Chemistry Reference
In-Depth Information
The starting cyanothioethyl thiolatemolecule (a) is converted to the thione (b) and
coupling of (b) with (c) yields the fundamental TTF-derivative brick (d). Reaction
with (e) renders (f) and the iterative reaction of (f) with (d) gives larger molecules
such as (g). Indeed the correct experimental conditions have to be met for each step.
As pointed out above, it is mandatory in order to proceed with the dendrimerization
to preserve the cyanoethyl
hook
.
Radial dendrimeric structures can be achieved by choosing a core, e.g., benzylic
bromides, because (g) will attach to the core at the bromine location, as schematized
in Fig. 2.16.
To finish this section and chapter we will briefly discuss the synthetic routes
pursued in order to obtain cyclothiophenes, fully
α
-conjugated macrocyclic
oligothiophenes, i.e., joining boths ends of linear
-thiophenes (Kromer
et al.
,
2000). One advantage of these macromolecules is that the mean diameter of the
cavity can be tuned by selecting the initial building blocks. In Section 4.2 we will
explore how such molecules, in particular cyclotrimeric terthiophenediacetylene,
self-assemble on a surface.
Figure 2.17 shows the simplified scheme of the cyclization reaction. The cho-
sen starting building block is butylated terthiophene. Symmetrical substitution of
thiophene units with butyl groups has proven to be ideal for maintaining sufficient
solubility and avoiding problems of regioisomerization. Thiophenes become io-
dated at the
α
-positions by elemental iodine and Hg(CH
3
COO)
2
in CHCl
3
to yield
diiodothiophenes (step a in Fig. 2.17). Diiodothiophenes become thiophenediynes
by palladium-catalyzed couplingwithTMSA (step b in Fig. 2.17).Macrocyclization
is obtained by controlled addition of pyridine solutions of the resulting oligothio-
phenes to a slurry of anhydrous CuCl and CuCl
2
in pyridine at RT (step c in Fig.
2.17). Separation of the mixtures is achieved by preparative HPLC. The fully
α
-
conjugated cyclo[
n
]thiophenes, after elimination of the triple bonds, are obtained
from the reaction of the cyclooligothiophenediacetylenes with Na
2
S (step d in Fig.
2.17). The full synthesis has been oversimplified here. Such experiments are in
reality quite complex.
The preparation of large clusters is certainly not exclusive to organic molecules.
Inorganic polyoxometallate clusters containing 154 molybdenum atoms per cluster
or more have been obtained (Muller
et al.
, 1998).
α
