Palladium(II) complexes with chelating biscarbene ligands in the catalytic Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1002/ejic.201200940

We studied the catalytic activity of palladium(II) complexes with chelating imidazolium and benzimidazolium ligands in the Suzuki-Miyaura cross-coupling reaction. The methylene-bridged systems with aryl substituents carrying sterically and electronically different groups (F, NO₂, OMe, H, Me, iPr) show good to excellent catalytic activities in the Suzuki-Miyaura cross-coupling reaction of aryl bromides. The p-methoxyphenyl-substituted bis(NHC)-palladium complex was the most active one under our reaction conditions, also in the context of a wide substrate scope. Several ortho- as well as para-substituted aryl bromides were coupled in excellent yields under mild reaction conditions. The catalytic activity of palladium(II) complexes with chelating bis(N-heterocyclic carbene) [bis(NHC)] ligands was tested in the Suzuki-Miyaura cross-coupling reaction. After an investigation of electronic as well as steric effects of substituents on the aromatic rings of the ligand, the most active complex was used to couple different aryl bromides under mild conditions in excellent yields. Copyright

Full text of DOI:10.1002/ejic.201200940

FULL PAPER
DOI: 10.1002/ejic.201200940
Palladium(II) Complexes with Chelating Biscarbene Ligands in the Catalytic
Suzuki–Miyaura Cross-Coupling Reaction
Maik Micksch[a] and Thomas Strassner*[a]
Keywords: Homogeneous catalysis / Carbene ligands / Palladium / Suzuki–Miyaura reaction / Cross-coupling
We studied the catalytic activity of palladium(II) complexes
with chelating imidazolium and benzimidazolium ligands in
the Suzuki–Miyaura cross-coupling reaction. The methylene-
bridged systems with aryl substituents carrying sterically and
electronically different groups (F, NO2, OMe, H, Me, iPr)
show good to excellent catalytic activities in the Suzuki–
Miyaura cross-coupling reaction of aryl bromides. The p-
methoxyphenyl-substituted bis(NHC)–palladium complex
was the most active one under our reaction conditions, also
in the context of a wide substrate scope. Several ortho- as
well as para-substituted aryl bromides were coupled in ex-
cellent yields under mild reaction conditions.
neral is usually explained by the strong σ-donating proper-
ties of the carbene ligands.[41,42] Examples are monodentate
phenyl-substituted NHC ligands,[43–47] palladium com-
plexes with pyridine coligands,[48] bis(oxazoline)-derived
NHC ligands,[49,50] and many other structures possessing an
NHC moiety.[32,51–67]
Chelating bis(NHC) complexes combine the strong do-
nating properties of the two carbenes, high steric demand,
and good stability even under harsh reaction condi-
tions.[68–73] Chelating NHC complexes also show excellent
catalytic activities in cross-coupling reactions at elevated
temperatures under air.[32,74–77] Owing to their high stability
bis(NHC)–palladium complexes have also been successfully
used in other palladium-catalyzed reactions such as the ac-
tivation of methane,[78–80] which requires oxidative condi-
tions and high reaction temperatures. In particular, N-phen-
ylbis(NHC)–palladium complexes substituted with elec-
tron-donating groups such as methoxy and butoxy groups
turned out to be very active catalysts.[81]
Introduction
Palladium-catalyzed cross-coupling reactions are one of
the most powerful carbon–carbon bond-forming pro-
cesses[1,2] and have recently been honored with the Nobel
Prize for chemistry.[3] The Suzuki–Miyaura cross-coupling
reaction has become a very important and widely used
method in fine chemical syntheses.[4–8] Examples include the
synthesis of 3-arylcoumarins,[9] 5-chloro-1-phenyltetra-
zoles,[10] isocoumarins[11] and 3(2H)-pyridazinones.[12] To
improve the catalytic efficiency in these transformations
various phosphane-based ligands have been investi-
gated,[13–17] but other ligands[18–23] or even ligand-free cata-
lytic systems[24–29] have also been published. Since Ard-
uengo[30] reported the isolation of a stable free N-heterocy-
clic carbene (NHC) in 1991, a large number of palladium–
NHC complexes has been reported. Even in-situ systems of
a palladium source [e.g., Pd(OAc)2] and imidazolium salts
show high catalytic activity in a variety of palladium-cata-
lyzed cross-coupling reactions.[31–34] Phosphane-based li-
gands are often air-sensitive and degrade at higher tempera-
tures.[35,36] Therefore, catalyst systems with these ligands
generally require the use of excess ligand.[13,37] Owing to
their high tolerance towards air and moisture, NHC ligands
were shown to be a promising alternative, although there
are several phosphane ligands (e.g., PtBu3) that reach the
same or even higher performance.[38–40]
We were therefore interested in whether electronic and
steric effects can also be observed in the Suzuki–Miyaura
cross-coupling reaction.
Results and Discussion
Preparation of Bis(NHC)–Palladium Complexes
The remarkably high catalytic activity of a large number
of palladium–NHC complexes in the Suzuki–Miyaura
cross-coupling reaction and C–C coupling reactions in ge-
To investigate electronic and steric effects of substituted
ligands, we prepared
a set of N-phenyl-substituted
bis(NHC)–palladium complexes with electron-donating,
electron-withdrawing and sterically demanding groups by
using the general synthetic route shown in Scheme 1.
Phenyl-substituted imidazoles 1–6 were prepared accord-
ing to literature procedures.[82] The reaction of 1–6 with di-
bromomethane in acetonitrile yielded the methylene-
bridged bis(imidazolium) salts 7–12 in good yields
[a] Fachrichtung Chemie und Lebensmittelchemie, Technische
Universität Dresden
Bergstr. 66, 01069 Dresden, Germany
Fax: +49-351-463-39679
E-mail: thomas.strassner@chemie.tu-dresden.de
Homepage: http://www.chm.tu-dresden.de/oc3/
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Biscarbene–Palladium(II) Complexes in the Suzuki–Miyaura Reaction
Scheme 1. Synthesis of the bis(NHC)–palladium complexes 13–18.
(Scheme 1). The bis(NHC)–palladium complexes 13–18 Table 1. Comparison of the catalytic activities of the bis(NHC)–
palladium complexes 13–20 in the Suzuki–Miyaura cross-coupling
were synthesized from the bis(imidazolium) salts with
Pd(OAc)2 in dimethyl sulfoxide (DMSO).
reaction.[a]
The investigated bis(NHC)–palladium bromide com-
plexes 13–18 include ligands with electron-donating (14) as
well as electron-withdrawing groups (15 and 16). To investi-
gate steric effects some complexes with sterically demanding
ortho-substituted phenyl groups (17 and 18) as well as non-
[Pd]
Yield[b] (after 1 h) Conv.[c] (after 24 h) Reaction time[c]
bulky ligands 19[83–85] and 20[86] were prepared (Figure 1).
[%]
[%]
[h]
13
14
15
16
17
18
19
44
95
77
63
79
83
92
95
22
35
100
100
97
100
100
98
100
24 (100)
2 (100)
48 (100)
24 (100)
24 (100)
72 (98)
5 (100)
72 (97)
20
97
Figure 1. N-Methyl-substituted bis(NHC)–palladium complexes 19
and 20.
Pd(OAc)2 + 8
Pd(OAc)2
not determined
not determined
We compared the catalytic activities of the investigated
bis(NHC)–palladium complexes 13–20 for the reaction of
4-bromotoluene with phenylboronic acid (Table 1). Prelimi-
nary experiments of the Suzuki–Miyaura cross-coupling re-
action with the investigated bis(NHC)–palladium com-
[a] Reaction conditions: aryl bromide (1 mmol), phenylboronic acid
(1.5 mmol), [Pd] (1 mol-%), K3PO4 (1.95 mmol), toluene (5 mL),
40 °C, argon. [b] GC–MS yields determined by using diethylene
glycol dibutyl ether as internal standard, average of two runs. [c]
For maximum conversion. Conversion of 4-bromotoluene was
measured in a separate run by using n-dodecane as internal stan-
plexes identified the solvent/base system toluene/K3PO4 as dard by GC–MS. Maximum conversion is given in parentheses.
superior to various combinations of other solvents (meth-
anol, ethanol, 2-propanol, dioxane, THF) and bases
(Cs2CO3, K2CO3, KOtBu, KOH, NaOMe).
dium complex 14. This is not surprising since the prepara-
With the exception of 13, all investigated bis(NHC)–pal- tion of 14 requires reaction temperatures from 60 to 130 °C,
ladium complexes achieve good to excellent yields under which makes the in-situ formation of complex 14 improb-
mild conditions within a short reaction time of 1 h. The able under the reaction conditions of the cross-coupling re-
best results were obtained with the p-methoxy-substituted action.
14 and the benzimidazole-derived complex 20, which led to
Another control experiment, the use of Pd(OAc)2 with-
yields of 95%, followed by the methyl-substituted complex out bis(imidazolium) salt led to a higher yield (35%) than
19 with a yield of 92%. After the yields for 1 h of reaction the reaction with the ligand precursor, in which the pres-
time, we examined the reaction time required for the full ence of the bis(imidazolium) salt 8 might decelerate the pre-
conversion of 4-bromotoluene. All complexes led to high activation of Pd(OAc)2.
conversions (Ͼ 96%) after 24 h; however, full conversion
In most cases, the substituents will exert an electronic as
was not observed with 18 and 20. Even longer reaction well as steric influence on the reaction. It is therefore hard
times of up to 72 h did not increase the conversion. Com- to distinguish which effect is responsible for observed ac-
plexes 14 and 19 required the shortest reaction times of 3 tivity differences or whether they influence the same step of
and 5 h, respectively. We studied the reaction rates in detail, the catalytic cycle. However, when comparing the results
and these experiments are discussed later.
for 13–16 the electronic effect should prevail (Table 1). It
We also tested the in-situ system of Pd(OAc)2 and bis- indicates that electron-donating groups show a positive ef-
(imidazolium) salt 8, but it shows a significantly lower yield fect, as the p-methoxy-substituted 14 leads to significantly
(22%) compared to the corresponding bis(NHC)–palla- better results than the p-fluoro- and p-nitro-substituted 15
Eur. J. Inorg. Chem. 2012, 5872–5880
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M. Micksch, T. Strassner
FULL PAPER
and 16, respectively. One reason might be that the electron- Table 2. Suzuki–Miyaura cross-coupling of 4-bromotoluene with
phenylboronic acid catalyzed by 14.[a]
withdrawing groups decelerate the oxidative addition of the
aryl bromide, which in turn results in lower yields.
The steric demand of the ligands certainly plays an im-
portant role in the case of the ortho-substituted complexes
17 and 18 and seems to have a negative effect as the com-
plexes with nonbulky alkyl ligands 19 and 20 are more
Entry
Temperature [°C]
Time [h]
GC yield[b] [%]
active. We suspect that the mesityl- and 2,6-diisoprop-
ylphenyl-substituted ligands 17 and 18, respectively, have
less of an electronic effect as we expect that the plane of
the phenyl ring is orthogonal and not conjugated with the
imidazole core. One way to evaluate the electron density at
the metal atom is by cyclic voltammetry experiments, which
have recently been conducted with 13, 19, and 20. It was
found that the electron density at the metal atom in 19 and
20 is lower than that in 13.[87] For 17 and 18 we need to
discuss steric and electronic effects, because one would ex-
pect a more active catalyst based on the positive inductive
effect, whereas the steric demand should lower the activity.
Based on these results it is hard to say which step of
the catalytic cycle is rate-determining or whether the rate-
determining step is the same for all catalysts. For the com-
plexes with electron-withdrawing groups (15 and 16), the
oxidative addition might be rate-determining, whereas for
more active complexes the oxidative addition seems not to
be rate-determining. A detailed investigation of the rate-
determining step for 14 is discussed later in the paper. Com-
plex 14 seems to be a good compromise between steric de-
mand and electronic effects, which leads to its high catalytic
activity.
1
2
3
4
5
6
7
8
9
111
80
60
40
40
40
40
32
r.t.
4
4
4
4
3
2
1
4
4
93[c]
98[c]
100
100
100
99
95
95
65
[a] Reaction conditions: aryl bromide (1 mmol), phenylboronic acid
(1.5 mmol), 14 (1 mol-%), K3PO4 (1.95 mmol), toluene (5 mL), ar-
gon. [b] GC–MS yields determined by using diethylene glycol di-
butyl ether as internal standard, average of two runs. [c] Formation
of palladium black.
obtained (Table 3, Entry 8). It can be concluded that 14
gives good to excellent yields (Table 3, Entries 3–8) even in
low concentrations, although an increased reaction tem-
perature and/or time might be required.
Table 3. Suzuki–Miyaura cross-coupling of 4-bromotoluene with
phenylboronic acid catalyzed by different concentrations of 14.
Optimal Reaction Temperature
Entry mol-% 14
Temperature [°C]
Time [h]
Yield [%]
1
2
3
4
5
6
7
8
1
1
40
40
40
40
40
60
40
60
2
3
2
6
2
72
2
72
99[a]
100[a]
86[b]
To optimize the reaction conditions, we first investigated
the reaction temperature (Table 2) for the previously de-
scribed reaction catalyzed by 14. Starting at room tempera-
ture up to refluxing toluene (111 °C), we found that 40 °C
seems to be the temperature of choice (Table 2, Entry 4).
Even with short reaction times (Table 2, Entries 5–7) excel-
lent yields were achieved. Temperatures higher than 60 °C
and lower than 40 °C resulted in lower yields (Table 2, En-
tries 1, 2, 8 and 9).
0.5
0.5
0.2
0.2
0.1
0.1
100[b]
84[b]
100[b]
74[b]
90[b]
[a] Reaction conditions: aryl bromide (1 mmol), phenylboronic acid
(1.5 mmol), 14 (as indicated), K3PO4 (1.95 mmol), toluene (5 mL),
argon, GC–MS yields determined by using diethylene glycol di-
butyl ether as internal standard, average of two runs. [b] Reaction
conditions: aryl bromide (2 mmol), phenylboronic acid (3 mmol),
14 (as indicated), K3PO4 (3.9 mmol), toluene (10 mL), argon, iso-
lated yields, average of two runs.
Stoichiometry of the Reactants
For comparison we had used 1 mol-% catalyst for all ex-
periments (Table 1), but were interested to see whether
We generally ran all cross-coupling reactions with a 1.5-
lower concentrations of 14 (Table 3) would also lead to full fold excess of phenylboronic acid (relative to 4-bromotolu-
conversion. With 1 mol-% a quantitative yield was achieved ene). To check whether smaller amounts of phenylboronic
after 3 h (Table 3, Entry 2), whereas it took 6 h with acid could also be used without a decrease of the reaction
0.5 mol-% of 14 (Table 3, Entry 4). The cross-coupling reac- rate, we ran the cross-coupling reaction with 0.8–2.0 equiv.
tion with 0.2 and 0.1 mol-% led to good yields of 84% and of phenylboronic acid with 1 mol-% of 14 at 40 °C. The
74%, respectively, after 2 h at 40 °C (Table 3, Entries 5 and conversions of the reactions were determined after fixed
7). To obtain a quantitative yield with 0.2 mol-% of 14 an periods of time (Figure 2a). We observed a strong depen-
increased reaction time and temperature were required dence of the reaction rate on the amount of phenylboronic
(Table 3, Entry 6). When the amount of catalyst was re- acid. Only reactions with Ն 1.3 equiv. of phenylboronic
duced to 0.1 mol-% of 14, only a good yield of 90% was acid led to full conversion of 4-bromotoluene. The reaction
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Biscarbene–Palladium(II) Complexes in the Suzuki–Miyaura Reaction
with 1.0 equiv. led to a conversion of 74% after 24 h; longer
reaction times did not increase the conversion.
Another important parameter in the reaction might be
the base used in the cross-coupling reaction. We checked
different bases and obtained the best results for K3PO4. We
therefore investigated in detail the optimal concentration of
the base and tested the cross-coupling reaction with
amounts of K3PO4 ranging from 1.7 to 2.3 equiv. (Figure 3)
and found the highest reaction rate for 1.95 equiv.
Figure 3. Conversions of 4-bromotoluene in the Suzuki–Miyaura
cross-coupling with different amount of K3PO4. Reaction condi-
tions: aryl bromide (2 mmol), phenylboronic acid (3 mmol), 14
(1 mol-%), K3PO4, n-dodecane (internal standard), toluene
(10 mL), argon.
Side Reactions
Unwanted side reactions often decrease the reaction
yield. We were, therefore, interested in whether typical side
reactions such as protodebromination,[91,92] homocoupling
of phenylboronic acid[55,93,94] and protodeboronation[94–99]
can be found for these catalysts. Therefore, we investigated
in detail 13, 14 and 18, which had shown different perform-
ances (Table 1).
Figure 2. (a) Conversion of 4-bromotoluene in the Suzuki–Miyaura
cross-coupling reaction with different amounts of phenylboronic
acid. (b) Initial reaction rates (v0) of Suzuki–Miyaura cross-cou-
pling reactions of 4-bromotoluene with different equivalents of
phenylboronic acid. Reaction conditions: aryl bromide (2 mmol),
phenylboronic acid, 14 (1 mol-%), K3PO4 (3.9 mmol), n-dodecane
(internal standard), toluene (10 mL), argon.
First, we investigated the conversion of 4-bromotoluene
in the Suzuki–Miyaura cross-coupling (Figure 4) and found
The correlation of the measured initial reaction rates (v0) that the measured aryl bromide conversions of 37 (13), 95
and the equivalents of phenylboronic acid are shown in Fig- (14), and 87% (18) were in good agreement with the ob-
ure 2b. The initial reaction rates were determined by the served product yields of 44 (13), 95 (14), and 83% (18) after
conversion after 15 min. This correlation shows a linear de- 1 h (Table 1).
pendence up to 1.5 equiv. Higher concentrations of phen-
We did not observe induction periods at the beginning
ylboronic acid did not increase the initial reaction rate. The of each reaction. Complexes 14 and 18 showed high initial
linear dependence of the initial reaction rate on the concen- reaction rates, and for the reaction with 14 no 4-bromotolu-
tration of phenylboronic acid indicates that the transmet- ene was detected in the reaction mixture after 90 min. Cer-
alation is the rate-determining step in the Suzuki–Miyaura tainly, 18 showed a lower reaction rate after 60 min, but
cross-coupling reaction with bis(NHC)–palladium complex after 24 h 98% conversion was obtained. Contrary to both
14. We postulate the transmetalation to be rate-determining other complexes, 13 gave a low, but constant, reaction rate
for the more active complexes (complexes that are not sub- from the beginning of the reaction until full conversion was
stituted with electron-withdrawing groups), which is also a reached after 24 h.
likely explanation for the negative effect of the sterically
demanding ligands (in 17 and 18) since the transmetalation cation of the absence of protodebromination in the Suzuki–
might be hindered by their steric demand (Table 1).[88–90]
Miyaura cross-coupling with the investigated bis(NHC)–
The agreement between conversions and yields is an indi-
Eur. J. Inorg. Chem. 2012, 5872–5880
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5875

M. Micksch, T. Strassner
FULL PAPER
Another possible side reaction, the homocoupling of
phenylboronic acid can be excluded as well, because no bi-
phenyl was detected in the reaction mixtures. To check for
protodeboronation reactions, we measured the conversion
of phenylboronic acid by determination of unreacted phen-
ylboronic acid, which (after an aqueous workup) was quan-
1
tified by H NMR with nitromethane as internal standard.
The conversions of the reactants and the product yields for
13, 14 and 18 are given in Table 4.
The conversions of the reactants and the product yields
are in agreement for 14 and 18, although we cannot exclude
that a small amount of phenylboronic acid is lost in the
cross-coupling reaction with 18. However, for 13 there is a
large difference between the amount of reacted 4-bromotol-
uene and phenylboronic acid. As no biphenyl could be de-
tected, the homocoupling of phenylboronic acid could be
ruled out, and protodeboronation is the most likely reason
for the loss of phenylboronic acid in the reaction mixture.
The results indicate that common side reactions of the
Suzuki–Miyaura cross-coupling do not take place in case
of the bis(NHC)–palladium complexes 14 and 18, but cer-
tainly for the less active complex 13 protodeboronation is a
likely side reaction, which might also contribute to its low
initial reaction rate.
Figure 4. Conversions of 4-bromotoluene in the Suzuki–Miyaura
cross-coupling with 13, 14 and 18. Reaction conditions: aryl brom-
ide (2 mmol), phenylboronic acid (3 mmol), [Pd] (1 mol-%), K3PO4
(3.9 mmol), n-dodecane (internal standard), toluene (10 mL), ar-
gon. Stated conversions were obtained after 60 min.
palladium complexes. To verify that no protodebromination
occurs, we ran the cross-coupling reaction with 1-bromo-
naphthalene (Scheme 2) as we would be able to detect the
corresponding product (naphthalene) by GC–MS.
Substrate Scope
To study the influence of substituents on the aryl brom-
ide we used different para-substituted aryl bromides for the
investigation of electronic effects as well as ortho-substi-
tuted aryl bromides for their steric effects (Table 5).
Complex 14 was able to catalyze the Suzuki–Miyaura
cross-coupling reactions of different aryl bromides with
electron-withdrawing as well as electron-donating groups in
the para position in excellent yields. 4-Bromotoluene
Scheme 2. Suzuki–Miyaura cross-coupling reaction of 1-bromo-
naphthalene and phenylboronic acid. Reaction conditions: aryl
bromide (1 mmol), phenylboronic acid (1.5 mmol), [Pd] (1 mol-%),
K3PO4 (1.95 mmol), toluene (5 mL), 40 °C, 2 h (for 14), 16 h (for
13 and 18), argon, isolated yield.
All complexes led to a quantitative yield of the corre- (Table 5, Entry 1), 4-bromoanisol (Table 5, Entry 2), and 4-
sponding product, and the reaction mixtures were analyzed bromoacetophenone (Table 5, Entry 3) were coupled in
by GC–MS. The quantitative yield and the observation that yields of 95–98% at 40 °C in 2 h (Table 5, Entries 1–3). Ow-
no naphthalene could be detected are evidence for the ab- ing to the low solubility of 1-bromo-4-nitrobenzene in tolu-
sence of protodebromination in the Suzuki–Miyaura cross- ene, a slightly increased reaction temperature was required
coupling with the investigated bis(NHC)–palladium com- (Table 5, Entries 5 and 6), which then afforded excellent
plexes.
yields too. For all studied aryl bromides with different para
Table 4. Comparison of the conversions of the reactants with product yields for 13, 14 and 18.
[Pd]
Conv. of 4-bromotoluene[a]
[mmol]
Conv. of phenylboronic acid[b]
[mmol]
GC yield of 4Ј-methylbiphenyl[c]
[mmol]
13
14
18
0.34
0.95
0.87
1.13
1.05
1.13
0.44
0.95
0.83
[a] Conversion of 4-bromotoluene was measured by GC–MS using n-dodecane as internal standard. [b] Conversion of phenylboronic
acid was measured by 1H NMR spectroscopy in a separate run by using nitromethane as internal standard. [c] GC–MS yields were
measured by using diethylene glycol dibutyl ether as internal standard.
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Biscarbene–Palladium(II) Complexes in the Suzuki–Miyaura Reaction
Table 5. Suzuki–Miyaura cross-coupling reactions of aryl bromides pling of chloro-substituted aryl bromides could be ac-
and phenylboronic acid.[a]
complished in good yields (Table 5, Entries 12–15). Even
the ortho-chloro-substituted aryl bromide led to the same
yield as the para-chloro-substituted aryl bromide. No side
products such as the corresponding biphenyl bromides or
terphenyls were observed.
Furthermore, we investigated the reaction parameters re-
quired for full conversion of the substrates. We were able to
obtain full conversion for all but one of the investigated aryl
bromides. para-Substituted aryl bromides generally needed
reaction times of 2–6 h for full conversion under standard
conditions. Only the sterically demanding bromomesitylene
(Table 5, Entries 10 and 11) did not reach full conversion
using our standard reaction conditions (1 mol-% 14,
1.5 equiv. of phenylboronic acid).
Conclusions
We report the high catalytic activity of bis(NHC)–palla-
dium complexes in the Suzuki–Miyaura cross-coupling re-
actions with aryl bromides. All investigated bis(NHC)–pal-
ladium complexes give good to excellent yields under mild
reaction conditions (40 °C). The p-methoxyphenyl-substi-
tuted 14, the methyl-substituted 19 and the benzimidazole-
derived 20 turned out to be the most active catalysts. Even
with low catalyst loadings, excellent yields were obtained.
We found that complexes carrying ligands substituted with
electron-donating groups led to higher yields, whereas steri-
cally demanding ligands gave lower yields in the Suzuki–
Miyaura cross-coupling of aryl bromides. Our results indi-
cate that the transmetalation is the rate-determining step in
the cross-coupling reaction, at least for the electron-rich,
sterically unhindered catalysts, which is in agreement with
a negative effect of sterically demanding ligands, because
the transmetalation reaction is hindered in these cases.
[a] Reaction conditions: aryl bromide (1 mmol), 14 (1 mol-%),
In addition, we were able to exclude common side reac-
tions like protodebromination, homocoupling of phenylbo-
ronic acid and protodeboronation for the most active com-
plex 14. Furthermore the substrate scope of the reaction of
aryl bromides catalyzed by 14 was investigated. Several
para- as well as ortho-substituted aryl bromides were cou-
pled in excellent yields. Electron-withdrawing as well as
electron-donating groups in the para position showed only
a small effect in contrast to groups in the ortho position.
For ortho-substituted and ortho,ortho-disubstituted aryl
bromides, an increased reaction temperature and time was
generally required to achieve excellent yields.
phenylboronic acid (1.5 mmol), K3PO4 (1.95 mmol), toluene
(5 mL), argon. [b] Isolated yields, average of two runs. [c] For maxi-
mum conversion, given in parentheses. Conversion of 4-bromotolu-
ene was measured in a separate run using n-dodecane as internal
standard by GC–MS. [d] Formation of palladium black.
substituents we obtained almost the same yield, so the very
active catalyst did not discriminate between the different
electronic effects of the substituents. This result is not sur-
prising as the transmetalation was found to be the rate-
determining step.
Certainly, the presence of ortho substituents at the aryl
bromide has an effect on the reaction rate. Although 4-bro-
motoluene was coupled in a 95% yield (Table 5, Entry 1),
under the same reaction conditions bromoxylene led to a
yield of 77% (Table 5, Entry 8) and bromomesitylene to a
yield of 62% (Table 5, Entry 10). This effect could also be
explained by the transmetalation being the rate-determining
step. However, high yields for these aryl bromides could be
obtained by increasing the reaction time and temperature
(Table 5, Entries 9 and 11).
Experimental Section
General Considerations: All chemicals were used as received. Tolu-
ene was distilled under argon from calcium hydride prior to use
and stored over molecular sieves. 1H, 13C and 19F NMR spectra
were obtained with a Bruker AC 300 P at 300.13, 75.45 and
282.40 MHz, respectively. Chemical shifts (δ) are given in parts per
million and are internally referenced to the solvent resonances. Ele-
mental analyses were performed by the microanalytical laboratory
The good selectivity toward bromo electrophiles allows
to distinguish between chloro and bromo substituents. Cou- of our institute with a Hekatech EA 3000 Euro Vector CHNSO
Eur. J. Inorg. Chem. 2012, 5872–5880
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
5877

M. Micksch, T. Strassner
FULL PAPER
elemental analyzer. 1-(4-Methoxyphenyl)imidazole (2),[82] 1-(4-ni-
trophenyl)imidazole (4),[82] 1-(2,4,6-trimethylphenyl)imidazole (5),
1-(2,6-diisopropylphenyl)imidazole (6),[82] imidazolium salts 8,[100]
10,[100] 12,[101] and complexes 13[81,102] 14,[81] 17,[83] 19,[83–85] and
20[86] were prepared according to literature procedures.
(5 mL) in an ACE pressure tube, CH2Br2 (0.3 mL, 4.0 mmol) was
added. A colorless precipitate formed after the solution had been
heated at 130 °C for 24 h. The precipitate was filtered and washed
with THF (3ϫ 5 mL). A colorless powder was obtained after the
1
product had been dried in vacuo (0.9 g, 72%). M.p. Ͼ 330 °C. H
NMR (300 MHz, [D6]DMSO, 25 °C): δ = 6.92 (s, 2 H, NCH2N),
7.60 (m, 4 H, ar), 7.91 (m, 4 H, ar), 8.42 (s, 4 H, ar), 10.35 (s, 2 H,
ar) ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): δ = 58.3, 117.2,
121.9, 122.9, 124.7 (d, J = 9.5 Hz), 130.9 (d, J = 2.5 Hz), 137.5,
162.4 (d, J = 247.9 Hz) ppm. 19F NMR (282.4 MHz, [D6]DMSO,
25 °C): δ = –110.76 (d, J = 5.8 Hz) ppm. C19H16Br2F2N4 (495.17):
calcd. C 45.81, H 3.24, N 11.24; found C 45.45, H 2.97, N 11.46.
1-Phenylimidazole (1): Aniline (18.6 g, 0.2 mol) in MeOH (50 mL)
was treated with 30% aqueous glyoxal (32.4 mL, 0.2 mol) at room
temperature for 10 min to form a yellow mixture. NH4Cl (21.4 g,
0.4 mol) was added followed by 37% aqueous formaldehyde
(32 mL, 0.4 mol). The mixture was diluted with MeOH (400 mL)
and heated to reflux for 1 h before H3PO4 (28 mL, 85%) was slowly
added. The resulting mixture was then stirred at reflux for another
8 h. After removal of the solvent, the dark residue was poured onto
ice (300 g) and neutralized with 40% aqueous KOH solution until
the solution was at pH = 9. The resulting mixture was extracted
with dichloromethane (3ϫ 300 mL). The organic phases were com-
bined and dried (MgSO4). The solvent was removed, and after dis-
tillation a yellow oil was obtained (24.69 g, 86%). 1H NMR
(300 MHz, CDCl3, 25 °C): δ = 7.12 (s, 1 H, NCHN), 7.32 (m, 1 H,
3,3Ј-Bis(2,4,6-trimethylphenyl)-[(1,1Ј-diimidazolium)methane]
Di-
bromide (11): In comparison to the previously published synthe-
ses,[83,101] the change of the solvent from toluene or THF, respec-
tively, to acetonitrile led to a significant increase in the product
yield: To a solution of imidazole 5 (0.9 g, 5.0 mmol) in acetonitrile
(5 mL) in an ACE pressure tube, CH2Br2 (0.3 mL, 4.0 mmol) was
added. A colorless precipitate formed after the solution had been
heated at 130 °C for 24 h. The precipitate was filtered and washed
with cooled THF (3ϫ 5 mL). A colorless powder was obtained
after the product had been dried in vacuo (1.1 g, 81%). M.p. 307 °C
3
ar), 7.48 (m, 2 H, ar), 7.60 (d, JH,H = 8.5 Hz, 2 H, NCHCHN),
7.73 (s, 1 H, ar), 8.22 (s, 1 H, ar) ppm. 13C NMR (75 MHz, CDCl3,
25 °C): δ = 118.0, 120.3, 126.9, 129.1, 129.9, 135.6, 136.2 ppm.
C9H8N2 (144.18): calcd. C 74.98, H 5.59, N 19.43; found C 74.61,
H 5.70, N 19.51.
1
(dec.). H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 2.05 (s, 12 H,
o-CH3), 2.35 (s, 6 H, p-CH3), 6.89 (s, 2 H, NCH2N), 7.19 (s, 4 H,
3
3
1-(4-Fluorophenyl)imidazole (3): 4-Fluoroaniline (11.1 g, 0.1 mol) in
MeOH (50 mL) was treated with 30% aqueous glyoxal (16.2 mL,
0.1 mol) at room temperature for 16 h to form a yellow mixture.
NH4Cl (10.7 g, 0.2 mol) was added followed by 37% aqueous form-
aldehyde (16 mL, 0.2 mol). The mixture was diluted with MeOH
(400 mL) and heated to reflux for 1 h. H3PO4 (14 mL, 85%) was
added slowly. The resulting mixture was then stirred at reflux for
another 8 h. After removal of the solvent, the dark residue was
poured onto ice (300 g) and neutralized with 40% aqueous KOH
solution until the solution was at pH = 9. The resulting mixture
was extracted with dichloromethane (3ϫ 300 mL). The organic
phases were combined and dried (MgSO4). The solvent was re-
moved, and the residue was subjected to sublimation in vacuo. Af-
ter purification by column chromatography on silica gel (ethyl acet-
ate), a white solid was obtained (12.1 g, 75%). M.p. 20 °C. 1H
NMR (300 MHz, CDCl3, 25 °C): δ = 7.11 (s, 1 H, ar), 7.18 (s, 1 H,
ar), 7.25 (m, 2 H, ar), 7.38 (m, 2 H, ar), 7.79 (s, 1 H, NCHN) ppm.
13C NMR (75 MHz, CDCl3, 25 °C): δ = 116.6 (d, J = 23.0 Hz),
118.5, 123.4 (d, J = 8.6 Hz), 130.5, 133.5 (d, J = 2.8 Hz), 135.7,
161.0 (d, J = 247.2 Hz) ppm. C9H7FN2 (162.17): calcd. C 66.66, H
4.35, N 17.27; found C 66.61, H 4.34, N 17.51.
ar), 8.10 (t, JH,H = 1.8 Hz, 2 H, NCHCHN), 8.38 (t, JH,H
=
3
1.8 Hz, 2 H, NCHCHN), 9.86 (d, JH,H = 1.4 Hz, 2 H, NCHN)
ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): δ = 16.9, 20.6,
67.0, 122.8, 124.8, 129.4, 130.8, 134.2, 139.1, 140.6 ppm.
C25H30Br2N4·H2O (544.08): calcd. C 53.35, H 5.71, N 9.93; found
C 53.21, H 5.72, N 9.93.
3,3Ј-Bis(4-fluorphenyl)-[1,1Ј-diimidazolin-2,2Ј-diylidene)methane]-
palladium(II) Dibromide (15): A solution of Pd(OAc)2 (122 mg,
0.55 mmol) and bisimidazolium salt 9 (302 mg, 0.61 mmol) in
DMSO (10 mL) was heated at 60 °C for 2 h, 80 °C for 2 h, 100 °C
for 2 h, and 130 °C for 1 h. The solvent was removed under reduced
pressure, and the colorless residue was washed with methanol (2ϫ
5 mL) and acetonitrile (2 ϫ 5 mL) and dried in vacuo (267 mg,
1
81%). M.p. Ͼ 330 °C. H NMR (300 MHz, [D6]DMSO, 25 °C): δ
= 6.52 (m, 2 H, NCH2N), 7.51 (m, 4 H, NCHCHN), 7.81 (m, 8
H, ar) ppm. Owing to the low solubility of 15, no 13C NMR spectra
could be obtained. C19H14Br2F2N4Pd (602.55): calcd. C 37.87, H
2.34, N 9.30; found C 38.06, H 2.03, N 9.65.
3,3Ј-Bis(4-nitrophenyl)-[1,1Ј-diimidazolin-2,2Ј-diylidene)methane]-
palladium(II) Dibromide (16): A solution of Pd(OAc)2 (117 mg,
0.52 mmol) and bisimidazolium salt 10 (302 mg, 0.58 mmol) in
DMSO (10 mL) was heated at 60 °C for 2 h, 80 °C for 2 h, and
100 °C for 2 h. The solvent was removed under reduced pressure,
and the yellow residue was washed with methanol (2ϫ 5 mL) and
acetonitrile (2ϫ 5 mL) and dried in vacuo (300 mg, 88%). M.p.
Ͼ 330 °C. 1H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 6.55 (m, 2
3,3Ј-Bisphenyl-[(1,1Ј-diimidazolium)methane] Dibromide (7): In
comparison with the previously published syntheses,[102] the change
of the solvent from toluene to acetonitrile led to a significant in-
crease of the product yield: To a solution of imidazole 1 (0.7 g,
5.0 mmol) in acetonitrile (5 mL) in an ACE pressure tube, CH2Br2
(0.3 mL, 4.0 mmol) was added. A colorless precipitate formed after
the solution had been heated at 110 °C for 24 h. The precipitate
was filtered and washed with THF (3ϫ 5 mL). A colorless powder
was obtained after the product had been dried in vacuo (0.6 g,
3
H, NCH2N), 7.93 (s, 4 H, NCHCHN), 8.14 (d, JH,H = 8.8 Hz, 4
H, ar), 8.58 (d, 3JH,H = 8.8 Hz, 4 H, ar) ppm. 13C NMR (75 MHz,
[D6]DMSO, 25 °C): δ = 63.4, 122.6, 122.8, 124.4, 126.5, 144.4
146.8, 160.2 ppm. C19H14Br2N6O4Pd (656.58): calcd. C 34.76, H
2.15, N 12.80; found C 34.65, H 1.76, N 13.04.
59%). M.p. 315 °C (dec.). 1H NMR (300 MHz, [D6]DMSO, 25 °C):
3
δ = 6.95 (s, 2 H, NCH2N), 7.67 (m, 6 H, ar), 7.84 (d, JH,H
=
3
7.4 Hz, 4 H, NCHCHN), 8.45 (dt, JH,H = 8.0, 1.6 Hz, 4 H,
NCHCHN), 10.41 (s, 2 H, NCHN) ppm. 13C NMR (75 MHz, [D6]-
DMSO, 25 °C): δ = 58.4, 121.6, 122.0, 123.1, 130.2, 130.0, 134.5,
137.4 ppm. C19H18Br2N4·0.5H2O (471.19): calcd. C 48.43, H 4.06,
N 11.89; found C 48.37, H 3.68, N 11.86.
(3,3Ј-Bis(2,6-diisopropylphenyl)-[1,1Ј-diimidazolin-2,2Ј-diylidene)-
methane]palladium(II) Dibromide (18): A solution of Pd(OAc)2
(177 mg, 0.79 mmol) and bisimidazolium salt 12 (551 mg,
0.87 mmol) in DMSO (14 mL) was heated at 60 °C for 3 h, 80 °C
for 3 h, and 110 °C for 2 h. The solvent was removed under reduced
pressure, and the colorless residue was washed with acetonitrile
(2ϫ 5 mL) and dried in vacuo (351 mg, 61%). M.p. 290 °C (dec.).
3,3Ј-Bis(4-fluorophenyl)-[(1,1Ј-diimidazolium)methane] Dibromide
(9): To a solution of imidazole 3 (0.8 g, 5.0 mmol) in acetonitrile
5878
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2012, 5872–5880

Biscarbene–Palladium(II) Complexes in the Suzuki–Miyaura Reaction
3
1H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 1.03 (d, JH,H
=
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3
6.6 Hz, 12 H, CH3), 1.27 (d, JH,H = 6.3 Hz, 6 H, CH3), 1.35 (d,
3JH,H = 6.4 Hz, 6 H, CH3), 2.34–2.65 (m, 4 H, CH), 7.92 (s, 2 H,
NCH2N), 6.51–6.59 (m, 2 H, ar), 7.17–7.61 (m, 8 H, ar) ppm. 13C
NMR (125 MHz, [D6]DMSO, 25 °C): δ = 22.9, 23.3, 24.9 25.0,
28.0, 28.4, 62.1, 121.2, 122.1, 123.5, 123.9, 127.6, 129.8, 136.0,
136.5, 144.6, 146.0, 144.6 ppm. C31H40Br2N4Pd (734.89): calcd. C
50.66, H 5.49, N 7.62; found C 50.30, H 5.48, N 7.30.
General Procedure for Suzuki–Miyaura Cross-Coupling Reactions
(with Determination of Isolated or GC–MS Yields): A 2-neck flask
containing a magnetic stir bar was charged with the bis(NHC)–
palladium complex, phenylboronic acid (183 mg, 1.5 mmol), and
powdered anhydrous K3PO4 (414 mg, 1.95 mmol). The flask was
capped with a septum, evacuated, and filled with argon three times.
The aryl bromide (1 mmol) dissolved in dry toluene (5 mL) was
added through the septum with a syringe, and the resulting mixture
was stirred as indicated. The solvent was removed under reduced
pressure. For the determination of GC–MS yields, the residue was
diluted with dichloromethane (5 mL) and filtered through a thin
pad of silica gel [elution with dichloromethane (100 mL)]. Diethyl-
ene glycol dibutyl ether (170 mg) was added as internal standard.
The isolated yields were determined after purification by column
chromatography (petroleum ether/ethyl acetate) on silica gel. For
the determination of the conversion of phenylboronic acid, the resi-
due after the removal of toluene was dissolved in ethyl acetate
(20 mL) and washed with saturated NH4Cl solution (20 mL). The
aqueous phase was extracted with EtOAc (2ϫ 15 mL). The organic
phases were combined and dried (Na2SO4). The solvent was re-
moved in vacuo. Nitromethane (61 mg) was added, the mixture was
dissolved in DMSO (2 mL), and analyzed by 1H NMR spec-
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complex, phenylboronic acid (183 mg, 1.5 mmol), and powdered
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5879

M. Micksch, T. Strassner
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Received: August 17, 2012
Published Online: October 17, 2012
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