1
48
K.S. Sindhu et al. / Journal of Catalysis 348 (2017) 146–150
Table 2
Optimization Studies.a
Entry
Base
Yieldb (%)
Entry
Base
Yieldb (%)
d
1
2
3
4
5
6
K
K
Cs
NaOH
KOH
Et
Cs
2
CO
PO
CO
3
67
45
84
49
62
nd
36
8
Cs
Cs
Cs
Cs
Cs
Cs
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
traces
65
41
82
48
e
3
4
9
f
2
3
10
11
g
h
12
3
N
13i
83
c
7
2
CO
3
a
Reaction conditions: aryl iodide (1 mmol), phenol (1.2 mmol), base (2 equiv.), FeCl
Isolated yield,
Reaction temp. 120 °C.
Reaction temp. 100 °C.
Reaction time 24 h.
3 2
ꢀ6H O (10 mol%), DMEDA (20 mol%), TBAB (1 equiv.), water (3 ml), 130 °C, 36 h.
b
c
d
e
f
Use of 5 mol% FeCl
Use of 15 mol% FeCl
Use of 10 mol% FeCl
3
ꢀ6H
2
O and 10 mol% of DMEDA.
g
h
i
3
ꢀ6H
ꢀ6H
2
O and 30 mol% of DMEDA.
O and 10 mol% of DMEDA.
.
3
2
Use of 3 equiv. of Cs
2
CO
3
spectrometry. The ICP-MASS analysis showed that catalytically
more active Pd, Cu and Ni were absent in our sample and a very
minute amount of trace metals such as Zn, Mn and Cr were present
in the sample (see Table 3).
We performed control experiments using different catalysts of
Zn, Mn and Cr to determine whether the trace amount of impuri-
ties was able to catalyze the reaction. In all cases, we found that
the trace amounts of impurities did not form the desired coupled
product which clearly demonstrates that this reaction was cat-
alyzed by iron. Moreover, we got similar yields of the products
iodonitrobenzene with 4-chlorophenol and 2-naphthol. It was
illustrated that iodobenzene and electron rich aryl iodides such
as 4-iodotoluene and 4-iodoanisole were also effectively coupled
with phenols affording the corresponding ethers in 82%, 56% and
51% yields (3h-3j). The O-nucleophile could also be varied consid-
erably, and when electron rich phenols such as p-cresol and p-
methoxyphenol were used, the coupling products were obtained
in excellent to good yields (3e and 3j). Electron deficient 4-
Chlorophenol gave 71% yield under our reaction conditions. When
phenol with strongly electron-withdrawing nitro moiety was used,
the cross-coupling did not occur. Furthermore, the reaction out-
come was not affected by the steric hindrance derived from the
presence of ortho-substituent on phenol, furnishing the target dia-
ryl ethers 3l in good yield (78%). In contrast, when 2-iodotoluene
was used, the coupling product was obtained in lower yield (3l,
49%). The coupling reaction provided 91% yield for the reaction
(
80%, 83% and 84% respectively) with different sources of FeCl
3
-
ꢀ
6H O (>96%, >97% and >99.9% purity). This again clearly demon-
2
strates that the reaction was catalyzed by iron and not by any
other trace metal. Thus, the optimal reaction conditions for the
O-arylation of phenol with 4-iodoacetophenone involved the use
of 10 mol% of FeCl
and 1 equiv. of TBAB in water under air at 130 °C.
3
ꢀ6H
2 2 3
O, 20 mol% of DMEDA, 2 equiv. of Cs CO
3
between aryl iodide having electron-withdrawing CF group at
Using the previous optimization conditions, the scope and lim-
itations of the catalytic system were explored. As shown in Table 4,
different aryl iodides were coupled efficiently with phenols to gen-
erate the desired diaryl ethers (3a–3p) in good yields. Electron
deficient aryl iodides such as 4-iodonitrobenzene and 4-
iodoacetophenone served as suitable partners for coupling with
various phenols affording excellent yields. Less reactive 4-
bromoacetophenone also coupled with phenol giving the corre-
sponding diaryl ether in 31% yield. However the catalytic system
failed to catalyze the reaction of aryl chlorides under similar con-
ditions. 1-Chloro-4-(4-nitrophenoxy)benzene (3f) and 2-(4-
nitrophenoxy)naphthalene (3g) were generated in 92% and 84%
yields respectively from the coupling reaction of 4-
the meta-position (3m) and while the coupling of iodobenzene
with meta-cresol gave the diaryl ether in 75% yield (3o). In addi-
tion, the coupling reaction between electron rich 1-iodo-3,5-
dimethylbenzene and 4-methoxy phenol proceeded smoothly fur-
nishing good yields of the product (3n, 83%). To our delight, the
coupling reaction between 3-iodopyridine and phenol occurred
successfully delivering the corresponding heteroaromatic ether in
good yields (3p, 78%). On the other hand, the coupling of iodoben-
zene with 3-pyridinol afforded the ether only in 51% yield (3p).
To sum up, we have developed the first efficient and environ-
mentally friendly procedure for CAO bond formation by cross-
coupling of phenols with aryl iodides by using a combination of
3 2
FeCl ꢀ6H O and DMEDA in the presence of TBAB. The reaction is
Table 3
ICP-MS analysis of the catalyst.
Pd
Cu
Ni
Co
Pb
Cd
Mn
Cr
Zn
(
ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
(ppm)
FeCl
3
ꢀ6H
2
O (>99.9%)
nd
nd
nd
nd
nd
nd
1.5
0.9
1.4
nd: Not detected.