Bi
O
O
3
5a
S
6
7
8
9
85
85
70
71
OEt
3
OEt
Me
6a
7a
Me
S
S
Bi
Bi
3
Cl
3
Cl
8a
9a
OMe
OMe
Table 2 Cross-Couplings with Different Triarylbismuthsa–d
S
Bi
Bi
3
[Pd]
+
Bi
Br
S
S
R
3
R
10
S
92
OMe
(3.5 equiv)
(1 equiv)
(3 equiv)
3
OMe
10a
Entry Triarylbismuth
2-Arylthiophene
Yield (%)
81
a Reaction conditions: BiAr3 (1 equiv, 0.25 mmol), 2-bromothiophene
(3.5 equiv, 0.875 mmol), Pd(OAc)2 (0.1 equiv, 0.025 mmol), Ph3P
(0.4 equiv, 0.1 mmol), K3PO4 (6 equiv, 1.5 mmol), DMF (3 mL), 90
°C, 1 h.
1
S
Bi
3
b Isolated yields based on three aryl coupling from triarylbismuth.
Thus, 3 equiv of cross-coupling product (0.75 mmol) corresponds to
100% yield.
1a
c All the products were characterized spectroscopically.
d In general, minor amounts of biaryl and 2,2¢-bisthiophene were
formed.
2
S
88
71
Bi
Bi
Me
3
Me
2a
3a
Cl
Cl
The above investigation prompted us to extend the study
of 2-bromothiophene with electronically divergent triaryl-
bismuths as summarized in Table 2.14 The general cou-
pling reactivity of various triarylbismuths with 2-
bromothiophene was found to be facile, furnishing the
corresponding functionalized 2-arylthiophenes in high
yields (Table 2, entries 1–10). Participation of both elec-
3
S
3
Synlett 2011, No. 9, 1324–1330 © Thieme Stuttgart · New York

1326
M. L. N. Rao et al.
LETTER
tron-rich and electron-deficient triarylbismuths with com- 2-arylthiophenes smoothly (Table 3, entries 1–11). The
parable coupling reactivity is noteworthy in these coupling reactivity with electronically divergent triaryl-
conversions. This observed reactivity is more facile in bismuths has proved to be promising with good to high
comparison with similar couplings with arylboronic acids yields of products. It is important to note that these prod-
under Suzuki-coupling conditions. The couplings with ucts are useful synthons for the preparation of trimerized
arylboronic acids have been reported to involve longer re- polyaromatic compounds.6a Furthermore, 5-acetyl-2-
action times even for one C–C bond formation.2c,d Thus, arylthiophenes are also useful in the synthesis of nano-
the present method using triarylbismuths as multicoupling sized molecular network systems.6a,b In addition, these
nucleophiles is advantageous.
products are also used in the synthesis of 5-aryl-5¢-
formyl-2,2¢-bithiophenes used in photophysical applica-
tions.15
To elaborate the scope of this process further, arylation of
functionalized 5-acetyl-2-bromothiophene was carried
out under the optimized conditions. As summarized in
Table 3,14 arylations of 5-acetyl-2-bromothiophene with
different triarylbismuths afforded functionalized 5-acetyl-
Table 3 Cross–Couplings of Functionalized 2-Bromothiophenes with Different Triarylbismuthsa–d
[Pd]
+
Bi
R1
R2
Br
R1
S
S
3
R2
(3.5 equiv)
(1 equiv)
(3 equiv)
R1 = Ac, COPh, CHO
Entry Triarylbismuth
2-Arylthiophene
Yield Entry Triarylbismuth
(%)
2-Arylthiophene
Yield
(%)
Ac
S
8
67
Ac
S
Bi
Cl
1
86
65
90
71
Bi
Me
3
Cl
3
Me
18a
11a
OMe
Cl
Cl
OMe
Ac
Ac
S
2
3
4
9
64
49
92
S
S
S
Bi
Bi
Bi
Bi
3
3
12a
19a
S
Ac
13a
Ac
10
11
Ac
S
S
S
3
3
20a
Ac
Bi
Bi
F
Bi
Bi
OMe
3
3
F
OMe
14a
21a
Ac
S
S
5
6
7
66
64
48
12
94
O
S
Me
O
3
O
3
Me
22a
15a
Ac
13
14
88
86
Bi
O
Bi
Bi
S
S
O
3
3
O
O
16a
23a
24a
CF3
CF3
F
Ac
S
Bi
3
F
3
17a
Synlett 2011, No. 9, 1324–1330 © Thieme Stuttgart · New York

LETTER
Synthesis of Functionalized 2-Arylthiophenes with Triarylbismuths
1327
Table 3 Cross–Couplings of Functionalized 2-Bromothiophenes with Different Triarylbismuthsa–d (continued)
[Pd]
+
Bi
R1
R2
Br
R1
S
S
3
R2
(3.5 equiv)
(1 equiv)
(3 equiv)
R1 = Ac, COPh, CHO
Entry Triarylbismuth
2-Arylthiophene
Yield Entry Triarylbismuth
(%)
2-Arylthiophene
Yield
(%)
H
S
O
S
15
66
22
77
Bi
F
Bi
O
O
3
3
F
O
O
32a
25a
H
S
16
85
23
24
81
74
S
O
Bi
O
Bi
O
O
O
3
3
26a
33a
H
CF3
CF3
S
17
18
19
61
75
85
Bi
Bi
O
S
O
Bi
Bi
O
O
3
3
27a
34a
H
S
S
25
26
27
56
71
83
Cl
S
O
3
S
3
Cl
O
35a
28a
S
H
S
S
Bi
OMe
Me
Bi
Bi
S
O
3
O
S
3
OMe
36a
29a
H
H
S
20
21
60
56
Bi
Bi
OMe
O
O
3
3
Me
OMe
30a
37a
H
S
O
3
31a
a Reaction conditions: BiAr3 (1 equiv, 0.25 mmol), 2-bromothiophene (3.5 equiv, 0.875 mmol), Pd(OAc)2 (0.1 equiv, 0.025 mmol), Ph3P (0.4
equiv, 0.1 mmol), K3PO4 (6 equiv, 1.5 mmol), DMF (3 mL), 90 °C, 1 h.
b Isolated yields based on three aryl coupling from triarylbismuth. Thus, 3 equiv of cross-coupling product (0.75 mmol) corresponds to 100%
yield.
c All the product were characterized by spectroscopic studies.
d In general, minor amounts of biaryl and 2,2¢-bisthiophene were formed.
Further studies were carried out with 5-benzoyl-2-bro- able molecular motifs for various biological and
mothiophene as the organic electrophilic coupling partner medicinal applications.16 Further couplings of 5-bromo-2-
with different triarylbismuth nucleophiles (Table 3, en- thienylaldehyde were carried out with different triarylbis-
tries 12–19).14 These reactions afforded efficient cross- muth reagents (Table 3, entries 20–25, entry 27).14 This
coupling with high yields in short reaction time. The 2- study allowed the successful arylations with various tri-
benzoyl-5-arylthiophene products obtained in these cou- arylbismuths furnishing the chemoselective formation of
plings are useful in materials applications3g and are valu- 5-aryl-2-thienylaldehydes in good yields. Importantly,
Synlett 2011, No. 9, 1324–1330 © Thieme Stuttgart · New York

1328
M. L. N. Rao et al.
LETTER
Table 4 Cross-Couplings of 5-Acetyl- and 5-Benzoyl-2-iodo-
these products are important precursors for the prepara-
tion of compounds useful for medicinal6e and photophys-
ical applications.3f,6f A coupling study using tris-2-
thienylbismuth also furnished the corresponding 2,2¢-
bisthiophene, albeit in moderate yield (Table 3, entry 26).
thiophene with Different Triarylbismuthsa–d (continued)
[Pd]
+ Bi
I
R1
R1
R2
S
S
3
R2
At this stage, it was of interest to explore the correspond-
ing couplings with 2-iodothiophene compounds.17 This
study has been carried out with different 2-iodothiophenes
and the results are summarized in Table 4.14 The coupling
reactions of 5-acetyl- and 5-benzoyl-derived 2-iodo-
thiophenes with different triarylbismuths gave the corre-
sponding coupling products in high yields (Table 4,
entries 1–10). The substituted 2-iodothiophenes demon-
strated excellent reactivity with triarylbismuth nucleo-
philes in short reaction time under these coupling
conditions.
(3.5 equiv)
(1 equiv)
(3 equiv)
R1 = Ac, COPh
Entry Triarylbismuth
2-Arylthiophene
Yield
(%)
7
87
Bi
S
3
O
23a
8
9
80
82
52
Bi
Bi
OMe
S
Table 4 Cross-Couplings of 5-Acetyl- and 5-Benzoyl-2-iodo-
thiophene with Different Triarylbismuthsa–d
O
3
OMe
29a
[Pd]
+ Bi
R1
R1
I
R2
S
S
3
R2
S
Cl
(3.5 equiv)
R1 = Ac, COPh
(1 equiv)
(3 equiv)
O
3
Cl
38a
Entry Triarylbismuth
2-Arylthiophene
Yield
CF3
CF3
(%)
10
S
Bi
O
3
27a
Ac
S
1
93
Bi
Me
a Reaction conditions: BiAr3 (1 equiv, 0.25 mmol), 2-iodothiophene
(3.5 equiv, 0.875 mmol), Pd(OAc)2 (0.1 equiv, 0.025 mmol), Ph3P
(0.4 equiv, 0.1 mmol), K3PO4 (6 equiv, 1.5 mmol), DMF (3 mL), 90
°C, 1 h.
3
Me
11a
b Isolated yields based on three aryl coupling from triarylbismuth.
Thus, 3 equiv of cross-coupling product (0.75 mmol) corresponds to
100% yield.
Ac
S
2
3
92
81
66
69
Bi
Bi
OMe
3
OMe
21a
c All the products were characterized by spectroscopic studies.
d In general, minor amounts of biaryl and 2,2¢-bisthiophene were
formed.
Ac
S
3
13a
Overall, the efficient coupling reactivity of triarylbis-
muths has been demonstrated with functionalized 2-bro-
mothiophenes and 2-iodothiophenes under the optimized
palladium-catalyzed conditions. The multicoupling and
the atom-efficient reactivity of triarylbismuths has been
established in couplings with three equivalents of both 2-
bromo- and 2-iodothiophene derivatives. It is noteworthy
that these couplings from triarylbismuths were effected
within one hour reaction time affording three equivalents
of coupling products This high reactivity of triarylbis-
muths is in sharp contrast to the corresponding reactivity
known with other organometallic reagents which involves
long reaction times even for a single C–C coupling under
heating.6 Thus, triarylbismuths serve as atom-efficient
multicoupling organometallic nucleophiles for the aryla-
tions of various 2-bromo- and 2-iodothiophenes under
palladium-catalyzed conditions.
Ac
S
S
4
5
Bi
F
3
F
14a
Ac
OMe
OMe
Bi
Bi
3
19a
Ac
6
S
71
Cl
3
Cl
18a
Synlett 2011, No. 9, 1324–1330 © Thieme Stuttgart · New York

LETTER
Synthesis of Functionalized 2-Arylthiophenes with Triarylbismuths
1329
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Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
We are grateful to the Department of Science and Technology
(DST), New Delhi for supporting this work under the Green Chemi-
stry Program (SR/S5/GC-11/2008). D.B. and R.J.D. thank IIT Kan-
pur and the Council of Scientific and Industrial Research (CSIR),
New Delhi, respectively, for research fellowships.
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(14) Representative Procedure: An oven-dried Schlenk tube
under a nitrogen atmosphere was charged with 2-bromo-
thiophene (0.875 mmol, 3.5 equiv, 0.143 g) followed by
BiPh3 (0.25 mmol, 1 equiv, 0.110 g), K3PO4 (1.5 mmol, 6
equiv, 0.318 g), Ph3P (0.10 mmol, 0.4 equiv, 0.026 g),
Pd(OAc)2 (0.025 mmol, 0.1 equiv, 0.0056 g) and anhyd
Synlett 2011, No. 9, 1324–1330 © Thieme Stuttgart · New York

1330
M. L. N. Rao et al.
LETTER
(15) Herbivo, C.; Comel, A.; Kirsch, G.; Raposo, M. M. M.
DMF (3 mL). The reaction mixture was stirred in an oil-bath
at 90 °C for 1 h. The contents were cooled to r.t., quenched
with H2O (10 mL) and extracted with EtOAc (2 × 20 mL).
The combined organic extracts were washed with H2O,
brine, dried (MgSO4) and concentrated under reduced
pressure. The crude product was purified by column
chromatography using 100–200 silica gel and petroleum
ether as eluent to obtain 2-phenylthiophene as a white, low
melting solid (0.097 g, 81%; isolated yield based on three
aryl coupling from triphenylbismuth).
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Synlett 2011, No. 9, 1324–1330 © Thieme Stuttgart · New York