Chemistry Reference
In-Depth Information
ArH + I
2
ArI + HI
Scheme 75
Formation of aryl iodide along with by-product hydrogen iodide by direct arene
iodination [
114
]
AcOH, rt
+
+
+I
2
+ Pb(OAc)
4
AcOH
Pb(OAc)
2
I
S
S
42%
Scheme 76
Use of I
2
in the presence of Pb(OAc)
4
for iodination of thiophene [
114
]
I
2
, H
5
IO
6
I
S
S
MW, EtOH
69%
Scheme 77
Use of I
2
in the presence of H
5
IO
6
for iodination of thiophene [
115
]
NIS
R
R
I
S
S
MeOH, AcOH
R = Me (69%), Ph (68%)
Scheme 78
Use of NIS in MeOH-AcOH for iodination of 2-substituted thiophenes [
117
,
118
]
The serious drawback of electrophilic iodination of heterocycles with iodine is
the formation of hydrogen iodide, which is both a strong reducing agent and a
strong acid (Scheme
75
)[
114
].
The destruction of thiophene by its treatment with iodine was noted already by
Meyer in 1884 [
77
]. Application of HgO to capture HI has been known from
Meyer's time and is still applied nowadays. Several other oxidants which can
convert hydroiodic acid into iodine have been described. Lead(IV) acetate serves
as useful oxidant to support iodination of thiophene with iodine (Scheme
76
)
[
114
]. The lead acetate was obtained in this case in situ from commercial red lead
(minium), Pb
3
O
4
[
114
].
Iodination of thiophene with iodine in the presence of orthoperiodic acid H
5
IO
6
used as oxidant was performed either with conventional heating, giving
2-iodothiophene (60
C, 69%), or with microwave irradiation in only 5 min
(Scheme
77
)[
115
].
Thiophene was converted into 2-iodothiophene in 58% yield in only 10 min
using iodine in the presence of silica-supported bismuth(III) nitrate pentahydrate
under solvent-free conditions at room temperature [
116
].
N
-Iodosuccinimide (NIS)
in acetic acid and methanol is also an effective agent for iodination of thiophenes
(Scheme
78
)[
117
,
118
].
