Aversatile palladium/triphosphane system for direct arylation of heteroarenes with chloroarenes at low catalyst loading

Article abstract of DOI:10.1002/anie.201002987

Put a ring on it: The use of an air-stable, robust palladium/tridentate phosphane catalyst in direct C-H and C-Cl activation reactions is reported (see scheme; DMAc=N,N-dimethylacetamide, TBAB=tetra-n-butylammonium bromide). Electron-rich, electron-poor, and polysubstituted furans (X=O), thiophenes (X=S), pyrroles (X=NR5), and thiazoles were arylated with chloroarenes in the presence of the catalyst.

Full text of DOI:10.1002/anie.201002987

Communications
DOI: 10.1002/anie.201002987
À
C H Activation
AVersatile Palladium/Triphosphane System for Direct Arylation of
Heteroarenes with Chloroarenes at Low Catalyst Loading**
David Roy, Sophal Mom, Matthieu Beaupꢀrin, Henri Doucet,* and Jean-Cyrille Hierso*
In memory of Keith Fagnou and Pascal Le Floch
Palladium-catalyzed cross-coupling reactions between
organometallic nucleophilic reagents and electrophilic
organic halides or pseudohalides emerged as powerful
dentate electron-rich catalysts,[15] and a catalytic system based
on a chelating diphosphane[16] have achieved a limited
number of intermolecular couplings[17] between mostly unsub-
stituted or electron-deficient aryl chlorides and heteroaro-
matic compounds. Despite this remarkable progress, more
sustainable catalytic systems, employing significantly less
palladium/ligand catalyst, for the coupling of a wide array
of diversely substituted aryl chlorides to heteroaromatic
compounds have not yet been reported. Herein, we disclose a
new air-stable, moisture and temperature tolerant palladium/
triphosphane system that is highly efficient for the direct
arylation of substituted furan, pyrrole, thiophene, and thi-
azole substrates. Notably, these findings represent an eco-
nomically attractive direct arylation of hetero- and dihetero-
aromatic substrates with chloroarenes by using less than
1 mol% of the palladium/ligand catalyst. This versatile
system highlights also the potential of tridentate ferrocenyl
polyphosphane ligands as air-stable, easy to handle auxiliaries
[1–2]
À
synthetic tools for the construction of C C bonds.
Such
catalytic coupling processes are applied to a wide array of
fields, which range from biological sciences to materials
chemistry.[3–4] Their applications to heteroaromatic substrates
set the stage for convergent synthetic routes to valuable
substituted heterocyclic structures.[5] Because of the necessity
of limiting costly and contaminant metallic reagents, the
research focus has shifted to the direct arylation of hetero-
À
À
aromatic substrates by the combined C H/C X activation
(X = halide or pseudohalide).[6] This type of methodology
presents the advantage of circumventing the preparation of
organometallic nucleophilic reagents. It also avoids stoichio-
metric formation of metallic side products, from which
undesired contamination could be appalling for pharmaceut-
ical, agrochemical, and related biological applications.
À
À
Reports of palladium-catalyzed direct arylations of
heteroaromatics have described the use of organic bro-
mides,[7] iodides,[8] triflates,[9] mesylates and tosylates,[10]
sulfamates and phosphates,[11] iodonium salts,[12] and potas-
sium trifluoroborates[13] as useful reagents. Organic chlorides
remained noticeably uncommon partners,[14] despite the fact
that among halides and pseudohalides, chlorides are arguably
the most useful single class of substrates because of their
straightforward access, their lower cost, and the wider
diversity of available compounds. However, chloroarenes
are most often unreactive under the conditions employed to
couple other more reactive starting materials. Few mono-
in demanding intermolecular C H/C Cl activations.
As a part of our program directed towards the develop-
ment of robust polydentate auxiliaries for various cross-
coupling reactions,[18] we probed various ferrocenyl polyphos-
phane ligands for direct arylation of different classes of
hetero- and diheteroaromatic compounds using demanding
bromoarenes.[19] In the course of these studies we observed
the apparent superior efficiency of ferrocenyl triphosphane
ligands over related mono-, di-, and tetraphosphanes. We
therefore focused our efforts on providing a sustainable and
efficient palladium-catalyzed method for the challenging
intermolecular direct arylation of functionalized chloroarenes
by using systems that incorporated the modified ferrocenyl
triphosphanes 1–4, and the diphosphane 5.
[*] S. Mom, M. Beaupꢀrin, Prof. Dr. J.-C. Hierso
Universitꢀ de Bourgogne, Institut de Chimie Molꢀculaire de
l’Universitꢀ de Bourgogne (ICMUB) UMR-CNRS 5260
9, avenue Alain Savary 21078 Dijon (France)
Fax: (+33)3-8039-3682
In earlier work, we determined that triphosphane 1 was
À
À
useful for demanding C H/C Br activation, but inefficient
E-mail: jean-cyrille.hierso@u-bourgogne.fr
D. Roy, Dr. H. Doucet
Universitꢀ de Rennes I, Institut Sciences Chimiques de Rennes
(ISCR) UMR-CNRS 6226
Campus de Beaulieu 35042 Rennes (France)
Fax: (+33)2-2323-6939
E-mail: henri.doucet@univ-rennes1.fr
[**] Support provided from the ANR program for Sustainable Chemistry
Development (ANR-09-CP2D-03 CAMELOT), the Rꢀgion Bourgogne
(PARI SMT8), Rennes Mꢀtropole, and the CNRS (3MIM program P4
on Heterochemistry), is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201002987.
6650
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Chemie
with electron-poor 4-chloroacetophenone.[19] Thus, as illus-
trated in Tables S1–S3 in the Supporting Information, neither
ligand 1 nor its related congeners 3–5 are as effective as
triphosphane 2 for the palladium-catalyzed direct arylation of
heteroaromatic compounds with functionalized chloroarenes.
Under the practical reaction conditions pioneered by Otha
([Pd(PPh3)4]),[14a] McClure (PdCl2/PCy3),[20] and Fagnou
(Pd(OH)2/C*),[21] deficient coupling reactions were obtained.
A catalytic system comprising Pd(OAc)2 and the constrained
ferrocenyl triphosphane ligand 2, as well as KOAc as a cheap
base and DMAc as the solvent, was optimized for in depth
studies of the coupling for a variety of heteroaromatic
compounds. Notably, the addition of tetra-n-butylammonium
bromide was found to be beneficial for the arylation of
chloroarenes; and we observed that ligand-free conditions
were totally inefficient. The scope of this procedure was
studied using various heteroaryl derivatives and chloroarenes
(Tables 1–4). The reactivity of 2-, 3-, or 4-chlorobenzonitrile
was found to be very similar for their coupling with 2-n-
butylfuran (Table 1, entries 1–3), high yields of the coupling
products 8a–8c were obtained. Excellent results were also
obtained in the presence of diversely functionalized chloro-
arenes such as 4-chloronitrobenzene, 3- or 4-chloropropio-
phenone, 4-trifluoromethylchlorobenzene, and 2-chloro-
benzaldehyde (Table 1, entries 4–10); very good yields were
obtained when only using 0.5 mol% catalyst. In some cases
0.1 mol% of catalyst allowed a fairly good conversion in
16 hours (Table 1, entries 6 and 10). Conversely, the coupling
of 4-chlorobenzoic acid methyl ester was unsatisfactory
(Table 1, entry 11).
With regards to the more challenging functionalization of
electron-poor furans, the reactivity of methyl 2-methylfuran-
3-carboxylate was examined (Table 2). With this furan
derivative, satisfactory to moderate yields of the desired
compounds 10a–10g were obtained, however, side products,
n-butyl 5-aryl-2-methylfuran-3-carboxylate derivatives, were
obtained in low yield. The highly oxygenated products 10a–
10g were isolated in 40 to 60% yield.
Table 1: Direct arylation of the electron-rich furan 6 with chloroarenes 7.[a]
Table 2: Direct arylation of electron-poor furan 9 with chloroarenes 7.[a]
Entry
7
Cat.
[mol%]
Product
Yield
[%]
Entry
1
7
Product
Yield [%]
58
1
2
7a
7b
0.5
0.5
8a
8b
83
85
7a
10a
10c
2
3
7c
7e
55
38
3
4
7c
7d
0.5
0.5
8c
8d
87
74
10e
10g
5
6
7e
7e
0.5
0.1
8e
8e
84
41
4
7g
42
7
8
7 f
0.5
0.5
8 f
72
82
[a] Same reaction conditions as described in the Table 1, with 0.5 mol%
Pd(OAc)2/2 as the catalyst.
7g
8g
Under the same optimized reactions, 2-n-alkylthiophenes
were found viable substrates for palladium-catalyzed direct
arylations.[7e,i] High yields of 5-arylation products were
obtained for the coupling of 4-chlorobenzonitrile, 4-chloro-
nitrobenzene, 4-trifluoromethylchlorobenzene, and 3- or 4-
chloropropiophenone with 2-n-butylthiophene (Table 3,
entries 1, 4, 5, 7, and 8). Conversely, compounds 12b and
12c were obtained in only low to moderate yields because of
the partial conversion of 2- or 3-chlorobenzonitrile (Table 2,
entries 2 and 3). The presence of sulfur-containing substrates
did not appear to poison the catalyst system, even at 0.5
mol% loading.
9
7h
7h
0.5
0.1
8h
8h
73
43
10
11
7i
0.5
8i
15
[a] Reaction conditions: Pd(OAc)2/ligand 2 (1:1), aryl chloride (1 mmol),
2-n-butylfuran (2 mmol), KOAc (2 mmol), DMAc 3 mL, TBAB (1 mmol),
1508C, 16 h under argon. Yields of isolated products reported (average
of two experiments). DMAc=N,N-dimethylacetamide , TBAB=tetra-n-
butylammonium bromide.
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6651

Communications
Table 3: Direct arylation of thiophenes 11 with chloroarenes 7.[a]
Table 4: Direct arylation of heteroarenes thiazole 16 and pyrrole 17 using
chloroarenes 7.[a]
Entry 11
7
Product
Yield [%]
90
1
2
11a
7a
12a
Entry Heteroarene
7
Product
Yield [%]
18a 92
1
2
16
16
7a
11a
7b
12b 17
7e
18e 83
3
4
11a
11a
7c
7d
12c
52
3
4
5
6
16
17
17
17
7 f
7a
7c
7e
18 f 62
19a 89
19c 63
19e 77
12d 80
5
6
11a
11a
7e
7e
12e
12e
63
90[b]
7
11a
11a
7 f
12 f
73
8
9
7h
12h 81
13a 69
11b 7a
11b 7b
7
8
17
17
7 f
19 f 50
19g 63
10
11
13b 25
7g
11b 7c
13c
62
[a] Same reaction conditions as described in the Table 1, with 0.5 mol%
Pd(OAc)2/2 as the catalyst.
12
13
11c
7a
14a
15a
79
52
1-methyl-2-formylpyrrole with several chloroarenes selec-
tively gives the desired coupling products 19a–19g in
excellent to fairly good yields (Table 4, entries 4–8). Elec-
tron-deficient para-, meta-, or ortho-substituted chloroarenes
displaying important functional groups can be employed.
In summary, we have reported the use of an air-stable,
easy to handle catalytic system that is efficient for the
coupling of functionalized chloroarenes to a variety of
heteroaromatic compounds at low palladium loadings. Elec-
tron-rich, electron-poor, and polysubstituted furans, thio-
phenes, pyrroles, and thiazoles were arylated by using catalyst
loadings ranging between 0.1 and 0.5 mol%. This catalytic
system tolerates important and useful functional groups,
which can be manipulated for accessing more sophisticated
heterocyclic molecules, such as formyl, nitriles, nitro, keto,
and ester groups in para, meta, or ortho positions. This scope
demonstrates the synthetic utility of this catalyst. Besides the
well-known interest of classical electron-rich monodentate
ligands, the present study highlights the usefulness of robust
tridentate ferrocenylphosphane catalytic auxiliaries in direct
11d 7a
[a] Same reaction conditions as described in the Table 1, with 0.5 mol%
Pd(OAc)2/2 as the catalyst. [b] 1 mol% Pd(OAc)2/2 as the catalyst.
We had observed that the direct 5-arylation of thiazoles
can be performed using 1 to 5 mol% of [PdCl(dppb)(C3H5)]
(dppb = bis(diphenylphosphanyl)butane) as the catalyst.[16]
The arylation of thiazoles with chloroarenes appears to be
generally easier than the direct arylation of furans or
thiophenes. As expected, the Pd(OAc)2/2 system used at 0.5
mol% loading efficiently catalyzes the coupling of 4-chlor-
obenzonitrile, or 3- or 4-chloropropiophenone with 2-n-
propylthiazole to give 18a, 18e, and 18 f, respectively, in
yields within the range of 62–92% (Table 4, entries 1–3). The
intermolecular palladium-catalyzed direct arylation of pyr-
roles using chloroarenes is of interest,[17c] and mostly intra-
molecular cyclizations[17a] or reactions using heteroaryl chlor-
ides[14a] have been reported. We observed that the coupling of
À
À
C H/C Cl activation reactions.
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In a typical experiment, the aryl chloride (1 mmol), heteroaromatic
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À
Keywords: C H activation · direct arylation · heterocycles ·
palladium · synthetic methods
.
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Communications
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