Methoxycarbonylation of iodobenzene in ionic liquids. A case of inhibiting effect of imidazolium halides

Article abstract of DOI:10.1002/adsc.200606142

The palladium(II) complexes, PdCl₂(cod) (1), PdCl ₂[P(OPh)₃]₂ (2), [bmim]₂[PdCl ₄] (3), and [bmpy]₂[PdCl₄] (4) (bmim = 1-butyl-3-methylimidazolium cation, bmpy = 1-butyl-4-methylpyridinium cation), were found to be active catalysts for the methoxycarbonylation of iodobenzene in ionic liquid (IL) media. The best results were obtained in pyridinium salts, [bmpy]X (X = Cl, Br, BF₄, PF₆) (76-100% yield). In methoxycarbonylation reactions carried out in imidazolium salts, [bmim]BF ₄ and [bmim]PF₆, the yield of benzoic acid methyl ester was slightly lower (50-78% yield), whereas in [bmim]X (X = Cl, Br) the reaction was totally retarded. The inhibiting effect was not observed when a Me group was present at C-2 (instead of a proton) on the imidazole ring. A palladium-carbene complex, Pd(bmimy)₂Br₂ (5), obtained in the reaction of a palladium catalyst precursor with [bmim]Cl, presented much lower activity than precursors 1-4. Its formation may explain the observed inhibition of methoxycarbonylation in a [bmim]Cl medium.

Full text of DOI:10.1002/adsc.200606142

FULL PAPERS
DOI: 10.1002/adsc.200606142
Methoxycarbonylation of Iodobenzene in Ionic Liquids. A Case
of Inhibiting Effect of Imidazolium Halides
a
a,
a
a
b
W. Zawartka, A. M. Trzeciak, * J. J. Ziółkowski, T. Lis, Z. Ciunik, and J. Pernak
a
Facultyof Chemistr y, Universityof Wrocław, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland
Fax : (+48)-71–375–7356; e-mail: ania@wchuwr.chem.uni.wroc.pl
Pozna n´ Universityof Technolog y, 2 Skłodowskiej-Curie Pl., 60-965 Poznan ´, Poland
b
Received: March 24, 2006; Accepted: June 13, 2006
Abstract: The palladium(II) complexes, PdCl (cod) [bmim]X (X=Cl, Br) the reaction was totallyretard-
A
H
R
U
G
2
(
1), PdCl [P
A
C
H
T
R
E
U
N
G
(OPh) ] (2), [bmim] [PdCl ] (3), and ed. The inhibiting effect was not observed when a
A
T
E
G
2
3 2
2 4
[
bmpy] [PdCl ] (4) (bmim=1-butyl-3-methylimida- Me group was present at C-2 (instead of a proton)
A
C
H
T
R
E
U
N
G
2 4
zolium cation, bmpy=1-butyl-4-methylpyridinium on the imidazole ring. A palladium-carbene complex,
cation), were found to be active catalysts for the me- Pd(bmimy) Br (5), obtained in the reaction of a pal-
thoxycarbonylation of iodobenzene in ionic liquid ladium catalyst precursor with [bmim]Cl, presented
IL) media. The best results were obtained in pyridi- much lower activitythan precursors 1–4. Its forma-
nium salts, [bmpy]X (X=Cl, Br, BF , PF ) (76– tion mayexplain the observed inhibition of methoxy-
AHCTREUNG
2
2
(
4
6
1
00% yield). In methoxycarbonylation reactions car- carbonylation in a [bmim]Cl medium.
ried out in imidazolium salts, [bmim]BF4 and
bmim]PF , the yield of benzoic acid methyl ester Keywords: ionic liquids; methoxycarbonylation; pal-
[
6
was slightlylower (50–78% i ey ld), whereas in
ladium; palladium carbenes
Introduction
few papers.[12–14] To the best of our knowledge, only
two examples have been presented of the application
Ionic liquids (IL), regarded as salts with insignificant of Pd
A
H
R
U
G
vapor pressures and melting points much below carbonylation of aryl halides with a 4–20-fold excess
1
008C, have recentlyfound widespread use as reac-
of PPh3 in [bmim]BF4 and [bmim]PF6 (bmim=1-
[1–10]
[12]
tion media for manycatal yt ic processes.
butyl-3-methylimidazolium cation).
Ionic liquids
Of the large number of ionic liquids, including also used as reaction media significantlyenhanced the al-
tetraalkylammonium halides (a), which melt at tem- koxycarbonylation of bromobenzene compared with
reactions carried out in methanol as solvent.[
12]
At
peratures below 1008C, the most frequentlyused are
those with imidazolium cations (b) and less often with higher excesses of PPh versus palladium quite good
3
pyridinium cations (c) (Scheme 1).[
1–11]
results have been obtained in the recyclability of cata-
Carbonylation reactions of aryl and benzyl halides lysts, both in hydroxycarbonylation and in methoxy-
[
12,13]
in ionic liquids have till now been described in just a carbonylation (Scheme 2).
The benzothiazole carbene-palladium complex,
quite active as a catalyst precursor in the butoxycar-
bonylation of iodobenzene and 4-bromoacetophenone
in molten TBAB (tetrabutylammonium bromide),
produced onlyverylow yi elds of products in reactions
carried out in butylpyridinium tosylate and in imida-
[
15]
zolium salts.
In our recent paper
[
16]
dealing with the methoxycar-
bonylation of iodobenzene catalyzed by Pd(0)-colloid
dissolved in different ionic liquids, we found that the
highest yields of esters were obtained in ionic liquids
with pyridinium cations, regardless of the kind of
À
À
À
anion present (Cl , BF4 , or PF ). Slightlyworse re-
6
Scheme 1.
sults were obtained for [bmim]BF and [bmim]PF ,
4
6
Adv. Synth. Catal. 2006, 348, 1689 – 1698
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1689

W. Zawartka et al.
FULL PAPERS
Scheme 2.
whereas [bmim]Cl used as the reaction medium acted Results and Discussion
as a strong inhibitor and caused total retardation of
[16]
the methoxycarbonylation process.
Those results, which experimentallydemonstrate a
Methoxycarbonylation in Different Ionic Liquids
strong inhibiting behavior of imidazolium chloride, in- The results of the methoxycarbonylation reaction of
spired us to undertake more detailed studies to ex- iodobenzene (PhI) in different ionic liquids (IL) are
plain this case.
collected in Table 1. Scheme 3 presents the structures
The main goal of the studies presented in this of and the abbreviations for the IL cations used.
paper was to correlate the molecular structures of
ionic liquids with their effect on the activityof palla-
dium catalyst precursors in the methoxycarbonylation [P
The following palladium(II) complexes were tested
as catalyst precursors: PdCl (cod) (1), PdCl2-
AHCTREUNG
2
A
H
R
U
G
A
H
R
U
G
3
2
of iodobenzene. The role of imidazolium salts of the [PdCl ] (4). The anionic complexes 3 and 4 were ob-
4
À
À
À
[
bmim]X type (X=Cl , Br , I ) was of particular in- tained from reactions of 1 with [bmim]Cl or [bmpy]Cl
terest. It is worth mentioning that [bmim]X-type ionic as the onlypalladium-containing products
liquids used in Heck coupling lead to an increase in (Scheme 4). The complex [bmim]2[PdBr ] (3a), an an-
AHCTREUNG
4
[
17]
both the reaction rate and the yield of products.
alogue of 3, was also obtained in the same way. Com-
The results presented in this paper comprise both plex 3 was obtained earlier from the reaction of PdCl
2
[
18]
investigations of methoxycarbonylation reactions as with [bmim]Cl.
The molecular structure of 3a is
well as identification of palladium complexes formed presented in Figure 1.
in reactions with ionic liquids. Two types of palladium
The above results allowed us to assume that com-
catalyst precursors have been studied: phosphane-free plexes 3, 3a, and 4 are formed in situ in the catalytic
and containing a triphenyl phosphite ligand.
methoxycarbonylation reaction carried out in
bmim]Cl, [bmim]Br, or [bmpy]Cl as the reaction
[
medium with 1 as catalyst precursor. The catalytic ac-
tivityof both precursors, 3 and 4, is similar to that of
1
, and in all cases no catalytic activity was observed
when [bmim]Cl and [bmim]Br were used as reaction
Table 1. Effect of ionic liquids (IL) on the yield (%) of methoxycarbonylation reaction of iodobenzene with PdCl (cod) (1),
A
H
R
U
G
2
[a]
[
bmim] PdCl (3), [bmpy] PdCl (4), PdCl [P
A
H
R
U
G
A
H
R
U
G
2
4
2
4
2
3
2
IL
PdCl
A
T
E
N
(cod) (1)
A
H
E
N
[bmim] PdCl (3)
A
H
R
U
G
PdCl (P
A
H
R
U
G
Pd(bmimy) Br (5)
A
H
E
N
2
2
4
2
4
2
3
2
2 2
A
C
H
T
R
E
U
N
G
[bmim]Cl
[bmim]Br
[dbmim]Br
[dmiop]Cl
[bmim]PF
0
0
0
0
70
3
0
8
0
80
0
0
14
A
C
H
T
R
E
U
N
G
A
C
H
T
R
E
U
N
G
62
27
58
51
60
92
68
92
74
26
78
71
89
76
93
88
[b]
A
C
H
T
R
E
U
N
G
30 (29; 18 )
A
C
H
T
R
E
U
N
G
78
56
94
96
89
82
50
74
100
6
A
C
H
T
R
E
U
N
G
[bmim]BF
4
A
C
H
T
R
E
U
N
G
[bmpy]Cl
[bmpy]Br
[bmpy]BF
7
A
C
H
T
R
E
U
N
G
16
22
55
A
C
H
T
R
E
U
N
G
74
4
A
C
H
T
R
E
U
N
G
[bmpy]PF
6
[
[
a]
b]
À5
À3
À2
À2
Reaction conditions: [Pd] 5·10 mol, [PhI] 8.9·10 mol, [NEt ] 2.1·10 mol, [MeOH] 2.4·10 mol, 3 h, 5 at, 908C, IL
3
1
.5 g.
Reaction yields in 2nd and 3rd runs.
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Methoxycarbonylation of Iodobenzene in Ionic Liquids
FULL PAPERS
the methoxycarbonylation product was also obtained
À
À
in reactions proceeding in [bmpy]X (X=Cl , Br ),
which suggests that the inhibiting effect of some imi-
dazolium salts is not related to the presence of halide
anions only.
The catalytic activity of 2 was found to be similar
to that observed for the phosphane-free palladium
precursor 1 and also to those of 3 and 4. Application
of imidazolium halides as the reaction medium practi-
callytotallyinhibited the cata lt yi c activityof
2,
whereas in pyridinium ionic liquids, [bmpy]X (X=
À
À
Cl , Br ), the methoxycarbonylation proceeded
smoothly.
It is well known that imidazolium salts mayreact
with palladium complexes to form N-heterocyclic car-
bene species that have been found to be good cata-
lysts for Heck, Suzuki, and amination reactions,[
but which have never been tested in methoxycarbony-
lation.
18–23]
Scheme 3.
Methoxycarbonylation with an N-Heterocyclic
Carbene-Palladium Complex
To learn about the possible contribution of N-hetero-
cyclic carbene-palladium species in the methoxycar-
bonylation, special test reactions were performed in
imidazolium halides substituted at C-2 with a Me
group, in [dmiop]Cl and [bdmim]Cl (Scheme 3). Such
imidazolium cations are unable to form carbene com-
plexes with PdÀCÀ2 bonds. As expected, the presence
Scheme 4.
of a Me group at C-2 did not affect the reactivityof
these ILs towards the precursor 1, and a 3-type com-
plex, [dmiop] [PdCl ] (3b) (Figure 2), was obtained.
A
H
R
U
G
2 4
The methoxycarbonylation of iodobenzene in both
media, [dbmim]Cl as well as [dmiop]Cl, proceeded
more effectivelythan in the corresponding non-substi-
tuted analogues, i.e., [bmim]Cl. Particularlygood re-
sults, ca. 70% of ester, were obtained in [bdmim]Cl
(
Table 1). It was also seen that, in spite of a rather
low yield of the methoxycarbonylation reaction in
dmiop]Cl (ca. 30% for precursors 1, 3, and 4), the
palladium catalyst was still reasonably active in two
[
Figure 1. The molecular structure and atom numbering
scheme of [bmim]
A
C
H
T
R
E
U
N
G
[PdBr ] (3a). Displacement ellipsoids are subsequent experiments producing 29% and 18% of
2 4
drawn at the 50% probabilitylevel and H atoms are shown
as small spheres of arbitraryradii. Unnumbered atoms are
symmetrically dependent via an inversion center. Selected
ester, respectively(Table 1).
A further test reaction of methoxycarbonylation
was studied with a carbene complex, Pd
A
H
R
U
G
o
geometric parameters (Š, ): PdÀBr1 2.440(1); PdÀBr2
(
5), as the catalyst precursor, synthesized according to
i
2
.397(1); Br1ÀPdÀBr2 89.59(3); Br1ÀPdÀBr2 90.41(3)
the Scheme 5, following a modified procedure de-
(
Symmetry codes: (i) 1–x, 1–y, 1–z).
[17]
scribed in the literature.
Complex 5 was found to be less active than the
other palladium precursors (Table 1). This is well il-
lustrated bythe results obtained in p yr idinium salts,
media. Contraryto these results, when halide anions
in the imidazolium salts were replaced bythe weakly
À
À
À
À
À
coordinating BF4 or PF6 anions, the methoxycarbo- [bmpy]X ( X=Br , BF4 , PF ). In these media the
6
nylation reaction proceeded quite well (Table 1). In yield of ester ranged from 7% to 55% when 5 was
[
bmpy]BF and [bmpy]PF the yield of ester was ca. used as a catalyst precursor, whereas all other precur-
4 6
90% (Table 1). It is worth noting that high yield of sors produced from 60% to 100% of the ester. A low
Adv. Synth. Catal. 2006, 348, 1689 – 1698
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W. Zawartka et al.
FULL PAPERS
Figure 2. The molecular structure and atom numbering scheme of [dmiop] [PdCl ]·2MeCN (3b). Displacement ellipsoids are
A
H
R
U
G
2 4
drawn at the 50% probabilitylevel and H atoms are shown as small spheres of arbitraryradii. Unnumbered atoms are s ym -
o
metricallydependent via an inversion center. Selected geometric parameters (Š, ): PdÀCl1 2.3111(7); PdÀCl2 2.3089(9);
i
OÀC6 1.389(3); OÀC7 1.441(3); Cl1ÀPdÀCl2 89.14(3); Cl1ÀPdÀCl2 90.86(3); C6-O-C7 112.7(2) (Symmetry codes: (i) 1–x, 1–
y, 1–z). Disordered parts (50% occupancy) in the acetonitrile molecules are represented by dashed lines.
5
was treated with HCl. Also in this case, the
1
H NMR spectrum presented three proton signals
characteristic of imidazolium cations (d=7.36, 7.48,
9.89).
According to literature data,[23,24] reduction of 5
can lead to a bis-carbene Pd(0) complex, but until
now we have not been able to confirm the formation
of a bis-carbene species when 5 is reacted with NEt3
(Scheme 6).
Scheme 5.
yield of ester, just 14%, was obtained in [bdmim]Br, The Mechanism of Catalyst Deactivation in the
a verygood reaction medium for all other precursors
Presence of Imidazolium Halides: The Case of
Phosphane-Free Catalyst Precursors 1, 3, 4, and 5
(
Table 1).
Keeping in mind the mechanism of the methoxycar-
bonylation reaction, in which Pd(0) plays an essential The results of the catalytic reactions collected in
role, one maysuppose that a Pd(II)-carbene complex
is less reactive because it does not easilytransform
Table 1 show that the anionic palladium complexes, 3
and 4, are verygood precursors of methox yc arbon yl a-
into catalytically active Pd(0) species. To test this hy- tion catalysts. Therefore, the formation of complex 3
pothesis we carried out a reduction of Pd(II) to Pd(0) in the first stage of a reaction catalyzed by 1 in a
in complexes 1 and 5 with NEt in methanol at 408C. [bmim]Cl medium does not explain the lack of activi-
3
Complex 1 underwent reduction within a few minutes tyof that s ys tem. Due to the better understanding of
and after 1 h practicallyall Pd(II) had been reduced
the nature of interactions between imidazolium hal-
to palladium black. Under the same reaction condi- ides and palladium catalyst precursors, the reactions
tions, complex 5 remained stable after 1 h, and only of complexes 3 and 4 with iodobenzene and iodoto-
traces of Pd(0) appeared after 1 h of heating at 608C. luene in the presence of NEt were studied. It was ex-
3
Complex 5 was also quite stable in a dihydrogen at- pected that Pd(II) in 3 and 4 would undergo in situ
mosphere (1 atm); however after 1 h heating at 608C reduction with NEt to Pd(0), which would then inter-
3
it decomposed partiallyto palladium black. The
act via oxidative addition with aryl halide to form a
1
H NMR spectra of the post-reaction solution showed new Pd(II) species containing an XÀPd(II)ÀPh frag-
the presence of an imidazolium cation with a proton ment. Instead, the formation of benzene was identi-
at the C-2 carbon. This points to the possibilityof
fied in the reaction mixture, and in the 13C NMR
transformation of the carbene complex 5 to a cationic spectrum two signals appeared, at d=167.9 and 166.4,
one like 3, as proposed in Scheme 6. Protonation of indicating the existence of palladium-carbene com-
the C-2 carbon in complex 5 was also confirmed when plexes. These results enabled us to propose a mecha-
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Methoxycarbonylation of Iodobenzene in Ionic Liquids
FULL PAPERS
Scheme 6. Transformations of a phosphane-free Pd(II) catalyst precursor under methoxycarbonylation reaction conditions in
bmim]Cl as the reaction medium.
[
nism (Scheme 6) in which the keystage is a reaction
The Mechanism of Catalyst Deactivation in the
of the proton of the imidazolium cation with the aryl Presence of Imidazolium Halides: The Case of a
ligand leading to its elimination as benzene (or tolu- Catalyst Precursor Containing a Phosphorus Ligand
ene). This means that an intermediate complex of (2)
Pd(II), formed as the product of oxidative addition of
iodobenzene to Pd(0), is decomposed under the influ- To elucidate the essence of the impact of the imidazo-
ence of [bmim]Cl and therefore the methoxycarbony- lium halide on a catalytic system with the precursor 2,
lation reaction is inhibited. Reactions of complex 3 we synthesized and isolated two palladium-aryl com-
with iodotoluene (at [(3)]:[RI]=1) in the presence of plexes with the formulas PdI(Ph)[P(OPh) ] (6) and
3
2
NEt were repeated using 5- and 10-fold excesses of [PdI(Ph)P(OPh) )] (7), which maycontribute to the
A
H
R
U
G
3
3 2
NEt in relation to complex 3. After 6 h, 10% and catalytic cycle. Complex 6 is the product of oxidative
3
17% of toluene were obtained, respectively. When addition of iodobenzene (PhI) to Pd in a monomolec-
complex 4 was used in reactions carried out under the ular complex of the formula Pd[P(OPh) ] :
A
H
R
U
G
3 4
same conditions, the formation of toluene was not ob-
served; however, when [dmiop]2[PdCl ] 3c was used,
A
H
R
U
G
4
ca. 4% of toluene was found.
Conclusions from the above observations are taken
into account in the mechanism proposed in Scheme 6,
which presents a possible pathwayfor the transforma-
The dimeric complex 7 was obtained in the reaction
tion of an IÀPd(II)ÀPh intermediate species to a less of 2 with PhI in the presence of NEt , i.e., under reac-
3
active carbene complex with simultaneous loss of the tion conditions close to those in the real methoxycar-
aryl ligand. Both these processes cause a decrease in bonylation process:
the catalytic activity of the system (Table 1).
Adv. Synth. Catal. 2006, 348, 1689 – 1698
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W. Zawartka et al.
FULL PAPERS
The formation of 7 was expected on the basis of
our earlier studies[ and the analogous synthesis of
25]
[
Pd(Br)
A
C
H
T
R
E
U
N
G
(CH Ph)P(OPh) ] . It maybe assumed that
A
H
R
U
G
2
3 2
the Pd(II) complex 2 is reduced with NEt in the
3
presence of traces of water to a Pd(0) species of the
composition Pd[P
A
H
R
U
G
A
H
R
U
tion of PhI to that complex leads to the formation of
the dimeric complex 7. A similar dimeric complex
with a P(3-Me-OC H ) ligand instead of P(OPh) , of
A
H
R
U
G
6
4 3
3
the formula {PdI(Ph)[P(3-Me-OC H ) ]} (7a), was
6
4 3 2
also synthesized in an analogous reaction:
Figure 4. Molecular structure and atom numbering scheme
of [bmim]{Pd(I) (Cl)[P(OPh) ]} (8). Displacement ellipsoids
A
H
R
U
G
2
3
are drawn at the 50% probabilitylevel and H atoms are
shown as small spheres with arbitraryradii. Disordered
parts with lower occupancy(40%) are represented by
o
dashed lines. Selected geometric parameters (Š, ): PdÀI1
2
.624(1); PdÀI2 2.613(1); PdÀI3 2.626(2); PdÀCl 2.351(4);
PdÀP 2.211(1); PÀPdÀI1 87.14(3); PÀPd -I2 93.49(3); PÀPdÀ
The molecular structure of 7a was determined by
X-raycr ys tallographic anal ys is (Figure 3).
À
À
À
À
À
À
I3 176.98(7); P Pd Cl 176.7(2); I1 Pd I3 91.49(7); I2 Pd
I3 87.98(7); I1ÀPdÀCl 89.6(2); I2ÀPdÀCl 89.8(2); I1ÀPdÀPÀ
Reactions of the palladium-aryl complexes 6, 7 and O1 À175.81(9); I2
ÀPdÀPÀO3 À122.41(9); I1ÀPdÀPÀO2
À52.70(9);
PdÀPÀO1ÀC11
À159.4(2);
I1ÀPdÀPÀO3
À À
7
a with [bmim]Cl were monitored bymeans of
P NMR measurements. After the addition of
3
1
À À
À
À
5
9.80(9); Pd P O2 C12 À63.9(2); I2 Pd P O1 1.99(9);
PdÀPÀO3ÀC13 40.3(2); I2ÀPdÀPÀO2 125.10(9).
[
bmim]Cl to the solution of 6 in CDCl , besides the
3
main signal at d=94.2 from the starting complex 6,
two new signals appeared, at d=96.4 and 12.1, which
could be assigned to two products. One was isolated nium salt [P(Ph)
and identified byX-raycr ys tallographyas an anionic ysis of the phosphonium salt was analyzed by GC-MS
complex of the formula [bmim]{Pd(I) Cl[P(OPh) ]} and assigned as OP(Ph)(OPh) .
8) (Figure 4), and the other one, characterized by
A
H
R
U
G
A
H
R
U
G
A
H
R
U
G
ACHTREUNG
2
3
3
(
Interaction of 6 with [bmim]Cl according to the re-
action pattern below leads to the elimination of both
3
1
means of P NMR (signal at d=12.1) and byMS
(
peak at m/z=387.9), was identified as the phospho- aryl (Ph) and P(OPh) ligands from the palladium co-
A
H
R
U
G
3
Figure 3. Molecular structure and atom numbering scheme of {PdI(Ph)[P(O-3-Me-C H ) ]} (7a). Displacement ellipsoids are
6
4
3
2
drawn at the 50% probabilitylevel and H atoms are shown as small spheres with arbitraryradii. Unnumbered atoms are
o
i
symmetrically dependent via an inversion center. Selected geometric parameters (Š, ): PdÀI 2.6661(9); PdÀI 2.7303(6);
i
i
PdÀP 2.212(1); PdÀC1 2.025(3); PdÀIÀPd 93.31(3); IÀPdÀI 86.69(3); IÀPdÀP 98.28(3); IÀPdÀC1 88.34(8); PÀPdÀC1
i
i
8
1
6.85(9); IÀPdÀC1ÀC2 83.1(2); PdÀPÀO1ÀC11 51.1(3); IÀPdÀPÀO1 À143.6(1); PdÀPÀO2ÀC12 À30.0(3); IÀPdÀPÀO2
i
00.8(1); PdÀPÀO3ÀC13 173.9(2); IÀPdÀPÀO3 À21.4(1) (Symmetry codes: (i) 1–x, 1–y, 1–z)
1694
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Adv. Synth. Catal. 2006, 348, 1689 – 1698

Methoxycarbonylation of Iodobenzene in Ionic Liquids
FULL PAPERS
ordination sphere and the formation of 8 and phos- aryl ligand to the phosphito one has a negative
phonium salt:
impact on the yield of methoxycarbonylation. We
cannot also rule out the possibilitythat Pd-ar yl com-
plexes react with [bmim]Cl with the formation of ben-
zene and carbene complexes, like it was found in the
phosphane-free system. However, careful analysis of
1
the H NMR spectra did not confirm the presence of
a carbene ligand when phosphite was present in the
system.
The elimination of the aryl ligand may explain the Conclusions
effect of methoxycarbonylation retardation caused by
[
bmim]Cl. Partial elimination of P
A
H
R
U
G
from 6 brings this system closer to the phosphane-free reaction with all the palladium precursors used, both
system discussed above.
phosphane-free, 1, 3, 4, and 5, and containing a P-
(OPh) ligand, 2. However, when a proton at the C-2
It is worth stressing that the imidazolium cation in
ACHTREUNG
3
this case facilitates the Ph ligand transfer from the IÀ carbon was substituted with a Me group, the inhibit-
PdÀPh fragment to P
A
T
E
N
phosphonium salt formation was observed during the and reaction yields were similar to those in imidazoli-
À
thermal decomposition of the [Pd(I)(Ph)
A
H
R
U
G
,
3
2
[
26]
À
complex.
The dimeric complexes 7 and 7a react with
bmim]Cl in a similar wayas complex 6, and in both that imidazolium halide participates in the decompo-
reactions phosphonium salts, [PPh(OPh) ]X and sition of the aryl-palladium complex, an important in-
PPh(3-Me-OC H ) ]X (m/z=430), respectively, were termediate in the catalytic process (Scheme 6). Aryl-
PF ).
6
In phosphane-free systems, it was indirectly shown
[
AHCTREUNG
3
[
6
4 3
obtained. Besides signals of the phosphonium salts, palladium complexes with phosphito ligands are also
3
1
the P NMR spectra of the reaction mixture showed decomposed byimidazolium halides, which facilitates
two signals of palladium complexes with phosphite li- the formation of phosphonium salts. The low catalytic
gands: at d=96.4 (less intensive) due to the cationic activityof the carbene-palladium complex indicates
complex 8 and at d=94.1 due to the other species. that the formation of this species in a catalytic
The same products were also formed in the reaction system, very probable in the presence of imidazolium
of 2 with [bmim]I, and the most reasonable formula halide, leads to a decrease in methoxycarbonylation
that can be proposed for the product that shows a reaction yield. Scheme 7 summarizes the inhibiting
signal at d=94.1 is [bmim]{PdI Cl [P(OPh) ]} 8a, action of [bmim]X ionic liquids observed in the cata-
A
H
R
U
G
x
3-x
3
analogous to 8. However, we were not able to isolate lytic systems under study (Scheme 7).
that complex from this reaction mixture.
Experimental Section
Methanol, Et N and diethyl ether were purified using stan-
3
[27]
dard procedures.
Iodobenzene and mesitylene (POCh,
Gliwice, Poland) were used without purification. Ionic liq-
uids were purchased from Fluka or synthesized[ and used
as obtained.
28]
Palladium complexes 1[29] and 2[30] were obtained accord-
ing to methods described in the literature.
A
H
R
U
G
A
H
R
U
Method 1:[18] PdCl (0.5g, 0.003 mol) and 1.0g (0.006 mol)
2
Our efforts to obtain the cationic complexes 8 and
a in other reactions described below were unsuccess-
ful. In the reaction of 2 with [bmim]Cl no new prod-
[
bmim]Cl were refluxed 24h at 1108C in CH CN and 3 pre-
3
8
cipitated when solution was cooled down; yield: 1.01g
(
67%).
ucts were obtained, and in the reaction of [bmim]2
Method 2: PdCl2(cod) ( 0.1g, 0.00035 mol) and 0.10g
A
H
E
N
A
C
H
T
R
E
U
N
G
[PdCl ] with P(OPh) onlycomplex 2 was formed.
A
C
H
T
R
E
U
N
G
4
3
(0.0007 mol) [bmim]CI were dissolved in 4.5mL of warm
The reaction of the Pd-aryl phosphito complexes 6, CH CN and stirred for ca. 15min, during which time the
3
7, and 7a with [bmim]X, leading to transfer of the color of the solution changed from yellow to red-brown.
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www.asc.wiley-vch.de
1695

W. Zawartka et al.
FULL PAPERS
Scheme 7. Deactivation mechanism of palladium catalyst precursors containing phosphorus ligand (left part) and phospho-
rus-free (right part) in imidazolium halide, [bmim]X, as methoxycarbonylation reaction medium.
The solution was cooled to room temperature and a solvent
was removed under reduced pressure. Crystallization of the
3H, -CH ); 13C NMR
(CDCl ): d=146.5 (s, -N-C-N-), 123.3,
3
A
H
R
U
G
3
122 (d, -CH=CH-, J=16.28 Hz), 118 (s, -N-CH ), 78.9 (t,
3
resultant powder from CH CN/toluene solution afforded 3
as dark red crystals; yield: 1.7g, (90%); anal. calcd. for
-N-CH -O-, J=9.15 Hz), 71.9 (s, -N-CH ), 35.9 (t, -O-CH -
3
2
3
2
CH -, J=6.10 Hz), 23.1 (s, -O-CH -CH -), 10.6 (s, -CH -
2
2
2
2
C H Cl N Pd: C 36.5, H 5.7, N 10.6%; found: C 36.6, H
CH3).
16
30
4
4
1
5
.6, N 10.5%; H NMR (CD CN): d=8.81 (s, 1H, -N-CH-
3
N-), 7.36; 7.39 (s, 2H, -CH=CH-), 4.17 (t, 2H, N-CH2-,
JH,H =7.27 Hz), 3.85 (s, 3H, N-CH ), 1.80 (qi, 2H, -CH -,
3
2
A
H
R
U
G
A
H
R
U
G
JH,H =7.47 Hz), 1.31 (sx, 2H, -CH -, J
=7.27 Hz), 0.92 (t,
2
H,H
1
3
3
5
H, -CH ); C NMR (CD CN): d=137.5 (d, -N-CH-N-, J=
5.8 Hz), 124.5, 122.0 (d, -CH=CH-, J=21.9 Hz), 50.1 (t, N-
The complex was obtained according to the procedure given
for 3 using 0.042 g (0.00015 mol) of PdCl2(cod) and 0.072 g
(0.0003 mol) of [bmpy]Cl; yield: 0.08 g (97%); anal. calcd.
3
3
ACHTREUNG
CH , J=21.66 Hz), 39.9 (s, N-CH -), 32.6, 19.9 (s, -CH -),
3
2
2
1
2
1
3.6 (d, -CH , J=18.21 Hz); UV-Vis (CD CN): l=220,
for C H Cl N Pd: C 36.5, H 5.7, N 10.6%; found: C 36.6,
3
3
20 32
4
2
1
50 nm; IR (KBr): n=3133, 3098, 3064 (=C-H), 2935, 1462,
H 5.6, N 10.5%; H NMR (CD CN): d=8.74, 7.86 (d, 2H,
3
À1
440, 1097, 864, 764 (C-H), 1632 (C=C), 1561 cm (C=N).
Ph-H, JH,H =6.64 Hz), 4.55 (t, 2H, N-CH -, J
=7.47 Hz),
=7.68 Hz),
2
H,H
2
1
.60 (s, 3H, N-CH ), 1.93 (qi, 2H, -CH -, J
3
2
H,H
.33 (sx, 2H, -CH -, J
=7.47 Hz), 0.92 (t, 3H, -CH );
2
H,H
3
A
C
H
T
R
E
U
N
G
[bmim] [PdBr ] (3a)
A
C
H
T
R
E
U
N
G
2 4
13
C NMR (CD CN); d=160.5 (s, CH -C-CH), 144.5 (d, -N-
3
3
The complex was obtained according to the procedure given
for 3, using 0.38 g (0.0007 mol) of PdBr2(cod) and 0.8 g
0.0036 mol) of [bmim]Br; yield: 0.47 g, (95%); anal. calcd.
for C H Br N Pd: C 27.3, H 4.3, N 7.9%; found: C 27.4,
CH=CH-, J=83.4 Hz), 129.6 (d, -N-CH=CH-, J=32.1 Hz),
61.5 (s, N-CH -), 33.8 (s, -C-CH ), 22.3, 19.9 (s, -CH -), 13.8
AHCTREUNG
2
3
2
(
(s, -CH ); UV-VIS (CD CN): l=226, 292 nm; IR (KBr):
3 3
n=3137, 3104, 3072 (C-H), 2973, 2960, 2940, 1466, 1447,
16
30
4
4
1
À1
H 4.1, N 8.1%; H NMR (CD CN): d=9.1 (s, 1H, -N-CH-
873, 768 ,628 (C-H), 1637 (C=C), 1564 cm (C=N).
3
N-), 7.43, 7.38 (s, 2H, -CH=CH-), 4.23(t, 2H, N-CH2-,
JH,H =7.27 Hz), 3.9 (s, 3H, N-CH ), 1.9 (qi, 2H, -CH -,
3
2
JH,H =7.47 Hz), 1.38 (sx, 2H, -CH -, J
=7.27 Hz), 0.93 (t,
2
H,H
Pd(bmimy) Br (5)
A
H
R
U
G
13
2 2
3
5
H, -CH ); C NMR (CD CN): d=137.5 (d, -N-CH-N-, J=
5.8 Hz), 124.5, 122.0 (d, -CH=CH-, J=21.9 Hz), 50.1 (t, N-
3 3
The complex was obtained according to modified procedure
reported in ref.[ To the solution of 0.051 g of Pd
17]
A
H
R
U
G
CH , J=21.66 Hz), 39.9 (N-CH -), 32.6, 19.9 (s, -CH -), 13.6
3
2
2
2
(
d, -CH , J=18.21 Hz); UV-VIS (CD CN): l=222, 272 nm;
10 mL of THF 0.106 g of [bmim]Br were added and the mix-
ture was stirred at 708C for 3 h. During that time the color
changed from red to yellow and two phases were formed.
The upper yellow phase containing 5 was separated and
THF was removed under reduced pressure to give yellow
product which was washed with hexane (9 mL), water (2”
10 mL), diethyl ether (5 mL) and dried; yield: 54%; anal.
calcd. for C H Br N Pd: C 35.4, H 5.2, N 10.3%; found: C
3
3
IR (KBr): n=3098, 3064 (=C-H), 2940, 1464, 1362, 1167,
À1
8
60, 768 (C-H), 1640 (C=C), 1560 cm (C=N).
A
C
H
T
R
E
U
N
G
[dmiop] [PdCl ] (3c)
A
C
H
T
R
E
U
N
G
2 4
The complex was obtained according to the procedure given
for 3 using 0.17 g (0.0006 mol) of PdCl2(cod) and 0.2 g
0.0012 mol) of [dmiop]Cl; yield: 0.47 g (95%); anal. calcd.
for C H Cl N Pd: C 38.9, H 6.0, N 10.1%; found: C 35.8,
AHCTREUNG
16
28
2
4
1
(
34.1, H 5.0, N 10.25%; H NMR (CDCl ): d=6.79 (s, 4H,
-CH=CH-), 4.41 (m, 4H, N-CH -), 4.06 (s, 6H, N-CH ),
3
1
8
34
4
4
2
1
H 5.4, N 11.2%; H NMR (CD CN): d=7.5, 7.4 (d, 2H,
2.03 (m, 4H, -CH -), 1.43 (m, 4H, -CH -), 0.98 (t, 6H,
3
2 2
-CH ); C NMR (CDCl ): d=169.0 (-N-CH-N-), 121.9,
3 3
13
-CH=CH-, JH,H =2.09 Hz), 5.5 (s, 2H, N-CH -O-), 3.8 (s,
2
3
3
H, N-CH ), 3.4 (t, 2H, -CH -O-, J
=6.69 Hz), 2.6 (s,
=7.32 Hz), 0.8 (t,
120.8 (-CH=CH-), 50.7 (N-CH ), 37.9 (N-CH -), 32.8, 20.1
(-CH -), 13.8 (-CH ).
2 3
3
2
H,H
3 2
H,=C-CH ), 1.5 (sx, 2H, -CH -, J
3
2
H,H
1696
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Methoxycarbonylation of Iodobenzene in Ionic Liquids
FULL PAPERS
PdI(Ph)[P
A
C
H
T
R
E
U
N
G
(OPh) ] (6)
Acknowledgements
3
2
[25]
To 0.13 g of Pd[P
A
C
H
T
R
E
U
N
G
(OPh) ]
in 1 mL of benzene 0.1 mL of
3
4
The authors thankMichał Szulmanowicz for preparation of
carbene complex 5, Andrzej Gniewek, MSc. Eng., for techni-
cal assistance in elaboration of X-ray data and MarekHoj-
niak, MSc for performing of GC-MS analyses. Financial sup-
port with grant PBZ-KBN-118/T09/2004 is gratefully ac-
knowledged.
PhI was added and the resultant red-brown solution was stir-
red for 1 h. Next, the solution was condensed in vacuum and
after addition of 1 mK of ethanol product 6 precipitated as
an orange powder. Anal. calcd. for C H O P IPd: C 54.2,
42
35
6 2
31
H 3.8%; found: C 52.8, H 3.6%; P NMR (CDCl ): d=
3
94.2.
A
C
H
T
R
E
U
N
G
{PdI(Ph)[P(OPh) ]} (7)
A
C
H
T
R
E
U
N
G
3 2
References
[
30]
To the suspension of 0.114 g of PdCl [P
A
H
R
N
(OPh) ]
in 2 mL
2
3 2
of benzene 0.15 mL of NEt were added and the resultant
mixture was stirred for 10 min under dinitrogen. Next,
3
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2
4
19
3
3
1
P NMR(CDCl ): d=89.1.
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C
H
T
R
E
U
N
G
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C
H
T
R
E
U
N
G
{PdI(Ph)[P(O-3-Me-C H ) ]} (7a)
6 4 3 2
1
This complex was obtained in the similar wayas 7 using
[
30]
0
.14 g of PdCl [P(O-3-Me-C H ) ]
and 0.15 mL of PhI.
The orange crystals of 7a were isolated next day; yield:
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2
6
4 3 2
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 1689 – 1698