Palladium-catalyzed C2 or C5 direct arylation of 3-formylthiophene derivatives with aryl bromides

Article abstract of DOI:10.1002/ejoc.200901213

The system Pd(OAc)₂/dppb was found to be an efficient catalyst precursor for the direct arylation of 3-formylthiophene derivatives. When using 3-formylthiophene, the 2-arylated thiophenes were obtained with regioselectivities of 76-86%, whereas the arylation of 3-formylthiophene diethyl acetal gave the 5-arylated thiophenes with regioselectivities of 6488%. These reactions were performed by using only 0.1 mol% of the catalyst. Moreover, this procedure has been found, to be tolerant to a variety of functional groups on the aryl bromide such as formyl, propionyl, benzoyl, nitrile, or nitro.

Full text of DOI:10.1002/ejoc.200901213

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200901213
Palladium-Catalyzed C2 or C5 Direct Arylation of 3-Formylthiophene
Derivatives with Aryl Bromides
Jia Jia Dong[a] and Henri Doucet*[a]
Keywords: Arylation / Homogeneous catalysis / C–H activation / Sulfur heterocycles / Palladium
The system Pd(OAc)2/dppb was found to be an efficient cata- gave the 5-arylated thiophenes with regioselectivities of 64–
lyst precursor for the direct arylation of 3-formylthiophene
derivatives. When using 3-formylthiophene, the 2-arylated
thiophenes were obtained with regioselectivities of 76–86%,
whereas the arylation of 3-formylthiophene diethyl acetal
88%. These reactions were performed by using only 0.1 mol-
% of the catalyst. Moreover, this procedure has been found
to be tolerant to a variety of functional groups on the aryl
bromide such as formyl, propionyl, benzoyl, nitrile, or nitro.
thiophenes in a 15:51 ratio. Bilodeau and co-workers exam-
ined the regioselectivity of the arylation of 3-methylthio-
phene with bromobenzene by using Pd[P(tBu)3]2 as the cat-
alyst. They obtained a mixture of the 2- and 5-phenylated
thiophenes in a 3.3:1 ratio (30% yield of 2-phenylation and
9% yield of the 5-phenylated thiophene).[7] The direct aryl-
ation of 3-methoxythiophene has been explored by
Borghese and co-workers.[8] With this reactant, 2-arylated
thiophenes were regioselectively obtained in yields of 28–
60%. In 1998, Lemaire and co-workers reported the direct
arylation of 3-formyl-, 3-cyano-, and 3-nitrothiophene with
aryl iodides.[9] In most cases, they obtained mixtures of 2-
arylated and 2,4-diarylated thiophenes together with an im-
portant amount of homocoupling of the aryl iodide. For
example, treatment of 3-formylthiophene with iodobenzene
in the presence of Pd(OAc)2 (8 mol-%), PPh3 (16 mol-%),
and K2CO3 as the base in DMF at 140 °C gave 2-phenyl-3-
formylthiophene in 35% yield.[10]
In summary, to the best of our knowledge, the regioselec-
tive C2 arylation of 3-formylthiophene derivatives via palla-
dium-catalyzed intermolecular C–H bond activation has
been reported in moderate yields by using a high catalyst
loading and only with iodobenzene. The direct C5 arylation
of 3-formylthiophene has not been described. Thus, it
would be useful to develop a simple procedure, employing
low catalyst loadings and a variety of functionalized aryl
bromides, allowing the direct arylation of 3-formylthio-
phenes to obtain either 2-aryl-3-formylthiophenes or 2-aryl-
4-formylthiophenes in a regioselective manner.
Introduction
Substituted thiophenes continue to attract the attention
of synthetic organic chemists due to their inherent bio-
logical activity. Conventional methods for the synthesis of
aryl thiophenes include metal-catalyzed cross-coupling re-
actions such as Suzuki-, Stille-, or Negishi-type reactions,[1]
which permit the coupling of aryl halides with organome-
tallic derivatives of thiophenes. Nevertheless, these pro-
cedures require the appropriate functionalization of one or
both the coupling partners, and they produce stoichiomet-
ric amounts of metallic salts as byproducts. Moreover, when
functionalized thiophene derivatives are employed, access
to the corresponding organometallic derivatives might be
tricky.
The direct coupling of functionalized thiophenes with
aryl halides by C–H bond activation/functionalization
should provide a cost-effective and environmentally attract-
ive procedure for the preparation of functionalized aryl
thiophenes. The selective C5 arylation of 2-substituted thio-
phenes by palladium-catalyzed C–H bond activation[2,3] has
been largely described in recent years.[4] The polyarylation
of 3-thiophenecarboxylic acid has been described by Miura
and co-workers.[5] On the other hand, the regiocontrolled
direct arylation of 3-substituted thiophenes has attracted
less attention.[6–10] In 2003, Sharp and co-workers reported
conditions that allow the selective regioselective arylation
of methyl 3-thiophene carboxylate.[6] The use of Pd(PPh3)4
in toluene gave selectively the 2-arylated thiophene, whereas
Pd2(dba)3 (dba = dibenzylideneacetone) in NMP (N-
methyl-2-pyrrolidone) gave a mixture of 2- and 5-arylated
Results and Discussion
[a] Institut Sciences Chimiques de Rennes, UMR 6226 CNRS –
Université de Rennes “Catalyse et Organometalliques”,
Campus de Beaulieu, 35042 Rennes, France
We initially directed our efforts towards the palladium-
catalyzed direct arylation of 3-formylthiophene with 4-bro-
mobenzonitrile (Scheme 1, Table 1). The reaction was found
to produce a mixture of 1a and 1b in a 79:21 ratio, using
Fax: +33-2-23236939
E-mail: henri.doucet@univ-rennes1.fr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200901213.
Eur. J. Org. Chem. 2010, 611–615
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
611

J. J. Dong, H. Doucet
SHORT COMMUNICATION
only 0.1 mol-% of Pd(OAc)2 as the catalyst in the presence phosphane ligands dppb [1,4-bis(diphenylphosphanyl)but-
of KOAc as the base and DMAc (N,N-dimethylacetamide) ane], dppe [1,2-bis(diphenylphosphanyl)ethane], dppf [1,1Ј-
as the solvent at 150 °C. In order to improve the regiocon- bis(diphenylphosphanyl)ferrocene], PPh3, or PCy3 (Table 1,
trol of the arylation, we performed a series of experiments Entries 12, 13, 15–18). GC and NMR analysis of the crude
by using various solvents, bases, and catalysts. Bases exhibit mixtures revealed that the reaction performed with
a considerable difference of efficiency and regioselectivity Pd(OAc)2 associated to dppb was very clean. With this cat-
for this reaction (Table 1, Entries 1–9). Carbonates or KF alyst, 1a was isolated in 57% yield (Table 1, entry 13).
led to the formation of C2-arylated thiophene 1a in lower Therefore, we selected this catalyst precursor to explore the
regioselectivities than KOAc or NaOAc. The NMR spectra scope and limitations of this reaction with the use of vari-
of the crude mixtures obtained by using KOAc were found ous aryl bromides.
to be cleaner than those obtained with the use of NaOAc.
3-Formylthiophene was coupled to a set of aryl bromides
Next, we examined the influence of the solvent. The use of (Table 2). The reactions performed with para-substituted
DMF or NMP resulted in low to moderate conversions of electron-deficient aryl bromides proceed conveniently in
4-bromobenzonitrile, and the regioselectivity of the aryl- most cases. High regioselectivities in favor of C2 arylation
ation was not improved (Table 1, Entries 10 and 11). In the were observed with the use of 4-bromobenzaldehyde, 4-bro-
presence of xylene, coupling products 1a or 1b were not mopropiophenone, 4-bromobenzophenone, or 4-bromo-
detected (Table 1, entry 14). The selectivity of the reaction nitrobenzene, resulting in yields of 55–61% for products
was also slightly dependent on the catalyst. We observed 2a–5a (Table 2, Entries 2–5). meta-Substituted aryl bro-
that [Pd(C3H5)Cl]2, in the absence of phosphane, exhibited mide, 3-bromobenzonitrile gave 6a in 58% yield (Table 2,
high regioselectivity (80%) towards product 1a (Table 1, Entry 6). A slightly lower regioselectivity in favor of C2
Entry 19). Similar regioselectivities (80–81% of 1a) were arylation was observed when ortho-substituted aryl bro-
obtained in the presence of Pd(OAc)2 associated to the mides were employed. 2-Bromobenzaldehyde and 2-bromo-
benzonitrile gave mixtures of regioisomers a/b in 76:24 and
77:23 ratios, respectively (Table 2, Entries 7 and 8). This is
certainly due to the steric hindrance of these aryl bromides.
Pyridines or quinolines are π-electron deficient heterocycles,
and therefore, their oxidative addition to palladium is, in
general, relatively easy. 3-Bromopyridine and 3-bromoquin-
olines gave regioisomers 10a and 11a in regioselectivities of
83 and 82% (Table 2, Entries 10 and 11).
Next, we examined the reactivity of 3-formylthiophene
diethyl acetal with 4-bromobenzonitrile (Scheme 1,
Table 3). We employed a similar set of reaction conditions
to those with 3-formylthiophene, and again, KOAc as the
Scheme 1.
Table 1. Arylation of 3-formylthiophene with 4-bromobenzonitrile: influence of the reaction conditions (Scheme 1).[a]
Entry
Solvent
Base
Catalyst
T [°C]
Conversion [%]
Ratio of 1a/1b
1
2
3
4
5
6
7
8
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMF
KOAc
KF
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
150
130
130
130
130
130
130
130
130
130
130
130
150
130
130
150
130
150
130
100
70
50
52
82
2
100
76
100
55
20
100
100
0
100
100
100
100
100
79:21
69:31
Cs2CO3
Na2CO3
K2CO3
K3PO4
NaOAc
KOAc
CsOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
mixture[b]
72:28
62:38
trace
79:21
78:22
71:29
75:25
9
10
11
12
13
14
15
16
17
18
19
Pd(OAc)2
Pd(OAc)2
NMP
mixture
81:19
DMAc
DMAc
xylene
DMAc
DMAc
DMAc
DMAc
DMAc
Pd(OAc)2/dppb[c]
Pd(OAc)2/dppb[c]
Pd(OAc)2/dppb[c]
Pd(OAc)2/dppe[c]
Pd(OAc)2/dppf[c]
81:19[e,f]
–
80:20
80:20[e]
81:19
[d]
Pd(OAc)2/2PPh3[d]
Pd(OAc)2/2PCy3
0.5 [Pd(C3H5)Cl]2
80:20[e]
80:20
[a] Conditions: [Pd] (0.1 mol-%), 4-bromobenzonitrile (1 mmol), 3-formylthiophene (2 mmol), base (2 mmol), solvent (3 mL), 16 h; conver-
sions and ratios of 1a/1b determined by GC and NMR spectroscopy. [b] The formation of an important amount of biphenyl-4,4Ј-
dicarbonitrile was observed. [c] 0.1 mol-% of diphosphane ligand. [d] 0.2 mol-% of PPh3. [e] 3-Formylthiophene (1.5 mmol). [f] Isolated
yield of 1a: 57%.
612
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Palladium-Catalyzed Arylation of 3-Formylthiophene Derivatives
Table 2. 2-Arylation of 3-formylthiophene with aryl bromides (Scheme 1).[a]
[a] Conditions: Pd(OAc)2 (0.1 mol-%), dppb (0.1 mol-%), aryl bromide (1 mmol), 3-formylthiophene (1.5 mmol), KOAc (2 mmol), DMAc
(3 mL), 150 °C, 16 h; isolated yields of regioisomers a.
base was found to give a good conversion of the aryl bro- predominant. The C2 carbon atom of 3-formylthiophene
mide and also a quite regioselective arylation (Table 3, En- diethyl acetal is more hindered than the C2 carbon atom of
tries 1–9). However, with this substrate, C5 arylation was 3-formylthiophene. The use of other solvents such as NMP
Table 3. Arylation of 3-formylthiophene diethyl acetal with 4-bromobenzonitrile, influence of the reaction conditions (Scheme 1).[a]
Entry
Solvent
Base
Catalyst
T [°C]
Conversion [%]
Ratio of 1a/1b
1
2
3
4
5
6
7
8
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMF
KOAc
KF
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
150
130
130
130
130
130
130
130
130
130
130
130
150
130
130
130
90
30
8
22
11
0
70
77
80
80
82
100
100
90
90
92
25:75
28:72
24:76
28:72
32:68
–
26:74
23:77
29:71
24:76
25:75
27:73
24:76[d]
25:75
24:76
25:75
Cs2CO3
Na2CO3
K2CO3
K3PO4
NaOAc
KOAc
CsOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
KOAc
9
10
11
12
13
14
15
16
Pd(OAc)2
NMP
Pd(OAc)2
DMAc
DMAc
DMAc
DMAc
DMAc
Pd(OAc)2/dppb[b]
Pd(OAc)2/dppb[b]
Pd(OAc)2/dppe[b]
[c]
Pd(OAc)2/2PPh3
0.5 [Pd(C3H5)Cl]2
[a] Conditions: [Pd] (0.1 mol-%), 4-bromobenzonitrile (1 mmol), 3-formylthiophene diethyl acetal (2 mmol), base (2 mmol), solvent
(3 mL), 16 h; conversions and ratios of 1a/1b determined by GC and NMR spectroscopy. [b] 0.1 mol-% of diphosphane ligand. [c] 0.2 mol-
% of PPh3. [d] 3-Formylthiophene diethyl acetal (1.5 mmol), isolated yield of 1b: 53%.
Eur. J. Org. Chem. 2010, 611–615
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613

J. J. Dong, H. Doucet
SHORT COMMUNICATION
Table 4. 5-Arylation of 3-formylthiophene diethyl acetal with aryl bromides (Scheme 1).[a]
[a] Conditions: 1) Pd(OAc)2 (0.1 mol-%), dppb (0.1 mol-%), aryl bromide (1 mmol), 3-formylthiophene diethyl acetal (1.5 mmol), KOAc
(2 mmol), DMAc (3 mL), 150 °C, 16 h; 2) HCl, THF, 25 °C 3 h. Isolated yields of regioisomers b.
or DMF did not improve the regioselectivity of the aryl- mylthiophenes. We have demonstrated that the protection
ation (Table 3, Entries 10 and 11). Again, the cleanest of the formyl function of 3-formylthiophene as an acetal
NMR spectra of the crude mixtures were obtained by using dramatically modifies the regioselectivity of the arylation in
Pd(OAc)2/dppb as the catalyst precursor, KOAc as the base, favor of the coupling on the C5 carbon atom. This pro-
and DMAc as the solvent at 150 °C (Table 3, Entry 13). cedure requires the use of only 0.1 mol-% of the palladium
Under these reaction conditions, the ratio of product 1a/1b catalyst and has proved to be tolerant to a variety of func-
was 24:76, and the yield of 1b was 53 %.
tional groups on the aryl bromide such as formyl, propi-
Then, we explored the arylation of 3-formylthiophene di- onyl, benzoyl, nitrile, or nitro. No prior preparation of an
ethyl acetal with several aryl bromides (Table 4). In the organometallic derivative is required for these couplings, re-
presence of aryl bromides electronically and sterically sim- ducing the number of required steps to obtain these aryl-
ilar to 4-bromobenzonitrile, such as 4-bromobenzaldehyde, ated thiophenes. Despite their interest, most of the products
4-bromobenzophenone or 4-bromopropiophenone, similar prepared by this method are new, indicating a relatively la-
regioselectivities (76–78% of 5-arylation) and yields were borious access to such compounds by using more tradi-
obtained (Table 4, Entries 2–4). In contrast, when we em- tional cross-coupling procedures. Moreover, this reaction is
ployed a sterically congested aryl bromide, such as, 2-bro- environmentally attractive, as the major byproducts are
mobenzonitrile, 2-bromobenzaldehyde, or 1-bromonaph- AcOH/KBr instead of the metallic salts encountered with
thalene, the regioselectivities of the arylations in favor of the use of more classical coupling procedures.
regioisomers 7b–9b were increased to 83–88% (Table 4, En-
tries 7–9). In the presence of 3-bromopyridine or 3-bromo-
quinoline, compounds 10b and 11b were obtained with a
regioselectivity of 64 and 70%, respectively (Table 4, En-
tries 10 and 11).
Experimental Section
General Procedure for the Reactions with 3-Formylthiophene: In a
typical experiment, the aryl bromide (1 mmol), 3-formylthiophene
(0.168 g, 1.5 mmol), and KOAc (0.196 g, 2 mmol) were introduced
in an oven-dried Schlenk tube, equipped with a magnetic stirring
Conclusion
In summary, we report herein a simple method for the
preparation of 2-aryl-3-formylthiophenes or 2-aryl-4-for- 0.001 mmol), and DMAc (3 mL) were added, and the Schlenk tube
bar. Then, Pd(OAc)2 (0.22 mg, 0.001 mmol), dppb (0.42 mg,
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Palladium-Catalyzed Arylation of 3-Formylthiophene Derivatives
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was purged several times with argon. The Schlenk tube was placed
in a preheated oil bath at 150 °C, and the reactants were allowed
to stir for 16 h. The solvent was removed by heating the reaction
vessel under vacuum, and the residue was charged directly onto a
silica gel column.
General Procedure for the Reactions with 3-Formylthiophene Diethyl
Acetal: Similar to the previous procedure, then, after cooling at
room temperature, THF (5 mL) and HCl solution (pH 2, 5 mL)
were added, and the mixture allowed to stir for 3 h. After separa-
tion and drying (MgSO4), the solvent was removed under vacuum,
and the residue was charged onto a silica gel column.
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cle): General procedure for the reactions of 3-formylthiophene or
3-formylthiophene diethyl acetal with aryl bromides and charac-
terization of 1a and 1b.
Acknowledgments
We thank the Centre National de la Recherche Scientifique and
“Rennes Metropole” for providing financial support.
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Received: October 26, 2009
Published Online: December 17, 2009
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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