Chemistry Reference
In-Depth Information
H
3
4
3
4
H
H
H
5
+
2
5
+
2
S
S
1
2
Fig. 1
The structures of 2
H
- and 3
H
-thiophenium ions
H
-H
+
S
H
+
+
H
S
S
S
S
S
H
H
S
S
3
Scheme 3
Formation of a trimer
3
of thiophene [
10
]
H
H
+
+
H
H
H
H
+
Cl
Cl
Cl
S
S
S
4
Scheme 4
Protonation of 2-chlorothiophene [
12
]
H
H
H
H
H
+
+
+
H
H
+
Cl
Cl
Cl
S
S
S
5
6
Scheme 5
Protonation of 2-chloro-5-methylthiophene [
12
]
The targeted protonation of several substituted thiophenes was achieved by their
treatment with HSO
3
F or AlCl
3
-HCl-CH
2
Cl
2
[
12
]. The thiophenium ions were
investigated by means of low-temperature NMR spectroscopy, and it was shown
that even substituted thiophenes are protonated exclusively at an
-position; thus,
2-chlorothiophene gives a C-5-protonated species
4
(Scheme
4
)[
12
].
2,5-Disubstituted thiophenes - 2,5-dichlorothiophene and 2-chloro-
5-methylthiophene - were also protonated by HSO
3
FandAlCl
3
-HCl-CH
2
Cl
2
,again
at an
ʱ
70
C
gave a mixture of 2- and 5-protonated thiophenes, i.e.
5
and
6
, in a 3:1 ratio (Scheme
5
)
[
12
]; however, in the AlCl
3
-HCl-CH
2
Cl
2
system,
5
was formed selectively [
12
].
ʱ
-carbon. For 2-chloro-5-methylthiophene, the protonation by HSO
3
Fat
2.2 Friedel-Crafts-Type Reactions
At the end of nineteenth century, several reports appeared in the most important
chemical journals of that time,
Justus Liebigs Annalen der Chemie
and
Chemische
