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1) DDQ, 135°C,96%
2)AlCl 3 /CuCl 2 , 99%
135°C
98%
33
32
34
Scheme 10 Synthesis of PAH 34 via intramolecular Diels-Alder reaction [ 31 , 32 ]
R 1
R 1
R 1
R 1
R 2
R 1
R 2
R 1
35
O
Cyclodehydrogenation
R 2
R 2
Diels-Alder
R 3
R 2
R 3
R 2
R 3
R 3
R 3
R 3
36
37
38
Scheme 11 General strategy for the synthesis of HBCs 38 via Diels-Alder reaction followed by
dehydrogenation
Over the last few decades M¨ llen and co-workers have perfected the
Diels-Alder/cyclodehydrogenation sequences to synthesize graphene-type
structures [ 10 ]. The general strategy is illustrated in Scheme 11 for the preparation
of hexa-substituted HBCs 38.
The advantage of this method is that it permits the selective synthesis of
substituted HBCs. Without any functional groups, HBCs are insoluble in most
organic solvents. The addition of solubilizing chains facilitated the characterization
of those molecules. In addition, the processability of such HBCs allows for their
application as functional materials. For example, alkyl substituted HBCs form
discotic mesophases, which show liquid crystalline behavior [ 33 ].
Employment of the Diels-Alder/dehydrogenation strategy facilitated the synthe-
sis of larger and larger graphene-type structures, such as the supernaphthalene 42
(Scheme 12 )[ 34 ].
M¨ llen and coworkers took the method to the next level by iterating the
Diels-Alder/dehydrogenation approach. This way the dendrimeric polyphenylenes
are synthesized by repeated Diels-Alder reaction involving ethynyl-substituted
tetraphenylcyclopentadienones 44 and a tetraethynylbiphenyl core 43. Up to three
generations of dendrimers have been synthesized by Diels-Alder/deprotection
repeating sequences. An example is shown in Scheme 13 [ 35 ]. With a similar
method, a 3-fold symmetric graphite disk consisting of 55 rings was prepared
[ 36 ]. The large PAHs could only be characterized by MALDI spectrometry as
their insolubility prevented the use of other analytical methods such as NMR
spectroscopy.
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