Potassium fluoride supported on natural nanoporous zeolite: A new solid base for the synthesis of diaryl ethers

Article abstract of DOI:10.1002/ejoc.201001447

An efficient and inexpensive synthesis of diaryl ethers has been developed. The process involves the nucleophilic aromatic substitution of electron-deficient aryl halides and phenols and is mediated by potassium fluoride/Clinoptilolite (KF/CP) in dimethyl sulfoxide (DMSO). The approach affords good to excellent yields of the arylated products without the need for additional cation capture. The solid base is also efficient in the Ullmann ether synthesis. A new, inexpensive solid base for the synthesis of diaryl ethers has been developed. Clinoptilolite, a natural zeolite with a high tendency for cation capture, combined with potassium fluoride, provides an efficientsolid base for the deprotonation of phenols in nucleophilic reactions. The same solid base acts as a reliable base for the copper-catalyzed synthesis of diaryl ethers. Copyright

Full text of DOI:10.1002/ejoc.201001447

FULL PAPER
DOI: 10.1002/ejoc.201001447
Potassium Fluoride Supported on Natural Nanoporous Zeolite: A New Solid
Base for the Synthesis of Diaryl Ethers
[a]
[a]
[a]
Mohammad A. Khalilzadeh,* Abolfazl Hosseini,* and Afsaneh Pilevar
Keywords: Heterogeneous reactions / Fluorides / C-O coupling / Nucleophilic substitution / Phenols / Zeolites
An efficient and inexpensive synthesis of diaryl ethers has
been developed. The process involves the nucleophilic aro-
matic substitution of electron-deficient aryl halides and phe-
nols and is mediated by potassium fluoride/Clinoptilolite
(KF/CP) in dimethyl sulfoxide (DMSO). The approach affords
good to excellent yields of the arylated products without the
need for additional cation capture. The solid base is also ef-
ficient in the Ullmann ether synthesis.
tion-metal oxides,[8] clay minerals,[9] and zeolites.[10] Zeolites
are particularly interesting in this context because of their
high capacity for cation exchange. Much of the research
Introduction
Diaryl ethers are valuable intermediates in organic syn-
thesis and have attracted significant attention due to their
role as structural units in a variety of naturally occurring
and medicinally important compounds.[ The nucleophilic
[11]
on zeolites has focused on synthetic materials; however, in
recent years considerable attention has been directed
towards natural zeolites. The main reason for the interest
1]
aromatic substitution (S Ar) reaction between activated
[12]
N
in natural zeolites is the increasing demand for low-cost
aryl halides and phenols is an important method for the
preparation of diaryl ethers;[ the coupling of activated aryl
fluorides with various phenols by using KF/Al O /18-
crown-6 described by Sawyer et al. is a representative exam-
ple of this chemistry.[ When compared to typical transi-
tion-metal-mediated processes for the formation of diaryl
ion-exchange materials, catalysts, and adsorbent materials
2]
[13]
in fields such as waste-water treatment, animal nutrition,
[14]
2
3
and aquacultural farms.
The abundance of natural zeo-
lites around the world also contributes to the interest in
these materials. Thus, naturally occurring nanoporous zeo-
lites such as Clinoptilolite (CP) have numerous potential
3]
ethers, the S Ar reaction is often found to be simpler,
[15]
N
applications in chemistry.
milder, and environmentally more benign. An interesting
variant on this approach involves the coupling of aryl silyl
ethers with aryl fluorides. Various methods have been used
to promote desilylation, many of which rely on the use of
costly or complex bases.[ In all of the cases discussed
above, the nature of the base used plays an important role
in the progress of the coupling reaction, and numerous
bases are typically screened in the process of optimizing
such reactions. Solid bases[ have become popular in these
cases, because they often exhibit useful properties, such as
enhanced reactivity and selectivity, and allow for straight-
forward workup procedures. Our interest in organic reac-
tions carried out under heterogeneous conditions prompted
us to investigate the utility of solid bases in carbon–oxygen
bond-forming reactions. In recent years, much research has
Clinoptilolite has been shown to selectively adsorb po-
[16]
tassium and larger alkali metal cations.
Bearing this in
mind, in the course of our work towards the development
of new catalytic methods for the preparation of diaryl
ethers, we discovered that Clinoptilolite impregnated with
KF (KF/CP) was a particularly effective base in the cou-
pling of phenols with activated aryl fluorides. To the best
of our knowledge, the use of KF/CP for diaryl ether synthe-
sis has not previously been reported. We therefore sought
4]
5]
to develop a novel, solid-base-mediated S Ar coupling
N
methodology involving the use of commercially available
Clinoptilolite doped with potassium fluoride.
Results and Discussion
[
6]
focused on the development of heterogeneous catalysts,
such as NaOH and KOH supported on alumina,[ transi-
7]
Our initial studies focused on the reaction between phe-
nol (1a) and 1-fluoro-4-nitrobenzene (2a) with the aim of
[
a] Department of Chemistry, Islamic Azad University, Qaemshahr identifying optimal reaction conditions (Table 1). To ident-
Branch,
ify general and cost-effective conditions, we conducted
P. O. Box 163, Qaemshahr, Iran
Fax: +98-123-224-0091
these reactions without any promoters, such as transition-
metal or phase-transfer catalysts. It was found that the na-
ture of the base and the solvent both have a profound influ-
ence on this reaction. The reaction of 1a with 2a mediated
E-mail: khalilzadeh73@yahoo.com
sabolfazl@yahoo.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001447.
Eur. J. Org. Chem. 2011, 1587–1592
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1587

M. A. Khalilzadeh, A. Hosseini, A. Pilevar
FULL PAPER
by either KF or Clinoptilolite alone did not proceed, with increased absorption of fluoride anions on Clinoptilolite.[17]
only a trace amount of product being obtained in MeCN Having developed a successful protocol for the aromatic C–
or dimethyl sulfoxide (DMSO) (Table 1, Entries 1–4). In O bond formation with KF/CP, we sought to assess the
contrast, the use of supported KF/CP led to the formation scope and generality of this reaction by coupling various
of the desired product 3a in high yield (Table 1, Entry 5). phenolic nucleophiles and nitro-activated aryl halides in the
Although unsupported K PO was a more effective base presence of KF/CP in DMSO (Table 2).
3
4
than unsupported KF in this S Ar reaction, it proved to
be less effective when supported on Clinoptilolite, affording
only a 32% yield of the desired ether (Table 1, Entries 6 and
N
Table 2. KF/CP-promoted coupling of nitro-activated aryl fluorides
with phenols.[
a]
7). It is likely that this is due to a strong binding interaction
between the phosphate ion and the Clinoptilolite, which
prevents the release of the phosphate ion into the bulk sol-
vent; this, in turn, results in limited deprotonation of the
phenol, giving rise to the poor yield observed.
Table 1. O-Arylation of phenol with 1-fluoro-4-nitrobenzene; opti-
mization of the reaction conditions.[a]
Entry
Base
Solvent Time Yield
Temp.
[°C]
Base
[g]
[b]
[c]
[h]
[%]
[
d]
1
2
3
4
5
6
7
8
9
KF
CP
KF
CP
KF/CP
MeCN
MeCN
DMSO
DMSO
MeCN
MeCN
11 trace
10 trace
4
4
reflux
reflux
110–115
110–115
reflux
reflux
reflux
reflux
0.22 (3.8)
0.4
0.22 (3.8)
0.4
0.62 (3.8)
0.56 (2.7)
1.4 (2.7)
0.62 (3.8)
0.62 (3.8)
0.62 (3.8)
0.62 (3.8)
[d]
Ͻ10
Ͻ5
75
10
11
10
8
3
K PO
4
45
32
68
K
3
4
PO /CP MeCN
KF/CP
KF/CP
KF/CP
KF/CP
toluene
DMF
DMSO 0.5
dioxane
1.5
97
110–115
110–115
reflux
1
0
100
48
11
8
[
a] Reaction conditions: ArF (2.4 mmol), phenol (2.6 mmol), sol-
vent (4–6 mL). [b] Average GC yields with n-octane as an internal
standard. [c] The amount of base in mmol is given in parentheses.
[d] Phenol (3.5 mmol) was used.
Having identified KF/CP as a suitable base, a brief sol-
vent screen was conducted (Table 1, Entries 5 and 7–11).
Good yields were obtained with acetonitrile and toluene,
but N,N-dimethylformamide (DMF) and DMSO proved to
be superior; use of the latter solvent, in particular, gave an
excellent yield (100%) in a comparatively short time. It is
noteworthy that the quantity of Clinoptilolite was also
found to be important: increasing the proportion of Clinop-
tilolite relative to the KF in the reaction mixture had an
adverse effect on the yield (Figure 1). This may arise from
[
a] Reaction conditions: ArF (2.4 mmol), phenol (2.6 mmol), KF/
CP (0.62 g), DMSO (4–6 mL). [b] Isolated yield. [c] A 1.5-fold ex-
cess of KF/CP was used.
The reaction of phenol with 2a proceeded smoothly and
in high yield in only 30 min; this reaction time is consider-
ably shorter than has previously been reported for the cou-
pling of these two compounds under alternative reaction
[
18]
conditions.
Under our conditions, electron-rich phenols
underwent smooth, high-yielding coupling reactions to af-
ford the appropriate diaryl ethers (Table 2, Entries 2–5). It
is interesting to note that the yield of 1-(4-methoxyphen-
Figure 1. Effect of the percentage of KF/CP on the outcome of the
reaction.
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Eur. J. Org. Chem. 2011, 1587–1592

Zeolite-Supported Potassium Fluoride
oxy)-4-nitrobenzene obtained by using our conditions was Table 3. KF/CP-promoted coupling of nitro-activated aryl chlor-
ides or aryl bromide with phenols.[
a]
similar to that obtained by using more elaborate procedures
involving deprotection of silylated phenols in situ (Table 2,
[
4c,4d]
Entry 3).
Nucleophiles bearing mildly electron-with-
drawing groups required somewhat longer reaction times
but reacted efficiently nevertheless (Table 2, Entries 6–9).
Importantly, these new conditions facilitate efficient and
high-yielding coupling reactions with sterically demanding
substrates (Table 2, Entries 10–12). It was also found that
with an extended reaction time, KF/CP effectively mediated
the coupling of the electron-deficient nucleophile 4-cyano-
phenol with 2a. Similarly, the reaction of phenols with
1-fluoro-2-nitrobenzene proceeded efficiently to afford the
desired products with yields comparable to those obtained
under ultrasonic reaction conditions (Table 2, Entries 14
[
19]
and 15).
Interestingly, yields did not significantly vary
upon scaling up the reaction, because substrate 3a was ob-
tained in similar yield when the reaction was performed on
multigram scales.
Due to their low cost and ready availability, aryl chlo-
rides are much more attractive substrates than the corre-
sponding fluorides for the preparation of diaryl ethers on
larger scales. We therefore investigated the efficiency of the
KF/CP system in the coupling reaction of electron-deficient
aryl chlorides with phenols. As summarized in Table 3, KF/ [a] Reaction conditions: ArCl(Br) (2.0 mmol), phenol (2.1 mmol),
KF/CP (1.24 g), DMSO (8 mL). [b] Isolated yield.
CP was an effective promoter of the coupling reaction, with
good or moderate yields being obtained with most of the
substrates examined. 1-Chloro-4-nitrobenzene was treated
We briefly investigated the possibility of using KF/CP
with phenol at 120 °C. The yield was improved to 74% as a fluorinating agent, but trial reactions with 1-chloro-4-
when the ratio of KF/CP to the substrates was increased by nitrobenzene failed to afford the desired product even after
a factor of two (Table 3, Entry 1). The coupling of 1-chloro- extended reaction times. Nevertheless, the efficiency of this
4-nitrobenzene with 4-methylphenol proceeded in good reaction protocol encouraged us to test the coupling of ani-
yield and within a reasonably short reaction time (Table 3, line and thiophenol with 2a, and we were pleased to find
Entry 2). Overall, the reaction efficiencies obtained by using that the coupling proceeded effectively to give the corre-
KF/CP were significantly better than those reported by sponding diarylated product in 62 and 98% yield, respec-
using other methods.[
4c,20]
The coupling of moderately elec- tively (Scheme 1).
We examined the reusability of KF/CP in the coupling
tron-deficient phenols also proceeded smoothly and with
good yields (Table 3, Entries 4 and 5). Steric hindrance had of phenol and 2a. The immobilized solid base was separated
a significant effect on the rates of coupling reactions involv- from the reaction mixture and reused. However, the activity
ing aryl chlorides. For instance, an extended reaction time decreased considerably, and the yield was reduced to 55%,
was required to obtain a reasonable yield (65%) in the cou- probably due to leaching of the KF from the zeolite surface.
pling of 2,6-dimethylphenol with 1-chloro-4-nitrobenzene
Table 3, Entry 6). Our conditions also proved suitable for with phenols encouraged us to study the efficiency of the
use with the less active electrophile 1-bromo-4-nitrobenzene KF/CP on the copper-catalyzed etherification. Even though
Table 3, Entry 7). Unactivated aryl halides did not undergo there are many systems that work reasonably well for the
coupling with phenol by using our standard protocol, with aryl ether synthesis, the use of moisture-sensitive Cs CO is
The unsuccessful reaction of unactivated aryl halides
(
(
2
3
[
21,22a–22g]
only trace amounts (less than 8%) of diphenyl ether being crucial to the success of the reaction.
Thus, non-
observed after extended reaction times. hygroscopic and easily recoverable bases that can be used
Scheme 1. KF/CP-promoted coupling of aryl halides with aniline or thiophenols (GC yields with n-octane as an internal standard).
1589
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M. A. Khalilzadeh, A. Hosseini, A. Pilevar
FULL PAPER
in this reaction are still in demand. In addition to the base, in Table 5. Various aryl iodides and phenols reacted to give
the type of ligand also has great influence on the outcome the corresponding diaryl ethers under mild reaction condi-
of Ullmann coupling reactions. In fact, only in the past few tions. We found that both electron-rich (Table 5, Entries 6
years has considerable effort been devoted to improving the and 7) as well as electron-deficient (Table 5, Entry 8) aryl
efficiency of this reaction through the use of copper salts iodides were suitable substrates for this reaction, which pro-
[
22]
with several ligands for the synthesis of diaryl ethers.
vided the corresponding diaryl ethers in good to excellent
Among a variety of multidentate chelating ligands used, yields. The coupling of sterically hindered phenols pro-
N- and O-bidentate chelators appear in the majority of the ceeded successfully to form the corresponding ethers in
copper-catalyzed coupling reaction protocols. Using a solid good yield (Table 5, Entries 3 and 5). The yields were dif-
base, we optimized the reaction conditions by varying the ferent with electron-rich and electron-deficient phenols. The
copper salts and applying a set of ligands, including N,N- presence of electron-releasing groups, such as methyl and
and N,O-chelators. A variety of multidentate chelating li- methoxy groups, in the phenol did not affect the outcome
gands were examined by using iodobenzene and p-cresol as of the reaction (Table 5, Entries 2 and 9 vs. 1), whereas an
model substrates, copper(I) salts as the precatalyst, DMSO electron-withdrawing group, such as a chloro group, in the
as the solvent, and KF/CP as the base. Many ligands that phenol decreased the yield to 65% (Table 5, Entry 10 vs. 1).
have proved to be successful in such a system were used. The presence of an electron-releasing group in the iodo-
Among these were 2-isonitrosoacetophenone (L1), 1,10- benzene decreased the yield of the product (Table 5, En-
phenanthroline (L2), and α-benzoin oxime (L3). Excellent tries 6 and 7). Similar to nucleophilic aromatic substitution,
yields were obtained in the presence of L1 and L2 at 110 °C the presence of electron-withdrawing groups in the aryl io-
within 12 h. A preliminary survey had shown that salts such dide moiety significantly increased the yield as well as the
as CuI and CuBr were effective copper catalysts, whereas rate of reaction, hence, a very strong electron-withdrawing
CuCl or Cu(OAc) displayed little activity (Table 4); the group, such as a nitro group at the para position of iodo-
2
2
CuI catalyst was especially effective when used in combina- benzene, drives the reaction to proceed even without cata-
tion with L1 or L2 as ligands. Screening of a range of sol- lyst through an S Ar mechanism instead of a copper-com-
N
vents showed DMSO to be much more efficacious than 1,4- plex-catalyzed coupling reaction (Table 5, Entry 8).
dioxane and acetonitrile. Although the formation of some
byproducts from the transformation of aryl halide into phe- Table 5. Coupling reaction of aryl iodide with phenols in the pres-
ence of L2–CuI catalyst.[
a]
nol has been reported by using cesium carbonate in similar
coupling reactions, no side reaction involving phenol or di-
phenyl ether was observed with KF/CP.[
23]
Table 4. Coupling of p-cresol with iodobenzene under different
[
a]
conditions.
Entry
Copper source
Ligand
Yield [%][b]
1
2
3
4
5
6
7
CuI
CuI
CuI
L1
L2
L3
L1
L2
L1
L1
92
94
82
62
66
21
18
CuBr
CuBr
Cu(OAc)
CuCl
2
2
[
(
a] Reagents and conditions: iodobenzene (0.1 mmol), p-cresol
0.25 mmol), Cu catalyst (0.01 mmol), ligand (0.02 mmol), and
I
KF/CP (0.186 g), DMSO (1 mL). [b] Isolated yield.
[
(
a] Reagents and conditions: phenol (0.25 mmol), aryl iodide
0.1 mmol), ligand (L1 or L2; 0.02 mmol), KF/CP (0.186 g), CuI
0.01 mmol), DMSO (1 mL). [b] Isolated yield. [c] The numbers in
Using the data obtained from the optimization process
described above, we conducted initial investigations into the
scope of the Ullmann coupling reaction with L1– or L2–
(
parentheses correspond to the yield obtained by using L1 as ligand.
CuI as the complex of choice; the results are summarized [d] Performed without copper complex.
1
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Zeolite-Supported Potassium Fluoride
were purified either by recrystallization (methanol) or by column
chromatography on silica gel (ethyl acetate/hexane).
Conclusions
We have developed a novel, convenient, scalable, high-
yielding synthesis of diaryl ethers based on the KF/CP-me-
diated coupling of phenols with activated aryl halides as
well as unactivated aryl halides using the Ullmann protocol.
Typical Procedure B. Coupling of Aryl Chlorides (Bromides) with
Phenols in DMSO: A mixture of the phenol (2.1 mmol), the appro-
priate aryl halide (2 mmol), and 35% (w/w) potassium fluoride/
Clinoptilolite (1.24 g) in DMSO (8 mL) was heated to 115–120 °C
Clinoptilolite is one of the most abundant naturally occur- (bath temperature) and stirred under air until the reaction was
ring zeolites, and is both commercially available and inex- complete (monitored by TLC or GC). The reaction mixture was
pensive. This methodology provides an efficient route to a cooled to room temperature, diluted with ethyl acetate, and filtered.
The resulting solution was placed in a separating funnel and
wide variety of substituted phenolic compounds. Moreover,
washed twice with water. The organic layer was dried with anhy-
the simple reaction procedure does not require the presence
drous sodium sulfate, filtered, and concentrated in vacuo. Products
were purified either by recrystallization (methanol) or by column
chromatography on silica gel (ethyl acetate/hexane).
of a crown ether and is tolerant towards moisture and oxy-
gen in the case of the S Ar reaction. The generally good to
N
excellent yields makes this new system an attractive alterna-
Typical Procedure C. Copper-Catalyzed Coupling of Aryl Iodides
with Phenols in DMSO: An oven-dried, screw-cap tube was
charged with p-cresol (0.25 mmol), iodobenzene (0.1 mmol), L2
tive to other S Ar procedures. Further applications of this
N
method are underway and will be reported in due course.
(
0.02 mmol), KF/CP (0.186 g), and CuI catalyst (0.01 mmol). The
tube was septum-capped, evacuated, and back-filled with argon.
Anhydrous DMSO (1 mL) was added, and the reaction mixture
was stirred at 110 °C for 18 h (the progress of the reaction was
monitored by GC). After complete disappearance of the iodo-
benzene, the reaction mixture was cooled to room temperature, di-
luted with ethyl acetate, filtered, and the crude product was purified
by column chromatography on silica gel (ethyl acetate/hexane) to
Experimental Section
General Remarks: The raw zeolite material was an Iranian commer-
cial Clinoptilolite (Afrandtooska Company) obtained from de-
posits in the region of Semnan (ca. 1 $ per kg). All starting materi-
als were used as purchased without further purification. Reactions
were performed under air. Thin-layer chromatography (TLC) was
performed by using 60 mesh silica gel Merck TLC plates. Flash
column chromatography was performed with Merck silica gel (230–
give 1-methyl-4-phenoxybenzene (171 mg, 94%) as a colorless li-
1
quid. H NMR (400 MHz, CDCl
3
): δ = 2.42 (s, 3 H), 7.01 (d, J =
8
8
.0 Hz, 2 H), 7.05–7.1 (m, 2 H), 7.12–7.18 (m, 1 H), 7.21 (d, J =
400 mesh). The yields reported are isolated yields and are the
1
3
.4 Hz, 2 H), 7.38 (t, J = 8.4 Hz, 2 H) ppm. C NMR (100 MHz,
): δ = 20.8, 118.5, 119.3, 122.9, 129.8, 130.4, 133.0, 154.9,
average of at least two runs. The reaction products were analyzed
with an HP GC 6890 instrument equipped with a DB-5 CP column
CDCl
1
1
3
+
57.9 ppm. HRMS: calcd. for C13
85.0974.
H13O [M + H ] 185.0966; found
(
30 mϫ0.32 mmϫ0.5 μm); GC program parameters: injector
50 °C; flow 2 mL/min; temperature program 40 °C (2 min); 10 °C/
min to 280 °C; hold 5 min. Melting points were determined with
2
Supporting Information (see footnote on the first page of this arti-
cle): Characterization data and copies of spectra for new com-
pounds.
1
an Electrothermal IA 9100 Digital Melting Point apparatus.
and 13C NMR spectra were recorded in [D
]DMSO with a Bruker
DRX-300 Avance spectrometer operating at 300.13 and
5.47 MHz, respectively. ESI mass spectra were acquired with a
H
6
7
Waters QToF 1 MS system equipped with an off-line nanospray Acknowledgments
ionization source.
The authors wish to thank the Islamic Azad University, Qaemsh-
ahr Branch for funding this research.
Preparation of KF/Clinoptilolite: Prepared by dissolving KF
5.5 g) in distilled water (10 mL) and Clinoptilolite (10 g). The mix-
(
ture was stirred for 1 h then, the water was removed at 60–70 °C
in a rotary evaporator under reduced pressure. The impregnated
Clinoptilolite was further dried at 70–80 °C in a vacuum drying
oven for 30 h. The hard, dry material, thus obtained, was powdered
with the help of either a hand held electric food mixer or a pestle
and mortar. The resulting KF/CP was kept in a desiccator until
required.
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3
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PO /Clinoptilolite: Prepared by a similar pro-
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olite (10 g).
3
PO (7 g) in distilled water (10 mL) and Clinoptil-
4
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drous sodium sulfate, filtered, and concentrated in vacuo. Products
Eur. J. Org. Chem. 2011, 1587–1592
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M. A. Khalilzadeh, A. Hosseini, A. Pilevar
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