Benzothiophene

1
Benzothiophene Physical properties and
structure Thianaphthene (1) also known as
benzothiophene was first obtained by difficult
synthesis in 1883 and in 1902 it was isolated
from coal tar Thianaphthene has little commercial
value , but derivatives in the form of thioindigo
dyes have great value and have been much
investigated Thianaphthene is colourless solid ,
m.p 32º , bp 220 221 .it is best presented as
a resonance hybrid of 1 and charged structure 1a
, 1b and 1c in order of decreasing important
2
Chemical Reaction The electrophilic substitution
of these system is much less regioselective than
that of indole(effectively complete selectivity
for attack at carbon 3 ) .for example nitration
gives mixture of more than half of the product is
the 3 nitro , 2,4,6 and 7 are also produced
3
Treatment of benzothiophene with halogene
4
Reduction The sulphur containing ring can be
opened in several ways the most useful being
desulphurization with Range nickel
Oxidation Thianaphthene doesn't react with
methyl iodide but with hydrogen peroxide in
refluxing acetic acid yield a 1,1 dioxide or
sulphone (2)
5
Addition reaction Dioxide is much more stable
than thiophene 1,1 dioxide .It under goes a
Diels Alder reaction with it self only at 220
cº Sulphur dioxide is lost giving 3 , the
structure of wich was proved by conversion to 4
6
The Thianaphthene 1,1 dioxide also give (5)
with butadiene , the 2,3 double bond of (2) has
the very interesting property of adding
7
Synthetic methods General method for preparation
of benzothiophene and benzofuran
8
2-
3 Thianaphthene is possibly prepared from ethyl
benzene in the vapour phase
9
4 I can also be prepared in 71 yield from
thiophenol and acetylene at 600 650 cº
10
5-
11
6 Diels Alder reaction
12
Benzofuran (coumarone) General properties Benzo
furan (1) some times called coumarone , is a
colourless liquid b.p.137cº which may be isoleted
from coal tar .Its structure has not been
investigated in detail ,but it has aromatic
properties and is best treated as a resonance
hybrid to which 1 is the major contributor
minor contribution are made by charged structure
such as (1a) and (1b). Benzofuran is much more
stable to chemical attack than furan.
13
Chemical reaction Oxidation Benzofuran is much
less easily opened than that of furan , but it
can be spilt by the two routes outlined .
14
2 Electrophilic subistitution . Benzofuran
display a lesser tendency for 3 subistitution
for- mylation of benzofuran reportedly gives only
the 2-formly derivative and nitric acid nitration
produced 2 nitrobenzofuran alater study using
dinitrogen tetroxide give 3 nitrobenzofurane as
amajor product togother with smaller percentage
of the 2 isomer .
15
Treatment of benzofuran with halogens result in
2,3 addition product with initial electrophilic
attacke taking place at c -2
This is in very marked contrast to indole and
thianphthene , where attack takes place
predominantly at position 3 , and has been
attributed to the much greater electronegative of
oxygen than that of nitrogen or sulphur .
Some benzofurans even form 2,3 epoxides with
perbenzoic acid .
16
Reduction Benzofuran is easily reduced by
hydrogen over Palladium or sodium and alcohol the
2,3 dihydroderivative ,which is also known as
coumaran
coumaran -2-one is the lacton of 2
hydroxyphenyl acetic acid
17
Synthesis of Benzofuran 1- A useful laboratory
synthesis starts with coumaran ,and is generally
applicable
18
2- An internal claisen reaction may be used to
bring about ring closure .
19
3- When the sodium derivative of etheyl
salicylate is used the product is ethyl coumaran
-3-one -2-carboxylate which can be decarboxylated
giving the best synthesis of coumaran -3-one
20
Heterocyclic Analogues of benzen with one hetero
atom The most important ring system of this class
, that of pyridine , can be derived from benzene
by the replacement of a carbon and hydrogen atom
by one of nitrogen .This change leaves the
aromatic structure of the rings essentially
unaltered .A similar replacement of carbon and
halogen atoms by one of oxygen cannot be
envistigated because of valency consideration
unless Oxygen atom subsequently bears appositive
charge .
21
This is the case with the pyrylium cation , which
has aromatic properties , Reduction of the
pyrylium cation could lead to the non aromatic
1,2 and 1,4 pyrans and derivatives of both are
known .Sulphur analognes of the pyrylium salt and
pyrans which are called thiapyrulium salt and
thiapyrans, respectively, and some unstable
tetravalent sulphur containing compound , the
thia (IV) benzenes , have been prepared recently .