KF/Clinoptilolite, an effective solid base in Ullmann ether synthesis catalyzed by CuO nanoparticles

Article abstract of DOI:10.1039/c3nj00834g

Employing KF/Clinoptilolite as an efficient base the cross-coupling reactions of various phenols with aryl iodides could be successfully carried out in the presence of copper oxide nanoparticles. The C-O coupling products were obtained in moderate to good yields (62-87%) for a variety of substrates. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014.

Full text of DOI:10.1039/c3nj00834g

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KF/Clinoptilolite, an effective solid base in Ullmann
ether synthesis catalyzed by CuO nanoparticles
Cite this: New J. Chem., 2014,
38, 42
Mohammad A. Khalilzadeh,*a Hoda Keipour,b Abolfazl Hosseinib and
Daryoush Zareyeea
Received (in Porto Alegre, Brazil)
25th July 2013,
Accepted 8th October 2013
DOI: 10.1039/c3nj00834g
www.rsc.org/njc
Employing KF/Clinoptilolite as an efficient base the cross-coupling by aqueous quenching. In the past decades, however, there has
reactions of various phenols with aryl iodides could be successfully been increasing interest in looking for new applications of clays,
carried out in the presence of copper oxide nanoparticles. The C–O natural, quite common and very cheap materials with superior
coupling products were obtained in moderate to good yields performances over the currently used materials.7 Among the salts
(62–87%) for a variety of substrates.
with potentially basic properties potassium fluoride is more
popular because of its cheapness and availability. A lot of supports
The synthesis of diaryl ethers has been an important objective in have been introduced for increasing the basicity of potassium
organic synthesis since they are important intermediates in the fluoride such as KF/ZnO,8 KF/Ca–Mg–Al,9 KF/CaO–Fe3O4,10
synthesis of biologically and pharmaceutically active molecules.1 KF/celite,11 KF/NaY zeolite,12 KF/montmorillonite,13 KF/CaF2,14
The classical Ullmann method for carbon–heteroatom bond KF/natural phosphate15 and KF/Ca–Al hydrotalcite16 mainly applied
formation is performed using stoichiometric amounts of copper for biodiesel production. Although, a majority of the above-
reagents at high temperature, which leads to environmental and mentioned solid bases are effective in organic transformations,
waste disposal problems at a larger scale.2 To overcome these they suffer from some drawbacks such as low basicity, use of
drawbacks, considerable attention has been recently focused on expensive solid supports and tedious preparation steps.
development of catalytic systems for this purpose.3 During the
Even though solid supported bases have found wide applica-
past few years, metal-catalyzed methods for the preparation of tions in organic transformation, there are only a few reports on
diaryl ethers, in particular, with aryl halide derivatives facilitated the Ullmann-type coupling reactions with solid bases.
by transition metal catalysts4 have been reported. Recent
Clinoptilolite (CP) is a naturally occurring zeolite mineral,
approaches in the use of copper salt nanoparticles have been with an open aluminosilicate framework structure, and a high
proved to be an effective alternative for the classical Ullmann internal surface area with broad applications in chemistry and
reaction under ligand-free conditions. The high surface area and other industries.17
reactive morphology of the CuO and copper salt nanoparticles
Clinoptilolite has high cation exchange capacity especially
made them successful catalysts in C–N, C–O, and C–S cross- for potassium cations.18 Thus the use of this property by
coupling reactions.5 Besides the importance of catalysts used in impregnation of potassium fluoride on CP causes a more free
cross-coupling reactions, the results established the significant role fluoride anion that is able to act as an effective base and it is
of the base in the outcome of the reaction. Among the bases used, obtained by simple mixing in water and evaporation without
solid bases6 have become popular due to their environmentally- the need for any preparation or preactivation.
friendly nature and unique properties. In contrast, homogeneous
Prompted by this finding, we have recently developed a
base catalysts inescapably involve in the emission of a large amount simple, cheap and effective solid base for C–O cross-coupling
of wastewater and the separation difficulty of the catalysts from reaction with potassium fluoride impregnated on Clinoptilolite.19
products owing to the emulsification and the saponification caused In continuation of our interest in exploring coupling methodology,
we report herein our results obtained with a nanosized CuO catalyst
for an efficient synthesis of diaryl ethers in the presence of KF/CP
as the base.
The reactions are successful at 120 1C in DMSO in the
presence of KF/CP under an inert atmosphere. The reaction of
phenol with iodobenzene was first studied as model substrates
a Department of Chemistry, Qaemshahr Branch, Islamic Azad University,
Qaemshahr, Iran. E-mail: m.khalilzadeh@hotmail.com; Fax: +98 1232211647;
Tel: +98 9111130400
b Department of Chemistry, Science and Research Branch, Islamic Azad University,
Mazandaran, Iran
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Table 1 Screening of reaction conditions for the O-arylation of phenol
with iodobenzenea
Table 2 Coupling reaction of aryl halides with phenols in the presence of
the CuO-NP catalysta
Entry
Aryl iodide
Phenol
Time (h)
Yieldb (%)
1
2
20
24
86
78
Entry
Catalyst
Solvent
Base
Yieldb (%)
1
2
3
4
5
6
7
8
9
CuSO4
Cu(OAc)2
CuO
CuBr
CuCl
CuO NP
CuO NP
CuO NP
CuO NP
CuO NP
DMSO
DMSO
DMSO
DMSO
DMSO
Toluene
Dioxane
DMSO
DMSO
DMSO
KF/CP
KF/CP
KF/CP
KF/CP
KF/CP
KF/CP
KF/CP
KF/CP
KF/CP
K3PO4/CP
28
30
22
42
38
12
18
3
24
75
78 (86)c
80d
16
4
5
6
7
24
18
18
18
68
85
70
70
10
a
Reaction conditions: phenol (1.5 mmol), catalyst (10 mol%), iodobenzene
(1 mmol), base (0.33 g, 2 equiv.) and solvent (5 mL) at 120 1C for
12 h. Isolated yield. Yield in parentheses refers to the isolated yield
after 20 h. 2.5 equiv. of KF/CP was used.
b
c
d
in the presence of CuO nanoparticles and KF/CP as the base
(Table 1).
To our pleasure, diphenylether was obtained in 78% yield
when the reaction was carried out at 120 1C in the presence of
10 mol% of copper oxide nanoparticles and 2 equiv. of KF/CP in
DMSO under nitrogen (Table 1, entry 8).
When the amount of base (KF/CP) was increased to 2.5 equiv.
the yield was inappreciably enhanced to 80% (Table 1, entry 9)
while the reaction time had more effect and the yield was
increased to 86% after 20 hours (Table 1, entry 8).
Other copper salts such as CuSO4, Cu(OAc)2, CuBr, CuCl and
CuO showed lower catalytic activity than CuO nanoparticles for
this reaction. Among the solvents used, DMSO afforded the
highest conversion. In addition, the reaction was more effective
with KF/CP as a base rather than K3PO4/CP.
To study the scope of the procedure, the reaction of other
phenols was studied next. The simplest case reaction of phenol
with iodobenzene afforded the corresponding diaryl ether in
high yield (Table 2, entry 1). Substituted halophenols underwent
reaction with iodobenzene in 68–78% yield (Table 2, entries 2–4).
Similar reactivity was observed with alkyl and methoxy substituted
phenols (Table 2, entries 5–11). When 1-naphthol was used for
the reaction, the corresponding naphthyl ether was obtained in
moderate yield (Table 2, entry 12). Phenols having electron-
donating groups showed greater reactivity in comparison to
those with moderate electron-withdrawing halophenols.
The coupling reaction of sterically hindered phenols was
slightly disfavoured. Hence, when employing ortho substituted
8
18
83
9
21
18
22
20
74
84
87
67
10
11
12
13
14
15
20
18
18
62
65
80
16
17
a
30
30
46
42
Reaction conditions: phenol (1.5 mmol), catalyst (10 mol%), iodo-
benzene (1 mmol), base (0.33 g, 2 equiv.) and solvent (5 mL) at 120 1C
under N2. Isolated yield.
b
Based on previous work20 a plausible mechanism for the
phenols, the reaction occurred with a lower yield than with less CuO NP catalyzed reductive Ullmann reaction of aryl halides is
hindered para substituted phenols (Table 2, entries 3–10). shown in Scheme 1. As shown in Scheme 1, the catalytic cycle
The reaction of phenols with substituted iodobenzenes was also may be initiated by adsorption of aryl halides on the surface of
studied. 1-Iodo-4-methoxybenzene and 1-iodo-4-methylbenzene the CuO nanoparticles through an oxidative addition. In the
provide the corresponding cross-coupled products in reasonable next step a proton was abstracted by a negatively charged
yields (Table 2, entries 13–15). Finally, it was found that the fluoride moiety from phenol to generate a phenoxide anion
protocol is also active in activating bromobenzene and afforded stabilized at the potassium surface. After that the reaction may
the corresponding ethers albeit with low yields compared to undergo anion substitution, followed by a reductive elimination
iodobenzene (Table 2, entries 16 and 17).
process on the surface of the CuO NPs.
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of Semnan. The catalyst was prepared according to the litera-
ture procedure.5b,19 All starting materials were purchased from
Aldrich or Fluka and were used without further purification.
A silica gel (60 mesh) was used for product purification. Melting
points were determined using an Electrothermal IA 9100 Digital
Melting Point apparatus. 1H and 13C NMR spectra were
recorded in [d6] DMSO using a Bruker DRX-300 Avance spectro-
meter operating at 300.13 and 75.47 MHz, respectively.
Typical procedure
A mixture of phenol (1.5 mmol), the appropriate aryl halide
(1 mmol), CuO nanoparticles (10 mol%) and 35% w/w potassium
fluoride/Clinoptilolite (0.33 g) in dry DMSO (5 mL) was heated at
120 1C. The mixture was stirred under a nitrogen atmosphere until
the reaction was complete (by TLC or GC monitoring). After
completion, the reaction mixture was cooled to room temperature
and filtered off to separate the solids (which was washed with 15 mL
EtOAc) and the combined solution was placed in a separatory
funnel and washed twice with water. The organic layer was dried
over anhydrous sodium sulfate, filtered, and concentrated in vacuo.
Products were purified by column chromatography on a silica gel
eluting with ethyl acetate–hexane mixtures.
Scheme 1 Proposed mechanism for CuO nanoparticle catalyzed O-arylation
of phenols with aryl halides.
SEM analysis
Copper nanoparticles were prepared according to a previously
reported procedure.5b The nanoparticles were characterized by
SEM analysis, and it gives a fairly well dispersed mixture of
small particles (o100 nm) (Fig. 1).
In summary, a simple protocol for the O-arylation of phenols
has been established through an effective new solid base. The
strategy described herein includes many characteristics that are
often considered as useful synthetic methods: solid phase
reaction using available nanoparticles is operationally simple
and scalable, employs starting materials that are commercially
available and cheap, generates an innocuous byproduct, provides
product diversity, and is compatible with a number of useful
functional groups. Moreover, this methodology provides an efficient
route to a wide variety of substituted phenolic compounds and it
also enabled phenyl bromides to perform the reaction with
moderate yields. In addition, an environmentally friendly solid
base can be used instead of a strong base such as Cs2CO3.
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The raw material was an Iranian commercial Clinoptilolite
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Fig. 1 SEM image of CuO nanoparticles.
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