Novel palladium imidazole catalysts for Suzuki cross-coupling reactions

Article abstract of DOI:10.1016/S1381-1169(03)00447-3

Novel palladium infidazole catalytic systems for the Suzuki cross-coupling reaction have been developed from commercially available and inexpensive imidazoles and palladium sources, and exhibit high activity and no homo-coupling.

Full text of DOI:10.1016/S1381-1169(03)00447-3

Journal of Molecular Catalysis A: Chemical 206 (2003) 77–82
Novel palladium imidazole catalysts for Suzuki
cross-coupling reactions
a
b
b,∗
Christopher J. Mathews , Paul J. Smith , Tom Welton
a
Syngenta, Jealotts Hill International Research Centre, Bracknell RG42 6ET, UK
b
Department of Chemistry, Imperial College of Science Technology and Medicine, Exhibition Road, London SW7 2AY, UK
Received 28 March 2003; received in revised form 28 March 2003; accepted 10 June 2003
Abstract
Novel palladium imidazole catalytic systems for the Suzuki cross-coupling reaction have been developed from commercially
available and inexpensive imidazoles and palladium sources, and exhibit high activity and no homo-coupling.
©
2003 Elsevier B.V. All rights reserved.
Keywords: Palladium; Cross-coupling reaction; N-donor ligands; Imidazoles; Suzuki; Aqueous catalysis
1
. Introduction
of P–C bonds is notorious and can result in deac-
tivation of the catalyst as well as the scrambling
of the coupling partners with the phosphine sub-
stituents [7]. This, in addition to their sensitivity to
moisture and aerial oxidation, their high toxicity,
often laborious synthesis and loss during product
extraction, has driven research into alternatives to
phosphines. One area where little has been reported
to date is the application of N-coordinated ligands
in palladium catalysed coupling reactions. Of those
reported most involve cyclometallated palladium
complexes incorporating either the imine [8,9], amine
[10], oxime [11], or oxazoline [12] moieties. The
remainder include either chelating bi- or terdentate
N-coordinating ligands such as diazabutanes [13,14],
bis(oxazolinyl)pyrrole [15] or dipyridyl [16] ligands,
respectively.
In recent years there has been a renaissance in pal-
ladium catalysed coupling reactions. This, in part, has
been due to the development of highly active and effi-
cient catalytic systems that have enabled activation of
aryl chlorides [1], room temperature catalysis [2] and
previously inhibited coupling reactions [3]. Significant
innovations have been made in the discoveries of novel
phosphine systems and in the modification of tradi-
tional phosphines. The replacement of the commonly
used triarylphosphine ligands with sterically-hindered
and electron-rich trialkylphosphines [4], phosphites
[5], and phospha-palladacycles [6] have all given rise
to highly active systems in palladium catalysed cou-
pling reactions.
There are, however, a number of problems that
are frequently encountered in the use of phosphine
ligands in catalysis. For example, the degradation
A series of methyl palladium(II) complexes have
been reported, incorporating N-coordinated bis- and
tris-imidazole chelates that were found to be catalyt-
ically active in the Heck reaction of 4-bromoaceto-
phenone with N-butyl acrylate [17]. More recently,
an example of a mixed imidazolylidene-imidazole
∗
Corresponding author. Tel.: +44-207-594-5763;
fax: +44-207-594-5804.
E-mail address: t.welton@ic.ac.uk (T. Welton).
1
381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00447-3

7
8
C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 77–82
X
1.2 mol% (CH CN) PdCl
3 2 2
4.8 mol% alkylimidazole
+
Solvent, NaCO3
R
B(OH)2
R
Scheme 1. The Suzuki coupling reaction of a halogenoarene with tolylboronic acid.
−
1
palladium complex and its application in Sonogashira
coupling reaction has been reported [18].
Although the Suzuki cross-coupling reaction
TON h
= 239) could be achieved within 20 min
◦
at 110 C. This compares very favourably to other
N-donor ligands that have been used. For instance, un-
der similar conditions, but with a higher concentration
of catalyst, a series of diazabutadienes gave yields of
between 60 and 99% (TON h between 60 and 99)
for the closely related reaction of 4-bromotoluene and
phenylboronic acid [13].
(Scheme 1) is one of the most powerful methodolo-
gies for the generation of new carbon–carbon bonds,
particularly in the synthesis of biaryls, it suffers from
a number of disadvantages that limit its application
−
1
[19]. The traditional triarylphosphine catalysts em-
ployed in the Suzuki reaction often suffer from low
activities, poor catalyst solubility and catalyst decom-
position. Further to this, the ubiquitous presence of
homo-coupled by-products that originate from either
the halogenoarene or the arylboronic acid coupling
with itself, necessitates additional purification steps.
We chose to investigate the use of simple, commer-
cially available imidazoles as ligands for palladium
catalysts for Suzuki cross-coupling reactions.
The (CH3CN)2PdCl2/4mim (where mim
=
1-methylimidazole) catalysed Suzuki reaction could
also be performed at room temperature, affording a
70.2% yield in 24 h. Although slower than the re-
action at higher temperatures, this can be useful for
thermally sensitive substrates. For the subsequent in-
vestigations in this project, we chose to perform the
◦
reactions at 110 C for 20 min.
Having established the reaction conditions, we con-
ducted an initial study of the effect of changing the
imidazole ligands on the reaction in dioxane (Table 2).
For comparison, the use of ‘ligand-free’ palladium
sources, the traditional phosphine based catalyst
Pd(PPh ) , and the independently prepared chloro-
2
. Results and discussion
The catalysts were prepared in situ from (CH3CN)2-
3
4
PdCl2 (1.2 mol.%) and the appropriate imidazole
4.8 mol.%) in the solvent used. After heating to give
tris(1-methylimidazole)palladium chloride {[(mim)3-
PdCl]Cl}, have been included.
(
a solution, an aqueous sodium carbonate solution,
the halogenoarene and tolylboronic acid were added
and the reaction heated to 110 C for 20 min. The
products were isolated by the addition of water and
subsequent extraction with hexane.
Imidazoles with an N–H bond did not afford any
coupled products and unreacted bromobenzene was
detected at the end of the reaction period (Table 2,
entries 3 and 5). All other imidazoles tested showed
◦
The reaction of bromobenzene with tolylboronic
acid was investigated in the first instance. This reac-
tion has the advantage that the product of the compet-
itive homo-coupling reaction (4,4 -dimethylbiphenyl)
can be readily identified in the product mixture. Initial
investigation of the reaction conditions (Table 1) using
Table 1
Investigation of Suzuki reaction conditions in dioxane
Temperature ( C) Yield (%)a
◦
−1 b
TON h
Entry Time
ꢀ
1
2
3
4
20 min 110
95.7 (94.2) 239 (236)
45.0 (47.2) 113 (118)
20 min
3 h
24 h
80
80
25
98.3 (96.7)
70.2 (73.9)
27 (27)
2 (3)
(CH3CN)2PdCl2 and 1-methylimidazole as the ligand
and dioxane as the solvent showed that the reaction
could be conducted at a wide range of temperatures.
At 80 C it afforded a modest yield of 45%, which
Results of repeat reactions in parentheses.
a
Isolated yield of 4-methylbiphenyl, based on bromobenzene.
◦
b
−1
TON h : number of moles of desired product per mole of
catalyst used per hour.
increased to 98.3% after 3 h. A similar yield (95.7%,

C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 77–82
79
Table 2
Effect of changing imidazoles in dioxane
−1 b
Homo (%)c
Entry
Catalyst/ligand
Yield (%)a
TON h
–
1
2
3
4
5
6
7
8
9
(CH3CN)2PdCl2/imidazole
(CH3CN)2PdCl2/1-methylimidazole
(CH3CN)2PdCl2/2-methylimidazole
(CH3CN)2PdCl2/1,2-dimethylimidazole
(CH3CN)2PdCl2/1,2,4,5-tetramethylimidazole
(CH3CN)2PdCl2/1-phenylimidazole
(CH3CN)2PdCl2/none
PdCl2/none
PdCl2/1-methylimidazole
[(mim)3PdCl]Cl
0 (0)
95.7 (93.7)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
0 (0)
1.9 (2.8)
3.5
239 (234)
–
95.5 (96.7)
95.9 (94.4)
63.5 (59.0)
43.2 (45.1)
54.9
94.0 (97.3)
96.5 (95.8)
28.6
239 (242)
240 (236)
159 (128)
108 (113)
137
235 (243)
241 (240)
29
0 (0)
0 (0)
5.23
1
1
0
1
Pd(PPh3)4
Results of repeat reactions in parentheses. Extensive catalyst decomposition observed during reaction, except those which did not afford
any product.
a
Isolated yields of 4-methylbiphenyl, based on bromobenzene.
b
c
−1
TON h : number of moles of desired product per mole of catalyst used per hour.
ꢀ
Homo-coupled product, 4,4 -dimethylbiphenyl, yield.
considerably greater reactivities than either the
ligand-free’ systems or the traditional Pd(PPh3)4 cat-
The (CH3CN)2PdCl2/4mim catalysed Suzuki reac-
tions of different halogenoarenes were also studied in
dioxane (Table 3). The results exhibit no appreciable
difference in yields between the activated and deacti-
vated bromo- and iodoarenes, all affording around a
95% yield. This indicates the high activity of this sys-
tem. However, only the electron-deficient, activated
4-chloroacetophenone cross-coupled to any apprecia-
ble extent, affording a 56.2% yield. Addition of the
chlorobenzene also resulted in instantaneous catalyst
decomposition of the (CH3CN)2PdCl2/4mim dioxane
‘
alyst under similar reaction conditions. Little differ-
ence was seen in the reactivity of the various multiply
methyl substituted imidazoles and 1-methylimidazole
itself (Table 2, entries 2, 4 and 5). However, a much re-
duced activity was observed for the 1-phenylimidazole
(Table 2, entry 6), so N-substitution can greatly ef-
fect the efficacy of the catalyst. We are continuing to
investigate this. Although homo-coupling was clearly
seen for the ‘ligand-free’ and Pd(PPh3)4 catalyst sys-
tems, it was not found in any of the reactions using
imidazole ligands.
One of the primary drawbacks of the Suzuki reac-
tion in synthesis is the production of a homo-coupled
by-product that can be difficult to separate from the
primary reaction product (Table 2, entries 7, 8 and 11)
Table 3
Scope of the Suzuki reaction with different halogenoarenes in
dioxane
−1 b
Entry
X
R
Yield (%)a
TON h
1
2
3
I
Br
Cl
H
H
H
95.5
93.7 (96.3)
4.9 (6.4)
239
234 (241)
12 (16)
[19]. It is, therefore, of great interest that no homo-
coupling was observed with any of the imidazole-based
catalysts, under any of the reaction conditions used.
This suggests that the catalysts formed in this sys-
tem do not catalyse the homo-coupling reaction. This
increased selectivity for the Suzuki product is an
important potential advantage of these systems.
4
5
6
Br
Br
Br
COCH3
CH3
OCH3
96.2
95.9
94.6
241
240
237
7
8
Cl
Cl
COCH3
OCH3
56.2 (54.7)
0
140 (137)
–
The [(mim)3PdCl]Cl shows very similar reactiv-
ity to the in situ generated catalyst (Table 2, entries
Repeat reactions in parentheses. Extensive catalyst decomposition
observed during reaction.
2
and 10), suggesting that such palladium imida-
a
Isolated yield of 4-methylbiphenyl, based on bromobenzene.
zole complexes are, at least, the immediate catalyst
precursor.
b
−1
TON h : number of moles of desired product per mole of
catalyst used per hour.

8
0
C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 77–82
Table 4
Investigation of Suzuki reaction solvent
imidazoles. The preliminary result of the aqueous
mediated Suzuki reaction with the water soluble,
phosphine free [(mim)3PdCl]Cl, therefore, represents
an exciting new area for aqueous Suzuki catalysis.
◦
Yield (%)a TON h
−1 b
Entry Temperature ( C) Solvent
1
2
3
4
110
110
65
Dioxane 95.7 (94.2) 239 (236)
Toluene 89.8 (92.3) 225 (231)
THF
36.8 (38.0)
92 (95)
c
3. Conclusions
100
Water
92.8 (94.1) 232 (235)
Repeat reactions in parentheses. Extensive catalyst decomposition
observed during reaction.
The novel palladium/imidazole catalyst system was
found to be active for Suzuki reactions in dioxane,
toluene and water, giving near quantative yields for
bromo- and iodoarenes irrespective of their function-
a
Isolated yield of 4-methylbiphenyl, based on bromobenzene.
TON h : number of moles of desired product per mole of
b
−1
catalyst used per hour.
c
◦
Palladium added as [(mim)3PdCl]Cl with no additional
ality. Similarly high yields were afforded at 25 C
1-methylimidazole.
for 24 h. A good yield was achieved for the acti-
vated 4-chloroacetophenone, although no significant
amount of coupling was observed for chlorobenzene.
While the reactivities demonstrated here do not com-
pete with those found for the most active systems that
have been recently developed, they have in no way
been optimised in terms of the most effective imida-
zole possible and the number of either commercially
available or simply synthetically accessible imida-
zoles is vast. Hence, the potential for generation of a
highly active catalyst is great.
solution. Extensive catalyst decomposition was also
observed at the end of each reaction. Whereas no
unreacted substrate was detected for the bromo-
and iodoarenes, in all cases unreacted chloroarene
was found. No homo-coupling was observed in any
reaction.
The reaction can also be performed in toluene with a
slight reduction in yield. In THF, a considerably lower
yield was afforded. This is in a large part due to the
◦
lower reflux temperature for THF (65 C) leading to a
A significant advantage of these catalysts is the
complete removal of homo-coupling. Hence, the
cross-coupled product could be isolated without any
further purification. Also, the catalytic systems were
readily prepared in situ from commercially available,
inexpensive and air stable imidazoles and palladium
sources, circumventing the need for synthesis of lig-
ands and/or palladium complexes. Extensive catalyst
decomposition was, however, observed in every reac-
tion and this problem needs to be addressed.
This preliminary investigation highlights the excit-
ing development of novel palladium/imidazole cat-
alytic systems, with potential applications in room
temperature and aqueous mediated Suzuki reactions.
Further investigations to improve the reactivity and to
extend the scope of these catalytic systems are cur-
rently in progress.
lower rate of reaction (cf. Table 1). Extensive catalyst
decomposition was observed in all the reactions. No
homo-coupling was observed in any case.
[(mim)3PdCl]Cl is soluble in water, leading to the
possibility of its use for aqueous Suzuki reactions.
The reaction of bromobenzene and tolylboronic acid
in water catalysed by [(mim)3PdCl]Cl gave a 92.8%
yield in 20 min (Table 4). In addition to efficient
catalyst-product separations, which are particularly
important for large-scale industrial processes, aque-
ous reaction media offer a number of economical and
environmental benefits [20]. Recently, there has been
extensive research into aqueous palladium catalysed
Suzuki reactions [21]. These systems commonly em-
ploy charged, water soluble, phosphine ligands, such
as sulfonated aryl phosphines or quaternary ammo-
nium salt derivatives of alkylphosphines. In some
cases neutral, water soluble, phosphines have been
prepared with glycosides or polymer ethylene glycol
supports. While all of these phosphines supply water
soluble catalysts, the need to prepare specialist lig-
ands adds greatly to the difficulty and expense of the
process, and limits the number of ligands that can be
used, whereas there are many commercially available
4. Experimental
4.1. Suzuki reactions with (CH CN) PdCl /4mim
3
2
2
In a typical (CH3CN)2PdCl2/4mim catalysed
Suzuki reaction in dioxane a flask was injected with

C.J. Mathews et al. / Journal of Molecular Catalysis A: Chemical 206 (2003) 77–82
81
3
(
1
CH3CN)2PdCl2 (2.36 mg (0.590 cm , 15.43 mM),
stirring for 18 h under N2. Hot filtration of the re-
sultant wine-red coloured solution through a ceolite
pad into stirred petroleum spirit (40–60 C) at room
.2 mol.%) and 1-methylimidazole (3.00 mg (0.02050
3
◦
cm , 1.784 M), 4.8 mol.%) under N2 from CH3CN
standard solutions. The CH3CN was removed in
vacuo for 0.5 h without heating affording an off-white
solid. The dioxane was injected and the mixture
heated at 110 C for 1 h with stirring, cooled to room
temperature and then stirred for a further 10 min.
Na2CO3 (161.2 mg, 1.52 mmol, 2 eq.) in deoxy-
genated water (0.80 ml), bromobenzene (119.3 mg
0.0800 cm ), 0.760 mmol, 1 eq.) and the tolylboronic
acid (113.8 mg, 0.837 mmol, 1.1 eq.) were added to
the yellow solution, which was then heated at 110 C
temperature afforded a yellow-orange solid. Recrys-
3
3
tallisation from CH3CN (200 cm ), DCM (300 cm )
3
and hexane (100 cm ) gave (CH3CN)2PdCl2 (1.78 g,
◦
65.3%) as a bright yellow powdery solid; mp
◦
129–131 C (decomp); Found: C, 18.63; H, 2.27;
N, 10.77; C4H N2Cl2Pd requires C, 18.55; H, 2.33;
6
−
1
N, 10.82%; νmax (cm ) (C≡N) 2305 vs (KBr); δH
3
6
(
(ppm) (270 MHz, d -DMSO) 2.06 (s, CH3, 3H); δC
6
(ppm) (270 MHz, d -DMSO) 118.0 (s, CH3CN, 1C)
◦
and 1.09 (s, CH3CN, 1C).
for 20 min. On completion of the reaction, the mixture
was rapidly quenched in an acetone-solid CO2 bath,
diluted with water (2 ml) and extracted with hexane
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[
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