An efficient palladium N-heterocyclic carbene catalyst allowing the suzuki-miyaura cross-coupling of aryl chlorides and arylboronic acids at room temperature in aqueous solution

Article abstract of DOI:10.1002/adsc.201300841

The application of a highly active Pd-NHC catalyst for the Suzuki-Miyaura cross-coupling of a broad variety of electronically activated and deactivated aryl chlorides with different arylboronic acids in excellent yields is described. This catalyst can be handled under aerobic conditions, is active with low catalyst loadings at room temperature and in an aqueous reaction medium and allows the application of cheap potassium carbonate as the base.

Full text of DOI:10.1002/adsc.201300841

UPDATES
DOI: 10.1002/adsc.201300841
An Efficient Palladium N-Heterocyclic Carbene Catalyst
Allowing the Suzuki–Miyaura Cross-Coupling of Aryl Chlorides
and Arylboronic Acids at Room Temperature in Aqueous
Solution
Fatemeh Rajabia,b,* and Werner R. Thiela,*
a
Fachbereich Chemie, TU Kaiserslautern, Erwin-Schrçdinger-Str. 54, 67663 Kaiserslautern, Germany
Fax : (+49)-631-205-4676; phone: (+49)-631-205-2752; e-mail: thiel@chemie.uni-kl.de
Department of Science, Payame Noor University, P. O. Box: 19395-4697, Tehran, Iran
b
Fax : (+98)-28-1334-4081; phone: (+98)-28-1336-6366; e-mail: f_rajabi@pnu.ac.ir
Received: September 17, 2013; Revised: February 12, 2014; Published online: April 15, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300841.
Abstract: The application of a highly active Pd-
NHC catalyst for the Suzuki–Miyaura cross-cou-
pling of a broad variety of electronically activated
and deactivated aryl chlorides with different aryl-
boronic acids in excellent yields is described. This
catalyst can be handled under aerobic conditions, is
active with low catalyst loadings at room tempera-
ture and in an aqueous reaction medium and allows
the application of cheap potassium carbonate as the
base.
pling catalysts. They stabilize and activate metal cen-
ters in the key catalytic steps of catalytic reactions
thus disfavoring palladium black formation through-
out the course of the transformation. Their strong s-
donor character facilitates the oxidative addition of
unreactive aryl halogenides and the bulkiness of cer-
tain substituents at the nitrogen atoms next to the car-
bene center enhances the reductive elimination of the
product.
Especially the groups of Glorius,[8] Beller,[9] Herr-
mann,[10] Nolan,[11] and Organ[12] have achieved signifi-
cant improvements in Suzuki–Miyaura reactions.
However most of these protocols suffer from high cat-
alyst loading, hazardous solvents, and harsh condi-
tions. Performing cross-coupling reactions in water as
the (co-)solvent should offer additional benefit in
terms of designing safer processes. This approach also
facilitates the separation of the non-polar biaryl prod-
Keywords: aqueous conditions; homogeneous catal-
ysis; N-heterocyclic carbenes; palladium; Suzuki–
Miyaura coupling
The palladium-catalyzed Suzuki–Miyaura cross-cou- ucts possessing low solubility in water.
pling reaction[1] is one of the most powerful methods
In 2012 Kçhler et al. reinvestigated the known ac-
for synthesizing biaryl motifs that are present in the celerating effect of water on Suzuki–Miyaura reac-
structure of several biologically active compounds tions in detail by comparing hydrophilic and hydro-
and in advanced materials.[2] Due to economic reasons phobic pre-catalysts and ruling out the role of the
low cost and readily available but poorly reactive aryl base and of Bu4N+ salts often applied as phase-trans-
chlorides are desirable as substrates for this coupling fer catalysts.[13] They came to the conclusion that, in
reaction. Consequently significant efforts have been aqueous systems, containing an immiscible aryl halide
devoted during the last decade to develop more reac- as an additional organic phase, the palladium(II) pre-
tive catalysts allowing the conversion of challenging catalyst will be reduced to Pd(0) in the aqueous phase
aryl chlorides.[3] A plethora of novel catalyst struc- by boronate, generated from the boronic acid and the
tures and ligand modifications have been examined base. However, the subsequent oxidative addition will
for this purpose,[4] among them, bulky and electron- take place in the organic phase requiring a phase
rich phosphines,[5] N-heterocyclic carbenes,[6] and pal- transfer of the reduced palladium species. For the fol-
ladacycles.[7] Due to their unique steric and electronic lowing transmetallation, the boronate has to be trans-
properties and their specific coordination chemistry ferred into the organic phase, which becomes acceler-
N-heterocyclic carbenes have in particular turned out ated by the presence of Bu4N+.
À
to provide highly reactive palladium based C C cou-
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Fatemeh Rajabi*[a,b] and Werner R. Thiel
UPDATES
Table 1. Cross-coupling of chlorobenzene and phenylboronic
acid catalyzed by 1 room temperature.[a]
Recently, we established a route for the synthesis of
4-(4-nitrophenyl)-1,3-bis-(2,4,6-trimethylphenyl)-4,5-
dihydro-3H-imidazolium chloride being a suitable
precursor for NHC ligands functionalized in the pe-
riphery and found that its palladium complex
1 (Scheme 1) shows fairly high catalytic activity in the
Suzuki–Miyaura cross-coupling of aryl bromides and
phenylboronic acids.[14] Structurally the palladium(II)
NHC complex 1 is closely related to so-called
PEPPSI catalysts mainly established by Organ et al.,
a class of catalysts exhibiting high activities in a series
of different coupling reactions.[10,15] In the present
contribution we show that this catalyst is also highly
active towards a broad variety of aryl chlorides.
To optimize the outcome of the transformation, an
extensive screening of the reaction conditions was car-
ried out by investigating various parameters including
catalyst concentration and the nature of the base and
solvent. The starting point for this optimization was
the use of chlorobenzene (1 mmol) and phenylboronic
acid (1.1 mmol) as the substrates, Cs2CO3 (1.2 mmol)
as base and 0.2 mol% of 1 for the evaluation of the
solvent influence (Scheme 2). In all cases, the reac-
Entry Solvent
Base
Cat. Loading Yield
A
[%]
1
2
3
4
5
6
7
8
1,4-dioxane
Cs2CO3 0.2
Cs2CO3 0.2
Cs2CO3 0.2
Cs2CO3 0.2
Cs2CO3 0.2
Cs2CO3 0.2
62
69
45
70
86
75
80
90
93
98
98
98
98
79
CH2Cl2
toluene
EtOH
i-PrOH
H2O
i-PrOH/H2O (1:2) Cs2CO3 0.2
i-PrOH/H2O (1:3) Cs2CO3 0.2
i-PrOH/H2O (1:3) Na2CO3 0.2
i-PrOH/H2O (1:3) K2CO3
i-PrOH/H2O (1:3) K2CO3
i-PrOH/H2O (1:3) K2CO3
i-PrOH/H2O (1:3) K2CO3
i-PrOH/H2O (1:3) K2CO3
9
10
11
12
13
14
0.2
0.1
0.05
0.02
0.01
[a]
Reaction conditions: 4 mL of solvent, PhCl (1 mmol),
PhB(OH)2 (1.1 mmol), base (1.2 mmol), 6 h, isolated
yields.
tions were carried out room temperature for 6 h in ubility of the base in aqueous media that will lead to
open air, and the solvents were used without any fur- generation of the aryl-transferring boronate, which
ther purification. The results are summarized in again is well-soluble in water according to the pKa
Table 1, entries 1–8.
value of phenylboronic acid.[16] While the use of pure
The results prove that there is catalytic activity al- water provides lower yields than the use of pure alco-
ready at room temperature in all solvents being inves- hols, mixtures of in particular isopropyl alcohol with
tigated. However, a clear trend related to solvent po- a high amount of water gives the best results. As
larity can be figured out: solvents of low polarity lead ruled out by Kçhler et al.,[11] biphasic mixtures of
to a decrease in performance while the use of alco- water and aryl halide intrinsically include phase-trans-
hols, water and mixtures of these solvents results in fer reactions which are ruled out by the addition of
excellent yields. This is a quite remarkable effect of a certain amount of an alcohol creating a one-phase
water. It can however be explained by the higher sol- system.
These results allowed investigation of cheap sodium
and potassium carbonate as the base, both of which
are only sparingly soluble in solvents other than
water. As expected all alkaline metal carbonates give
very good conversions, however, potassium carbonate
showed an outstanding performance (Table 1, en-
tries 8–10). The chance to avoid well soluble but ex-
pensive cesium carbonate is a clear advantage of the
aqueous reaction medium employed here. To demon-
strate the efficiency of catalyst 1, the amount of the
catalyst was reduced (Table 1, entries 10–14). The pal-
ladium loading could be decreased to 0.02 mol% and
still the cross-coupling reaction afforded a 98% isolat-
ed yield of biphenyl. However, lowering the catalyst
loading down to 0.01 mol% finally resulted in a de-
creased yield (79%) of the desired product. During
the last years, a few reports were published wherein
catalysts of comparable reactivity towards aryl chlor-
ides were described working in protic solvents. In
Scheme 1. Chemical structure of the palladium NHC com-
plex 1.
2008 Cazin et al. showed that with [PdACHTUNGRTENNUG(m-Cl)ClACHTUNGTRENNUNG(IPr)]2
[IPr=bis(2,6-diisopropylphenyl)imidazol-2-ylidene]
Scheme 2. Suzuki–Miyaura cross-coupling of aryl chlorides
and phenylboronic acid.
catalysts loadings as low as 0.1 mol% are possible.
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Adv. Synth. Catal. 2014, 356, 1873 – 1877

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An Efficient Palladium N-Heterocyclic Carbene Catalyst
Yields of more than 80 were obtained after 24 h at Miyaura cross-couplings between further aryl chlor-
room temperature with EtOH as the solvent and alk- ides and different aryl boronic acids such as 1-naph-
oxides or NaOH as the base. The catalyst is almost thylboronic and 2-thiopheneboronic acid. Both sub-
unproductive in dioxane.[17] A similar catalyst bearing strates undergo the coupling with excellent yields of
a weakly bound methylimidazole ligand giving yields the desired biaryl products (Table 3).
of 85–99% with sterically not hindered substrates in
Thiophene-based materials play an important role
a mixture of water and THF was published by Shao as components in optoelectronic technologies includ-
et al. in 2011.[18] Karimi et al. described in 2011 a solu-
ble palladium-containing coordination polymer con-
Table 3. Cross-coupling of aryl chlorides with 1-naphthylbor-
taining bifacial dicarbene ligands, which allows reduc-
onic and 2-thiopheneboronic acid catalyzed by 1 at room
tion of the catalyst loadings down to 0.05 mol% for
activated aryl chlorides.[19] After 24–30 h yields of 75–
95% could be obtained. Triazolylidene palladium
complexes reported by Fukuzawa et al. gave yields up
to 99% after 1 h in EtOH (KO-t-Bu) with catalyst
loadings of 1 mo%.[20]
temperature.[a]
Given the high reactivity of the Pd-NHC catalyst,
its utility in the cross-coupling of a series of different
activated and non-activated chlorobenzenes with phe-
nylboronic acid was examined under the conditions
optimized above (Scheme 2 and Table 2).
Chlorobenzenes bearing electron-withdrawing as
well as electron-donating substituents readily reacted
with phenylboronic acid at room temperature in
aqueous media (Table 2, entries 1–6). Often palladi-
um-catalyzed Suzuki–Miyaura coupling reactions of
aryl chlorides are inefficient as long as the substrate
does not contain an electron-withdrawing substituent.
Here this is not the case: catalyst 1 shows excellent
activity independent from the nature of the substitu-
ent and the position it is attached to the benzene ring.
The high catalytic activity of 1 under the given re-
action conditions encouraged us to explore Suzuki–
Table 2. Cross-coupling of chlorobenzenes and phenylboron-
ic acid catalyzed by 1 at room temperature.[a]
[a]
[a]
Reaction conditions: 4 mL of i-PrOH/H2O (1:3), catalyst
Reaction conditions: 4 mL of i-PrOH/H2O (1:3), catalyst
1 (0.0002 mmol), ArCl (1 mmol), PhB(OH)2 (1.1 mmol),
K2CO3 (1.2 mmol), 6 h, isolated yields.
1 (0.0002 mmol), ArCl (1 mmol), ArB(OH)2 (1.1 mmol),
K2CO3 (1.2 mmol), 6 h, isolated yields.
Adv. Synth. Catal. 2014, 356, 1873 – 1877
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Fatemeh Rajabi*[a,b] and Werner R. Thiel
UPDATES
via flash chromatography on silica gel (hexane/EtOAc, 90/
ing organic light emitting diodes (OLEDs), organic
field effect transistors (OFETs) and organic solar
cells (OSCs).[21] To the best of our knowledge, there
are only a few catalysts that effectively couple thio-
pheneboronic acids with non-activated aryl chlor-
ides.[22]
10). The yields were determined by a combination of weigh-
1
ing the product and H NMR spectroscopy (see the Support-
ing Information) to examine the purity.
Finally we compared the catalytic performance of
our system to data reported in the literature in terms
of catalytic activity by calculating the turnover fre-
quencies (TOFs, see the Supporting Information,
Table S1). Taken into account that many of the
Suzuki–Miyaura couplings with aryl chlorides were
performed at elevated temperatures – usually 80–
1008C – it becomes clear that the performance of
compound 1 is outstandingly high under the given re-
action conditions. There are only a few reports on
Suzuki–Miyaura couplings of aryl chlorides running in
neat water or aqueous solutions performed at room
temperature in the literature: M.-J. Jin et al. reported
the use of an oxaminato palladium catalyst heterogen-
ized on an inorganic support working at 508C.[23]
Here the yields are comparable but the palladium
loading is a factor of 50 higher although diffusion lim-
itation caused by the porous support has to be taken
into account.
In summary, we found excellent catalytic activities
for the Suzuki–Miyaura cross-coupling of a broad va-
riety of aryl chlorides and arylboronic acids using
a palladium catalyst bearing a functionalized NHC
ligand. The reactions were run under air in aqueous
media at room temperature and require low loadings
of the catalyst. These results represent a significant
improvement in the field of the Suzuki–Miyaura
cross-coupling. We are presently exploring the use of
the nitrophenyl moiety at the NHC ligand for cova-
lently grafting the catalyst onto an organic or inorgan-
ic support in order to investigate its performance and
fate under heterogeneous conditions. The synthesis of
the enantiomerically pure ligand is underway.
Acknowledgements
The authors would like to thank the Alexander von Hum-
boldt-Foundation for the donation of a research fellowship to
F.R.
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