An Ullmann C-O coupling reaction catalyzed by magnetic copper ferrite nanoparticles

Article abstract of DOI:10.1002/adsc.201200600

Herein, an efficient method for the Ullmann C-O coupling reaction between various kinds of phenols and aryl halides, including amino, ketone, cyano, methyl, methoxy, fluoro, chloro and bromo derivatives, is described. The catalyst used, copper ferrite (CuFe₂O₄) nanoparticles, are easily made, air-stable, and of low cost. The catalyst can be recycled easily just by using an external magnet. Even in the presence of sensitive substituents, the reaction proceeds successfully to provide the desired products in high yields without protection of other functional groups. Copyright

Full text of DOI:10.1002/adsc.201200600

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DOI: 10.1002/adsc.201200600
À
An Ullmann C O Coupling Reaction Catalyzed by Magnetic
Copper Ferrite Nanoparticles
Shuliang Yang,a,b Cunqi Wu,a Hua Zhou,a Yanqin Yang,a,b Yongxia Zhao,a,b
Chenxu Wang,a,b Wei Yang,a,* and Jingwei Xua,*
a
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of
Science, Changchun 130022, Peopleꢀs Republic of China
Fax : (+86)-431-8526-2643; phone: (+86)-431-8526-2649; e-mail: yangwei@ciac.jl.cn or jwxu@ciac.jl.cn
Graduate School of Chinese Academy of Sciences, Beijing 100039, Peopleꢀs Republic of China
b
Received: July 9, 2012; Revised: August 20, 2012; Published online: December 12, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200600.
Abstract: Herein, an efficient method for the Ull-
mann C O coupling reaction between various kinds
conditions. Nevertheless, for palladium, its drawbacks
such as contamination to products, high cost, toxicity,
moisture-sensitive nature and the non-commercial so-
phisticated phosphine ligands seriously limit its appli-
cations in industrial and large-scale manufacture.[4a,d,g]
Besides, recovery of the expensive catalysts is still
a problem.[6] At the same time, a blemish for the
copper catalytic system is that these cuprous salts,
such as CuBr, CuCl, etc, are often air-sensitive. To
keep these cuprous salts stable, more harsh conditions
are often required. Although there are reports
making use of the relatively stable cupric salts such as
CuO nanoparticles, the recycle process is quite com-
plex.[7] Herein, we report an efficient CuFe2O4-cata-
À
of phenols and aryl halides, including amino,
ketone, cyano, methyl, methoxy, fluoro, chloro and
bromo derivatives, is described. The catalyst used,
copper ferrite (CuFe2O4) nanoparticles, are easily
made, air-stable, and of low cost. The catalyst can
be recycled easily just by using an external magnet.
Even in the presence of sensitive substituents, the
reaction proceeds successfully to provide the de-
sired products in high yields without protection of
other functional groups.
À
Keywords: catalyst recycling; C O cross-coupling;
À
copper ferrite (CuFe2O4); magnetic particles; Ull-
mann reaction
lyzed Ullmann C O cross-coupling of phenols with
both aryl iodides and bromides for the synthesis of
diaryl ethers. The characteristics of this catalyst, such
as easy recovery of catalyst using an external magnet,
efficient recycling, and the high stability make it very
attractive for further applications.
As a pervasive motif, the diaryl ether moiety exists in
The optimization of the reaction conditions was
many kinds of biologically active natural products, im- first carried out with phenol 1a and iodobenzene 1b.
portant pharmaceutical compounds and polymers.[1] The influences of the reaction parameters are sum-
Among them, the most conspicuous example is vanco- marized in Table 1. The reaction did not take place in
mycin,[2] which is used only after treatment with other the absence of catalyst (Table 1, entry 1) while only
antibiotics has failed when a patient is infected by a 23% yield was obtained in the absence of the ligand
Gram-positive bacteria. The traditional approach to (Table 1, entry 2). Among the screened ligands, dike-
À
diaryl ethers is through the Ullmann-type C O cou- tone ligand 2,2,6,6-tetramethylheptane- 3,5-dione (L2)
pling procedure. This method, however, often suffers resulted in the best performance (Table 1, entries 3–
from some drawbacks, such as stoichiometric amounts 14). Among the tested organic and inorganic bases,
of copper reagents and the high temperatures re- Cs2CO3 was found to be the most suitable base
quired (often higher than 2108C).[3]
(Table 1, entries 15–21). For the solvent, NMP result-
In order to overcome these problems, many transi- ed in the highest yield among the surveyed solvents
À
tion metal-catalyzed C O bond coupling reaction sys- (Table 1, entries 22–26). After a series of tests, the re-
tems have been proposed. The main advances lie in action conditions of 1358C, 10 mol% ligand and
the applications of palladium and copper.[4,5] With the 2.0 equivalents Cs2CO3 provided the optimal result
aid of different ligands and metal salts, this has al- (Table 1, entries 27–32). Finally, when the reaction
lowed the diaryl ether to be obtained under milder time was prolonged to 24 h under such conditions,
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Shuliang Yang et al.
COMMUNICATIONS
Table 1. Optimization.
Table 1. (Continued)
[a]
Reaction conditions: 1a (0.5 mmol), 1b (1.0 mmol),
CuFe2O4 (5 mol%), ligand (10 mol%), base (1.0 mmol),
solvent (1.0 mL), 12 h; GC yields.
Ligand L2 was decreased to 5 mol%.
Cs2CO3 was decreased to 1.0 equivalent.
Time was prolonged to 24 h.
[b]
[c]
[d]
[e]
The CuFe2O4 was stored open to air for 2 months prior
to use.
Cs2CO3 (2.0 equiv.) stirred in NMP (1.0 mL) at 1358C
for 24 h under an argon atmosphere.
With the optimized conditions, the substrate scope
was subsequently investigated (Table 2). Both elec-
tron-donating and electron-withdrawing groups at the
para position of aryl iodides reacted with phenols
smoothly and gave the corresponding aryl ethers in
good yields (Table 2, entries 1–4). For phenols, elec-
tron-donating substituents, such as methoxy and
methyl, promoted the reactions (Table 2, entries 5 and
6). Although the electron-withdrawing cyano group
decreased nucleophilicity of phenol, a yield of 81%
was also obtained with a catalyst amount of 10 mol%
(Table 2, entry 7).
In our previous study, we found that ortho-substi-
tuted aryl halides were slightly less reactive than
phenol was completely transformed with 99% yield ortho-substituted phenols in O-arylation reactions of
(Table 1, entry 33).
phenols.[5f] Similarly, an isolated yield of 82% was
After the CuFe2O4 nanoparticles had been stored achieved with 5 mol% catalyst for the reaction be-
open to air for 2 months and then used, a yield of tween 2-phenylphenol and iodobenzene while, on the
91% was still achieved, which confirmed the high air- other hand, 10 mol% catalyst were necessary for 2-io-
stability of CuFe2O4 (Table 1, entry 34). On the other dotoluene and phenol (Table 2, entries 8 and 9).
hand, two other magnetic species, CoFe2O4 and
Both 1-naphthol and 2-naphthol afforded good
NiFe2O4, were also explored and only a trace of the yields (Table 2, entries 10 and 11). It was noteworthy
diphenylether product was obtained (Table 1, en- that the reaction was highly chemoselective. For ex-
tries 35 and 36). Therefore, the optimal reaction con- ample, the mono-coupling product was selectively
ditions were: phenols (0.5 mmol), aryl halide produced when 4,4’-diiodobiphenyl was employed as
(2.0 equiv), CuFe2O4 (5 mol%), L2 (10 mol%) and a coupling partner. Reaction of p-bromoiodobenzene
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An Ullmann C O Coupling Reaction Catalyzed by Magnetic Copper Ferrite Nanoparticles
À
À
Table 2. CuFe2O4-catalyzed Ullmann C O couplings of aryl
iodides with phenols.
Table 3. CuFe2O4-catalyzed Ullmann C O couplings of aryl
bromides/chlorides with phenols.
[a]
Reaction conditions (unless otherwise stated): phenols
(0.5 mmol), aryl halides (1.0 mmol), CuFe2O4 (5 mol%),
ligand (10 mol%), Cs2CO3 (1.0 mmol, 2.0 equiv.), NMP
(1.0 mL), 1358C, under argon atmosphere, 24 h; isolated
yields.
20 mol% CuFe2O4 and 40 mol% L2 were used.
According to the GC analysis.
[b]
[a]
Reaction conditions (unless otherwise stated): phenols
[c]
(0.5 mmol), aryl halides (1.0 mmol), CuFe2O4 (5 mol%),
ligand (10 mol%), Cs2CO3 (1.0 mmol, 2.0 equiv.), NMP
(1.0 mL), 1358C, under argon atmosphere, 24 h; isolated
yields.
report, in which the mixtures of N- and O-arylated
products were obtained if the catalyst had not been
chosen appropriately.[8]
[b]
10 mol% CuFe2O4 and 20 mol% L2 were used.
Aryl bromides underwent similar reactions as aryl
and phenol selectively resulted in 97% 1-bromo-4- iodides. The electron-withdrawing substituents on aryl
phenoxybenzene, which could be ascribed to the dif- bromides at the ortho to para positions provided the
À
À
ferent reactivity of the C Br and C I bonds (Table 2, corresponding O-arylated products with excellent
entries 12 and 13). In addition, the reaction between yields (Table 3, entries 1 and 2). The reaction of 4-me-
3-aminophenol and iodobenzene gave the desired O- thoxyphenol with 4-bromobenzonitrile gave a 93%
arylation product with excellent yields under our con- isolated yield, significantly higher than that of a rhodi-
ditions, and no N-arylated products were detected by um (NHC)-catalyzed reaction, which afforded only
GC-MS analysis on the crude reaction mixture a trace of product in 24 h.[9] Even the inactivated 1-
(Table 2, entry 14). This differs from the former bromo-4-methoxybenzene was also coupled with 4-
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55

Shuliang Yang et al.
COMMUNICATIONS
Figure 1. Magnetization (M) as a function of field (H) for
CuFe2O4 nanoparticles at 300 K. The inset displays the
CuFe2O4 was dispersed in NMP (a) and attracted by the
outer magnet (b).
Figure 3. X-ray diffraction (XRD) pattern of the freshly syn-
thesized CuFe2O4 nanoparticles (line a) and after 6th run
(line b, 10 mol% catalyst was used). The bottom of the
image indicates the JCPDS data (JCPDS File Card No. 34-
0425) for CuFe2O4.
The M–H plots showed that CuFe2O4 nanoparticles
exhibited ferrimagnetic behavior with a coercivity
about 750 Oe and a saturation magnetization about
29 emugÀ1 at 300 K (Figure 1). The magnetic particles
could be concentrated on one side easily when
a magnet was closely placed outside (inset of the
Figure 1), which facilitated the separation and reutili-
zation of the catalyst.
The reusability of the catalyst was investigated
using the coupling of iodobenzene with phenol. After
each cycle, the mixture solution was extracted with
EtOAc for GC analysis. The catalyst that sticked to
the stir bar was washed with deionized water and fi-
nally dried under high vacuum for 3 h before per-
forming the next run with the same substrate loadings.
When 10 mol% catalysts was used, a 92% yield was
still obtained at the 6th run (Figure 2). The X-ray dif-
Figure 2. Cycle performance of the magnetic CuFe2O4 nano-
particles; GC yields, 5 mol% catalyst (left column of each
run) and 10 mol% catalyst were used (right column of each
run), separately.
methoxyphenol smoothly when the catalyst loading fraction (XRD) patterns (Figure 3) indicated that the
was increased to 20 mol% (Table 3, entry 4). The re- crystal structure of the CuFe2O4 nanoparticles was
À
action selectively took place at the C Br bond over intact after 6 runs, which not only explained the excel-
the C F and C Cl bonds when two different halide lent recycle results, but also reconfirmed the high sta-
groups were present (Table 3, entries 6 and 7). Inter- bility of this magnetic catalyst.
À
À
estingly, when 1,3,5-tribromobenzene was used, only
When 5 mol% catalyst was used (Figure 2), the cat-
one site was coupled (Table 3, entry 8). The remaining alytic efficiency decreased with the recycled runs,
À
C Br bonds were very useful for late-stage modifica- which might be attributed to the loss of catalyst
tions. On the other hand, only a trace of product was during the reaction and recovery processes. Actually,
obtained from coupling with chlorobenzene even it was likely that the use of 5 mol% catalyst was very
when using greater amounts of catalyst (Table 3, close to the lower limit under our experimental condi-
entry 9). Finally, the heterocyclic substrates 2-chloro- tions, so a slight loss of the catalyst had a significant
pyridine, 2-bromopyridine and 3-bromopyridine were influence on the catalytic results. The catalyst losses
active in this system, providing 65%, 99% and 92% included (i) a slight particles aggregation, which could
yields, respectively (Table 3, entries 10–12).
be observed by the naked eyes, (ii) the decomposition
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Adv. Synth. Catal. 2013, 355, 53 – 58

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An Ullmann C O Coupling Reaction Catalyzed by Magnetic Copper Ferrite Nanoparticles
of part CuFe2O4 nanoparticles, which floated at the
EtOAc-H2O interface and could not be reused (Fig- method for the Ullmann C O coupling reaction be-
ure S2, in the Supporting Information); and (iii) pro- tween various kinds of phenols and aryl halides. The
In summary, we have developed an efficient
À
gressive loss during the recovery process.
CuFe2O4 nanoparticles could catalyze the coupling re-
The reaction between iodobenzene and phenol did actions with good to excellent yields under our opti-
not proceed through uncatalyzed pathways such as an mized conditions. Even in the presence of sensitive
SNAr mechanism since the diphenyl ether was not substituents (i.e., MeCO, CN, and NH2), this reaction
formed without the addition of the CuFe2O4. In addi- proceeded successfully to the desired products in high
tion, an aryne was not an intermediate of the reac- yields without the requirement of protection on these
tions because Cs2CO3 was not strong enough to functional groups. CuFe2O4 nanoparticles are easily-
[10]
À
remove an ortho hydrogen from the Ar X. No iso- made, air-stable, of low cost, and magnetic. They can
mers were obtained when 4-methyliodobenzene react- be easily recycled with an external magnet.
ed with phenol, which supported the absence of aryne
intermediates. Moreover, very strong bases, such as t-
BuOK or t-BuONa, were required to produce the aryl Experimental Section
[11]
À
radical in the C H activation reactions. The result
that a 76% yield was still obtained with the addition Preparation of the CuFe2O4 Nanoparticles Catalysts
of a strong free radical capturer –2,2,6,6-tetramethyl-
CuFe2O4 nanoparticles were prepared according to the pre-
piperidinoox (TEMPO, 2.0 equiv.) – confirmed that
the aryl radical was not the necessary intermediate.
According to the regioselectivity experiments
(Table 2, entry 13 and Table 3, entry 7), the order of
reactivity is I>Br>Cl. Based on this reactivity order
and the experimental results, a mechanism through
oxidative addition followed by a reductive elimination
vious report.[13] Simply, Fe
and Cu
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
75 mL deionized water. Then, 15 mL citric acid solution
(containing 7.161 g citric acid) were added dropwise into the
above mixed equeous solution. The obtained mixed aqueous
solution was evaporated to a gel with vigorous stirring at
908C, followed by decomposition at 4008C for 1 h, and calci-
nation in air at 7008C for 4 h. After cooling to room temper-
ature, the grey material was ground to powder using an
agate mortar for later use. The identity of the CuFe2O4
nanoparticles was ascertained by powder XRD.
À
process was assumed. Firstly, Ar X reacted with
À
À
a CuFe2O4 cluster, forming an [Ar CuFe2O4 X] clus-
ter (Scheme 1, a1), which reacted with the phenol to
form a2. Then this intermediate led to the product
through a reductive elimination step (Scheme 1).
The nanoparticles may facilitate the oxidative cou-
pling reaction with an aryl halide due to the easier
transfer of electrons and higher surface area in com-
parison to bulk metal salts. The ligand could increase
the catalyst solubility and stability and reduce aggre-
gation of the CuFe2O4 nanoparticles. For Cu-catalyzed
À
General Procedure for the C O Coupling Reactions
A 25-mL Schlenk tube was flame-dried under vacuum and
filled with argon after cooling to room temperature. To this
tube were added phenols (0.5 mmol), aryl halides (if solid,
1.0 mmol), CuFe2O4 (5–20 mol%), ligand (10–40 mol%),
and Cs2CO3 (1.0 mmol, 2.0 equiv.). The tube was then evacu-
ated and backfilled with argon (3 cycles). A dry NMP solu-
tion (1.0 mL) of aryl halides (if liquid, 1.0 mmol) was loaded
into a plastic syringe. After the tube was purged with argon,
this solution was injected into bottom of the tube using
a long needle syringe. The mixture was stirred under an
argon atmosphere in sealed Schlenk tubes at the preheated
oil bath for 24 h. After the reaction had cooled down to
room temperature, the mixture was filtered through a short
plug of silica gel and washed with 60 mL diethyl ether and
saturated NaCl solution (3ꢂ20 mL). The combined organic
phase was dried over MgSO4 and then concentrated under
vacuum. The product was purified through flash column
chromatography on 200–300 mesh silica gel with petroleum
ether/ethyl acetate as eluent with a suitable ratio according
to the TLC experiments. The identity and purity of the
À
C O coupling reactions, Cu(I) instead of Cu(II) was
typically the true active species.[5n,12] The function of
Cu(II) in the CuFe2O4, whether due to the diketone
ligand acting as the reducing agent or a synergetic
effect existing between Cu and Fe, requires further in-
vestigation.[13]
1
product were ascertained by GC-MS, DSC, FT-IR, H and
13C NMR spectroscopy.
Acknowledgements
We are grateful to the State Key Laboratory of Electroanalyt-
ical Chemistry for financial support.
Scheme 1. A plausible mechanism.
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