Application of Suzuki cross-coupling reaction catalyzed by ligandless palladium chloride in the synthesis of liquid crystals

Article abstract of DOI:10.1080/00397910701397292

Palladium chloride-catalyzed Suzuki cross-coupling reaction was applied to the preparation of highly pure multiring liquid crystals with a biphenyl unit. The optimal reaction condition is the combination of 0.5 mol% PdCl2, pyridine, and K3PO4, which was a

Full text of DOI:10.1080/00397910701397292

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Application of Suzuki Cross‐Coupling Reaction
Catalyzed by Ligandless Palladium Chloride in
the Synthesis of Liquid Crystals
a
a
b
a
Mei Huang , Jianhua Cheng , Xiaochun Tao , Mingwang Wei & Dong
a
Shen
a
Laboratory of Chemical Physics, East China University of Science and
Technology, Shanghai, China
b
Laboratory of Organometallic Chemistry, East China University of Science
and Technology, Shanghai, China
Published online: 03 Jul 2007.
To cite this article: Mei Huang , Jianhua Cheng , Xiaochun Tao , Mingwang Wei & Dong Shen (2007)
Application of Suzuki Cross‐Coupling Reaction Catalyzed by Ligandless Palladium Chloride in the Synthesis of
Liquid Crystals, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 37:13, 2203-2208, DOI: 10.1080/00397910701397292
To link to this article: http://dx.doi.org/10.1080/00397910701397292
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Synthetic Communications , 37: 2203–2208, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701397292
Application of Suzuki Cross-Coupling
Reaction Catalyzed by Ligandless Palladium
Chloride in the Synthesis of Liquid Crystals
Mei Huang and Jianhua Cheng
Laboratory of Chemical Physics, East China University of Science and
Technology, Shanghai, China
Xiaochun Tao
Laboratory of Organometallic Chemistry, East China University of
Science and Technology, Shanghai, China
Mingwang Wei and Dong Shen
Laboratory of Chemical Physics, East China University of Science and
Technology, Shanghai, China
Abstract: Palladium chloride–catalyzed Suzuki cross-coupling reaction was applied to
the preparation of highly pure multiring liquid crystals with a biphenyl unit. The optimal
2 3 4
reaction condition is the combination of 0.5 mol% PdCl , pyridine, and K PO , which
was able to catalyze the cross-coupling of substituted aryl bromides with substituted
trans-cyclohexylphenylboronic acids to give pure products in 38–87% yields.
Keywords: ligandless palladium catalysis, trans-cyclohexyl biphenyl liquid crystals,
Suzuki cross-coupling
INTRODUCTION
Palladium-catalyzed Suzuki cross-coupling reaction of arylboronic acid
with aryl halide has been widely used for selective construction of
Received in Japan December 7, 2006
Address correspondence to Dong Shen, Laboratory of Chemical Physics, East
China University of Science and Technology, Shanghai 200237, China. E-mail:
shen@ecust.edu.cn
2
203

2
204
M. Huang et al.
[
1]
carbon–carbon bonds in organic synthesis.
phosphine ligand, typically Pd(PPh ) , were usually employed in the
Palladium catalysts with
3
4
reactions. One disadvantage of using phosphine ligand catalysts is side
reactions due to aryl–aryl exchange between phosphine ligands and arylpal-
ladium halides and self-coupling of aryl groups of the arylboronic acids
[2]
when the cross-coupling is very slow. The side products, especially the
one from homocoupling of arylboronic acids, are sometimes very difficult
to separate from the cross-coupling products because of the similar polarity
and solubility of these compounds. Additionally, Pd(PPh ) is expensive
3
4
and thermally and air sensitive. These drawbacks limited the application of
the reaction to the preparation of highly pure cross-coupling products such
[
3–5]
as liquid crystals.
suppress the phosphine ligand–related side reactions and improve the effi-
The use of ligandless palladium catalysts could
[6]
ciency of the Suzuki cross-coupling.
effective solvent for PdCl -catalyzed Suzuki reactions in the synthesis of
In addition, pyridine was an
2
[7]
some simple biphenyl compounds. In this article, PdCl -catalyzed Suzuki
2
reactions were applied to the preparation of multiring liquid crystals with
high purity (.99.5%).
RESULTS AND DISCUSSION
0
The PdCl -catalyzed reaction of 4-bromobenzonitrile with 4,4 -propyl-trans-
2
cyclohexylphenylboronic acid was chosen for the model study to determine
the optimal quantity of PdCl . As shown in Table 1, the best result was
2
obtained when 0.5 mol% of PdCl was used for the reaction. Although the
2
yield was higher with more PdCl , the purity was lower, even after purification
2
by column chromatography and recrystallization. HPLC showed that the
amount of side products increased with increasing PdCl . So, 0.5 mol% of
2
Table 1. Effect of PdCl
2
quantity
a
Yield (%)
b
Entry
PdCl
2
(mol%)
Purity (%)
A1
A2
A3
A4
0.3
0.5
0.7
1.0
68
85
93
97
99.8
99.8
99.2
99.0
a
0
Reaction conditions: 4-bromobenzonitrile (2.0 mmol), 4,4 -propyl-trans-cyclohex-
ylphenylboronic acid (3.0 mmol), K CO (4.0 mmol), pyridine (4 mL), reflux 6 h
2
3
under argon.
b
Detected by HPLC.

Suzuki Cross-Coupling Reaction
2205
PdCl was used for our preparation of liquid crystals to meet industrial purity
2
standard (.99.5%).
A variety of aryl bromides and arylboronic acids were selected for the
Suzuki cross-coupling reaction to prepare our desired compounds. The results
are summarized in Table 2. As compared with other substrates, 4-bromobenzo-
0
nitrile (1, 2) that had electron-drawing substituent CN and 2-fluoro-4-4 -pentyl-
trans-cyclohexylphenylboronic acid (8) that had electron-drawing substituent F
at the 2 position gave higher yields (.80%). This agreed with the mechanism of
[
8]
Suzuki cross-coupling. Aryl bromides containing multifluoro substituents
5, 6) gave rather lower yields, probably due to the high volatility of multifluo-
(
rinated compounds and high activity of F in multifluorinated aryl compounds.
[9]
F was substituted, and complex defluoro by-products were found. When the aryl
bromide had electron-donating or fluoro substituents, K PO was working better
3
4
Table 2. PdCl
a
2
-catalyzed Suzuki cross-coupling of aryl bromides and arylboronic
acids
c
Arylboronic acid Yield Phase transition tempera-
0
d
Comp.
Aryl bromide R
4-CN
R
(%)
tures (lit.) (8C)
0
1
4,4 -Propyl- trans-
cyclohexyl
85
Cr. 133 N . 210 (Cr.
[
3]
132 N 230 Iso.)
Cr. 96 N . 210 (Cr. 94 N
0
2
4-CN
4-Et
4,4 -Pentyl- trans-
cyclohexyl
87
79
[
4]
219 Iso.)
Cr. 36 S 146 N 165 Iso.
(Cr. 34 S 146 N 164
b
3
0
4,4 -Pentyl- trans-
cyclohexyl
[
4]
Iso.)
Cr. 97 N 152 Iso. (Cr.
b
4
0
4-F
4,4 -Pentyl- trans-
cyclohexyl
78
54
38
62
82
[
12]
102 N 153.9 Iso.)
Cr. 52 N 106 Iso (Cr 55 N
b
5
0
3,4-Difluoro
4,4 -Pentyl- trans-
cyclohexyl
[
3]
108 Iso.)
Cr. 26 N 53 Iso. (Cr.
b
6
0
3,4,5-Trifluoro
4,4 -Pentyl- trans-
cyclohexyl
[
3]
30.4 N 58 Iso.)
Cr. 53 S . 210 (Cr. 54 S
0
0
7
8
4,4 -Propyl- trans- 4,4 -Pentyl- trans-
[
10]
cyclohexyl cyclohexyl
0
4,4 -Propyl- trans- 2-F; 4,4 -Pentyl-
251 N 311 Iso.)
Cr. 55 S 112 N . 210 (Cr.
0
[
10]
cyclohexyl
trans-cyclohexyl
55 S 112 N 291)
a
Reaction conditions: aryl bromide (2.0 mmol), arylboronic acid (3.0 mmol), K CO3
2
(
4.0 mmol), PdCl
2
(0.5 mol%), pyridine (4 mL), reflux 6 h under argon.
PO as base was used.
Purified by column chromatography and recrystallization, purity more than 95.5%
b
4.0 mmol K
3
4
c
(
detected by GC or HPLC).
d
Cr.: crystalline phase, N: nematic phase, S: smectic phase, Iso.: isotropic phase.

2
206
M. Huang et al.
as a base than K CO for the cross-coupling reaction. The reactions should be
2
3
stopped as the arylbromides were consumed completely (6 h, detected by thin-
layer chromatography, TLC), otherwise the self-coupling of the arylboronic
acids (excess arylboronic acids were used in reactions) would increase and
make the purification of the products very difficult.
All of the products have liquid-crystalline property and exhibit N or N, S
phases. The compounds with CN or four rings have rather high clearing temp-
eratures, so they can be used in liquid-crystalline mixtures to increase the
clearing temperatures. Because fluorinated liquid crystals have the advantages
[
9,10]
of low viscosity and high voltage holding ratio,
5
the compounds with F (4,
, 6, 8) can be used in thin film transistor (TFT) liquid-crystalline displays.
In summary, palladium chloride was used as a catalyst in pyridine for the
Suzuki cross-coupling reaction of arylboronic acids with aryl bromides to give
multiring liquid crystals with high purity (.99.5%).
EXPERIMENTAL
1
All products are known and confirmed by transition temperatures, H NMR
spectra, and MS spectra. Transition temperatures were determined with a
1
XP-201 polarizing micromelting-point apparatus, H NMR spectra were
measured on a Brucker AM 500 (500-MHz) spectrometer using TMS as
internal standard; MS spectra were recorded on a Micromass LCT; gas chrom-
atography (GC) and high pressure liquid chromatography (HPLC) analysis
were performed using GC9790 and Agilent 1100. Solvent pyridine was
[11]
dried according to the method described in the literature.
Aryl bromides
and arylboronic acids were provided by Yantai Xianhua Chem.-Tech. Co.,
Ltd.; other starting materials are commercially available.
General Procedure of PdCl -Catalyzed Ligandless Suzuki
2
Cross-Coupling Reaction
A mixture of aryl bromide (2.0 mmol), arylboronic acid (3.0 mmol), K CO or
3
2
K PO (4.0 mmol), PdCl (0.5 mmol%), and pyridine (4 mL) was refluxed
3
4
2
under argon until complete consumption of the aryl bromide, as determined
by TLC analysis, and then cooled down to room temperature. After the
solvent was recovered under vacuum, the residue was purified by chromato-
graphy on silica gel using petroleum ether as eluant to give the product as a
white solid, which was further purified by recrystallization.
Spectral Data of the Selected Products
0 0
-(4-Pentyl-trans-cyclohexyl)-4 -cyano-1,1 -biphenyl (1): White crystal,
4
[3]
phase-transition temperature (8C): Cr. 96 N .210 (lit. Cr. 94 N 219).

Suzuki Cross-Coupling Reaction
2207
1
H NMR (CDCl , 500 MHz) d ¼ 0.90 (t, J ¼ 7.1 Hz, 3H), 1.02–1.11 (m, 2H),
3
1
.21–1.36 (m, 9H), 1.44–1.53 (m, 2H), 1.85–1.97 (m, 4H), 2.48–2.57
(
m, 1H), 7.32 (d, J ¼ 8.3 Hz, 2H), 7.52 (d, J ¼ 8.1 Hz, 2H), 7.66–7.71
þ
þ
(
m, 4H). MS(EI m/z) ¼ 331.4 (100, M ), 332.4 (15.92, M þ 1), 218.2
þ þ þ
5 11 3 6 5 11 4 7
(
30.56, M -C H C H ), 205.2 (86.74, M -C H C H ), 192.2 (28.70, M -
C H C H ).
5 11 5 8
0 0
-(4-Pentyl-trans-cyclohexyl)-4 -fluoro-1,1 -biphenyl (4): White crystal,
4
[12]
phase-transition temperature (8C): Cr. 97 N 152 Iso. (lit.
Cr. 102 N 153.9
1
Iso.). H NMR (CDCl , 500 MHz) d ¼ 0.90 (t, J ¼ 7.1 Hz, 3H), 1.03–1.10
3
(
m, 2H), 1.21–1.36 (m, 9H), 1.44–1.52 (m, 2H), 1.86–1.95 (m, 4H), 2.47–
.54 (m, 1H), 6.98–7.13 (m, 2H), 7.28 (t, J ¼ 8.2 Hz, 2H), 7.46
2
1
9
(
d, J ¼ 8.2 Hz, 2H), 7.51–7.54 (m, 2H). F NMR (CDCl , 470 MHz)
3
þ
d ¼ 2177.34 (m, 1F). MS (EI m/z) ¼ 324.4 (100, M ), 325.4 (16.16,
þ
þ
þ
M þ 1), 211.2 (55.62, M -C H C H ), 198.2 (51.50, M -C H C H ),
5
11
3
6
5 11 4 7
þ
85.2 (30.04, M -C H C H ).
5 11 5 8
1
0
0
0
0
4
-(4-Pentyl-trans-cyclohexyl)–3 ,4 ,5 -trifluoro-1,1 -biphenyl (6): White
[3]
solid, phase-transition temperature (8C): Cr. 26 N 53 Iso. (lit. Cr. 30.5 N
1
5
1
2
8 Iso.). H NMR (CDCl , 500 MHz) d ¼ 0.90 (t, J ¼ 7.1 Hz, 3H), 1.01–
3
.10 (m, 2H), 1.22–1.34 (m, 9H), 1.42–1.51 (m, 2H), 1.88–1.92 (m, 4H),
.49–2.52 (m, 1H), 7.18–7.15 (m, 2H), 7.29 (t, J ¼ 7.7 Hz, 2H), 7.42
1
9
(
d, J ¼ 8.1 Hz, 2H). F NMR (CDCl , 470 MHz) d ¼ 2135.36 (m, 2F),
3
þ
þ
2
(
164.19 (tt, 1F). MS (EI m/z) ¼ 360 (100, M ), 361 (10.25, M þ 1), 247
þ þ þ
5 11 3 6 5 11 4 7
19.00, M -C H C H ), 234 (78.50, M -C H C H ), 229.2 (48.19,M -
C H C H ).
5 11 5 8
ACKNOWLEDGMENT
This work was supported by Ministry of Education, P. R. China.
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