Copper (I) chloride catalyzed synthesis of diaryl ethers in ionic liquids under mild conditions

Article abstract of DOI:10.1081/SCC-120024748

The reaction of different aryl halides with aryl oxides catalyzed by CuCl in 1-butyl-3-methyl-imidazolium tetrafluoroborate [bmim][BF₄] and 1-butyl-3-methyl-imidazolium hexafluorophosphate [bmim][PF₆] ionic liquids to give diaryl ethers in 54-90% yields has been studied under mild reaction conditions.

Full text of DOI:10.1081/SCC-120024748

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Copper (I) Chloride Catalyzed Synthesis of Diaryl Ethers
in Ionic Liquids Under Mild Conditions
a
a
a
a
S. M. S. Chauhan , Nidhi Jain , Anil Kumar & K. A. Srinivas
a
Department of Chemistry, University of Delhi, Delhi, India
Version of record first published: 15 Aug 2006.
To cite this article: S. M. S. Chauhan , Nidhi Jain , Anil Kumar & K. A. Srinivas (2003): Copper (I) Chloride Catalyzed Synthesis
of Diaryl Ethers in Ionic Liquids Under Mild Conditions, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 33:20, 3607-3614
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 20, pp. 3607–3614, 2003
Copper (I) Chloride Catalyzed Synthesis of
Diaryl Ethers in Ionic Liquids Under
Mild Conditions
S. M. S. Chauhan,* Nidhi Jain, Anil Kumar, and K. A. Srinivas
Department of Chemistry, University of Delhi,
Delhi, India
ABSTRACT
The reaction of different aryl halides with aryl oxides catalyzed by
CuCl in 1-butyl-3-methyl-imidazolium tetrafluoroborate [bmim][BF4]
6
and 1-butyl-3-methyl-imidazolium hexafluorophosphate [bmim][PF ]
ionic liquids to give diaryl ethers in 54–90% yields has been studied
under mild reaction conditions.
Key Words: Copper(I) chloride; Diaryl ethers; Ionic liquid.
*Correspondence: S. M. S. Chauhan, Department of Chemistry, University of
Delhi, Delhi 110 007, India; E-mail: smschauhan@chemistry.du.ac.in.
3
607
DOI: 10.1081/SCC-120024748
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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Chauhan et al.
INTRODUCTION
Diaryl ethers are widely distributed in alkaloids, polyphenolics,
antibiotics and many other biologically active compounds as well as in
[
1,2]
many synthetic organic compounds.
phenols using copper salts as catalyst is an important reaction for
The reaction of aryl halides with
[
3–5]
the synthesis of diaryl ethers. These reactions are generally performed
ꢀ
at high reaction temperatures (200–300 C), in presence of strong
base and toxic solvents (pyridine, collidine, DMF etc.) and for long
reaction times to give low yields of diaryl ethers. Various attempts
have been made by replacing strong bases with milder bases such as
[7]
Cs CO , phosphazene and by changing the catalysts such as Pd, Ni
and mixed metal complexes to improve the yields of diaryl ethers.
The ambient room temperature ionic liquids, particularly those based
on 1,3-dialkylimidazolium cations are promising new reaction media
because of their unique chemical and physical properties. They are
moisture stable, have no vapor pressure and their unique solvating
[
3]
[6]
2
3
[
8]
[
9]
[
10]
properties make them better recyclable and green solvent.
been used as solvent in various organic transformations such as acyla-
They have
[
11]
[12]
[13]
[14]
tion,
hydrogenation,
Diels-Alder,
Heck reactions,
Robinson
[
15]
[16]
annulation and Knoevenagel reaction,
Suzuki cross-coupling
as well as other organic transformations.
The N-alkylation of indole and O-alkylation of 2-naphthol have also
and
[18,19]
[17]
Michael additions,
[
20]
been reported in ionic liquids
but there are no reports on arylation
of phenols with aryl halides. Herein, we report the reaction of aryl halides
with aryl oxides catalyzed by copper (I) chloride to synthesize diaryl
ethers in ionic liquids under mild reaction conditions.
RESULTS AND DISCUSSION
The optimum amount of phenoxide was found to be approximately
.5 times the aryl halide and gave diphenyl ether in 78% yield. However,
2
Scheme 1.

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Diaryl Ethers in Ionic Liquids
3609
the yield of diphenyl ether decreased to 56% (Table 1) when the reaction
of potassium phenoxide and bromobenzene in 1-butyl-3-methylimidazo-
lium tetrafluoroborate [bmim][BF ] containing CuCl was carried out in
4
stoichiometric amount. In the absence of cuprous chloride the formation
of diphenyl ether was not observed by the reaction of potassium
phenoxide and bromobenzene under these conditions indicating that
CuCl plays an important role in the reaction. The reaction of potassium
phenoxide and bromobenzene in conventional solvents such as DMF and
dioxane gave diphenyl ether in 20 and 12% yields respectively under same
conditions. This highlights that the ionic liquid plays an important role
in enhancing the rate of reaction. Further, the yield of diphenyl ether
reduced to 23% when the reaction was carried out under oxygen
atmosphere and this may be due to the consumption of copper (I)
chloride by oxygen. Different phenyl halides were taken for the synthesis
of diphenyl ether (Entries 1, 2, and 3, Table 1) under similar conditions
and their reactivities were found to be in the order PhI>PhBr>PhCl, in
[
21]
agreement with copper containing systems reported elsewhere.
The aryl oxides with electron donating substituents at p-position
(Entries 4 and 6, Table 1) gave higher yields and required milder reaction
conditions than unsubstituted aryl oxides. Similarly the activated aryl
halides with electron withdrawing substituents at o- and p-positions
(
yield of corresponding diaryl ethers. Further, C-C coupled side product
Entries 9, 10, and 11, Table 1) required less time (7–8 h) and gave higher
0
(
nitrochlorobenzene and potassium phenoxide in [bmim][BF ] (Entry 9,
4,4 -dinitrobiphenyl) was not observed in CuCl catalyzed reaction of p-
4
[
22]
Table 1) as was observed in DMF. It may be due to the preference of
ionic mechanism over radical mechanism in noncoordinating anion based
ionic liquid. The reaction of sodium phenoxide with bromobenzene in
[bmim][BF ] under similar conditions also gave 76% yield of diphenyl
4
ether indicating that the reaction is equally facile with sodium phenoxide.
The related ionic liquid 1-butyl-3-methyl-imidazolium hexafluoro-
phosphate [bmim][PF ] is also found to be an equally efficient solvent
6
for diaryl ether synthesis under these conditions (Entries 4 and 10, Table 1).
However it has been found that the reaction does not take place in
[bmim][Br] and the ionic liquid gets destroyed during the course of
reaction. The recovered ionic liquid [bmim][BF ] after the extraction of
4
the product with ethyl acetate:hexane (3 ꢁ 10 mL, 1:1 v/v) and finally with
ethyl acetate (10 mL) was dried under vaccum. The recovered ionic liquid
containing CuCl catalyst was further used for recycle experiments.
Additional potassium phenoxide (2.5 eq.) and bromobenzene (1.0 eq.)
were added to recovered ionic liquid and reaction was perfomed
under similar conditions as for fresh ionic liquid. The results of recycle

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610
Chauhan et al.

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Diaryl Ethers in Ionic Liquids
3611
Table 2. Recycle experiments of recovered ionic liquid [bmim][BF ] containing
4
a
CuCl in the reaction of potassium phenoxide and bromobenzene.
Run
Yield (%)
1
78
2
72
3
61
4
28
b
a
Reaction conditions: bromobenzene (0.4 mmol), potassium phenoxide
1.0 mmol), recovered [bmim][BF ] (2.0 mL) containing CuCl, 10 h.
(
b
4
Yields are based on GC-analysis.
experiments are given in Table 2. It was found that very low yields of
diphenyl ether were obtained after third cycle and further recycling of
ionic liquid becomes difficult.
In conclusion, the present procedure provides a facile and efficient
method for CuCl catalyzed synthesis of diaryl ethers in imidazolium ionic
liquids with noncoordinating anions. Simple work-up procedure, high
yield, nontoxicity and reusability of ionic liquids makes this procedure
a useful and attractive green alternative to the conventional methods in
the synthetic organic chemistry.
EXPERIMENTAL
1
The H NMR spectra were recorded on Brucker Heaven (300 MHz)
spectrophotometer using TMS as internal standard and CDCl as solvent
3
and chemical shift were expresssed in ppm. The IR spectra were recorded
using KBr pellets on Perkin Elmer Spectrum 2000 infrared and ꢀ
were
max
ꢂ1
expressed in cm . GC analysis was performed on Shimadzu GC-14B
using packed column SE-30. The melting points were taken on Thomas
HOOVER capillary melting point apparatus and all the melting points
and boiling points are uncorrected. Ionic liquids [bmim][BF ] and
[
4
23,24]
[bmim][PF ] were synthesized by the literature procedures
were characterized by IR and NMR spectroscopic data.
and
6
A mixture of bromobenzene (0.07 mL, 0.4 mmol), potassium
phenoxide (125 mg, 1.0 mmol), CuCl (40 mg, 0.4 mmol) in [bmim][BF ]
4
ꢀ
(
2.0 mL) was heated at 100 C with stirring for 10 h under nitrogen
atmosphere. The reaction was monitored by GC and TLC and the
product was extracted with ethyl acetate:hexane (3 ꢁ 10 mL, 1:1 v/v).
The product was purified by column chromatography to give diphenyl
ether in 72% yield and was characterized by comparison with authentic
1
sample (b.p., GC) and other spectroscopic data such as H NMR, IR, and
EI-MS. Similarly, other diaryl ethers were prepared by the reaction

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of corresponding potassium phenoxide and aryl halide in ionic liquid
containing CuCl and the yields are given in Table 1.
ACKNOWLEDGMENTS
We are grateful to Department of Biotechnology (DBT) for financial
support. The authors (NJ) and (AK) are grateful to Council of Scientific
and Industrial Research (CSIR) and University Grant Ccommission
(
UGC), New Delhi, respectively for research fellowships.
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