Chemistry Reference
In-Depth Information
CuCl
2
LDA
Li
H
2n
H
H
H
H
S
S
S
n
n
Figure 2.2. Scheme of oligomerization of
α
-thiophenes.
synthesis, although it would mean the ruin of the chemical industry. For instance,
perylene can be more efficiently obtained from phenanthrene using anhydrous HF
as a condensing agent (Calcott
et al.
, 1939).
An additional very expensive synthetic route, certainly not indicated for industrial
production although scientifically sublime, is the generation of a single molecule by
chemical reaction of two precursor molecules induced with a scanning tunnelling
microscope (STM). We will see in Section 4.2 the example of the synthesis of
biphenyl from C
6
H
5
I on a Cu(111) surface, thus reproducing Ullmann's reaction
at the single molecule level (Ullmann
et al.
, 1904).
Finally, just a fewwords dedicated to the synthesis of polyphenylenes, extremely
important polymers, and in particular substituted polyphenylenes such as PPV,
which exhibit superb thermal and chemical resilience, semiconducting properties
upon doping and applications such as OLEDs. Contrary to their linear acenes coun-
terparts, long polyphenylenes can be obtained e.g., by Bergman's method consisting
in the thermal cycloaromatization of enediynes (Lockhart
et al.
, 1981).
2.2 Pentagons
Thiophene, resulting from the substitution of a carbon by a sulfur atom in an
all-carbon pentagon is a prominent representative of the MOMs building blocks.
Although found in coal, in the laboratory it can be prepared by heating sodium
succinate with phosphorus trisulfide or by passing C
2
H
2
or C
2
H
4
into boiling sul-
fur.
-
n
T, can be obtained in good yields using a simple
approach based on lithiation and oxidative coupling of organolithium compounds,
schematized in Fig. 2.2.
The experimental procedure is exemplified here with the synthesis of
α
-thiophene oligomers,
α
α
-6T
(
n
=
6) from
α
-4T (
n
=
4) (Kagan & Arora, 1983). The reaction of two equiv-
alents of
α
-2T (
n
=
2) with one equivalent of LDA and then with one equivalent
of CuCl
2
yields
-2T is obtained from thiophene in a similar
way. Then, the reaction of two equivalents of
α
-4T in high yields.
α
α
-4T with one equivalent of LDA
followed by one equivalent of CuCl
2
produces
α
-6T (
n
=
6). Stoichiometry is thus
α
important. In more detail,
-6T is prepared in the following way.
n
-BuLi is added
