Jafarzadeh et al.
1809
Table 1. Iodination of anisole with Fe(NO ) ·9H O in the pres-
Scheme 1. Iodination of arenes with iron nitrate in the presence
3
3
2
of tungstophosphoric acid at room temperature.
ence of various heteropoly acids and other solid acids in CH Cl
2
2
at room temperature for 8 h.
R
R
Entry Catalyst
Conv. (%) GC yield (%)a
.
I2, Fe(NO3)3 9H O
1
2
None
36
78
36
77
2
b
I
H PW O
3
12 40
H PW O , CH Cl
2
3
12 40
2
3
H PW O
100
92
99
3
12 40
r.t., 8–24 h
R'
R'
4
H3PMo12O40
92
c
5
H PW O :H PMo O
97
96
3
12 40
3
12 40
Yield 9%–92%
6
Cs2.5H0.5PW O
83
82
12
40
R = OCH , CH , OH, NH , NHCOCH
3
R' = OCH , CH , NO , Br, H
3
3
2
7
Cs2.5H0.5PMo O
75
75
12
40
3
3
2
8
(NH ) HPW O
12 40
72
71
4
2
9
Fe0.85H0.45PMo O
80
79
74, 7d
12
40
drated heteropoly acids. Primarily, iodination of anisole, as a
model compound, was performed in the presence of several
HPAs and some other solid acids in dichloromethane at
room temperature for 8 h (GC results are presented in Ta-
ble 1). The result shows that in the absence of the catalyst,
the reaction proceeds with low yield, but in the presence of
HPAs, this conversion proceeded efficiently under similar
reaction conditions. In this reaction, anisole was converted
to p-iodoanisole in good yield and excellent regioselectivity
10
H6P2Mo18O62
82
e
11
12
13
14
Montmorillonite-K10
85
89
83
88
85
89
82
+
e
Amberlyst-15H
Silica gele
f
H PW O
>86
3
12 40
Note: Reaction conditions: anisole (1 mmol), I (0.6 mmol),
2
Fe(NO ) ·9H O (0.4 mmol), catalyst (0.1 mmol), CH Cl (1 mL).
3
3
2
2
2
a
b
c
Product: 4-IC H OMe.
Catalyst: 0.05 mmol.
HPW and HPMo in equal molar ratio.
By-product: 2-IC H OMe.
One gram of solid acid was used.
Catalyst: after recovery and reuse.
6
4
(
100%) without the formation of diiodinated derivatives. To
show that tungstophoric acid is more effective than other
HPAs and solid acids, we chose H PW O , which is a very
d
6
4
e
f
3
12 40
efficient and environmentally friendly catalyst (6), for fur-
ther iodination studies.
of H P Mo O . The formation of the resulting by-product
6
2
18 62
Heterogeneous acid-catalyzed systems
is due to the amorphous shape and high catalytic activity in
Dawson-type HPAs (Table 1, entry 10). Also, solid acids
show only surface-type activity, so their catalytic activities
are less than those of H PM O (M = W, Mo) (Table 1,
Heteropoly acids and solid acids are not dissolved in di-
chloromethane, thus the iodination reaction takes place in
heterogeneous conditions. The acid catalysis of heteropoly
compounds in the solid state is classified into “surface-type”
and “bulk-type” catalysis. Catalytic reactions involving polar
molecules (here, anisole) occur not only at the surface, but
also in the bulk solid HPAs. This shows good parallelism be-
tween catalytic activity (bulk-type) and acidity (surface-
type) (13). Under bulk-type conditions, all of the acid sites
are available to the reactant and therefore high catalytic ac-
3
12 40
entries 11–13). One of the most desirable properties for a
catalyst is the ease of its recovery. We have found that
H PW O can be easily recovered and recycled simply by
3
12 40
filtering (by diethyl ether) and drying (Table 1, entry 14).
The recovery had only slightly decreased their catalytic ac-
tivity (because of the reduction of tungsten atoms) without
structural degradation.
tivity could be expected. Cs and NH salts (scarcely soluble
4
in polar solvents) usually show only surface-type catalysis.
However, solid HPAs containing a cation of low ionic radii
to charge ratio (H ) readily absorb small polar molecules
and tend to exhibit pseudoliquid behaviour (bulk-type), and
are soluble in polar solvents. The diffusion of reactant mole-
cules in the solid is faster than the reaction; the solid bulk
forms a pseudoliquid phase in which the catalytic reaction
can proceed (14). Therefore, the heteropoly salts containing
Effect of oxidant
We also investigated other metal nitrates for the iodination
of anisole under similar reaction conditions in dichloro-
methane at room temperature for 8 h (GC results are pre-
sented in Table 2). The results revealed that in the absence
of the oxidant (Table 2, entry 1), and also with an excess of
the catalyst (tungstophosphoric acid) (Table 2, entry 2), the
iodination reaction did not proceed after 8 h. Therefore, we
have concluded that in the presence of oxidant, most of the
metal nitrates were effective oxidants for this reaction, but
iron(III) and bismuth(III) nitrates gave better results. These
results were expected regarding their reduction potential.
+
Cs, NH , and Fe (Table 1, entries 6–9) showed less catalytic
4
activity than HPAs containing W and Mo (Table 1, entries
3
–5).
On the other hand, although Keggin-type HPAs
H PM O , M = W, Mo) are much stronger acids than the
(
3
12 40
Dawson-type HPAs (H P Mo O ) (6), but Dawson-type
Effect of solvent
6
2
18 62
heteropoly acids, because of their amorphous shape (ellip-
soidal shape of the polyanion), show higher catalytic acivity
than Keggin-type HPAs, and this is because of their crystal-
line shape (spherical shape) (14). Consequently, we can see
a low selectivity in the iodination of anisole in the presence
The iodination of anisole was also conducted in different
solvents at room temperature for 8 h (GC results are pre-
sented in Table 3). The results show that in the absence of
solvent (Table 3, entry 1) the iodination reaction proceeds
with low yield after 1 h at room temperature. The iodination
©
2005 NRC Canada