Hindered aryl bromides for regioselective palladiumcatalysed direct arylation at less favourable C5-carbon of 3-substituted thiophenes

Article abstract of DOI:10.3762/bjoc.10.123

The use of the congested aryl bromide 2-bromo-1,3-dichlorobenzene as coupling partner allows to modify the regioselectivity of the arylation of 3-substituted thiophene derivatives in favour of carbon C5. The coupling of this aryl bromide with a variety of 3-substituted thiophenes gave in all cases the desired 5-arylation products in moderate to good yields using only 0.5 mol % of a phosphinefree and air-stable palladium catalyst. Then, from these 5-arylthiophenes, a second palladium-catalysed C-H bond functionalization at C2 of the thiophene ring allows the synthesis of 2,5-diarylthiophenes with two different aryl units.

Full text of DOI:10.3762/bjoc.10.123

Hindered aryl bromides for regioselective palladium-
catalysed direct arylation at less favourable
C5-carbon of 3-substituted thiophenes
Rongwei Jin, Charles Beromeo Bheeter and Henri Doucet*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1239–1245.
Institut Sciences Chimiques de Rennes, UMR 6226 CNRS-Université
de Rennes "Organometalliques, Material and Catalysis", Campus de
Beaulieu, 35042 Rennes, France. Fax: +33 (0)2-23-23-69-39; Tel:
+33 (0)2-23-23-63-84
doi:10.3762/bjoc.10.123
Received: 24 February 2014
Accepted: 24 April 2014
Published: 27 May 2014
Email:
Henri Doucet* - henri.doucet@univ-rennes1.fr
Associate Editor: I. Marek
* Corresponding author
© 2014 Jin et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
aryl bromides; atom economy; C–H bond activation; palladium;
regioselectivity; thiophenes
Abstract
The use of the congested aryl bromide 2-bromo-1,3-dichlorobenzene as coupling partner allows to modify the regioselectivity of the
arylation of 3-substituted thiophene derivatives in favour of carbon C5. The coupling of this aryl bromide with a variety of 3-substi-
tuted thiophenes gave in all cases the desired 5-arylation products in moderate to good yields using only 0.5 mol % of a phosphine-
free and air-stable palladium catalyst. Then, from these 5-arylthiophenes, a second palladium-catalysed C–H bond functionaliza-
tion at C2 of the thiophene ring allows the synthesis of 2,5-diarylthiophenes with two different aryl units.
Introduction
As thiophenes bearing aryl substituents are known to be present Suzuki or Stille cross-coupling reactions. Moreover, direct aryl-
in several bioactive molecules and are used as precursors of ma- ation avoids the preliminary preparation of organometallics
terials, the regioselective introduction of aryls on thiophenes is reducing the number of steps to prepare these arylthiophenes.
an important research area in organic synthesis. The coupling of
thiophene derivatives with aryl halides via a C–H bond acti- The regioselective arylation via a Pd-catalysed C–H bond acti-
vation/functionalisation [1-12] provides an environmentally vation at carbon C5 of 2-substituted thiophenes has been largely
attractive and cost-effective procedure for the preparation of a described in recent years [13-22]. On the other hand, the
variety of arylated thiophenes. For such coupling reactions, the Pd-catalysed direct arylation of 3-substituted thiophenes has
major byproduct is a base associated to HX, instead of metallic attracted much less attention (Scheme 1, top) [23-31]. With
salts which are produced under the more classical Negishi, such thiophene derivatives, in most cases, either C2-arylated
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Beilstein J. Org. Chem. 2014, 10, 1239–1245.
thiophenes or mixtures of C2- and C5-arylated products have thiophene derivatives especially to provide 5-arylthiophenes
been obtained. For example, Sharp et al. described conditions remains a challenging reaction.
for the regioselective arylation at carbons C2 or C5 of methyl
3-thiophenecarboxylate [23]. The reaction performed with Our goal was to promote arylation at carbon C5 of a range of
Pd(PPh3)4 as the catalyst in toluene led selectively to the 3-substituted thiophenes without the use of a blocking group at
2-arylated thiophene; whereas, the use of Pd2(dba)3 in NMP carbon C2. To our knowledge, ortho-substituents on aryl bro-
afforded a mixture of 2- and 5-arylated thiophenes in a 15:51 mides have not been employed as directing groups for palla-
ratio. Lemaire and co-workers have reported the C2-arylation of dium-catalysed direct arylation of 3-substituted thiophenes. The
3-formyl-, 3-cyano- and 3-nitrothiophenes with iodobenzenes use of congested aryl bromides for such couplings would
[24,25]. Forgione, Bilodeau et al. reported that the reaction of certainly modify the regioselectivity in favour of arylation at the
3-methylthiophene with bromobenzene using Pd[(P(t-Bu)3]2 as less hindered thiophene position. Herein, we wish to report on
the catalyst affords a mixture of 2- and 5-phenylated thio- the influence of such ortho-substituents on aryl bromides on the
phenes in a 3.3:1 ratio [26]. Fagnou and co-workers reported regioselectivity of the palladium-catalysed direct arylations of
that the coupling of 3-n-hexylthiophene with 4-bromonitroben- 3-substituted thiophenes.
zene also led to a mixture of C2- and C5-arylation products in a
1.3:1 ratio [27]. Then, they blocked one position on the thio- Results and Discussion
phene ring using a chloro-substituent in order to selectively We first studied the palladium-catalysed direct arylation of
arylate positions C2 or C5. The direct arylation of 3-methylthiophene using tert-butyl 2-bromobenzoate as the
3-methoxythiophene, which was studied by Borghese and coupling partner (Scheme 2). Based on previous results [19],
co-workers afforded regioselectively the C2-arylated thio- DMA was initially chosen as the solvent and KOAc as the base.
phenes in moderate to high yields [28]. Finally, in the course of The reactions were performed at 150 °C under argon in the
their studies on sp3 C–H bond activation, Baudoin and Pierre presence of 0.5 mol % Pd(OAc)2 as the catalyst. However,
recently reported that in the presence of an extremely bulky under these conditions, a poor regioselectivity was observed as
substituent at C3 of a thiophene derivative, the C5-arylated the desired C5-arylation product 1b was only obtained in 34%
compounds were selectively obtained in good yields [29]. In selectivity together with 66% of C2-arylation product 1a. More-
summary, due to the presence of two reactive C–H bonds in over, a moderate conversion of this aryl bromide was observed
3-substituted thiophenes (with position C2 generally slightly and purification by silica gel chromatography afforded a mix-
more reactive than position C5), the control of the regio- ture of 1a and 1b. A slightly lower selectivity in favour of the
selectivity of the palladium-catalysed direct arylation of such formation of C5-arylation product 2b was observed using
Scheme 1: Regioselectivity of coupling reactions of 3-substituted thiophenes with aryl halides.
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Scheme 2: Regioselectivity of the arylation of 2-methylthiophene with ortho-substituted aryl bromides.
2-bromonitrobenzene as the coupling partner, as a large amount selectively the desired 5-arylation product 8b. Only traces of the
of 2,5-diarylated product 2c was also produced with a 2a:2b:2c C2-arylated thiophene 8a and a low amount of 2,5-diarylated
ratio of 34:21:45. However, a complete conversion of product 8c were detected by 1H NMR and GC–MS analysis of
2-bromonitrobenzene was observed. 2-Bromoaniline was found the crude mixture. Moreover a high conversion of 86% of this
to be unreactive and was recovered. It should be noted that no aryl bromide was observed using only 0.5 mol % Pd(OAc)2 as
amination reaction of 2-bromoaniline due to self-coupling was the catalyst.
observed. From 2-(trifluoromethyl)bromobenzene, a very
similar mixture of regioisomers than with 2-bromonitrobenzene Then, we studied the scope of the coupling of 2-bromo-1,3-
was obtained, as 4a, 4b and 4c were formed in a 24:31:45 ratio. dichlorobenzene, using other 3-substituted thiophene deriva-
The use of 2-bromobenzaldehyde was not successful, as the tives (Scheme 3, Table 1). Both 3-chlorothiophene and ethyl
desired product 5b was only obtained with 10% selectivity. thiophene-3-carboxylate led to the desired 5-arylation products
From more hindered 2-bromobenzaldehyde diethyl acetal, the 9b and 10b with 89% selectivity and in 44% and 65% yields,
selectivity in favour of desired product 6b was slightly higher respectively (Table 1, entries 1 and 2). A slightly higher regio-
(34%), although still not synthetically useful. Next, we selectivity in favour of the C5-arylation was observed in the
employed 2-chlorobromobenzene as the coupling partner. presence of 3-acetylthiophene, as 11b was obtained with 92%
However, again the desired product 7b was only formed with selectivity (Table 1, entry 3). From ethyl thiophen-3-ylacetate,
15% selectivity. It should be noted that with these aryl bro- 12b was formed with 90% selectivity and in 62% yield
mides, in all cases, mixtures of regioisomers a and b were (Table 1, entry 4). On the other hand, from methyl (E)-3-thio-
obtained after column chromatography. On the other hand, the phen-3-ylacrylate, 3-bromothiophene or 3-formylthiophene,
use of 2-bromo-1,3-dichlorobenzene allowed to obtain very mixtures of unidentified products were obtained.
Scheme 3: Direct arylation of 3-substituted thiophenes with 2-bromo-1,3-dichlorobenzene.
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Beilstein J. Org. Chem. 2014, 10, 1239–1245.
Table 1: Direct arylation of 3-substituted thiophenes with 2-bromo-1,3-dichlorobenzene (Scheme 3).a
Entry
Heteroarene
Major product
Ratio a:b:c
Yieldb of regioisomer b (%)
1
3:89:8
44
9b
2
3
4
3:89:8
0:92:8
2:90:8
65
57
62
10b
11b
12b
aConditions: Pd(OAc)2 (0.5 mol %), aryl bromide (1 mmol), thiophene derivative (2 mmol), KOAc (2 mmol), DMA (3 mL), 150 °C, 20 h; bisolated yields
of regioisomers 9b–12b.
The reactivity for arylation at carbon C2 of the previously arylations of a 3-substituted thiophene offer a simple access to a
prepared 2-(2,6-dichlorophenyl)-4-methylthiophene (8b), via a variety of 2,5-diarylthiophenes with two different aryl units.
second Pd-catalysed C–H bond activation, was also investi-
gated (Scheme 4). 4-Bromobenzonitrile and 8b in the presence Conclusion
of 0.5 mol % Pd(OAc)2 and KOAc as the base gave the desired In summary, we have demonstrated that the use of the
coupling product 13 in 82% yield. A similar reactivity was congested coupling partner 2-bromo-1,3-dichlorobenzene
observed with 4-bromobenzaldehyde and 3-bromoacetophe- allows to direct the arylation to the unfavourable C5 position of
none affording 14 and 15 in 80% and 85% yields, respectively. 3-substituted thiophenes. These less favoured regioisomers can
Finally, more congested 2-bromobenzonitrile was reacted with be selectively obtained in moderate to good yields using a range
8b to provide 16 in 88% yield. These regioselective sequential of 3-substituted thiophenes, as chloro, ester, acetyl or ethyl
Scheme 4: Pd-catalysed C2-arylation of 8b with aryl bromides.
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acetate substituents are tolerated. Moreover, the sequential ture of products 4a, 4b and 4c was obtained in 66% (0.160 g)
catalytic C5 and C2 arylations, allow the preparation of 2,5- yield. 1H NMR (400 MHz, CDCl3) δ 6.88 (s, 1H), 6.83 (s, 1H),
diarylthiophenes with two different aryl units in two steps. The 1.95 (s, 3H). 4a was also observed: 1H NMR (400 MHz,
major byproduct of these couplings is KBr/AcOH instead of CDCl3) δ 7.20 (d, J = 5.1 Hz, 1H), 6.81 (d, J = 5.1 Hz, 1H). 4c
metallic salts as with more classical coupling procedures. For was also observed: 1H NMR (400 MHz, CDCl3) δ 7.75–7.65
these reasons, this process gives an economically viable access (m, 2H), 7.55–7.35 (m, 6H), 6.82 (s, 1H), 1.95 (s, 3H).
to C5-arylated 3-substituted heteroaromatics.
2-(4-Methylthiophen-2-yl)benzaldehyde (5b): After column
Experimental
chromatography (pentane/diethyl ether 95:5), a mixture of prod-
General
ucts 5a and 5b was obtained in 51% (0.103 g) yield. 1H NMR
All reactions were perfomed in Schlenk tubes under argon. (400 MHz, CDCl3) δ 10.14 (s, 1H), 7.92 (d, J = 7.8 Hz, 1H),
DMA analytical grade was not distilled before use. Commer- 7.60–7.35 (m, 3H), 6.97 (s, 1H), 6.81 (s, 1H), 2.25 (s, 3H). 5a
cial aryl bromide derivatives were used without purification. was also observed: 1H NMR (400 MHz, CDCl3) δ 9.85 (s, 1H),
1H NMR (400 MHz), 13C NMR (100 MHz) spectra were 7.95 (d, J = 7.6 Hz, 1H), 7.56 (t, J = 7.6 Hz, 1H), 7.44 (t, J = 7.6
recorded in CDCl3 solutions on Bruker GPX (400 MHz). GC Hz, 1H), 7.36 (d, J = 7.6 Hz, 1H), 7.27 (d, J = 5.0 Hz, 1H), 6.89
was performed on a Shimadzu GC-2014 and GC–MS was (d, J = 5.0 Hz, 1H), 2.02 (s, 3H).
performed on a Shimadzu QP-2010S. Chemical shifts are
reported in ppm relative to CDCl3 (1H: 7.26 and 13C: 77.0). 2-(2-Chlorophenyl)-4-methylthiophene (7b): After column
Flash chromatography was performed on silica gel chromatography (pentane/diethyl ether 95:5), a mixture of prod-
(230–400 mesh).
ucts 7a and 7b was obtained in 64% (0.133 g) yield. 1H NMR
(400 MHz, CDCl3) δ 7.11 (s, 1H), 6.90 (s, 1H), 2.24 (s, 3H). 7a
was also observed: 1H NMR (400 MHz, CDCl3) δ 6.87 (d, J =
General procedure for direct arylations
The aryl bromide (1 mmol), thiophene derivative (2 mmol), 5.0 Hz, 1H), 2.04 (s, 3H).
KOAc (2 mmol), Pd(OAc)2 (0.005 mmol, 1.1 mg) and DMA
(3 mL) were introduced in a Schlenk tube under argon equipped 2-(2,6-Dichlorophenyl)-4-methylthiophene (8b): From 2-bro-
with a magnetic stirring bar. The Schlenk tube was placed in an mo-1,3-dichlorobenzene (0.226 g, 1 mmol) and 3-methylthio-
oil bath pre-heated at 150 °C, and the reaction mixture was phene (0.196 g, 2 mmol), 8b was obtained in 63% (0.153 g)
allowed to stir for 20 h. After cooling, the crude reaction mix- yield as an oil after column chromatography (pentane).
ture was analysed by gas chromatography and 1H NMR to 1H NMR (400 MHz, CDCl3) δ 7.31 (d, J = 7.5 Hz, 2H), 7.15 (t,
determine the conversion of the aryl bromide and the regio- J = 7.5 Hz, 1H), 6.98 (s, 1H), 6.75 (s, 1H), 2.25 (s, 3H); 13C
selectivity of the arylation. The solvent was removed by heating NMR (50 MHz, CDCl3) δ 137.4, 136.6, 136.4, 133.1, 130.8,
under vacuum, then the residue was charged onto a silica gel 129.7, 128.0, 122.1, 15.8; Anal. calcd for C11H8Cl2S (243.15):
column.
C, 54.34; H, 3.32; found: C, 54.19; H, 3.17. Traces of 8a and 8c
were also detected by GC–MS analysis of the crude mixture.
tert-Butyl 2-(4-methylthiophen-2-yl)benzoate (1b): After
column chromatography (pentane/diethyl ether 98:2), a mixture 4-Chloro-2-(2,6-dichlorophenyl)thiophene (9b): From 2-bro-
of products 1a and 1b was obtained in 24% (0.066 g) yield. mo-1,3-dichlorobenzene (0.226 g, 1 mmol) and 3-chlorothio-
1H NMR (400 MHz, CDCl3) δ 6.84 (s, 1H), 2.20 (s, 3H). 1a phene (0.237 g, 2 mmol), 9b was obtained in 44% (0.116 g)
was also observed: 1H NMR (400 MHz, CDCl3) δ 7.14 (d, J = yield as an oil after column chromatography (pentane).
5.0 Hz, 1H), 6.81 (d, J = 5.0 Hz, 1H), 1.97 (s, 3H).
1H NMR (400 MHz, CDCl3) δ 7.32 (d, J = 7.5 Hz, 2H),
7.23–7.17 (m, 2H), 6.83 (d, J = 1.2 Hz, 1H); 13C NMR (50
4-Methyl-2-(2-nitrophenyl)thiophene (2b): After column MHz, CDCl3) δ 137.4, 136.5, 131.7, 130.4, 128.8, 128.2, 124.9,
chromatography (pentane/diethyl ether 95:5), a mixture of prod- 121.4; Anal. calcd for C10H5Cl3S (263.57): C, 45.57; H, 1.91;
ucts 2a and 2b was obtained in 60% (0.131 g) yield. 1H NMR found: C, 45.67; H, 1.90. Traces of 9a and 9c were also
(400 MHz, CDCl3) δ 7.63 (d, J = 8.0 Hz, 1H), 6.91 (s, 1H), 2.19 detected by GC–MS analysis of the crude mixture.
(s, 3H). 2a was also observed: 1H NMR (400 MHz, CDCl3) δ
7.83 (d, J = 8.0 Hz, 1H), 7.22 (d, J = 5.0 Hz, 1H), 6.83 (d, J = Ethyl 5-(2,6-dichlorophenyl)thiophene-3-carboxylate (10b):
5.0 Hz, 1H), 1.99 (s, 3H).
From 2-bromo-1,3-dichlorobenzene (0.226 g, 1 mmol) and
ethyl thiophene-3-carboxylate (0.312 g, 2 mmol), 10b was
4-Methyl-2-(2-trifluoromethylphenyl)thiophene (4b): After obtained in 65% (0.196 g) yield as an oil after column chroma-
column chromatography (pentane/diethyl ether 90:10), a mix- tography (pentane/diethyl ether 98:2). 1H NMR (400 MHz,
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Beilstein J. Org. Chem. 2014, 10, 1239–1245.
CDCl3) δ 8.16 (s, 1H), 7.37 (d, J = 7.5 Hz, 2H), 7.34 (s, 1H), 2H), 7.63 (d, J = 7.5 Hz, 2H), 7.34 (d, J = 7.5 Hz, 2H), 7.19 (t, J
7.22 (t, J = 7.5 Hz, 1H), 4.29 (q, J = 7.5 Hz, 2H), 1.31 (t, J = 7.5 = 7.5 Hz, 1H), 6.81 (s, 1H), 2.36 (s, 3H); 13C NMR (50 MHz,
Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 162.6, 137.3, 136.6, CDCl3) δ 191.6, 140.6, 137.9, 136.4, 136.1, 134.8, 134.6,
133.8, 133.7, 131.8, 130.3, 129.3, 128.2, 60.8, 14.3; Anal. calcd 133.2, 132.3, 130.0, 129.1, 128.2, 15.5; Anal. calcd for
for C13H10Cl2O2S (301.19): C, 51.84; H, 3.35; found: C, 51.99; C18H12Cl2OS (347.26): C, 62.26; H, 3.48; found: C, 62.09; H,
H, 3.17. Traces of 10a and 10c were also detected by GC–MS 3.50.
analysis of the crude mixture.
1-{3-[5-(2,6-Dichlorophenyl)-3-methylthiophen-2-
1-[5-(2,6-Dichlorophenyl)thiophen-3-yl]ethanone (11b): yl]phenyl}ethanone (15): From 3-bromoacetophenone
From 2-bromo-1,3-dichlorobenzene (0.226 g, 1 mmol) and (0.199 g, 1 mmol) and 2-(2,6-dichlorophenyl)-4-methylthio-
1-thiophen-3-ylethanone (0.252 g, 2 mmol), 11b was obtained phene (8b, 0.486 g, 2 mmol), 15 was obtained in 85% (0.307 g)
in 57% (0.154 g) yield as an oil after column chromatography yield as a white solid after column chromatography pentane/
(pentane/diethyl ether 95:5). 1H NMR (400 MHz, CDCl3) δ diethyl ether 85:15). 1H NMR (400 MHz, CDCl3) δ 8.04 (s,
8.10 (s, 1H), 7.37 (d, J = 7.5 Hz, 2H), 7.34 (s, 1H), 7.22 (t, J = 1H), 7.84 (d, J = 7.5 Hz, 1H), 7.65 (d, J = 7.5 Hz, 1H), 7.45 (t, J
7.5 Hz, 1H), 2.49 (s, 3H); 13C NMR (50 MHz, CDCl3) δ 191.0, = 7.5 Hz, 1H), 7.34 (d, J = 7.5 Hz, 2H), 7.17 (t, J = 7.5 Hz, 1H),
141.3, 136.7, 135.4, 132.5, 130.6, 129.4, 127.4, 127.2, 26.4; 6.79 (s, 1H), 2.58 (s, 3H), 2.31 (s, 3H); 13C NMR (50 MHz,
Anal. calcd (%) for C12H8Cl2OS (271.16): C, 53.15; H, 2.97; CDCl3) δ 196.8, 137.1, 136.4, 135.5, 134.0, 133.9, 132.5,
found: C, 53.31; H, 3.07. Traces of 11c were also detected by 132.4, 131.7, 131.6, 128.8, 127.9, 127.8, 127.1, 126.1, 25.7,
GC–MS analysis of the crude mixture.
14.1; Anal. calcd for C19H14Cl2OS (361.29): C, 63.19; H, 3.91;
found: C, 63.04; H, 3.99.
Ethyl [5-(2,6-dichlorophenyl)thiophen-3-yl]acetate (12b):
From 2-bromo-1,3-dichlorobenzene (0.226 g, 1 mmol) and 2-[5-(2,6-Dichlorophenyl)-3-methylthiophen-2-yl]benzoni-
ethyl thiophen-3-ylacetate (0.340 g, 2 mmol), 12b was obtained trile (16): From 2-bromobenzonitrile (0.182 g, 1 mmol) and
in 62% (0.195 g) yield as an oil after column chromatography 2-(2,6-dichlorophenyl)-4-methylthiophene (8b, 0.486 g,
(pentane/diethyl ether 95:5). 1H NMR (400 MHz, CDCl3) δ 2 mmol), 16 was obtained in 88% (0.303 g) yield as an oil after
7.32 (d, J = 7.5 Hz, 2H), 7.22 (s, 1H), 7.16 (t, J = 7.5 Hz, 1H), column chromatography (pentane/diethyl ether 95:5). 1H NMR
6.89 (s, 1H), 4.12 (q, J = 7.5 Hz, 2H), 3.60 (s, 2H), 1.19 (t, J = (400 MHz, CDCl3) δ 7.77 (d, J = 7.8 Hz, 1H), 7.63 (t, J = 7.8
7.5 Hz, 3H); 13C NMR (50 MHz, CDCl3) δ 170.9, 136.7, 136.6, Hz, 1H), 7.57 (d, J = 7.8 Hz, 1H), 7.46 (t, J = 7.8 Hz, 1H), 7.40
133.5, 132.8, 130.2, 129.9, 128.1, 124.2, 60.9, 36.2, 14.2; Anal. (d, J = 7.8 Hz, 2H), 7.24 (t, J = 7.8 Hz, 1H), 6.88 (s, 1H), 2.27
calcd for C14H12Cl2O2S (315.22): C, 53.34; H, 3.84; found: C, (s, 3H); 13C NMR (50 MHz, CDCl3) δ 136.9, 135.7, 135.6,
53.21; H, 3.70. Traces of 12a and 12c were also detected by 135.1, 133.5, 132.4, 131.4, 131.3, 130.9, 130.8, 128.9, 127.2,
GC–MS analysis of the crude mixture.
127.1, 117.0, 112.8, 13.9; Anal. calcd for C18H11Cl2NS
(344.26): C, 62.80; H, 3.22; found: C, 63.01; H, 3.40.
4-[5-(2,6-Dichlorophenyl)-3-methylthiophen-2-yl]benzoni-
trile (13): From 4-bromobenzonitrile (0.182 g, 1 mmol) and Acknowledgements
2-(2,6-dichlorophenyl)-4-methylthiophene (8b, 0.486 g, This research was supported by a CEFIPRA fellowship. We
2 mmol), 13 was obtained in 82% (0.282 g) yield as an oil after thank the Centre National de la Recherche Scientifique and
column chromatography (pentane/diethyl ether 95:5). 1H NMR “Rennes Metropole” for providing financial support.
(400 MHz, CDCl3) δ 7.63 (d, J = 7.5 Hz, 2H), 7.56 (d, J = 7.5
References
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