Diethyl carbonate as a solvent for ruthenium catalysed C-H bond functionalisation

Article abstract of DOI:10.1039/b913115a

The ruthenium catalysed direct functionalisation of arene C-H bonds by aryl halides is reported. Reactions were performed in diethyl carbonate (DEC) instead of N-methylpyrrolidone (NMP), the solvent of choice used in most ruthenium catalysed C-H bond transformations. The use of diethyl carbonate facilitates the workup procedure thus reducing the amount of waste water. The slight loss of activity due to the use of diethyl carbonate is counterbalanced by the improvement of the catalyst efficiency achieved by a judicious choice of additives. Several arenes containing an N-heterocycle as a directing group have been diarylated. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b913115a

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www.rsc.org/greenchem | Green Chemistry
Diethyl carbonate as a solvent for ruthenium catalysed C–H bond
functionalisation†
Percia Arockiam, Valentin Poirier, Ce´dric Fischmeister,* Christian Bruneau and Pierre H. Dixneuf*
Received 2nd July 2009, Accepted 11th August 2009
First published as an Advance Article on the web 8th September 2009
DOI: 10.1039/b913115a
The ruthenium catalysed direct functionalisation of arene C–H bonds by aryl halides is reported.
Reactions were performed in diethyl carbonate (DEC) instead of N-methylpyrrolidone (NMP),
the solvent of choice used in most ruthenium catalysed C–H bond transformations. The use of
diethyl carbonate facilitates the workup procedure thus reducing the amount of waste water. The
slight loss of activity due to the use of diethyl carbonate is counterbalanced by the improvement of
the catalyst efficiency achieved by a judicious choice of additives. Several arenes containing an
N-heterocycle as a directing group have been diarylated.
with the requirement of sustainable chemistry, catalysts with
Introduction
enhanced activity operating under milder conditions in non-
Transition metal catalysed functionalisation of C–H bonds1
toxic solvents are strongly desired. In addition, as is the case in
has received much attention over the last few years as a
most of the ruthenium catalysed directed arylations, the polar
greener alternative to conventional cross-coupling reactions.2 As
and high boiling point NMP is the solvent of choice, although
compared to cross-coupling reactions, direct arene C–H bond
catalytic C–H bond transformations were recently performed in
arylation with aryl halides does not require the synthesis of metal
toluene.5b So far, the use of more eco-friendly carbonate solvents
containing substrates (Li, B, Sn, Zn, Mg, etc.) and does not
produce by-products besides acid formation. Functionalisation
has not been reported for such transformations. Consequently,
we focused our efforts to perform C–H bond functionalisations
of (hetero)arene C–H bonds is now possible under various
in carbonate solvents which are known to be non-toxic and
catalytic conditions requiring in some cases a directing group.
more environmentally acceptable.13 Some examples of their use
Palladium3 and rhodium4 have been extensively used for the
in homogeneous catalysis were recently reported by Bo¨rner
functionalisation of sp2 C–H bonds, however recent examples
et al. for asymmetric hydrogenation and allylic substitution
have emerged using ruthenium,5 nickel,6 rhenium,7 copper,8
reactions.14 Several other catalytic transformations15 including
iron,9 iridium10 and manganese catalysts.11 A very efficient
olefin metathesis16 have also been reported in carbonate solvents.
in situ generated ruthenium catalyst based on ruthenium(II)-
acetate operating in N-methylpyrrolidinone (NMP) as solvent
and allowing the direct arylation with inexpensive aryl chloride
Here, the net benefit is not only the replacement of a solvent by
a less toxic one, but the use of diethyl carbonate also reduces
waste products by strongly facilitating the workup procedure.
partners under very short reaction times has just been reported,12
Herein we show for the first time that metal catalysed C–H bond
but as in most metal catalysed C–H bond functionalisation
functionalisation can be performed in diethyl carbonate and new
a reaction temperature as high as 120 ◦C was required to
ruthenium(II) based catalytic systems with improved efficiencies
ensure high catalytic efficiency. Thus, in order to enable the
can allow these reactions to be performed at temperatures as low
as 80 ◦C.
transformation of usually reluctant substrates and to comply
Results and discussion
UMR 6226-CNRS-Universite´ de Rennes 1 Sciences Chimiques de
Rennes. Catalyse et Organome´talliques Campus de Beaulieu, 35042
The directed C–H bond functionalisation of phenylpyridine was
selected as our model reaction as it is one of the benchmark
substrates used to evaluate the efficiency of new catalysts
(Scheme 1). In a typical experiment, phenylpyridine was reacted
Rennes cedex, France. E-mail: cedric.fischmeister@univ-rennes1.fr,
pierre.dixneuf@univ-rennes1.fr
† Electronic supplementary information (ESI) available: Experi-
mental procedures, spectroscopic and analytical data. See DOI:
10.1039/b913115a
Scheme 1 Ruthenium catalysed arylation of phenylpyridine.
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Table 1 Arylation of phenylpyridine with [RuCl2(p-cymene)]2/KOAc
With this efficient system in hand we then focused on the
nature of the carboxylate salt as it was demonstrated that
carboxylates play a crucial role in the C–H bond activation
via a ligand or base assisted proton abstraction.18 This type
of mechanism was recently invoked for palladium by Lafrance
and Fagnou3f and ruthenium5b C–H bond functionalisation. In
both cases a carboxylic acid was initially used and supposed to
in situ generate a metal–carboxylate complex. Having obtained
very good results using potassium acetate with amide additives,
we evaluated a similar procedure using potassium pivalate and
acetamide A or pivalamide C as additives. The results depicted
in Table 2 show that at 120 ◦C the catalytic system generated
in situ from [RuCl2(p-cymene)]2 and KOPiv is as efficient as the
three-components system based on [RuCl2(p-cymene)]2, KOAc
and amide additives. However in that case amide additives did
not improve the efficiency of the KOPiv system (Table 2 and
Table 1, entries 3, 6 and 8).
and various additivesa
Entry Solvent Additiveb
t/h Conv. (%)c M : Dd 1 (%)e
1
2
3
4
5
6
7
8
9
10
11
12
13
NMP
DEC
DEC
DEC
DEC
DEC
DEC
DEC
DEC
DEC
DEC
DEC
DEC
—
—
—
1
5
9
4
4
4
4
2
2
2
4
4
4
100
83
100
87
0 : 100 95
40 : 60
0 : 100 96
30 : 70
29 : 71 65
0 : 100 95
—
NMP (10 mol%)
NMP (20 mol%)
—
94
A
B
C
D
E
F
100
100
100
100
93
62
63
0 : 100
—
0 : 100 96
0 : 100 94
30 : 70 68
46 : 54
55 : 45
60 : 40
—
—
—
G
H
60
a Phenylpyridine (0.5 mmol), PhCl (1.25 mmol, 2.5 equiv.), [RuCl2(p-
cymene)]2 (2.5 mol%), KOAc (10 mol%), 2 mL of solvent, K2CO3
(3 equiv.), 120 ◦C. b 20 mol%. c Conversion determined by gas chro-
matography using tetradecane as internal standard. d Ratio monoary-
lated : diarylated (1) products determined by gas chromatography.
e Isolated yields.
The efficiency of the new ruthenium carboxylate catalysts
has been evaluated at lower temperatures using acetamide and
pivalamide as additives in 20 mol% amounts (Table 3). At
Table 2 Arylation of phenylpyridine at 120 ◦C in DEC with [RuCl2(p-
with an excess of phenyl chloride in the presence of ruthenium
acetate catalyst prepared in situ from [RuCl2(p-cymene)]2 and
KOAc.17 The efficiency of the reaction was reflected by the
phenylpyridine conversion and the ratio of mono (M) and
disubstituted (D) products (Scheme 1).
The catalytic system based on [RuCl2(p-cymene)]2/KOAc was
first tested in diethyl carbonate at 120 ◦C. We were pleased
to observe that the reaction proceeded to completion in 9 h
affording the diarylated product 1 in 96% yield (Table 1, entry 3).
Since the same reaction required only 1 h in NMP as solvent
(Table 1, entry 1), we evaluated the influence of NMP as an
additive in diethyl carbonate. Thus, the reactions were performed
with 10 and 20 mol% of NMP in diethyl carbonate resulting in
improved conversions (Table 1, entries 2, 4 and 5). Encouraged
by these results, several other amide and carbamate additives
were tested (Fig. 1).
cymene)]2/KOPiv catalysta
Entry
1
Additiveb
—
t/h
Conv. (%)c
M : Dd
1 (%)f
2
1
4
2
1
1
100
100
100
100
100
100
0 : 100
6 : 94
96
7 : 93e
0 : 100
10 : 90
10 : 90
2
3
A
C
94
—
a Phenylpyridine (0.5 mmol), PhCl (1.25 mmol, 2.5 equiv.), [RuCl2(p-
cymene)]2 (2.5 mol%), KOPiv (10 mol%), K2CO3 (3 equiv.), DEC (2 mL),
120 ◦C. b 20 mol%. c Conversion determined by gas chromatography
using tetradecane as internal standard. d Ratio monoarylated : diarylated
(1) products determined by gas chromatography. e [RuCl2(p-cymene)]2
(1 mol%), DEC (2 mL), K2CO3 (3 equiv.), additive (4 mol%), 120 ◦C.
f Isolated yields.
Table 3 Arylation of phenylpyridine in DEC at 100 ◦C and 80 ◦C with
[RuCl2(p-cymene)]2/KO2CRa
Entry T/◦C KO2CRb
Additive t/h Conv. (%)c M : Dd
1
100
KOAc
—
A
4
10
16
4
6
4
53
98
100
91
100
100
100
100
24
64
70
100
98
100
100
100
55 : 45
15 : 85
0 : 100
29 : 71
0 : 100
7 : 93
C
2
3
100
80
KOPiv
KOAc
—
A
C
—
A
C
4
4
1 : 99
1 : 99
16
16
10
20
14
18
14
14
59 : 41
46 : 54
47 : 53
0 : 100
30 : 70
0 : 100
34 : 66
20 : 80
Fig. 1 Additives tested in the [RuCl2(p-cymene)]2/KOAc catalysed
arylation of phenylpyridine.
This screening unambiguously showed the superiority of
amides as additives. In particular acetamide A and pivalamide
C allowed complete diarylation in 4 h and 2 h leading to 95 and
96% isolated yield of 1, respectively, i.e. in a short reaction time
considering the use of phenyl chloride (Table 1, entries 6 and 8).
N-Methyl acetamide D also increased the catalyst performances
but not to a sufficient extent with regard to its toxicity and was
therefore discarded (Table 1, entry 9). These results show that
amides act as catalyst partners in diethyl carbonate.
4
80
KOPiv
—
A
C
a Phenylpyridine (0.5 mmol), PhCl (1.25 mmol, 2.5 equiv.), [RuCl2(p-
cymene)]2 (2.5 mol%), KOAc or KOPiv (10 mol%), K2CO3 (3 equiv.),
DEC (2 mL). b 20 mol%. c Conversion determined by gas chro-
matography using tetradecane as internal standard. d Ratio monoary-
lated : diarylated (1) products determined by gas chromatography.
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Table 4 [RuCl2(p-cymene)]2/KOPiv catalysed arylation of phenylpyridine in DECa
Entry
1
Ar–X
T/◦C
t/h
C (%)b
Product
M : Dc
Y (%)d
97
100
80
16
61
100
100
1 : 99
1 : 99
2
120
24
100
8 : 92
90
3
4
100
80
24
61
100
100
1 : 99
1 : 99
92
—
100
16
100
100
0 : 100
82
80
5
120
100
24
16
4 : 96
20 : 80
6
7
8
120
100
16
10
100
100
1 : 99
96
94
38e
3 : /97
120
80
16
61
100
100
8 : 92
7 : 93
a Phenylpyridine (0.5 mmol), ArCl (1.25 mmol, 2.5 equiv.), [RuCl2(p-cymene)]2 (2.5 mol%), KOPiv (10 mol%), K2CO3 (3 equiv.), DEC (2 mL).
b Conversion determined gy gas chromatography using tetradecane as internal standard. c Ratio monoarylated : diarylated (1) products determined
by gas chromatography. d Diarylated product isolated yield. e Product partially decomposed during purification on SiO2 or Al2O3.
100 ◦C, 1 was obtained quantitatively in 6 h using the catalytic
system based on KOAc/pivalamide (Table 3, entry 1) and in
only 4 h with KOPiv (Table 3, entry 2). Lowering further the
reaction temperature to 80 ◦C resulted in a loss of activity but it
is noteworthy that 1 could still be obtained in a reasonable time
of 20 h with the KOAc/pivalamide system (Table 3, entry 3)
and in 18 h with KOPiv only (Table 3, entry 4). In both cases,
the benefit of using amide additives was only observed with
the system based on acetate. Further studies are ongoing to
understand the origin of this amide effect. It can be postulated
that the steric hindrance of the pivalate ligands may prevent
the coordination of amides to the ruthenium site whereas this
coordination is not prevented by acetate ligands.
The two component system [RuCl2(p-cymene)]2 and KOPiv in
diethyl carbonate was selected to further extend the scope of the
reaction to various aryl halide substrates. As depicted in Table 4,
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Table 5 Arylation of N-containing heteroaromaticsa
Entry
1
Substrate
PhX
T/◦C
t/h
Conv. (%)b
Product
M : Dc
Y (%)d
X = Cl
X = Br
X = Br
120
120
120
24
24
68
52
78
94
—
—
—
70
2
3
X = Cl
X = Cl
120
80
10
24
100
100
4 : 96
7 : 93
91
89
X = Cl
X = Cl
120
100
16
61
100
43
0 : 100
1 : 99
96
a Heteroaromatic derivative (0.5 mmol), PhCl (1.25 mmol, 2.5 equiv.), [RuCl2(p-cymene)]2 (2.5 mol%), KOPiv (10 mol%), DEC (2 mL), K2CO3
(3 equiv.). b Conversion determined gy gas chromatography using tetradecane as internal standard. c Ratio Monoarylated/diarylated (1) products
determined by gas chromatography, (diarylated product isolated yield). d Diarylated product isolated yield except for entry 1.
the reaction proceeded with high efficacy with electron-
donating or electron-withdrawing substituted aryl chlorides.
With electron-donating p-substituted aryl derivatives the reac-
tion proceeded in 16 h or 24 h at 100 ◦C◦(Table 4, entries 1
and 3) and could even be performed at 80 C (Table 4, entries
1 and 3). The sterically hindered o-chlorobenzene required
higher reaction temperature to reach full conversion in 24 h
with high selectivity for the bis-arylated product 3 (Table 4,
entry 2). The transformation of aryl halides bearing electron-
withdrawing groups was also found to proceed with high efficacy.
For instance, the ester substituted derivative reacted under mild
conditions in 16 h to provide 5 in high yield (Table 4, entry 4). The
more electron-withdrawing cyano substituted derivative could
also be transformed efficiently, however this reaction required
24 h heating at 120 ◦C (Table 4, entry 5).
The ruthenium carboxylate catalysed C–H bond function-
alisation in diethyl carbonate was also applicable to het-
eroaromatic halides leading to an efficient transformation of
2-chlorothiophenes and a bromo pyridine derivative. In partic-
ular the mixed thiophene–pyridine compound 8 and the tris-
pyridine derivative 9 were obtained with high conversions at
100 ◦C, 120 ◦C and 80 ◦C (Table 4, entries 7 and 8).
of phenyl pyrazole could be achieved at 80 ◦C in 24 h providing
11 in 89% yield (Table 5, entry 2). Phenyloxazoline could also
be arylated at low temperature using phenyl chloride but higher
temperature was necessary to ensure high conversion in 16 h
providing 12 in high yield (Table 5, entry 3).
As mentioned earlier, in all the experiments presented herein,
the use of diethyl carbonate strongly facilitated the workup pro-
cedure and also contributed to the reduction of wastes. Indeed,
diethyl carbonate can easily be removed by distillation and
recycled whereas efficient removal of N-methylpyrrolidinone,
the solvent of choice for ruthenium catalysed C–H bond
functionalisations, requires numerous water washings.
Conclusion
In summary, we have presented two new catalytic systems for
sp2 C–H bond functionalisation with enhanced performances
operating in diethyl carbonate, an eco-friendly solvent. These
catalysts based on the easily available [RuCl2(p-cymene)]2 and
potassium carboxylates allow the efficient transformation of
inexpensive aryl halides at temperatures as low as 80 ◦C. De-
pending on the nature of the carboxylate used, amide additives
were shown to boost the catalyst activity to a large extent. The
activity of the [RuCl2(p-cymene)]2/KOAc system was strongly
increased by the addition of amides whereas the [RuCl2(p-
cymene)]2/KOPiv system was not influenced by the addition
of the same amides. The use of diethyl carbonate instead of
NMP also contributed to the reduction of wastes by avoiding
fastidious water extractions source of large amounts of waste.
The [RuCl2(p-cymene)]2/KOPiv catalyst was then consid-
ered for the arylation of other N-heterocyclic substrates of
interest as pharmaceutical building blocks or dyes (Table 5).
a-Benzoquinoline was found difficult to arylate and the use of
the bromide derivative was in this particular case necessary to
obtain high conversion and yield at 120 ◦C in 24 h (Table 5, en-
try 1). On the other hand, phenyl pyrazole and phenyloxazoline
were more efficiently arylated at lower temperatures. Arylation
1874 | Green Chem., 2009, 11, 1871–1875
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Int. Ed., 2009, 48, 6045. For Ru(0) C–H bond functionalisation,
Experimental
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Typical experimental procedures, spectroscopic and analytical
data are provided as supplementary data.†
Acknowledgements
The authors are grateful to the CNRS and the French Ministry
of Research, and to the European Network IDECAT (NOE
NMP3-CT-2005-011730) for support. We also thank the Centre
Regional de Mesures Physiques de l’Ouest for HRMS analysis.
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During the refereeing process of this manuscript, the ruthenium
catalyzed alkylation of arenes with alkyl halides was reported; (h) L.
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