Chemistry Reference
In-Depth Information
Br
2
Δ
Δ
3
2
1
Scheme 1 Synthesis of HBC via condensation of PAH fragments by Clar et al. [
17
]
AlCl
3
Zn/ZnCl
2
8%
1-3%
O
5
4
6
Scheme 2 Synthesis of HBC by Halleux et al. [
18
]
2.1 Pioneering Synthesis of PAHs
Polycyclic aromatic hydrocarbons (PAHs) were accessed originally by extraction
from oil, tar, or coal as combustion byproducts. Hence, the first examples of
syntheses of PAHs relied on reproducing such conditions with relatively harsh
conditions such as high temperature. Therefore, products were obtained as mixtures
of various isomers. Already at that time issues such as insolubility made the
purification and characterization of the product a challenge.
Early work from Scholl et al. provided the first selective syntheses and character-
izations of extended PAHs. One example is the synthesis of hexabenzocoronene
(HBC) via dimerization of dibenzo-
-naphthene by Clar et al. (Scheme
1
)
[
17
]. The synthesis proceeds through bromination to a perbromide, eliminating
HBr upon heating. This observation was rationalized by the formation of the dimer
2 which, upon heating (melting) to 482
C, formed the insoluble and extremely
stable HBC 3.
In another synthesis of HBC 6 proposed by Halleux et al. [
18
] the molecule was
synthesized via two different methods to prove unambiguously the structure of the
desired product. One strategy was the cyclodehydrogenation of hexaphenylbenzene
(4) which is a method still used by M¨ llen and co-workers. The second method was
the condensation of dibenz[
peri
]anthrone 5 with Zn/ZnCl
2
(Scheme
2
)[
18
].
The addition of organometallic compounds to benzoquinone derivatives was an
early approach for the synthesis of PAHs. The diol obtained was subsequently
aromatized and cyclodehydrogenated to give the PAH [
19
-
22
]. An example of
such an approach is depicted in Scheme
3
. Clar et al. synthesized a cata-
hexabenzocoronene 9 by this method.
1
,
9
;
2
,
3
