A highly efficient, recyclable catalyst for C-C coupling reactions in ionic liquids: Pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(II)

Article abstract of DOI:10.1021/jo052098b

A hemilabile pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(II) has been synthesized. It is an excellent catalyst for Heck and Suzuki cross-coupling reactions in ionic liquids.

Full text of DOI:10.1021/jo052098b

mental and theoretical investigations revealed that this disad-
vantage could be prevented if the chelating coordination groups
containing P, O, or N donor atoms were attached to the carbene
as hemilabile arms resulting in more stable and efficient
catalysts.6-9
A Highly Efficient, Recyclable Catalyst for C-C
Coupling Reactions in Ionic Liquids:
Pyrazolyl-Functionalized N-Heterocyclic Carbene
Complex of Palladium(II)
Recently, there have been a few reports of metal complexes
containing pyridyl-functionalized NHCs and their applications
as catalyst precursors for C-C coupling,7 copolymerization of
CO/norbornene,8 and polymerization of olefins.9 Their excellent
properties encouraged us to develop new types of phosphine-
free hemilabile catalysts. Since it is well-known that the
pyrazolyl ring is more weakly coordinating to metal centers than
the pyridyl ring,10 we reasoned that transition-metal complexes
of pyrazolyl-functionalized NHCs may facilitate oxidative
additions in a catalytic cycle and may form more efficient
catalyst precursors than those from a corresponding pyridyl ring.
However, these kinds of hemilabile compounds have not been
reported.
Ruihu Wang, Brendan Twamley, and Jean’ne M. Shreeve*
Department of Chemistry, UniVersity of Idaho,
Moscow, Idaho 83844-2343
jshreeVe@uidaho.edu
ReceiVed October 6, 2005
Organic reactions that employed palladium(II) complexes of
pyridyl-functionalized hemilabile NHCs as catalysts were typi-
cally carried out in nonreusable organic solvents, such as
toluene, THF, DMF, and dioxane,7-9 and the expensive catalysts
could not be recovered and recycled. However, when the
reactions were performed in ionic liquids (ILs), important
advantages in immobilization and recycling of the catalyst were
available.11 The capacity of ILs to solvate both polar and
nonpolar species allows the dissolution of a wide range of
organic, inorganic, and organometallic compounds favoring the
formation of a homogeneous catalytic system. The organic
products can be readily separated from the palladium(II)
catalysts in the ionic liquids through simple distillation or
extraction with ether or hexane. Most importantly, the carbene
ligands do not readily dissociate from the metal center.3,7
Therefore, this means that an excess of ligand is unnecessary
and the catalyst can easily be immobilized in ILs. However,
many palladium(II)-catalyzed coupling reactions are carried out
in ILs in the presence of bulky phosphine ligands.12 The latter
are sensitive to oxygen and moisture, which can lead to catalyst
A hemilabile pyrazolyl-functionalized N-heterocyclic carbene
complex of palladium(II) has been synthesized. It is an
excellent catalyst for Heck and Suzuki cross-coupling
reactions in ionic liquids.
The initial isolation of a free carbene in 19911 has encouraged
much interest in the chemistry of N-heterocyclic carbenes
(NHCs) and their metal complexes.2 NHCs behave like typical
strong σ-donor ligands with negligible π-acceptor abilities.
Because these electronic characteristics are similar to those of
phosphines,3,4 they have played the same roles as well-studied
phosphine ligands in catalytic cycles. In addition, NHCs show
superior performance in many aspects over traditional phosphine
ligands, including ready preparation, air and moisture stability,
nontoxicity, low loading, and high efficency.4 Many metal
complexes of NHCs behave as highly efficient catalysts in
organic synthesis. However, facile reductive elimination with
hydrocarbyl ligands from Ni(II) and Pd(II) NHC complexes was
observed in some cases.5 The process suggested a potentially
important route to deactivation of catalysts. Subsequent experi-
(6) (a) Yang, C.; Lee, H. M.; Nolan, S. P. Org. Lett. 2001, 3, 1511. (b)
Danopoulos, A. A.; Winston, S.; Gelbrich, T.; Hursthouse, M. B.; Tooze,
R. P. Chem. Commun. 2002, 482. (c) Tulloch, A. A. D.; Winston, S.;
Danopoulos, A. A.; Eastham, G.; Hursthouse, M. B. J. Chem. Soc., Dalton
Trans. 2003, 699. (d) Batey, R. A.; Shen, M.; Lough, A. J. Org. Lett. 2002,
4, 1411. (e) Corma, A.; Garc´ıa, H.; Leyva, A. Tetrahedron 2004, 60, 8553.
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(b) Tulloch, A. A. D.; Danopoulos, A. A.; Tooze, R. P.; Cafferkey, S. M.;
Kleinhenz S.; Hursthouse, M. B. Chem. Commun. 2001, 1247 (c) Magill,
A. M.; McGuinness, D. S.; Cavell, K. J.; Britovsek, G. J. P.; Gibson, V.
C.; White, A. J. P.; Williams, D. J.; White, A. H.; Skelton, B. W. J.
Organomet. Chem. 2001, 617, 546. (d) Gru¨ndemann, S.; Albrecht, M.;
Kovacevic, A.; Faller, J. W.; Crabtree, R. H. J. Chem. Soc., Dalton Trans.
2002, 2163.
(8) Chen, J. C. C.; Lin, I. J. B. Organometallics 2000, 19, 5113.
(9) Wang, X.; Liu, S.; Jin, G. X. Organometallics 2004, 23, 6002.
(10) Canty, A. J.; Lee, C. V. Organometallics 1982, 1, 1063. (b) Fornies,
J.; Martin, A.; Sicilia, V.; Martin, L. F. Chem. Eur. J. 2003, 9, 3427.
(11) (a) Chauvin, Y.; Mussmann, L.; Olivier, H. Angew Chem., Int. Ed.
Engl. 1995, 34, 2698. (b) Welton, T. Chem. ReV. 1999, 99, 2071. (c)
Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (d)
Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. ReV. 2002, 102, 3667.
(e) Selvakumar, K.; Zapf, A.; Beller, M. Org. Lett. 2002, 4, 3031.
(12) (a) Carmicheal, A. J.; Earle, M. J.; Holbrey, J. D.; McCormac, P.
B.; Seddon, K. R. Org. Lett. 1999, 1, 997. (b) Xu, L.; Chen, W.; Ross, J.;
Xiao, J. Org. Lett. 2001, 3, 295. (c) Vallin, K. S. A.; Emilsson, P.; Larhed,
M.; Hallberg, A. J. Org. Chem. 2002, 67, 6243.
(1) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc. 1991,
113, 361.
(2) (a) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem.
ReV. 2000, 100, 39. (b) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41,
1290. (c) Huang, J.; Stevens, E. D.; Nolan, S. P.; Petersen, J. L. J. Am.
Chem. Soc. 1999, 121, 2674. (d) Campeau, L. C.; Thansandote, P.; Fagnou,
K. Org. Lett. 2005, 7, 1857.
(3) (a) Garrou, P. E. Chem. ReV. 1985, 85, 171. (b) Weskamp, T.; Kohl,
F. J.; Hieringer, W.; Gleich, D.; Herrmann, W. A. Angew. Chem., Int. Ed.
1999, 38, 2416.
(4) (a) Herrmann, W. A.; Schwarz, J.; Gardiner, M. G.; Spiegler, M. J.
Organomet. Chem. 1999, 575, 80. (b) Lee, M. T.; Hu, C. H. Organometallics
2004, 23, 976. (c) Whitcombe, N. J.; Hii, K. K.; Gibson, S. E. Tetrahedron
Lett. 2001, 57, 7449.
(5) (a) Cavell, K. J.; McGuinnes, D. S. Coord. Chem. ReV. 2004, 248,
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10.1021/jo052098b CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/01/2005
426
J. Org. Chem. 2006, 71, 426-429

TABLE 1. Heck Cross-Coupling Reactions of Aryl Halides with
SCHEME 1. Synthesis of Pyrazolyl-Functionalized
Imidazolium Chloride and Catalyst 2
n-Butyl Acrylate in ILs under Various Reaction Conditionsa
entry
X
base
Et3N
Et3N
Et3N
Et3N
Et3N
Na2CO3
NaOAc
Et3N
Na2CO3
NaOAc
time (h)
(cycle) yieldb
1
2
I
I
I
I
I
I
I
Br
Br
Br
8
8
8
8
1
8
8
12
12
12
(1) 91
(4) 92
(1) 91
(1) 89
(1) 83
(1) 90
(1) 91
(1) 49
(1) 29
(1) 32
(2) 91
(5) 87
(2) 83
(2) 91
(2) 88
(2) 91
(2) 93
(2) 54
(2) 24
(2) 29
(3) 89
(6) 93
(3) 73
(3) 87
(3) 85
3c
4d
5
6
7
8
9
10
a All reactions were carried out using 1 mmol of aryl halides, 1.25 mmol
of n-butyl acrylate, 1.5 mmol of base, 2 mol % of catalyst, and 3 g of ILs
at 120 °C. b Isolated yield (%). c The reaction was exposed to air. d Per-
formed with 0.2 mol % of catalyst.
decomposition. Thus, the development of phosphine-free recy-
clable catalysts is of great importance.13,14
As an extension of our research work in recyclable phosphine-
free catalytic systems,14 we were interested in the use of
conventional and functionalized room-temperature ionic liquids
as reaction media. We now report the synthesis of a palladium
(II) catalyst precursor from a pyrazolyl-functionalized hemilabile
NHC, {[Pd(Me)(3-mesityl-1-(pyrazolylmethylene)imidazolium)]
chloride (2)}, which exhibited good activity in Heck and Suzuki
cross-coupling reactions in an ionic liquid, 1-butyl-3-methylimi-
dazolium hexafluorophosphate [(BMIm)(PF6)].
between them being 76.1° and 63.9°, respectively. The chloride
ion is trans to the carbene carbon atom with the Cl(1)-Pd(1)
bond distance and the C(11)-Pd(1)-Cl(1) bond angle at
2.3699(7) Å and 176.31(8)°, respectively.
Compound 2 is stable in air and moisture and is moderately
soluble in [BMIm][PF6]. The catalytic activity of 2 was first
examined in Heck cross-coupling reactions. Initially, we screened
the base for the Heck reaction at 120 °C. The reactions of
iodobenzene and n-butyl acrylate occurred completely with a
catalyst loading of 2 mol % in the presence of different bases,
such as triethylamine, Na2CO3, and NaOAc. No side products
were formed and only n-butyl (E)-cinnamate was obtained in
an excellent isolated yield. When the reaction was carried out
for 1 h, iodobenzene was completely converted into the target
product, which suggested that the pyrazolyl-functionalized
palladium(II) catalyst is highly efficient for Heck reactions. It
should be emphasized that the catalyst still remains active after
the initial coupling reaction of iodobenzene and n-butyl acrylate
in the presence of triethylamine. The reaction was performed a
total of six times (Table 1, entries 1 and 2) using the same
catalyst, and high yields were obtained consistently. Moreover,
the reactions of bromobenzene and n-butyl acrylate under
identical conditions showed that triethylamine is an ideal base
in the absence of other additives. Sodium carbonate and NaOAc
have also been employed successfully, but lower isolated yields
were obtained. In our work, the Heck reactions were run with
triethylamine as a base. The observations were in agreement
with the catalytic reactions of pyridyl-functionalized hemilabile
NHCs in THF7b and other phosphine-free catalysts in ILs.16 It
should be mentioned that palladium black or an insoluble solid
formed when the coupling reaction of iodobenzene and n-butyl
acrylate was run under similar conditions in an open system.
Although no unreacted iodobenzene was observed at the end
of the time period and a high isolated yield was found, the yield
in the second run is slightly lower (entry 3). Therefore, all of
the reactions were carried out in an atmosphere of nitrogen.
The stability and recyclability of the catalyst at very low
concentrations were also examined. The reactions proceeded
well at a catalytic loading of 0.2 mol %. In this case, excellent
The pyrazolyl-functionalized imidazolium salt 1 was synthe-
sized by reaction of 1-chloromethylpyrazole hydrochloride,
NaH, and 1-mesitylimidazole in DMF at 60 °C (Scheme 1).
Since attempts to prepare the methylpalladium(II) catalyst 2 by
in situ deprotonation of 1 with an external strong base followed
by reaction with Pd(cod)MeCl failed, a strategy based on use
of a silver (I) N-heterocyclic carbene complex as a ligand
transfer reagent for the synthesis of the desired palladium(II)
complex was adopted.6b,7a,c,8,1515 The formation of 2 was
1
confirmed by the absence of the H NMR resonance for the
imidazolium C2-H at 9.93 ppm, where the signal of its precursor
(1) is found. In the 13C NMR spectrum, the signal for the carbene
carbon atom of 2 appears at 170.6 ppm which is a characteristic
peak for a metal carbene complex.7a,9 The chemical shifts for
the methylpalladium group were found at -0.21 ppm in the 1H
NMR and at -12.4 ppm in the 13C NMR spectra, respectively,
which are similar to those reported for methylpalladium(II) NHC
compounds.6b,7a
Crystals of 2 were obtained by slow evaporation of the CH2-
Cl2 solution. As shown in Figure 1 (Supporting Information),
the palladium(II) center has a distorted square-planar geometry.
Ligand 1 serves as a chelating ligand coordinating to palladium-
(II) through its carbene carbon and pyrazolyl nitrogen. The
resulting six-membered ring is puckered to release conforma-
tional strain. The imidazolium ring is not coplanar with the
mesityl ring and the pyrazolyl ring with the dihedral angles
(13) (a) Park, S. B.; Alper, H. Org. Lett. 2003, 5, 3209. (b) Gianfranco,
B.; Sandro, C.; Giancarlo, F. Synlett 2002, 439. (c) Consorti, C. S.; Zanini,
M. L.; Leal, S.; Ebeling, G.; Dupont, J. Org. Lett. 2003, 5, 2881. (d) Nakoji.
M.; Kanayama, T.; Okino, T.; Takemoto, Y. Org. Lett. 2001, 3, 3328. (e)
Zhao, D. B.; Fei, Z. F.; Geldbach, T. J.; Scopelliti, R.; Dyson, P. J. J. Am.
Chem. Soc. 2004, 126, 15876.
(14) (a) Xiao, J. C.; Twamley, B.; Shreeve, J. M. Org. Lett. 2004, 6,
3845. (b) Jin, C. M.; Twamley, B.; Shreeve, J. M. Organometallics 2005,
24, 3020. (c) Xiao, J. C.; Ye, C. F.; Shreeve, J. M. Org. Lett. 2005, 7,
1963.
(16) Gerritsma, D. A.; Robertson, A.; McNulty, J.; Capretta, A.
(15) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972.
Tetrahedron Lett. 2004, 45, 7629.
J. Org. Chem, Vol. 71, No. 1, 2006 427

TABLE 2. Heck Cross-Coupling Reactions of Aryl Halides with
TABLE 3. Recyclable Heck Cross-Coupling Reactions of Aryl
Halides with Styrenea
n-Butyl Acrylate under Optimized Reaction Conditionsa
R )
entry
R
X
(cycle) yieldb
H
Me
MeO
F
NO2
1
2
3
4
5c
6
7
8
9
Me
I
I
I
I
(1) 92
(1) 94
(1) 93
(1) 95
(1) 86
(1) 80
(1) 72
(1) 77
(1) 77
(2) 89
(2) 90
(2) 91
(2) 90
(2) 88
(2) 72
(2) 77
(2) 84
(2) 73
(3) 94
(3) 91
(3) 91
(3) 94
(3) 81
(3) 76
(3) 68
(3) 74
(3) 81
cycle
1
89
2
93
3
90
4
91
5
93
yieldb
MeO
NO2
F
a All reactions were carried out using 1 mmol of aryl halides, 1.25 mmol
of styrene, 1.5 mmol of Et3N, 2 mol % of catalyst, and 3 g of ILs at 120
°C for 8 h. b Isolated yield (%).
I
NO2
F
CF3
Br
Br
Br
Br
TABLE 4. Recyclable Suzuki Coupling Reactionsa
CH3CO
a All reactions were carried out using 1.0 mmol of aryl halides, 1.25
mmol of n-butyl acrylate, 1.5 mmol of Et3N, 2 mol % of catalyst, and 3.0
g of ILs at 120 °C for 8 h. b Isolated yield (%). c 2-iodothiophene acts as
a substrate.
R
H
Me
MeO
F
NO2
cycle
1
90
2
3
88
4
90
5
87
stability and high activity have been achieved as shown in Table
1 (entry 4). Furthermore, the reaction could be recycled three
times without evident loss of catalytic efficiency. As shown in
Table 2, the reactions of n-butyl acrylate with aryl iodides
containing electron-withdrawing groups, such as fluoro and
nitro, or electron-donating groups, such as methyl and methoxyl,
progressed smoothly under optimized coupling conditions. The
corresponding n-butyl (E)-cinnamates were obtained in very high
yields with complete regioselectivity at the â-position. In
addition, no cis olefin products were observed. A heteroaromatic
compound, 2-iodothiophene, also reacted with n-butyl acrylate
to give the desired product in a satisfactory yield (entry 5). The
side product of the reaction, 1,1′-bisthiophene, has not been
observed in the system.13a To determine the scope of this catalyst
with aryl bromide substrates, the reactions of n-butyl acrylate
with electron-poor 1-bromo-4-nitrobenzene, 4-bromofluoroben-
zene, 4-bromobenzene trifluroride, and 4-bromoacetophenone
were run under identical conditions, and satisfactory yields were
obtained (entries 6-10). All coupled products were easily
separated from the catalyst and ILs by extraction with ether.
After extracting the products from ionic liquids which contained
the palladium(II) catalyst, the resulting solution was washed
with water to remove ammonium salts and dried under vacuum
before reusing.
yieldb
91c
a All reactions were carried out using 1.0 mmol of aryl iodide, 1.1 mmol
of phenylboronic acid, 2.0 mmol of Na2CO3, 1 mL of H2O, 2 mol % of
catalyst, and 3.0 g of ILs at 110 °C for 1 h. b Isolated yield (%). c Containing
∼3% biphenyl.
was added as cosolvent. Excellent results were observed in the
reactions of aryl iodides with phenylboronic acid with Na2CO3
and the catalyst loading of 3% at 110 °C for 1 h. The ILs
containing the palladium (II) catalyst were recycled five times
using different reactants, and no detectable loss of activity was
observed (Table 4). Moreover, no appreciable difference in
yields between activated and deactivated iodoarenes was found,
thus demonstrating the high activity of the catalytic system.
The excellent catalytic activity and recyclability of 2 in ILs
arise from its strong Pd(II)-carbene bond and the weak
Pd(II)-nitrogen bond. On one hand, the weak coordination of
the pyrazolyl nitrogen to Pd(II) made it easy to dissociate from
the Pd(II) center to form the Pd(0) intermediate. Such dynamic
behavior generates a vacant coordination site that allows
complexation of substrates during the catalytic process facilitat-
ing oxidative addition of organic halides. Concomitantly, the
strong σ bond between the carbene electron pair and Pd(II)
stabilizes and activates the zerovalent palladium center. This is
especially true when an alkyl group coordinates to the Pd(II)
center in the hemilabile ligands, where the alkyl group might
accelerate catalysis through olefin insertion into the Pd-alkyl
bond with subsequent â-hydride elimination.7 Consequently, the
catalyst may be activated. On the other hand, the pyrazolyl ring
can coordinate to the palladium(II) center after the catalytic
reaction which will result in recovery of the catalyst. Perhaps
most importantly, ionic liquids, as an excellent supporting
medium, can further immobilize the catalyst and enhance the
recovery and recycling of the palladium(II) catalyst.18 This
mechanism can also be used to explain excellent efficiency in
Suzuki cross-coupling reactions.
To test the scope and recyclability of the catalyst further in
[BMIm][PF6], a more versatile and practical method was applied
to the coupling reactions between iodoarenes and styrene under
similar conditions (Table 3). When iodobenzene was treated
with styrene with a catalyst loading of 2 mol % at 120 °C for
8 h, the desired coupling product was obtained in an 89%
isolated yield. After separation of the product and the recovery
of ionic liquids containing the palladium(II) catalyst, different
substrates were charged into the reaction system. The reactions
still proceeded well and no unreacted aryl iodide was detected
at the end of the reaction period. The ILs containing the
palladium(II) catalyst were recycled five times with different
reactants, and no loss of catalytic activity was detected.
(17) (a) McNulty J.; Capretta A.; Wilson J.; Dyck J.; Adjabeng G.;
Robertson A. Chem. Commun. 2002, 1986. (b) Liu, S. F.; Fukuyama, T.;
Sato, M.; Ryu, I. Synlett 2004, 1814.
(18) Clavier, H.; Audic, N.; Guillemin, J. C.; Mauduit, M. J. Organomet.
Chem. 2005, 690, 3585.
To evaluate the present reaction system further, Suzuki
coupling reactions were also evaluated according to the reported
protocol.17 In this system, Na2CO3 was used as a base rather
than Et3N. To increase the solubility of the inorganic salt, water
428 J. Org. Chem., Vol. 71, No. 1, 2006

128.5, 123.0, 121.4, 106.3, 62.6, 54.8, 20.5, 16.7, -12.4; IR (KBr
pellet) 3500 (s, br), 1637 (s), 1561 (m), 1446 (w), 1354 (s), 1288
(w), 1198 (vs), 1143 (m), 1094 (w), 1058 (s), 984 (vw), 764 (m),
615 (m), 570 (w), 511 (w) cm-1. Anal. Calcd for C17H21ClN4Pd‚
(422.05): C, 48.24; H, 5.00; N 13.24. Found: C, 48.04; H, 5.04;
N, 13.17. Colorless single crystals of 2 suitable for X-ray diffraction
were obtained by slow evaporation in CH2Cl2. Compound 2
decomposed before melting.
In summary, we have synthesized a new type of palladium-
(II) complex from a pyrazolyl-functionalized hemilabile NHC.
The Pd(II) compound not only exhibits high stability and
efficient activity as catalyst precursor for Heck and Suzuki cross-
coupling reactions in ionic liquids, but it can also be recovered
and recycled at least three times without significant loss in
activity. This work has demonstrated that organic reactions
catalyzed by a functionalized hemilabile Pd(II) complex proceed
efficiently in ionic liquids.
General Procedure for Heck Reactions in Ionic Liquid.
Palladium(II) complex 2 (8.44 mg, 0.02 mmol) was dissolved in
[BMIm][PF6] (3 g), and the solvent was degassed under reduced
pressure at 60 °C for 1 h. Aryl halide (1.0 mmol), olefin (1.25
mmol), and base (1.5 mmol) were subsequently added at 25 °C.
The resulting mixture was stirred for an appropriate time under
nitrogen at 120 °C. The product was extracted from the reaction
mixture with ethyl ether (3 × 3 mL). The combined organic layer
was concentrated in vacuo. The residue was purified by flash
chromatography on silica gel to give the desired product. The
products were confirmed by comparison with literature spectro-
scopic data. The IL solutions containing Pd(II) catalyst were washed
three times with water (3 × 3 mL) to remove excess of base and
its salt, dried under reduced pressure at 60 °C for 4 h to remove
traces of ethyl ether and water, and employed for the next cycle.
General Procedure for the Suzuki Reactions in Ionic Liquid.
The aryl iodide (1.0 mmol), phenylboronic acid (0.13 g, 1.1 mmol),
and Na2CO3 (0.21 g, 2.0 mmol) and water (1 mL) were added to
the ionic liquid solution of Pd(II) catalyst 2 (8.44 mg, 0.02 mmol).
The resulting mixture was stirred for 1 h at 110 °C. The product
was extracted from the reaction mixture by addition of ethyl ether
(4 × 3 mL). The combined organic layer was concentrated in vacuo.
The residue was purified by flash chromatography on silica gel to
give the desired product. The identity of the products was confirmed
by comparison with literature spectroscopic data. The ILs containing
the Pd(II) catalyst were recycled as above.
Experimental Section
Synthesis of 3-Mesityl-1-(pyrazolylmethylene)imidazolium
Chloride Hydrate (1). 1-(Chloromethyl)pyrazole hydrochloride
(1.52 g, 10.0 mmol) was added slowly to a solution of sodium
hydride (0.40 g, 10.0 mmol) in dry DMF (50 mL) at 0 °C and then
stirred for 30 min. 1-Mesitylimidazole (2.05 g, 11 mmol) in 10
mL of DMF was added to the reaction mixture and heated at 60
°C for 48 h. After the mixture was cooled to 25 °C, the inorganic
salt was removed by filtration and washed with acetone several
times. After removal of the solvent in the combined washings under
reduced pressure, the residue was purified by flash chromatography
on silica gel to give a colorless solid: yield 1.9 g (63%); 1H NMR
δ 9.93 (s, 1H), 8.28 (d, J ) 2.0 Hz, 1H), 8.17 (d, J ) 2.0 Hz, 1H),
7.95 (d, J ) 1.8 Hz, 1H), 7.68 (d, J ) 1.8 Hz, 1H), 7.15 (s, 2H),
6.73 (s, 2H), 6.42 (t, J ) 1.9 Hz, 1H), 2.34 (s, 3H), 1.97 (s, 6H);
13C NMR δ 141.7, 140.3, 138.2, 134.0, 131.6, 130.9, 129.2, 124.3,-
122.4 107.0, 61.7, 20.5, 16.7; IR (KBr pellet) 3560 (w), 3146 (m),
2967 (s), 2876 (m), 1610 (m), 1583 (s), 1515 (s), 1467 (m), 1447
(m), 1351 (s), 1192 (s), 1057 (s), 936 (w), 781 (m), 701 (w), 654
(w), 615 (m), 571 (w), 511(m). Anal. Calcd for C16H21ClN4O
(320.14): C, 59.90; H, 6.60; N 17.46. Found: C, 60.17; H, 6. 53;
N, 17.71. Compound 1 decomposed before melting.
Synthesis of [Pd(Me)(3-mesityl-1-(pyrazolylmethylene)imi-
dazolium) Chloride (2). A mixture of 1 (0.60 g, 2.0 mmol) and
silver(I) oxide (0.24 g, 1.05 mmol) in dichloromethane (20 mL)
was stirred at 25 °C for 12 h. The reaction mixture was filtered
through Celite and washed twice with CH2Cl2 (10 mL). The solvent
volume was reduced to ∼10 mL under vacuum. The Pd(cod)MeCl
(0.53 g, 2 mmol) in CH2Cl2 (10 mL) was added to the resulting
solution and stirred at 25 °C for 12 h. It was filtered through Celite
and washed twice with CH2Cl2 (2 × 10 mL). The combined solution
was evaporated under reduced pressure to leave a white solid which
was recrystallized from CH2Cl2/Et2O (3 mL/10 mL), washed with
hexane (2 × 5 mL), and dried in vacuo to produce a white power:
Acknowledgment. We gratefully acknowledge the support
of the National Science Foundation (Grant No. CHE-0315275),
AFOSR (Grant No. F49620-03-1-0209), and ONR (Grant No.
N00014-02-1-0600). The Bruker (Siemens) SMART APEX
diffraction facility was established at the University of Idaho
with the assistance of the NSF-EPSCoR program and the M. J.
Murdock Charitable Trust, Vancouver, WA.
Supporting Information Available: Experimental procedures
and spectroscopic data for new compounds, packing diagram of
compound 2, and X-ray crystallographic data for 2 (CIF). This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
yield 0.65 g (77%); H NMR 8.06 (d, J ) 2.0 Hz, 1H), 7.69 (d, J
) 2.0 Hz, 2H), 7.29 (d, J ) 1.8 Hz, 1H), 7.00 (s, 2H), 6.56 (s,
2H), 6.40 (t, J ) 2.2 Hz, 1H), 5.74 (s, 1H), 2.29 (s, 3H), 2.00 (s,
6H), -0.21 (s, 3H); 13C NMR 170.6, 141.1, 138.1, 134.4, 131.8,
JO052098B
J. Org. Chem, Vol. 71, No. 1, 2006 429