Chemistry Reference
In-Depth Information
Scheme 14 Synthesis of HBC in old times [
98
-
100
]
4.1 Hexa-Peri-Hexabenzocoronene
The optimization of synthesis for parent HBC has taken a long time, and a number of
precursors were developed to construct HBC core in the past (Scheme
14
). The first
synthesis of parent HBC was reported by Clar in 1958 [
98
]. The bromination of
2.3,7.8-dibenzoperinaphthene 129 in benzene gave a dark brown precipitate in which
three bromine atoms were added. The precipitate was heated at 153
C and resulted in
the formation of tetrabenzoperopyrene 130. The absorption spectrum of the dilute
HBC solution in trichlorobenzene exhibits three typical bands: a low-energy
a
-band
at around 440 nm, a
-band at 360 nm, and a
p
-band at 387 nm. Orange-yellow
phosphorescence with a very long lifetime in a frozen solid solution of HBC in
trichlorobenzene at low temperature was also observed. These properties are typical
for PAHs. Two alternative approaches were then reported by Halleux et al., the
condensation and the concomitant cyclization of 2.3,7.8-dibenzoperinaphthone 133
with Zn/ZnCl
2
as well as oxidative cyclodehydrogenation of hexaphenylbenzene 134
by an AlCl
3
/NaCl melt which afforded HBC in low yields [
99
]. A new synthesis of
HBC was reported by Schmidt and coworkers in 1986; reaction of quinone 131 with
phenyllithium afforded diol 132, which was cyclized in AlCl
3
/NaCl melt followed by
aromatization with Cu at 400
C to give HBC in 0.4% yield [
100
].
Although efforts have been made to obtain HBC from a variety of precursors, the
above-mentioned methods all suffered from low overall yield and complicated
experimental workup. Later, a milder and more effective approach developed in
M¨ llen's group led to a golden age of HBC synthesis and applications. As shown in
Scheme
15
, the substituted hexaphenyl benzene 136 was synthesized by Co
2
(CO)
8
-
promoted cyclotrimerization of diphenylacetylene 135, which subsequently
underwent oxidative cyclodehydrogenation to give the fused HBC derivatives
137 in high yields. The key step in this reaction sequence is the oxidative
cyclodehydrogenation of branched hexaphenylbezene, which can be achieved by
Cu(II) salts such as CuCl
2
and Cu(OTf)
2
combined with AlCl
3
. Milder Lewis acid
FeCl
3
has also proved to be effective to promote solely oxidative cyclization of
b
