DOI: 10.1002/asia.201801812
Synthesis of Cyclopenta-HBCs and their Regioselective
[
a]
Abstract: Hexa-peri-hexabenzocoronenes with a bay-fused
five-membered ring are synthesized from fluorenyl precur-
sors. The key oxidative cyclodehydrogenation step is ac-
companied by regioselective chlorination that is enhanced
by methylation at the cyclopenta-ring or increased reac-
tion concentration. The CpHBC products undergo mild
electrophilic aromatic bromination, without catalyst, to
Figure 1. Tetra(tert-butyl)-hexa-peri-hexabenzoncoronene (HBC, 1) and its cy-
clopenta-annulated derivative, CpHBC (2a).
afford adducts suitable for p-extension by cross-coupling.
none (3) was the starting material chosen and was synthesized
[
10]
Annulation of five-membered rings onto polycyclic aromatic
hydrocarbons (PAHs) has proven to be an effective method to
distort the electronic properties and structure of the aromatic
via an adapted method of Thiery et al.
After Sonogashira
cross-coupling with TIPS-acetylene to give 4a, then deprotec-
tion with TBAF, the alkyne 4b was obtained (Scheme 1).
Alkyne 4b was subjected to a Diels–Alder cycloaddition with
cyclopentadienone 5 to yield the central arene ring. The
[
1]
system, either by enforcing rigidity, as in the case of trux-
[
2]
ene, or inducing curvature, as with the buckybowl suma-
[
3]
1
nene. Additionally, the benzylic methylene carbon offers a
new reactive site, allowing for further modification, such as the
formation of cyclopentadienyl-type ligands able to form metal-
H NMR spectrum of 6 contains several regions of broad sig-
nals attributed to the restricted rotation of the fluorenonyl
moiety. A distinct singlet at 7.52 ppm assigned to the single
proton of the central ring was indicative a successful reaction.
Fluorenone 6 was hydrogenated efficiently to afford fluorenyl-
[
4]
locenes, of importance as Ziegler–Natta catalysts, or for
tuning of the electronic/supramolecular properties, such as the
modification of polyfluorenes to yield more photochemically
1
benzene 7 which exhibited a highly convoluted H NMR spec-
[
5]
stable polymers. Our group has reported the synthesis of cy-
trum with a number of broad signals, similar to those of 6; the
appearance of two inequivalent, geminally-coupled benzylic
methylene protons at ca 3.8 ppm is consistent with restricted
rotation of the fluorenyl group at room temperature.
[
6]
clopentatriphenylene, and as an expansion of this, our sights
were set on annulation to the larger PAH, hexa-peri-hexaben-
zocoronene (HBC), an extensively studied, well-defined ‘nano-
graphene“ which participates in robust p-stacking interac-
A key step in the synthesis of HBCs is cyclodehydrogenation
of an oligophenylene precursor such as a hexaphenylbenzene,
[
7]
tions leading to long-range order as applied in organic elec-
[
8]
tronics. Although a cyclopentadiene bis-ortho-annulated by
two HBCs with the trivial name ”superfluorene“ has been re-
[
9]
ported, we endeavoured to ”bridge“ a bay region with a
methylene group to give a cyclopenta-HBC (CpHBC, 2a,
Figure 1) as a step toward systematically modulating the struc-
ture and reactivity of the HBC core.
Although the five-membered ring could be incorporated fol-
lowing cyclodehydrogenation to the HBC core, we chose to
have the five-membered ring present from the beginning by
incorporating a fluorene moiety. To this end, 4-bromofluore-
[
a] T. B. J. Hall, B. R. Hoggard, Dr. C. B. Larsen, Dr. N. T. Lucas
MacDiarmid Institute for Advanced Materials and Nanotechnology
Department of Chemistry
University of Otago
Dunedin 9016 (New Zealand)
E-mail: nigel.lucas@otago.ac.nz
Supporting information and the ORCID identification number(s) for the au-
This manuscript is part of a special issue on chemistry in New Zealand. A
link to the Table of Contents of the special issue will appear here when the
complete issue is published.
Scheme 1. Synthesis of fluorenyl precursors and CpHBC (2a).
Chem. Asian J. 2019, 00, 0 – 0
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