Palladium-catalysed direct arylations of heteroaromatics bearing dicyanovinyls at C2

Article abstract of DOI:10.1016/j.tetlet.2012.10.008

The palladium-catalysed direct 5-arylation of heteroaromatics bearing dicyanovinyls at C2 with aryl bromides via C-H bond functionalisation allows the synthesis of a variety of new conjugated systems in only one step. The nature of the solvent was found t

Full text of DOI:10.1016/j.tetlet.2012.10.008

Tetrahedron Letters 53 (2012) 6801–6805
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Tetrahedron Letters
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Palladium-catalysed direct arylations of heteroaromatics bearing dicyanovinyls
at C2
Younes Bouazizi a,b, Kassem Beydoun a, Anis Romdhane b, Hichem Ben Jannet b, Henri Doucet a,
⇑
a Institut Sciences Chimiques de Rennes, UMR 6226 CNRS, Université de Rennes, ‘Catalyse et Organometalliques’, Campus de Beaulieu, 35042 Rennes, France
b Laboratoire de Chimie Hétérocyclique, Produits Naturels et Réactivité (LR11ES39), Equipe: Chimie Bioorganique et Produits Naturels, Faculté des Sciences de Monastir, 5000 Monastir,
Tunisia
a r t i c l e i n f o
a b s t r a c t
Article history:
The palladium-catalysed direct 5-arylation of heteroaromatics bearing dicyanovinyls at C2 with aryl bro-
mides via C–H bond functionalisation allows the synthesis of a variety of new conjugated systems in only
one step. The nature of the solvent was found to be crucial to perform such arylations. In DMAc, complete
decomposition of the dicyanovinyl derivatives was observed, whereas they were found to be stable in
cyclopentyl methyl ether.
Received 2 August 2012
Revised 28 September 2012
Accepted 2 October 2012
Available online 10 October 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Heteroaromatics
Aryl bromides
C–H activation
Catalysis
Palladium
Mixed bi(hetero)aryl derivatives display a set of bioactive or
physical properties and their preparation constitutes an active field
of research in organic chemistry. Among them, some bi(hetero)aryls
bearing dicyanovinyls on the heteroaromatic moiety have been
found to display useful optical properties.1 In most cases the bi(het-
ero)aryl motif was prepared by palladium-catalysed cross-coupling
reactions between aryl halides and heteroarene derivatives such as
In recent years, several interesting results for the palladium-
catalysed coupling of aryl halides with heteroaromatic derivatives
via C–H bond functionalisation have been reported and provide
attractive procedures for the preparation of arylated heteroaro-
matics.5–9 These coupling reactions provide only HX associated to
a base as the by-product and therefore are very interesting both
in terms of atom-economy and inert wastes. However, if the palla-
dium-catalysed direct arylation of heteroaromatics bearing acetyl,
formyl, nitrile, silyl, ester, methylalcohol, amide or even amino as
the functional groups has been described,7–10 on the other hand,
a
5-formylthiophene-2-boronic acid. Then, the Knoevenagel
condensation of malononitrile with the appropriate aldehyde
produces the dicyanovinyl derivatives (Scheme 1, top).1–3
An example of the synthesis of organic semiconductors via Stille
coupling of 5-bromo-2-dicyanovinylthiophene with 2,7-di(tribu-
tyltin)-9,9-bis-n-hexylfluorene has also been described by Lui
and co-workers.4
However, these methods are not very convenient for three rea-
sons; (i) organometallic derivatives have to be prepared for the
cross-coupling reactions; (ii) these reactions are not environmen-
tally attractive as they provide an organometallic or a salt (MX)
as the by-product; (iii) as the malononitrile is generally introduced
after the cross-coupling reaction, the introduction of various func-
tional groups on the aryl requires two steps. Therefore, to over-
come these limitations, a straightforward functionalisation of
heteroaromatics bearing dicyanovinyls is highly desirable.
Previous work
NC
NC
R
R
O
[Pd]
O
MZ or X
R
CN
CN
Y
Y
Y
Y
+
NC
CN
MZ or X
X = Cl, Br or I
MZ = ZnX, MgX, SnR'3 or B(OR')2
This work
O
R
NC
Y
[Pd]
CN
+
H
Y
R
NC
CN
Br
Y = O, S or NMe
⇑
Corresponding author. Tel.: +33 2 23 23 63 84; fax: +33 2 23 23 69 39.
E-mail address: henri.doucet@univ-rennes1.fr (H. Doucet).
Scheme 1.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.008

6802
Y. Bouazizi et al. / Tetrahedron Letters 53 (2012) 6801–6805
NC
the use of vinyl substituents has attracted much less attention.11,12
This might be due to the possible competitive Heck reaction and
also to the moderate stability of some vinyl derivatives.13 To our
knowledge, only a few examples of the intermolecular direct aryla-
tions of such heteroaromatics have been reported. Trisubstituted
alkenes [2-(2,2-diarylvinyl)-thiophenes] have been employed by
Mori and co-workers to prepare 5-arylthiophene derivatives.11
We have also recently reported the palladium-catalysed C5 aryla-
tion of furans or thiophenes bearing enal, enone or acrylate func-
tions at carbon C2.14
NC
CN
Br
H
S
[Pd]
CN
1
S
2
base
NC
+
NC
Scheme 2. Arylation of 4-bromobenzonitrile with 2-thiophen-2-ylmethylenemal-
ononitrile 1.
To our knowledge, the intermolecular direct arylation of thio-
phenes, pyrroles or furans bearing dicyanovinyl function has not
been described, although it would allow a more simple access to
such arylated heteroaromatics (Scheme 1, bottom). Therefore, the
discovery of effective conditions, for the direct coupling of such
heteroarenes with aryl halides, would be a considerable advantage.
Here, we wish to report on the palladium-catalysed reaction of
some heteroaromatics bearing dicyanovinyl function at C2 with a
set of aryl bromides.
highest yields were obtained with 4-bromoacetophenone or 3-bro-
mobenzonitrile. With these two reactants, 4 and 7 were isolated in
68% and 61% yields, respectively (Table 2, entries 2 and 5). Lower
yields were obtained in the presence of 3- or 4-(trifluoro-
methyl)bromobenzenes and also from ethyl 4-bromobenzoate
due to moderate conversions of these aryl bromides (Table 2, en-
tries 1, 3 and 4). It should be noted that in the presence of the elec-
tron-rich aryl bromide, 4-bromoanisole, no formation of the
desired coupling product was detected and the aryl bromide was
recovered unreacted.
Next, we studied the reactivity of 2-furan-2-ylmethylenemal-
ononitrile 8 (Table 3). Quite similar results than with 1 were ob-
tained. Again a regioselective arylation at C5 was observed.
However, the best yields were obtained in the presence of 4-(tri-
fluoromethyl)bromobenzenes and from ethyl 4-bromobenzoate.
With these two reactants, 10 and 11 were obtained in 70 and
72% yields, respectively (Table 3, entries 2 and 3). Even the more
congested aryl bromide, 2-bromobenzonitrile gave the desired
coupling product 14 in good yield (Table 3, entry 6).
Finally, we explored the reactivity of 2-(1-methylpyrrol-2-
ylmethylene)-malononitrile 15 (Table 4). To our knowledge, so
far no examples of preparation of 2-(1-methylpyrrol-2-ylmethyl-
ene)-malononitrile substituted at C5 by aryls have been described
in the literature. Only one example of such motif bearing a 2-thie-
nyl substituent has been reported.21 From malononitrile and a 2-
formylthienylpyrrole the condensation product was obtained by
Raposo and co-workers. Then, its properties as non linear optical
chromophore were studied.
Again, we observed a regioselective arylation at C5 of the pyr-
role derivative. However, lower yields than with 1 or 8 were ob-
tained due again to partial conversions of the aryl bromides.
From 4-(trifluoromethyl)bromobenzene, 16 was isolated in 50%
yield (Table 4, entry 1). On the other hand, 2-(trifluoromethyl)bro-
mobenzene, 3,5-bis(trifluoromethyl)bromobenzene or 3- or 4-bro-
mobenzonitriles gave 17–20 in moderate yields due to partial
conversions of the aryl bromides (Table 4, entries 2–5). With this
heteroarene, the best yield of 58% was obtained from 3-(trifluoro-
methyl)-4-nitrobromobenzene (Table 4, entry 6).
For this study, we initially employed 0.5 mol % of Pd(OAc)2 as the
catalyst, KOAc as the base and DMAc as the solvent at 110 °C for the
coupling of 2-thiophen-2-ylmethylenemalononitrile 1 with 4-bro-
mobenzonitrile, as we previously observed that these conditions
are generally effective for the direct arylation of several heteroaro-
matics.14,15 However, the use of these conditions resulted in the
decomposition of the dicyanovinyl derivative 1 and no trace of
the target product 2 was detected (Table 1, entry 1). Similar results
were obtained at 70 °C using 0.5 mol % PdCl(C3H5)(dppb) as the cat-
alyst (Table 1, entry 3). On the other hand, the use of cyclopentyl
methyl ether (CPME) as the solvent for this reaction, using
1 mol % PdCl(C3H5)(dppb) as the palladium source at 125 °C, affor-
ded selectively the desired C5 arylated product 2 in 70% conversion
and 57% yield (Table 1, entry 5). No formation of other regioisomers
or diarylated thiophenes was detected. This selectivity is consistent
with a ‘concerted metallation deprotonation’ mechanism.16 We had
previously observed that the use of CPME as the solvent allows the
palladium-catalysed direct arylations with some quite unstable
heteroaromatic derivatives, including dithienylperfluorocyclopent-
enes, in high yields.17 It should be noted that this solvent presents
several advantageous features such as limited miscibility in
water and low formation of peroxides. Moreover CPME can be
manufactured by the addition of MeOH to cyclopentene. This
process produces no apparent waste.18 We also performed reac-
tions in CPME or xylene using 1 mol % Pd(OAc)2 as the catalyst;
however, low yields in 2 were obtained (Table 1, entries 6 and 7)
(see Scheme 2).
Then, we examined the coupling of 2-thiophen-2-ylmethylene-
malononitrile 1 with a variety of aryl bromides (Table 2). The
In summary, we report here that a range of heteroaromatics
bearing a dicyanovinyl at C2 undergoes palladium-catalysed cou-
pling via C–H bond activation/functionalisation reaction at C5
with electron-deficient aryl bromides. It should be noted that this
protocol, which employs a moderate loading of an air stable cata-
lyst and a cheap base, is compatible to a range of functions,
including reactive ones, such as acetyl, ester, nitrile or nitro on
the aryl bromide. Such functional group tolerance allows the
modification of the electronic structure of such derivatives. For
all these reactions, CPME 99+%, which can be considered as a
‘greener’ solvents than DMAc or DMF, which are generally
employed for palladium-catalysed direct arylations, was used
without any purification. The major by-products of these cou-
plings are KBr/AcOH instead of metallic salts with more classical
coupling procedures. For these reasons, this process should give
a more economically viable and environmentally attractive access
to these products.
Table 1
Influence of the reaction conditions for palladium-catalysed direct arylation of 4-
bromobenzonitrile with 2-thiophen-2-ylmethylenemalononitrile 1 (Scheme 2)19,20
Entry Catalyst (mol %)
Solvent Base
Temp.
(°C)
Yield in 2
(%)
1
2
3
Pd(OAc)2 (0.5)
Pd(OAc)2 (0.5)
PdCl(C3H5)(dppb)
(0.5)
PdCl(C3H5)(dppb) (1)
PdCl(C3H5)(dppb) (1)
Pd(OAc)2 (1)
DMAc
DMAc
DMAc
KOAc
K2CO3 70
KOAc
110
0
0
0
70
4
5
6
7
CPME
CPME
CPME
Xylene
KOAc
KOAc
KOAc
KOAc
125
125
125
100
30
70 (57)a
20b
Pd(OAc)2 (1)
6b
Conditions: 4-bromobenzonitrile (1 equiv), 2-thiophen-2-ylmethylenemalononit-
rile 1 (2 equiv), base (2 equiv) under argon, 12 h.
a
36 h.
20 h.
b

Y. Bouazizi et al. / Tetrahedron Letters 53 (2012) 6801–6805
6803
Table 2
Palladium-catalysed 5-arylation of aryl bromides with 2-thiophen-2-ylmethylenemalononitrile 119,20
NC
NC
PdCl(C3H5)(dppb)
1 mol%
R
CN
H
S
R
CN
1
+
S
2-7
KOAc, CPME,
125 °C, 24-36 h
Br
Entry
1
Product
Entry
4
Product
NC
NC
NC
NC
F3C
CN
CN
CN
S
S
F3C
3 53%
6 48%
NC
CN
S
S
2
3
5
O
7 61%
4 68%
NC
CN
S
EtO2C
5 55%
Conditions: PdCl(C3H5)(dppb) (0.01 mmol), aryl bromide (1 mmol), 2-thiophen-2-ylmethylenemalononitrile 1 (2 mmol), KOAc (2 mmol), CPME (6 mL), 24–36 h, 125 °C,
isolated yields.
Table 3
Palladium-catalysed 5-arylation of aryl bromides with 2-furan-2-ylmethylenemalononitrile 819,20
NC
NC
PdCl(C3H5)(dppb)
1 mol%
R
CN
H
O
R
CN
8
+
O
9-14
KOAc, CPME,
125 °C, 24-36 h
Br
Entry
1
Product
Entry
Product
NC
NC
NC
NC
F3C
CN
CN
CN
O
O
4
5
6
NC
9 45%
12 66%
NC
CN
O
O
2
3
F3C
10 70%
13 42%
NC
NC
CN
CN
CN
O
O
EtO2C
14 59%
11 72%
Conditions: PdCl(C3H5)(dppb) (0.01 mmol), aryl bromide (1 mmol), 2-furan-2-ylmethylenemalononitrile 8 (2 mmol), KOAc (2 mmol), CPME (6 mL), 24–36 h, 125 °C, isolated
yields.

6804
Y. Bouazizi et al. / Tetrahedron Letters 53 (2012) 6801–6805
Table 4
Palladium-catalysed 5-arylation of aryl bromides with 2-(1-methylpyrrol-2-ylmethylene)-malononitrile 1519,20
NC
NC
PdCl(C3H5)(dppb)
R
CN
H
1 mol%
N
R
CN
15
N
KOAc, CPME,
125 °C, 24-36 h
+
16-21
Br
Entry
Product
Entry
Product
NC
NC
NC
F3C
CN
CN
CN
N
N
1
4
F3C
16 50%
F3C
CF3
19 41%
NC
CN
N
N
2
3
5
6
NC
NC
17 37%
20 40%
NC
NC
F3C
CN
CN
N
N
O2N
18 33%
21 58%
Conditions: PdCl(C3H5)(dppb) (0.01 mmol), aryl bromide (1 mmol), 2-(1-methylpyrrol-2-ylmethylene)-malononitrile 15 (2 mmol), KOAc (2 mmol), CPME (6 mL), 24–36 h,
125 °C, isolated yields.
5. Ohta, A.; Akita, Y.; Ohkuwa, T.; Chiba, M.; Fukunaga, R.; Miyafuji, A.; Nakata, T.;
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Supplementary data
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K. J. Org. Chem. 2010, 75, 1047–1060; (f) Derridj, F.; Roger, J.; Djebbar, S.;
Doucet, H. Org. Lett. 2010, 12, 4320–4323; (g) Dong, J. J.; Roger, J.; Verrier, C.;
Martin, T.; Le Goff, R.; Hoarau, C.; Doucet, H. Green Chem. 2010, 12, 2053–2063;
(h) Shibahara, F.; Yamaguchi, E.; Murai, T. Chem. Commun. 2010, 46, 2471–
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Commun. 2011, 47, 1872–1874.
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Parisien, M.; Valette, D.; Fagnou, K. J. Org. Chem. 2005, 70, 7578–7584; (b)
Beccalli, E. M.; Broggini, G.; Martinelli, M.; Sottocornola, S. Synthesis 2008, 136–
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.10.
008.
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the aryl bromide (1 mmol), heteroaromatic 1, 8 or 15 (2 mmol), KOAc (0.196 g,
2 mmol) and PdCl(C3H5)(dppb)22 (0.006 g, 0.01 mmol) were dissolved in CPME
(6 mL) under an argon atmosphere. The reaction mixture was stirred at 125 °C
for 24-36 h. Then, the solvent was evaporated and the product was purified by
silica gel column chromatography.
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(400 MHz, CDCl3): d 7.86 (s, 1H), 7.82 (d, J = 8.5 Hz, 2H), 7.79 (d, J = 4.3 Hz, 1H),
7.77 (d, J = 8.5 Hz, 2H), 7.55 (d, J = 4.3 Hz, 1H). 13C NMR (75 MHz, CDCl3): d
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19. General procedure for the direct arylation of
2-(thiophen-2-
ylmethylene)malononitrile 1, 2-(furan-2-ylmethylene)malononitrile 8 and 2-
((1-methylpyrrol-2-yl)methylene)malononitrile 15: In a typical experiment,