Investigation on the coordination mode of IL-supported diols used as phosphine-free ligands for palladium catalyzed heck reaction

Article abstract of DOI:10.1002/cjoc.201300461

The coordination mode of IL-supported diols used as phosphine-free ligands for palladium catalyzed Heck reaction has been investigated by tuning their compositions. The difference in coordination of these IL-supported diols with metal Pd in Heck reaction was related to the changes of the cation rings, leading to the different activities of Pd catalyst in the reaction. The experimental results indicated that the activities of Pd catalyst were affected mainly by π-electron density of the cation rings. Compared to pyridinium and piperidinium cations, imidazolium cations showed the best coordination to metal Pd. In the meantime, C-2 hydrogen and the length of alkyl side chains had impacts on the coordination as well. Copyright

Full text of DOI:10.1002/cjoc.201300461

COMMUNICATION
DOI: 10.1002/cjoc.201300461
Investigation on the Coordination Mode of IL-Supported Diols
Used as Phosphine-Free Ligands for Palladium
Catalyzed Heck Reaction
Yueqin Cai,a Gonghua Song,*,b and Xiao Chena
a School of Chemistry and Molecular Engineering, East China University of Science and Technology,
Shanghai 200237, China
b Shanghai Key Laboratory of Chemical Biology, Institute of Pesticides & Pharmaceuticals,
East China University of Science and Technology, Shanghai 200237, China
The coordination mode of IL-supported diols used as phosphine-free ligands for palladium catalyzed Heck reac-
tion has been investigated by tuning their compositions. The difference in coordination of these IL-supported diols
with metal Pd in Heck reaction was related to the changes of the cation rings, leading to the different activities of Pd
catalyst in the reaction. The experimental results indicated that the activities of Pd catalyst were affected mainly by
π-electron density of the cation rings. Compared to pyridinium and piperidinium cations, imidazolium cations
showed the best coordination to metal Pd. In the meantime, C-2 hydrogen and the length of alkyl side chains had
impacts on the coordination as well.
Keywords homogeneous catalysis, acylation, Heck reaction, phosphine-free ligands
Introduction
ability of ionic liquids, electron-donating groups, for
example, imidazole,[28] nitrile group,[29] pyridyl moi-
ety,[30] pyrazolyl,[31] and so on,[32,33] have been investi-
gated as phosphine-free ligands. However, utilities of
functionalized ionic liquids as phosphine-free ligands in
Heck reaction, especially the studies on coordination
mode of functionalized ionic liquids with palladium
catalysts have not been much assessed yet. Therefore,
there is a need to look for a new class of functionalized
ionic liquids with an intention to study their application
as the ligands in Heck reaction.
On the other hand, it is all known that one of the
major attractions of utilizing ionic liquids in place of
organic solvents is their ‘designer solvent’ property:
different ionic liquids with different physical and
chemical properties can be made up by combinations of
different cations and anions.[34] Thus it is reasonable to
optimize the performance of catalysts by a systematic
variation of the composition of the ionic liquids.
We have been investigating functionalized ILs of
the type IL-supported diols,[35-38] which were proved to
be efficient phosphine-free ligands for Heck reaction
under aerobic condition. Despite these advances, op-
portunities to study relationships between the structure
of IL-supported diol ligands and palladium catalyst ac-
tivity have been rare. A closer look at the nature of
ligands coordination mode with metal Pd is necessary
Ionic liquids (ILs), which were reviewed firstly by
Welten,[1] are of great interest in many areas in the past
ten years.[2-14] Many of the studies on ionic liquids have
been focused on environmentally benign solvents for
synthesis and catalyst.[15-17] This claim usually rests on
the fact that ionic liquids are thermally stable, and recy-
clable under ambient conditions.[18-20] The development
of ionic liquids for use as solvents offers advantages for
a wide variety of chemical processes, which might be
the best possible solvents to try a reaction in them due
to their unique characters.[21-25] Heck reaction, one of
the most important methodologies for C－C bond for-
mation in general organic synthesis, was also studied in
ionic liquids to investigate catalytic processes.[21-26] The
results showed that the reactions were affected mainly
by the following factors, Pd-NHC species formed dur-
ing the reaction, anions, cations, as well as alkyl chains
of the cation, meaning that the composition of the ionic
liquids usually determines the performance of catalyst
in the reaction. Despite considerable advances in the
field, however, notable limitations remain for which
improved methods will have an immediate impact on
the chemistry community. Most of investigated ionic
liquids are ones without functionalized groups,[27] as
solvents for the reaction. For improving the ligation
*
E-mail: ghsong@ecust.edu.cn; Tel.: 0086-021-64253452; Fax: 0086-021-64253452
Received June 16, 2013; accepted July 25, 2013; published online September 16, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300461 or from the author.
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Chin. J. Chem. 2013, 31, 1250—1256

Investigation on the Coordination Mode of IL-Supported Diols Used as Phosphine-Free Ligands
for us to understand how IL-suppoted diols affect the
catalyst performances in the Heck reaction. We felt that
modification of IL-supported diol cations would be an
ideal way for detailed studies, which permits incre-
mental adjustment of metal electron density and steric
profile. With this approach, we extended our efforts to
develop novel IL-supported diols (1－9) composed of
different cations including pyridinium, piperidinium,
and imidazolium (Scheme 1), which were used as the
phospine-free ligands in Heck reaction of aryl halides
with acrylates. Compared to the IL-supported diols pub-
lished before,[35-38] unique structural design of varies
cations with different alkyls was introduced into the
ionic liquids. The difference in their coordination mode
of cations with metal Pd in Heck reaction derived from
tuning the composition of IL-supported diols was inves-
tigated in detail. The goal of this study is also to de-
scribe an investigation into the cause of high stability
and activity of catalysts in Heck reaction in the presence
of IL-supported diols.
A mixture of 2,2-bis(bromomethyl)-propane-1,3-
diol (1 mmol) and 1-alkylimidazole/1-alkylpyridine/
1,2-dimethylimidazole/1-methyl piperidine (3 mmol)
was stirred vigorously at 150 ℃ for 8 h. After cooling
to room temperature, the crude product was washed
with acetonitrile. The resulting solid collected by filtra-
tion was treated with water (5 mL) as well as KPF6 (2
mmol) and the reaction mixture was stirred at room
temperature for 1 h. After filtration, the white solid was
washed with ethanol and dried in vacuo to give the de-
sired product.
The NMR, MS (ESI), and FT-IR data for the prod-
ucts 2,2-bis((1-ethylimidazolium)methyl) propane-1,3-
diol hexafluorophosphate (2): White solid (0.42 g, 72%);
FT-IR (KBr) ν: 3620, 3590, 3180, 1560, 1460, 1350,
1
1160, 1060, 839 cm−1; H NMR (DMSO-d6, 500 MHz)
δ: 1.42 (t, J＝5 Hz, 6H, CH2-CH3), 3.11 (d, J＝5 Hz,
4H, CH2-OH), 4.20 (q, J＝10 Hz, 4H, N-CH2-CH3),
4.20 (s, 4H, N-CH2), 5.32 (t, J＝5 Hz, 2H, OH), 7.62 (s,
2H, NCH), 7.84 (s, 2H, NCH), 9.03 (s, 2H, N(H)CN);
13C NMR (DMSO-d6, 400 MHz) δ: 15.47, 44.82, 45.74,
49.31, 59.17, 122.41, 124.41, 137.47; MS (ESI) m/z:
439.2 (M−PF6)＋, 147.1 (PF6＋2H)＋.
Scheme 1
2,2-Bis((1-butylimidazolium)methyl) propane-1,3-
diol hexafluorophosphate (3): White solid (0.46 g, 72%);
FT-IR (KBr) ν: 3600, 3160, 2970, 2930, 1570, 1450,
_
N
+
+
N
2PF6
N
N
R
R
1
1170, 1020, 845 cm−1; H NMR (DMSO-d6, 500 MHz)
OH OH
δ: 0.88 (t, J＝5 Hz, 6H, (CH2)3-CH3), 1.23－1.28 (m,
4H, (CH2)2CH2-CH3), 1.76－1.79 (m, 4H, CH2-CH2-
CH2-CH3), 3.11 (d, J＝5 Hz, 4H, CH2-OH), 4.18 (t, J＝
5 Hz, 4H, N-CH2-(CH2)2 CH3), 4.24 (s, 4H, N-CH2),
5.31 (t, J＝5 Hz, 2H, OH), 7.64 (s, 2H, NCH), 7.84 (s,
2H, NCH), 9.05 (s, 2H, N(H)CN); 13C NMR (DMSO-d6,
400 MHz) δ: 13.76, 19.23, 31.77, 45.70, 49.16, 49.45,
59.35, 122.72, 124.47, 137.78; MS (ESI) m/z: 495.2
(M−PF6)＋, 351.3 (M−2PF6＋H), 175.1 [(M−2PF6)/2]＋.
2,2-Bis((1-hexylimidazolium)methyl) propane-1,3-
diol hexafluorophosphate (4): White solid (0.45 g, 65%);
FT-IR (KBr) ν: 3610, 3180, 3060, 2960, 2930, 2860,
1570, 1460, 1170, 1010, 839 cm−1; 1H NMR (DMSO-d6,
500 MHz) δ: 1.54 (t, J＝5 Hz, 6H, (CH2)5-CH3), 1.95－
1.96 (m, 12H, (CH2)2-(CH2)3-CH3), 3.80 (d, J＝2.5 Hz,
4H, CH2-OH), 4.03－4.04 (m, 4H, CH2-CH2-(CH2)3-
CH3), 4.88 (t, J＝5 Hz, 4H, N-CH2-(CH2)4-CH3), 4.96
(s, 4H, N-CH2), 6.02 (t, J＝5 Hz, 2H, OH), 8.36 (s, 2H,
NCH), 8.54 (s, 2H, NCH), 9.82 (s, 2H, N(H)CN); 13C
NMR (DMSO-d6, 400 MHz) δ: 14.28, 22.34, 25.58,
29.73, 30.99, 45.78, 49.35, 49.42, 59.16, 122.69, 124.46,
137.78; MS (ESI) m/z: 551.3 (M−PF6)＋, 407.9 (M−2PF6
＋H), 203.2 [(M−2PF6)/2]＋.
1 R = Me
4 R = n-He 5 R = n-Oc
2 R = Et
3 R = n-Bu
N
N
+
+
_
N
N
2PF6
OH
OH
6
_
+
+
N
R
R
N
2PF6
OH OH
7 R = H
8 R = CH3
+
N
+
N
_
2PF6
OH OH
9
2,2-Bis((1-octylimidazolium)methyl) propane-1,3-
diol hexafluorophosphate (5): White solid (0.58 g, 77%);
FT-IR (KBr) ν: 3610, 3170, 2920, 2850, 1570, 1460,
Experimental
1
1180, 1010, 837 cm−1; H NMR (DMSO-d6, 500 MHz)
General procedure for synthesis of ionic liquid
ligands
δ: 0.84 (t, J＝5 Hz, 6H, (CH2)7-CH3), 1.24－2.25 (m,
20H, (CH2)2-(CH2)5-CH3), 1.77 － 1.81 (m, 4H,
CH2-CH2-(CH2)5-CH3), 3.10 (d, J＝5 Hz, 4H, CH2-OH),
4.17 (t, J＝5 Hz, 4H, N-CH2-(CH2)6-CH3), 4.24 (s, 4H,
2,2-Bis((1-methylimidazolium)methyl) propane-1,3-
diol hexafluorophosphate (1) was synthesized according
to the Ref. [35] (Scheme 1).
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Cai, Song & Chen
COMMUNICATION
N-CH2), 5.31 (t, J＝5 Hz, 2H, OH), 7.63 (s, 2H, NCH),
7.84 (s, 2H, NCH), 9.04 (s, 2H, N(H)CN); 13C NMR
(DMSO-d6, 400 MHz) δ: 14.01, 22.10, 25.93, 28.79,
28.94, 29.78, 31.62, 45.75, 49.38, 49.44, 59.26, 122.71,
124.46, 137.74; MS (ESI) m/z: 607.4 (M−PF6)＋, 463.4
(M− 2PF6＋H), 231.2 [(M−2PF6)/2]＋.
3). The combined organic extracts were dried with
Na2SO4, filtered and concentrated under vacuum to
give the desired product. H NMR (DMSO-d6, 500
1
MHz) δ: 1.25 (t, J1＝7 Hz, J2＝8 Hz, 3H, CH3), 4.19
－4.21 (m, 2H, CH2), 6.40 (d, J＝16 Hz, 1H, CH),
7.35－7.50 (m, 5H, ArH), 8.65 (d, J＝16 Hz, 1H, CH);
GC-MS m/z (%): 176 (M＋, 25), 147 (20), 131 (100),
103 (45), 77 (30), 51 (20).
The remaining liquid phase composed of PdCl2,
ligand and the formed salt of K2CO3 was used directly
without further treatment for the next run through
treated with phenyl iodide (1.02 g, 5 mmol), ethyl
acrylate (1.00 g, 10 mmol), K2CO3 (0.69 g, 5 mmol),
and CH3CN (1.0 mL) again.
2,2-Bis((1,2-dimethylimidazolium)methyl) propane-
1,3-diol hexafluorophosphate (6): White solid (0.40 g,
68%); FT-IR (KBr) ν: 3605, 3160, 2980, 1590, 1570,
1
1450, 1430, 1300, 1171, 1015, 840 cm−1; H NMR
(DMSO-d6, 500 MHz) δ: 2.64 (s, 6H, CH3), 3.11 (d, J＝
1.5 Hz, 4H, OH-CH2), 3.76 (s, 6H, N-CH3), 4.19 (s, 4H,
N-CH2), 5.41 (s, 2H, OH), 7.50 (s, 2H, NCH), 7.68 (s,
2H, NCH); 13C NMR (DMSO-d6, 400 MHz) δ: 9.94,
35.38, 46.59, 47.53, 58.13, 122.58, 122.84, 146.55; MS
(ESI) m/z: 439.2 (M−PF6)＋, 295.2 (M−2PF6＋H), 147.1
(PF6＋2H)＋.
Results and Discussion
Firstly, several IL-supported diols with imida-
zolium cations, namely 1, 2, 3, 4, and 5 were synthe-
sized from N-alkylimidazole and 2,2-bis-(bromometh-
yl)-propane-1,3-diol followed by anion exchange with
KPF6 (Scheme1).
2,2-Bis((1-pyridinium)methyl)
propane-1,3-diol
hexafluorophosphate (7): White solid (0.39 g, 71%);
FT-IR (KBr) ν: 3608, 3060, 2880, 1592, 1575, 1455,
1432, 1301, 1170, 1015, 837 cm−1; 1H NMR (DMSO-d6,
500 MHz) δ: 3.16 (s, 4H, OH-CH2), 4.78 (s, 4H,
N-CH2), 5.58 (s, 2H, OH), 8.13－8.24 (m, 4H, m-CH),
8.66－8.69 (m, 2H, p-CH), 8.91 (d, J＝5 Hz, 4H, o-CH);
13C NMR (DMSO-d6, 400 MHz) δ: 47.10, 58.32, 60.72,
128.19, 128.48, 128.85, 146.69, 146.79; MS (ESI) m/z:
405.1 (M−PF6)＋, 262.0 (M−2PF6＋2H).
The molecular structures of alkylimidazolium ILs, 1,
1
2, 3, 4, and 5 have been investigated by H NMR and
1
FT-IR spectra. In the H NMR studies (Figure 1), only
the sample of 4, exhibited different proton signals of
imidazolium ring and OH group from that of other
compounds, which shifted to downfield by about δ 1.0.
Whereas, 1, 2, and 3 had the similar proton chemical
shifts to 5 in terms of aforementioned proton absorp-
tions as the composition of the alkyl side chain was
changed. This feature is not in accord with previous
investigations, where the proton resonances of the imid-
azolium cations showed the changes as the ionic liquids
were changed, probably due to anion-cation interac-
tions.[39,40] A major difference between our work and the
previous work is that the ionic liquids we studied have
diol functionalized groups, namely imidazolium IL-
supported diols. Thus, in addition to anion-cation inter-
actions, the anion-diol interactions may also affect the
shifts. As a result, all the above intramolecular interac-
tions between anion and cation/diol should be consid-
2,2-Bis((4-methylpyridinium)methyl) propane-1,3-
diol hexafluorophosphate (8): White solid (0.40 g, 69%);
FT-IR (KBr) ν: 3601, 3040, 2981, 2855, 1580, 1564,
1450, 1425, 1310, 1168, 1019, 843 cm−1; H NMR
1
(DMSO-d6, 500 MHz) δ: 2.64 (s, 6H, CH3), 3.13 (s, 4H,
OH-CH2), 4.69 (s, 4H, N-CH2), 5.57 (s, 2H, OH), 8.02
－8.03 (m, 4H, Ar-H), 8.73－8.74 (m, 4H, Ar-H); 13C
NMR (DMSO-d6, 400 MHz) δ: 21.95, 47.05, 58.26,
59.82, 128.68, 145.60, 160.25; MS (ESI) m/z: 433.1
(M−PF6)＋, 287.2 (M−2PF6−H), 144.1 (PF6−H).
2,2-Bis((1-methylpiperidinium)methyl) propane-1,3-
diol hexafluorophosphate (9): White solid (0.18 g, 31%);
FT-IR (KBr) ν: 3605, 2987, 2886, 1500, 1445, 1423,
1
1306, 1164, 1010, 845 cm−1; H NMR (DMSO-d6, 500
MHz) δ: 3.02 (s, 6H, CH3), 3.37－3.46 (m, 12H,
CH2-CH2-CH2) 3.79 (s, 4H, OH-CH2), 3.90 (d, J＝5 Hz,
4H, N-CH2), 4.23－4.24 (m, 4H, N-CH2), 4.55－4.56
(m, 4H, N-CH2), 5.63 (d, J＝4 Hz, 2H, OH); 13C NMR
(DMSO-d6, 400 MHz) δ: 19.95, 20.92, 43.55, 46.81,
61.69, 63.65, 77.05; MS (ESI) m/z: 445.4 (M−PF6)＋,
147.1 (PF6＋2H)＋.
General procedure for Heck reaction and recovery
of catalyst
In a typical experiment, a mixture of 1.02 g phenyl
iodide (5 mmol) with 1.00 g ethyl acrylate (10 mmol)
in CH3CN/H2O (V/V) 1.0/1.0 mL was added ligand
(1.0 mol%), 0.69 g K2CO3 (5 mmol), and 0.009 g
PdCl2 (1.0 mol%). The mixture was reacted at 80 ℃.
On completion monitored by TLC and GC, the reac-
tion mixture was extracted with ethyl ether (10 mL×
Figure 1 1H NMR spectra of 1, 2, 3, 4, and 5.
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Chin. J. Chem. 2013, 31, 1250—1256

Investigation on the Coordination Mode of IL-Supported Diols Used as Phosphine-Free Ligands
understanding on the changes in coordination modes of
ered when the proton signals of imidazolium IL-sup-
ported diols were investigated. They compete with each
other, and the downfield shift of the proton signals of 4
is indicative of the result of these two interactions inside
molecular.
the ligands relevant to the catalytic reaction is required.
We then compared the performance of 1, 2, 3, 4, and 5
as the phosphine-free ligands in the coupling of phenyl
iodide and ethyl acrylate. In our catalysis system, these
ILs ligands exhibited satisfactory activity owing to more
available coordinating sites (Table 1, Entries 1－5). In
contrast, worse activity was obtained under the same
catalytic conditions without ligands (Table 1, Entry 17).
It has been reported that the diol was an excellent
O,O-ligand for the metal derived from the ethylene
glycol group.[41] The results by Calo et al.[41-43] have
indicated that imidazolium skeleton could generate
N-heterocyclic carbene (NHC) in situ, which provided
superior reactivity and better yields in the coupling re-
action. Importantly, the optimal result (reaction time 25
min) was achieved when phenyl iodide reacted with 2.0
equiv. of ethyl acrylate at 120 ℃ in the presence of 4
(1.0 mol%) using K2CO3 as a base in DMF only with
1.0 mol% of PdCl2 loading (Table 1, Entry 4). When the
reaction was run in the presence of other alkylimid-
azolium ligands, 1, 2, 3, and 5, longer time of 30－45
min was obtained (Table 1, Entries 1, 2, 3, and 5). The
reaction time decreased with the changes of the alkyl
side chain from methyl, ethyl, n-butyl, to n-hexyl. Much
to our surprise, this tendency became to increase when
n-octyl group was present. These may imply that the
alkyl side chains play an important role in the transfor-
mation. As we know, the alkyl group has electron-rich
nature, and longer alkyl chain results in greater electron
donating ability. The electronic environment of π-sys-
tem on the cation may be varied by the alkyl side chain.
The superiority of 4 in our study is proposed higher
π-electron density on the cation ring, relative to n-hexyl,
leading to more strongly coordination to Pd. This means
π-system effect displays unprecedented effect on the
catalyst activity besides NHC species and O,O-ligand
from the diol in the Heck reaction when imidazolium
IL-supported diols are applied as ligands. Nevertheless,
in spite of strong donor ability of n-octyl group, the re-
action in the case of 5 occurred at a slower rate than that
of 4. This may suggest that the overall course/progress
of the reaction may not be solely governed by the donor
ability of the alkyl group. The peculiar behavior of 5 is
explained by hypothesizing that the presence of longer
alkyl chain in 5 prevents the formation of NHC species
as well as the coordination of diol to Pd, and reduces the
activity of the catalyst. The alkyl side chain influence
may account for the longer reaction time with 5.
The FT-IR spectra of alkylimidazolium ILs were
shown in Figure 2. The analysis showed that the most
obvious feature in this set of spectra was changes of the
peaks at 3600 cm−l and 3000－3200 cm−1 as the differ-
ent alkyl side chains on the cation rings were present.
On the one hand, the absorption peak at 3600 cm−l does
not show the same feature in 3, 4, and 5 as in 1 and 2,
where splitting was present. It is known that the band at
3600 cm−1 is attributed to O－H stretch in the ethylene
glycol group.[39] Because of the interaction between an-
ion PF6 and OH group in the ethylene glycol group, the
absorption peak at 3600 cm−l in 1 and 2 splits. However,
the longer alkyl chains, n-butyl, n-hexyl, and n-octyl,
are present in 3, 4, and 5 separately, which could inter-
fere with the anion-diol interaction, leading to single
absorption peaks at 3600 cm−l of 3, 4, and 5. This means
the alkyl side chain interactions may affect O－H
stretch owing to the unique structure of imidazolium
IL-supported diols. On the other hand, the bands at
about 3100 cm−1 as C-2 hydrogen-stretching fre-
quency[39] in 4 shifting to about 3050 cm−1, did not shift
in 1, 2, 3, and 5. It was reported that the interaction in-
volving hydrogen bonding through the C-2 hydrogen
were found in the imidazolium cations,[39] which af-
fected their IR spectra. Moreover, the alkyl side chain
on the imidazolium cations may interact with C-2 hy-
drogen or diol. It suggests that above mentioned red
shift of 4 is due to all these interactions of anion/
diol/alkyl or anion/cation/alkyl to explain this observa-
tion.
1
2
3
4
5
120
100
80
60
40
20
0
4000 3800 3600 3400 3200 3000 2800 2600 2400
Wavenumber/cm-1
For comparative purposes, we also prepared IL-
supported diols, 6, 7, 8, and 9 starting from 1,2-dimeth-
ylimidazole/alkylpyridine/N-methylpiperidine using the
same synthetic pathway as alkylmidazolium ILs
(Scheme 1), which were used as ligands in the Heck
reaction as well. The results showed that the catalytic
efficiency of the alkylimidazolium ILs was generally
found to be superior to that of the pyridinium/piperi-
dinium cation containing ILs except 8. It is noteworthy
Figure 2 FT-IR spectra of 1, 2, 3, 4, and 5.
The above studies disclose that the relative contribu-
tion of these phenomena to 1H NMR and FT-IR spectra
is strictly related to the composition and structure of
ionic liquid. Significantly, it can be seen that compound
4 shows unusual characters, probably implying unique
activity in the reaction. To address this question, a better
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Cai, Song & Chen
COMMUNICATION
that the reaction times were 50 min for 7, 30 min for 8,
and 70 min for 9 respectively with Pd participation (Ta-
ble 1, Entries 7, 8, and 9). In our opinion, a high elec-
tron density of π-system in a small volume is considered
the key factor to enhance coordination to Pd, improving
the catalyst activity. Compared to alkylmidazolium ring,
pyridinium ring has lower π-electron density because of
its larger volume, resulted in slower reaction rate. Simi-
larly, in the case of 9 without π-system on the piperi-
dinium ring, the longest time derived from its less
available coordinating sites to Pd. With weakly coordi-
nating ability of 9, the resulting catalyst Pd-9 is less
thermodynamically stable and gives the product with
very slow efficiency. Taken together, those described
above demonstrate the importance of π-system on reac-
tivity. In addition, the lower activity of 6, 7, 8, and 9
may be caused by lack of C-2 hydrogen on the piperi-
dinium/pyridinium ring, so the Pd-NHC complex cannot
be generated during the reaction.
of IL-supported diol ligands and the catalyst activity.
This assessment is also supported by the following
studies. On the one hand, the reaction took place over
shorter time periods in the case of 8 than that of its ana-
logue 7, which may be caused by π-system effect. A
substitution of the pyridinium ring in position 4 with
CH3, resulted in an increase of the π-electron density,
leading to higher activation of 8. On the other hand, to
demonstrate the influence of the C-2 hydrogen on the
overall catalytic efficiency of the ILs, the reaction was
performed in the presence of 6, of which C-2 hydrogen
was replaced by CH3 group. A drastic decrease in the
catalytic power was observed and longer reaction time
of 30 min was obtained despite the fact that it has the
similar imidazolium skeleton to 1, 2, 3 and 4 (Table 1,
Entry 6). These signified the important role of the C-2
hydrogen in imparting catalytic activity.
Table 2 Heck reactions of haloarenes with acrylatesa
Table 1 Heck reaction of phenyl iodide with ethyl acrylatea
X
PdCl2-4
COOR2
+
I
K2CO3/CH3CN/H2O
COOR2
R1
PdCl2-ligand
+
COOEt
K2CO3/Solvent
COOEt
R1
Entry
R1
R2
X
Time/h
Yieldb/%
Entry
Ligand
Solvent
DMF
Time/min
Yieldb/%
1
2
H
H
Me
Et
I
I
I
I
I
I
I
I
I
I
I
I
25
25
97
98
78
76
98
99
83
80
68
64
70
66
96
95
69
68
23
20
1
1
2
3
4
5
6
7
8
9
4
4
45
40
30
25
35
55
50
30
70
150
55
55
55
55
60
60
75
97
96
96
98
97
97
96
95
97
40
97
97
97
97
96
95
54
2c
3c
4c
5c
6
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
H2O
3
p-OCH3
p-OCH3
p-NO2
p-NO2
p-CH3
p-CH3
o-CH3
o-CH3
o-CF3
Me
Et
130
130
20
4
5
Me
Et
6
20
7
Me
Et
90
7
8
90
8
9
Me
Et
200
200
180
180
40
9
10
11
12
13
14
15
16
17
18
10d
11e
Me
Et
CH3CN/H2O
o-CF3
12f 4 (1th run) CH3CN/H2O
13f 4 (2th run) CH3CN/H2O
14f 4 (3th run) CH3CN/H2O
15f 4 (4th run) CH3CN/H2O
16f 4 (5th run) CH3CN/H2O
p-NO2
p-NO2
p-CH3
p-CH3
p-NO2
p-NO2
Me Br
Et Br
Me Br
Et Br
Me Cl
Et Cl
40
300
300
300
300
17
－
DMF
a
The molar ratios of phenyl iodide/ethyl acrylate/K2CO3/are 1∶
a PdCl2 1.0 mol%, ligand 4 1.0 mol%, haloarene 5 mmol, acrylate
10 mmol, K2CO3 5 mmol, CH3CN/H2O (V/V) 1.0/1.0 mL. b Iso-
lated yields.
2∶1, PdCl2 1.0 mol%, ligand 1.0 mol%, temperature 120 ℃,
DMF 2.0 mL. b Isolated Yields. c Pd precipitations are not found.
d
H2O 2.0 mL, temperature 100 ℃. e CH3CN/H2O (V/V) 1.0/1.0
f
mL, temperature 80 ℃. Reuse of catalyst in CH3CN/H2O from
Encouraged by these results, for further investigation
of the diol-functionalized ILs as the ligands, the cou-
pling reaction of phenyl iodide and ethyl acrylate was
performed at 100 ℃ using 4 as the ligand in water
1 to 5 runs, temperature 80 ℃.
These investigations show a strong association be-
tween π-system, C-2 hydrogen, and the length of alkyl
1254
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© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2013, 31, 1250—1256

Investigation on the Coordination Mode of IL-Supported Diols Used as Phosphine-Free Ligands
considering environmental protection (Table 1, Entry
10). Unfortunately, the polymer of ethyl acrylate was
formed during the reaction because of bad solubility of
hydrophobic substrates in water, thus leading to lower
yield of 40% even though reaction time increased to 150
min. Considering solubility factor, CH3CN was used as
co-solvent to reduce the polymerization reaction. When
the reaction occurred at 80 ℃ in H2O/CH3CN (V/V,
1.0/1.0), the catalyst PdCl2-4 showed high activity and
selectivity with no Pd precipitation. Consequently, we
planned to assess the feasibility of recovery and reuse of
the catalyst PdCl2-4 in H2O/CH3CN because of advan-
tageous physicochemical properties of ILs like nonvola-
tile, thermally stable properties, and ease of recyclabil-
ity.[44] In the reaction, we observed that the product was
soluble in Et2O but PdCl2-4 was insoluble in Et2O,
which enabled us to extract the product from the reac-
tion mixture with a minimal amount of Et2O whereupon
the catalyst was retained in the reaction flask and could
be reused. Thus, PdCl2-4 was recovered and reused for
five subsequent fresh batches of reaction without sig-
nificant decrease in the catalytic activity (Table 1, En-
tries 12－16). The good performance of the catalyst is
ascribed to the strong chelating coordination of ligand 4
with the metal center, which makes catalyst to exhibit
good thermal and oxidative stability in practice.
by the ligand 4 with more available coordinating sites,
the palladium catalyst showed good stability and could
be reused at least five runs without any activity loss and
Pd leaching.
Acknowledgement
We acknowledge the Natural Science Foundation of
China (Grant 20676033), China Postdoctoral Science
Foundation (Grant 20070410169) and Shanghai Lead-
ing Academic Discipline Project (Project Number: B507)
for financial support.
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Chin. J. Chem. 2013, 31, 1250—1256
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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Chin. J. Chem. 2013, 31, 1250—1256