Green, Efficient and Reusable Bis(imidazolium) Ionic Liquids Promoted Pd-Catalyzed Aqueous Suzuki Reaction for Organic Functional Materials

Article abstract of DOI:10.1007/s10562-018-2452-y

Abstract: Glucopyranoside-substituted asymmetric bis(imidazolium) ionic liquids were used to promote environmentally friendly, efficient and reusable Pd-catalyzed Suzuki reaction between arylboronic acids and 2-bromofluorene, 2,7-dibromo-9,9′-dialkylfluorene. Over 30 carbazole-based fluorene core potential organic functional materials were synthesized. In addition, recyclability of catalyst system was inspected on the Suzuki reaction observing nearly no loss in catalytic activity up to five cycles. Graphical Abstract: Development of green, efficient and recycle glucopyranoside substituted bis(imidazolium) ILs promoted Pd-catalyzed aqueous Suzuki reaction for OFMs. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2452-y

Catalysis Letters
https://doi.org/10.1007/s10562-018-2452-y
Green, Efficient and Reusable Bis(imidazolium) Ionic Liquids Promoted
Pd-Catalyzed Aqueous Suzuki Reaction for Organic Functional
Materials
Zhonggao Zhou1 · Yangyang Yuan1 · Yongrong Xie1 · Mei Li1
Received: 14 March 2018 / Accepted: 10 June 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Glucopyranoside-substituted asymmetric bis(imidazolium) ionic liquids were used to promote environmentally friendly,
efficient and reusable Pd-catalyzed Suzuki reaction between arylboronic acids and 2-bromofluorene, 2,7-dibromo-9,9′-
dialkylfluorene. Over 30 carbazole-based fluorene core potential organic functional materials were synthesized. In addition,
recyclability of catalyst system was inspected on the Suzuki reaction observing nearly no loss in catalytic activity up to five
cycles.
Graphical Abstract
Development of green, efficient and recycle glucopyranoside substituted bis(imidazolium) ILs promoted Pd-catalyzed aque-
ous Suzuki reaction for OFMs.
Keywords Green chemistry · Bis(imidazolium) ionic liquids · Carbohydrate · Pd-catalyzed · Suzuki reaction · Organic
functional materials
1 Introduction
Electronic supplementary material The online version of this
Small molecules and polymers containing spirofluorene, flu-
article (https://doi.org/10.1007/s10562-018-2452-y) contains
orene, carbazole, fluorine and triphenylamine groups have
supplementary material, which is available to authorized users.
recently attracted obviously attention as popular organic
functional materials (OFMs), especially in organic light-
emitting diodes (OLED), organic photovoltaic (OPV), and
perovskite solar cells (PVSCs) research areas [1–8]. Suzuki
reaction can be run in organic, organic/aqueous biphase
system, water, even solvent-free, which depending on the
* Zhonggao Zhou
zhgzhou@foxmail.com
1
College of Chemistry and Chemical Engineering,
Gannan Normal University, Ganzhou 341000,
People’s Republic of China
Vol.:(0123456789)
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Z. Zhou et al.
nature of the catalyst [9, 10]. The bulk of OFMs including
spirofluorene core, fluorene core, phenoxazine core, truxene
core, carbazole core, thiophene core and miscellaneous core
were nearly all obtained by Pd/phosphine catalyzed efficient
Suzuki coupling under anhydrous and oxygen-free environ-
ment [2, 3, 11–18]. Ionic liquids (abbreviated as ILs) are
emerging materials with essential importance for energy
storage, synthesis, and catalysis, usually air-stable and easy
handle, the chemical and physical properties was well under-
stood [19–24]. Liu reported PEG chain functionalized ILs for
efficient Pd-catalyzed Suzuki reaction at the Pd loading of
1.0 mol% [10]. Shi reported N-heterocyclic carbine (NHC)
and phosphine-chelated ILs with PEG chain for Pd-cata-
lyzed Suzuki reaction at the palladium loading of 0.1 mol%
[25]. Bazgir reported NHCs–Pd complex supported on ILs
functionalized GO for Suzuki reaction at the Pd loading
of 0.1 mol% [26]. Cai reported mono(imidazolium) ILs
functionalized reduced GO for Suzuki reactions at the Pd
loading of 0.5 mol% [27]. Carbohydrate units are bulky,
multiple stereo centers, biocompatible, naturally abundant
molecules that provide a variety of functions [28, 29]. Give
catalyst chirality and water solubility [30–33]. Several
carbohydrate-substituted mono-NHCs–Pd complexes have
bulky, rigid, and usually exhibit excellent catalytic activity
at the Pd loading of 0.05–1.0 mol% [34–39]. We reported
several glucopyranoside-substituted mono(imidazolium) ILs
for efficient Pd-catalyzed Suzuki reactions at the Pd loading
of 0.1–0.05 mol% previously [40, 41]. Aliphatic chain or
aromatic ring bridged, glucopyranoside-substituted asym-
metric bis(imidazolium) ILs have not been reported, and the
catalytic activity may be very interesting.
with this catalytic system. Finally, recyclability of this cata-
lyst system was inspected and nearly no loss of catalytic
activity up to five cycles.
2 Results and Discussion
2.1 Synthesis of Bis(imidazolium) ILs
The carbohydrate-containing mono(imidazolium) ILs usu-
ally obtained from N-carbohydrate substituted imidazole
and halogenoarene or haloalkane [35, 38, 42]. The sym-
metric bis(imidazolium) ILs can be easily obtained by one-
step method also [19]. While asymmetric bis(imidazolium)
ILs usually obtained via two-step with two different inter-
mediates. Scheme 1 show the synthesis of aliphatic chain
and aromatic ring bridged glucopyranoside-substituted
bis(imidazolium) ILs 3a and 3b by two methods (Scheme 1,
method A, 3a, 54%, 3b, 98%, method B, 3a, 96%, 3b, 97%).
Compounds 3a and 3b were water-soluble, air stable and can
be stored at room temperature in air atmosphere for more
than 12 months without any decomposition. In addition,
the NMR spectra suggested that it was β-anomer due to the
neighboring group participation [36]. The isolate yield of
3a bridging with aryl just 54% by method A caused our
attention, we found a mixture of α and β epimers can be
encountered from the in-situ NMR spectra, unfortunately,
the α anomer is unstable during purification and just β ano-
mer obtained herein, similar to our previous results [41].
Two independent NCHN proton chemical shift of 3a and
3b located at relative high field (9.63 and 9.31, 9.70 and
9.24 ppm, Fig. S1), compared with raw materials (usually
between 10 and 12 ppm, Fig. S2), indicating that the second
imidazole ring increases the shielding effect and electron
cloud density around two NCHN protons [43]. The elec-
tronic and steric effect of imidazole ring directly affect the
catalytic activity of ionic liquids, next, the catalytic activ-
ity were investigated in Pd-catalyzed Suzuki reaction using
mono- and bis(imidazolium) ILs as auxiliary ligands.
Herein, we report an efficient, green and reusable
bis(imidazolium) ILs promoted Pd-catalyzed aqueous
Suzuki reaction for OFMs. Firstly, two glucopyranoside-
substituted hydrophilic bis(imidazolium) ILs (Scheme 1,
3a and 3b) were obtained by two synthetic methodologies
(Scheme 1, method A and B). Secondly, the best catalytic
activity of glucopyranoside-substituted bis(imidazolium)
ILs in Pd-catalyzed Suzuki reaction was optimized in detail.
Thirdly, a series of OFMs (over 30 examples) were prepared
Scheme 1 Synthesis of asymmetric bis(imidazolium) ILs via different methods
1 3

Green, Efficient and Reusable Bis(imidazolium) Ionic Liquids Promoted Pd-Catalyzed Aqueous…
Scheme 2 Pd-catalyzed Suzuki
L
Pd/
B
(HO)2
Br
reaction of 2-bromofluorene and
phenylboronic acid
Base
Sovlent
Temperature
Table 1 Effect of various bases, solvents and time on the Suzuki reac-
Table 2 Effect of various catalysts on the Suzuki reaction
Entry Pd/ligand
tion
Pd (mol%) Time (h) Conver-
Entry
Bases
Solvent
Time (h)
Conver-
sion (%)a
sion (%)a
1
2
3
4
5
6
7
8
Pd(OAc)2:3a=2:1.5
0.1
0.1
0.1
0.1
0.05
0.03
0.01
0.01
0.01
1.5
1.5
1.5
1.5
1.5
1.5
3.0
3.0
3.0
3.0
3.0
87
77
97
89
95
93
98
92
83
91
83
1
2
3
4
5
6
7
8
NaOH
KOH
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (95%)
EtOH (80%)
EtOH (50%)
EtOH (30%)
EtOH
0.5
0.5
0.5
1.5
1.5
1.5
3.0
3.0
0.5
0.5
0.5
1.5
1.5
1.5
1.5
1.5
91
94
76
86
74
77
88
68
95
98
85
88
65
83
80
85
Pd(OAc)2:3a=1:1
Pd(OAc)2:3a=2:1.1
Pd(OAc)2:3a=2:0.9
Pd(OAc)2:3a=2:1.1
Pd(OAc)2:3a=2:1.1
Pd(OAc)2:3a=2:1.1
Pd(OAc)2:3a=2:1
Pd2(dba)3:3a=2:1.1
K2CO3
K3PO4
Na2CO3
Cs2CO3
t-BuONa
Et3N
KOH
KOH
KOH
KOH
KOH
KOH
KOH
9
10
11
9
Pd(OOCCF3)2:3a=2:1.1 0.01
PdCl2:3a=2:1.1 0.01
10
11
12
13
14
15
16
Reaction conditions: 2-bromofluorene (0.5 mmol), phenylboronic
acid (0.75 mmol), Pd/L catalyst, KOH (1.0 mmol), EtOH (50%)
(3.0 mL), reflux, in air
H2O
MeOH
iPrOH
nBuOH
aAverage of two runs, measured by GC–MS
KOH
Other inorganic bases (K2CO3, K3PO4, Na2CO3 and Cs2CO3)
and organic bases (t-BuONa and Et3N) resulted in relative
lower conversion even prolong reaction time (Table 1,
entries 3–9), and KOH was the best base (Table 1, entry
2). Taking into account the caloric power of polar solvents
[47], we then running the Suzuki reaction in mixed polar sol-
vents (ethanol and water) with several volume ratios (v/v),
which dramatically enhanced the reactivity, probably due to
higher solubility of the catalysis and product in mixed sol-
vent media, interestingly, ethanol/water=1/1 (v/v) stood out
as the most suitable solvent, and obtained 98% conversion
(Table 1, entry 10). However, pure ethanol (Table 1, entry
12) and neat water (Table 1, entry 13) just afforded 88 and
65% conversion. 9H-Fluoren-2-ol was formation from 2-bro-
mofluorene when used neat water as solvent (Table 1, entry
13), the phenomenon is similar to the report of Buchwald
et al. [48] We also investigated various polar solvents, such
as MeOH, iPrOH and nBuOH, unfortunately, these com-
monly used solvents for Suzuki reaction showed diminished
conversions (Table 1, entries 14–16).
Reaction conditions: 2-bromofluorene (1.0 mmol), phenylbo-
ronic acid (1.5 mmol), catalyst Pd(OAc)2:3a=2:1, 0.1 mol%, base
(2.0 mmol), solvent (3.0 mL), reflux in air
aAverage of two runs, measured by GC–MS
2.2 Initial Optimization of Pd‑Catalyzed Suzuki
Reaction
Imidazolium ILs constitute an important home of auxiliary
ligands in transition metal catalysis, simultaneously, weak
interaction have been emerged between NHCs and solvent
[44]. Pd source, ILs, base, and solvent directly combined
is a classic pattern for the Suzuki reaction [45, 46]. Subse-
quent the application of bis(imidazolium) ILs as ligand in
Pd-catalyzed Suzuki reactions were assessed.
We choose the pattern reaction between 2-bromofluorene
and phenylboronic acid as the initial research (Scheme 2).
Firstly, we investigated the effects of various bases and
results summarized in Table 1. Base on the solubility experi-
ment and our previous results [41], the catalytic activity may
be better in polar solvent systems, especially in green solvent
system [23]. To our delight, in the presence of Pd(OAc)2/3a
(molar ratio = 2:1), 2-bromofluorene reacted with phe-
nylboronic acid in ethanol (95%) afforded 91% conversion
(Table 1, entry 1) with 0.1 mol% catalyst loading. The strong
base KOH can afford 94% conversion (Table 1, entry 2).
With the best base and solvent in hand, we further focused
on the molar ratio of Pd/3a, catalyst loading and Pd source
(Table 2) [49]. We first identified the Pd/3a molar ratio, and
2:1.1 Pd/3a ratio afforded the best conversion (Table 2, entry
3), which was higher than others (Table 2, entries 1, 2, and
4). Next, the model reaction was investigated with various
catalyst loading including 0.05 and 0.03 mol%, and observed
1 3

Z. Zhou et al.
Table 3 Effect of different ligands on Pd(OAc)2-catalyzed Suzuki
95 and 93% conversion after 1.5 h (Table 2, entries 6–7). If
prolong the time of reaction to 3.0 h, 98% conversion was
obtained with 0.01 mol% catalyst loading (Table 2, entry
7). Pd2(dba)3, Pd(OOCCF3)2 and PdCl2 were screened and
without improvement in conversion (Table 2, entries 9–11).
The activity of Pd(OAc)2 with different imidazole, mono-
and bis(imidazolium) IL-type ligands including 1a, 1b, 2,
4a, 4b, 5a, 5b and 5c were tested (Fig. 1; Table 3). The
conversion is significantly affected with auxiliary ligands,
the imidazole 2, afforded lowest 35% conversion (Table 3,
entry 3), mono(imidazolium) ILs-type ligands 1 afforded
62–65% conversion (Table 3, entries 1 and 2), more rigid
and steric demand mono(imidazolium) ILs-type ligands 4
afforded moderated 73–77% conversion (Table 3, entries 6
and 7), and bis(imidazolium) ILs-type ligands 5 afforded
over 80% conversion (Table 3, entries 8–10).
reactions
Entry
Ligand
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
1a
12
12
3
3
3
6
6
12
12
12
3
65
62
35
98
90
77
73
84
82
80
15
1b
2
3a
3b
4a
4b
5a
9
10
11
5b
5c
Without
Reaction conditions: 2-bromofluorene (0.5 mmol), phenylboronic
acid (0.75 mmol), catalyst Pd(OAc)2/L=2:1.1, 0.01 mol %, KOH
(1.0 mmol), EtOH (50%) (3.0 mL), reflux 3.0 h in air
The bis(imidazolium) IL 3a with 1,4-bis-benzyl bridg-
ing two imidazole, show more steric and rigid than 1,3-bis-
propyl bridging ligand 3b, and maybe formed two functional
Pd-NHCs freely chelating centers with Pd(OAc)2. The possi-
ble reason is the flexible propyl linkage two imidazole rings
of 3b makes them work together and formed one Pd–NHCs
catalytic center, afforded a decreased 90% conversion
(Table 3, entry 5). Our results indicate that catalytic activ-
ity related to the Pd–NHCs which have different electronic,
rigid and steric demand with aliphatic chain and aromatic
ring bridged bis(imidazolium) ILs ancillary ligand [42].
aAverage of two runs, measured by GC–MS
2.3 Suzuki Reactions of Variety of Aryl Halides
and Aryl Boronic Acids
With the optimized “Pd/bis(imidazolium) ILs catalysis”
in hand, a range of OFMs (33 examples) were synthesized
by the reaction between arylhalides (2-bromofluorene,
2,7-dibromofluorene, 2,7-dibromo-9,9′-diethylfluorene,
2,7-dibromo-9,9′-dibutylfluorene) and arylboronic acids
containing nitro, trifluoromethyl, fluorine, triphenylamine
and carbazole moieties (Figs. 2, 3). The results revealed that
arylboronic acids containing substituted with electron-with-
drawing and electron-donating groups all reacted smoothly
Fig. 1 The structures of imidazole, mono- and bis(imidazolium) ionic liquids
1 3

Green, Efficient and Reusable Bis(imidazolium) Ionic Liquids Promoted Pd-Catalyzed Aqueous…
Me
Fig. 2 2-Arylfluorene deriva-
tives obtained from Suzuki
reaction (reaction condi-
Me
tions: Pd(OAc)2/3a (2:1.1),
Me
0.01 mol%, 0.21 mmol ml−1
8a
8b
8c
, 98%
, 98%
Me
, 96%
arylboronic acid, 2.0 mol of
KOH, ethanol (50%), with
0.17 mmol ml−1 2-bromoflu-
orene, reaction maintained at
100 °C in oil bath, stirred about
1.5 h in air, the isolated yields
are presented)
Me
8d
8g
8e
8f
, 96%
, 95%
, 98%
, 97%
, 99%
F
F
F
F
F
F
F
8h
8k
8n
8i
, 98%
, 95%
, 92%
CF3
F
3C
OH
CF3
8l
, 97%
, 96%
8j
O
CHO
Et
O
CHO
8m
8o
, 94%
, 93%
S
CN
O
O
8r
, 96%
, 93%
, 91%
8p
8q
Ph
N
N
N
8s
8t
8u
, 93%
, 94%
, 97%
with excellent isolated yields (over 91%). The Suzuki reac-
tion of 2-bromofluorene and heteroarylboronic acids (includ-
ing 2-thienylboronic acid and 3-pypridylboronic acid) gave
corresponding products in very low isolated yields (<5 and
<10%, respectively).
2,7-diaryl-9,9′-alkylfluorene derivatives cannot dissolve in
ice ethanol/water, a simple mechanical filtration and solvent
wash method was used to purified the OFMs.
2.4 Reusability of Catalyst in Suzuki Reaction
The catalyst Pd residue in materials used in devices is a
very crucial issue, a tricky question for Pd/phosphine cat-
alyzed Suzuki coupling is the catalyst decomposition via
reductive, and remove the residue catalyst Pd from materi-
als aroused the attention of scientists also [50]. However,
the comparatively weak π-acceptor and strong σ-donor
properties of bis(imidazolium) ILs bear stronger coordina-
tion characteristics of phosphines [51], and the chelating
bis(imidazolium) ILs can increases the stability of metal
catalysis and avoid remove metal residue.
The reusability of catalyst system (Pd(OAc)2/3a = 2:1.1,
0.01 mol%) was tested with the coupling of 2-iodofluorene
and phenylboronic acid using KOH at 100 °C. The product
2-phenyl-9H-fluorene was soluble in non-polar solvents, in
contrast, the catalyst (Pd(OAc)2/3a) was soluble in polar
solvents, the product of Suzuki reaction was easily puri-
fication with hexane using simple extraction. The catalyst
(Pd(OAc)2/3a) combined with polar solvent was reused
directly. As shown in Table 4, our catalyst system could be
recycled five times with slightly decrease in catalytic activ-
ity, and the main reason may be associated with the decom-
position of catalytic species bis-NHCs–Pd.
Based on the solubility of each component, the
Pd/bis(imidazolium) ILs catalysis and KOH dis-
solved in ethanol/water, 2-arylfluorene derivatives and
1 3

Z. Zhou et al.
Fig. 3 2,7-Diaryl-9,9′-
Me
Me
Me
Me
alkylfluorene derivatives
obtained from Suzuki
reaction (reaction condi-
tions: Pd(OAc)2/3a (2:1.1),
0.01 mol%, 0.42 mmol ml−1
arylboronic acid, 2.0 mol of
KOH, ethanol (95%), with
0.17 mmol ml−1 2,7-dibromo-
9,9′-alkylfluorene, reaction
maintained at 100 °C in oil
bath, stirred about 1.5 h in air,
the isolated yields are pre-
sented)
Me
Me
9a
9c
9b
9d
, 93%
, 95%
, 98%
F
3C
CF3
Me
Me
F
3C
CF3
Me
Me
, 97%
Me
Me
F
F
Et Et
Et Et
F
Me
Me
F
9e
9f
,
96%
, 94%
H
H9
H9
H9
C4 9 C4
Me
Me
H
H9
C4 9 C4
, 95%
9g
9h
, 94%
H
F
F
C4 9 C4
H
H9
C4 9 C4
F
3C
CF3
F
F
F
F
9i
, 96%
, 98%
9j
H
H9
C4 9 C4
F
3C
CF3
H
C4 9 C4
S
S
F
3C
CF3
9k
9l
, 92%
, 97%
3 Conclusion
In summary, aliphatic chain and aromatic ring bridged glu-
copyranoside-substituted asymmetric bis(imidazolium) ILs
derivative from d-glucose were synthesized in two meth-
ods, and used in Pd-catalyzed environmentally friendly,
efficiently and recycle aqueous Suzuki reaction. The aro-
matic ring bridged asymmetric bis(imidazolium) ILs maybe
formed two functional Pd–NHCs freely chelating centers,
which is the main reason for high catalytic activity. Novel
carbazole-based fluorene core OFMs, 2-arylfluorene deriv-
atives (21 examples) and 2,7-diaryl-9,9′-dialkylfluorene
derivatives (12 examples) were synthesized in high isolated
yields (90–99%). Overall, we have provides an efficiently,
environmentally friendly, trouble-free catalyst system for the
preparation of OFMs.
Table 4 Reusability of Pd(OAc)2/3a catalyzed Suzuki reactions
Entry
Time (min)
Conversion (%)a Yield (%)b
1
2
3
4
5
10
10
10
15
30
98
98
97
96
93
97
96
95
93
92
Reaction conditions: 2-iodofluorene (3.0 mmol), phenylboronic
acid (4.5 mmol), catalyst Pd(OAc)2/3a=2:1.1, 0.01 mol %, KOH
(6.0 mmol), EtOH (50%) (10.0 mL), reflux in air
aAverage of two runs, measured by GC–MS
bIsolated yield, purified by column chromatography
1 3

Green, Efficient and Reusable Bis(imidazolium) Ionic Liquids Promoted Pd-Catalyzed Aqueous…
24. Qiao Y, Ma W, Theyssen N, Chen C, Hou Z (2017) Chem Rev
Acknowledgements This work was financially supported by the
National Natural Science Foundation of China (No. 21562002), the
Key Laboratory of Jiangxi University for Functional Materials Chem-
istry (No. FMC16201), and Gannan Normal University.
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