G Model
CCLET-2708; No. of Pages 5
Y.-P. Zhang et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
3
(a)
Cu2p
935.2
(b)
Fe2p
708.2
9400
9200
9000
8800
8600
8400
8200
10000
934.3
Cu2p3/2
8000
6000
4000
2000
953.8
Cu2p1/2
C1s
284.2
O1s
532.0
0
970
965
960
955
950
945
940
935
930
0
200
400
600
800
1000
Binding energy (eV)
Binding energy (eV)
Fig. 3. XPS spectra of sample CuO-Fe3O4: (a) survey spectrum, (b) Cu2p spectrum in CuO-Fe3O4.
PhOH
Table 1
Optimization of the reaction conditions.a
OH
Br
O
catalyst
+
Fe3O
4
base, solvent
Fe3O4
HO Cu Oph
CuO
Entry
Catalyst
Solvent
Base
Yieldd (%)
a
1
2b
3
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
CuO-Fe3O4
Fe3O4
DMF
K3PO4
K3PO4
KOH
23
2
DMF
Ar
X
DMF
55
12
94
43
9
HX
4
DMF
K2CO3
Cs2CO3
Et3N
5
DMF
6
DMF
Oxidative Addition
O
c
7
DMF
CH3ONa
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Ph
Ar
Ph
8
CH3CN
NMP
37
3
OCu
Ar
O
Fe3O4
b
9
Reductive Elimination
10
11
12
13
14c
CH3CH2OH
DMSO
Pyridine
DMF
12
13
10
0
HX
Scheme 1. Plausible mechanism for the CuO-Fe3O4-catalyzed O-arylation of
CuO-Fe3O4
DMF
89
phenols with aryl halides.
a
Unless otherwise stated, all reactions were run for 24 h at reflux, bromobenzene
(2.0 mmol), phenol (2.2 mmol), solvent (15.0 mL), catalyst (0.2 mmol), TBAB
4-hydroxybenzaldehyde (Table 2, entries 6 and 7) decreased the
yields of the product. Finally, a better quantitative yield was
obtained when 4-bromobenzaldehyde reacted with p-cresol
(Table 2, entry 10). With aryl halide components, the present
catalytic system was also effective for the coupling reaction of aryl
halides with phenols. Iodobenzene afforded the corresponding
diaryl ethers in good to excellent yields (Table 2, entries 11 and 18);
whereas chlorobenzene only produced a 32% yield (Table 2, entry
12). The yields were excellent in the aryl halides with electron-
withdrawing groups, such as nitro or formyl (Table 2, entries 13, 14
and 16). However, aryl halides with electron-withdrawing groups
decreased the yield (Table 2, entry 15), in comparison with
chlorobenzene. With the existence of steric hindrance (Table 2,
entry 17), the reaction also proceeded smoothly. In addition,
heterocyclic substrates also reacted in this system. For example, the
reaction of phenol and 2-bromine pyridine led to good yields
(Table 2, entry 19).
Next, we considered the problem of the catalyst recovery. After
the completion of the coupling reaction, we used a magnet to
remove the catalyst from the reaction medium. The solid was
washed with EtOAc, ethanol and water and dried at 80 8C for 12 h.
The isolated catalyst was used for the next reaction with Cs2CO3 as
base in DMF as solvent at 145 8C, the results are summarized in
Table 3. Based on these results, it was shown that the catalyst
retained its high catalytic activity in these repeating cycles.
According to published research work [26], the possible
mechanism for O-arylation proposed in Scheme 1 may involve
(0.2 mmol), base (4.0 mmol), 145 8C.
b
Reaction carried out without TBAB.
c
Reaction carried out with 1.0 mmol Cs2CO3.
d
Isolated yield after column chromatography.
Cs2CO3 as base, we obtained the best result (compare entries 1, 3, 4,
6 and 7), furnishing the product in 94% yield (Table 1, entry 5).
Subsequently, when the reaction was carried out in other solvents,
such as CH3CN and NMP, very poor yields of the diaryl ether were
achieved (Table 1, entries 8–12). However, DMF as solvent gave the
best yield (Table 1, entry 5). When Fe3O4 was used as catalyst, the
reaction failed (Table 1, entry 13). In addition, when the reaction
was carried out with 1.0 mmol Cs2CO3, we also obtained 89% yield
(Table 1, entry 14). Since the base Cs2CO3 was much more
expensive, the amount of base was 1.0 mmol in the following work.
Using the above optimized conditions, we explored the scope of
this methodology with the CuO-Fe3O4-catalyzed Ullmann-type
reaction for a variety of phenols with aryl halides. The results are
summarized in Table 2 and shows that electron-rich substituted
phenols lead to the desired product in good to excellent yields
(Table 2, entries 2 and 5). Also, even in the presence of an ortho
substituted phenol or a meta-position phenol (which is capable
of providing a steric bias) the reaction proceeded smoothly
(Table 2, entries 3, 4, 8 and 9). However, the yieldswithelectron-rich
phenols were different from electron-deficient ones. Those
bearing strong electron-withdrawing groups seemed difficult
to undergo the O-arylation. For example, 4-nitrophenol and
Please cite this article in press as: Y.-P. Zhang, et al., Impregnated copper on magnetite as catalyst for the O-arylation of phenols with