Chemistry Reference
In-Depth Information
HNO
3
/(CF
3
CO)
2
O
NO
2
S
S
Scheme 87
Use of HNO
3
in (CF
3
CO)
2
O for nitration of thiophene [
136
]
Br
Br
O
2
N
Br
Br
Br
O
2
N
Br
(CF
3
CO)
2
O/HNO
3
+
+
or
+
NO
2
O
2
N
O
2
N
O
2
N
NO
2
NO
2
S
S
S
S
S
S
58%
8%
14%
6%
Scheme 88
Nitration of 3-bromothiophene [
136
]
NO
2
Me
4
NNO
3
,Tf
2
O
DCM
+
MeO
2
C
MeO
2
C
NO
2
MeO
2
C
S
S
S
62%
38%
Scheme 89
Use of Me
4
NNO
3
in Tf
2
O for nitration of methyl thiophene-2-carboxylate [
137
]
NO
2
Tf
2
O
-5 to 35
o
C
-
+
+
+
NH
3
NO
3
R
R
R
NO
2
S
S
S
R = COMe 61%
R = COMe 39%
R = CHO 60%
R = CHO 40%
Scheme 90
Use of ionic liquid EtNH
3
+
NO
2
promoted by Tf
2
O for nitration of 2-acyl thiophenes
[
138
]
A mixture of nitric acid and trifluoroacetic anhydride nitrated thiophene in 78%
yield (Scheme
87
)[
136
].
3-Bromothiophene under these conditions gave a mixture with the main product
being 2-nitro-3-bromothiophene (58%) together with 2-nitro-4-bromothiophene
(8%), and two products of dinitration (Scheme
88
)[
136
].
Nitronium triflate (TfONO
2
) which is readily generated in dichloromethane
from tetramethylammonium nitrate (Me
4
N
+
NO
3
) and triflic anhydride, can be
used as a nitrating agent. Its application for the nitration of methyl thiophene-2-
carboxylate afforded 5-nitro- and 4-nitro-substituted isomeric products in 91%
combined yield (Scheme
89
)[
137
].
Nitration of 2-acetylthiophene and thiophene-2-carbaldehyde in ionic liquid
EtNH
3
+
NO
2
promoted by Tf
2
O gave very high yields of mixtures containing
5-nitrated and 4-nitrated thiophenes in a ratio of 3:2 (Scheme
90
)[
138
]. Formed
in situ, nitronium triflate (TfONO
2
) provides efficient nitration under mild condi-
tions, and, moreover, the ionic liquid can be recovered and reused.
