Chemistry Reference
In-Depth Information
O
2
N
SO
2
Ph
O
2
N
SO
2
Ph
ArSNa
+
SAr
ArS
MeOH
S
S
S
Scheme 131 cine
-Substitution of 3-nitro-4-phenylsulfonylthiophene [
187
]
O
2
N
NO
2
O
2
N
NaSAr
MeOH, reflux
Ar = Ph; 2,4,6-Me
3
C
6
H
2
S
S
SAr
Scheme 132 tele
-Substitution at 2,5-dimethyl-3,4-dinitrothiophene [
188
]
In the asymmetrically substituted 3-phenylsulfonyl-4-nitrothiophene, either or
both functional groups can play the role of leaving group.
cine
-Substitution is
operative also in this case; thus, both 4-nitro-2-arylsulfanylthiophene and
2-arylsulfanyl-4-phenylsulfonylthiophene are formed (Scheme
131
)[
187
].
2,5-Dimethyl-3,4-dinitrothiophene is not able to form
cine
-substitution products
and
undergoes
tele
-substitution
in
its
reaction with
arylthiolates
(Scheme
132
)[
188
].
4 Substitution in Thiophenes Through Radical Reactions
Radical substitution reactions and their mechanisms and applications have been
reviewed several times [
189
,
190
]. Thiophene participates well in radical reactions.
There are reviews describing both unimolecular radical nucleophilic substitutions
(S
RN
1) [
191
] and homolytic aromatic substitutions (HAS) of thiophenes [
192
]. The
formation of thiophene radicals from peroxides, thienylamines and iodothiophenes
has been discussed [
192
].
Radical substitutions are chain reactions involving single-electron transfer (SET).
They proceed through initiation, propagation and termination stages. The following
procedures can be used for the initiation step [
193
]: photostimulation [
194
,
195
], the
addition of solvated electrons or electron transfer by electrochemical methods [
196
].
Among the various methods of generation of radicals able to attack thiophene,
aprotic diazotisation of heterocyclic amines and the photolysis of iodoheteroarenes
in thiophene [
197
] give moderate to excellent yields of substituted thiophenes. Both
methods have their advantages and disadvantages and lead to mixtures of 2- and
3-substituted thiophenes with the ratio dependent on the reaction conditions and the
substrate [
197
].
Though radical arylations of thiophene have been largely replaced in recent
years by transition metal-catalysed arylation processes, modern demands for
