Angewandte
Communications
Chemie
Graphene-like Hydrocarbons
Cyclization of Pyrene Oligomers: Cyclohexa-1,3-pyrenylene
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Dominik Lorbach , Ashok Keerthi , Teresa Marina Figueira-Duarte, Martin Baumgarten,
Manfred Wagner, and Klaus Müllen*
Abstract: First synthesis of the macrocycle cyclohexa(1,3-
pyrenylene) is achieved in six steps starting with pyrene, leading
to a non-aggregating highly twisted blue-light-emitting mate-
rial. The cyclodehydrogenation of the macrocycle offers
a promising synthesis route to holey-nanographene.
a highly strained macrocycle. This is verified through the
single crystal analysis of the model compound 7’-(tert-butyl)-
4,4’:10’,4’’-terpyrene obtained from coupling reaction of 4,10-
diiodo-7-tert-butylpyrene with 4-pyrene boronic acid (Fig-
[
9]
ure S1 in the Supporting Information). The 1,3-coupling of
pyrenes geometrically and electronically resembles that of
[1c,2a,10]
S
hape-persistent macrocycles have become an important
meta-phenylenes leading to cyclohexa-m-phenylenes.
class of molecules owing to their unique optical and electronic
properties and their role as building blocks of three-dimen-
sional nanostructures, discotic liquid crystals, extended tub-
ular channels, supramolecular guest-host complexes, and
Therefore, we can take advantage of 1,3-substitution of
pyrenes to synthesize the PyMC6. Pyrene (Py) was first
mono-tert-butylated to afford 2-tert-butylpyrene (1), which
was then treated with bromine (two equivalents) in CH Cl at
2
2
[
1]
porous organic solids. Seminal work on the synthesis of
À788C to provide selectively the 1,3-dibromo-7-tert-butylpyr-
[7a]
cyclohexa-m-phenylenes was carried out by Staab and Binnig
ene (2) in 89% yield (Scheme 1).
[2]
in the 1960s and recently functionalized cyclohexa-m-
Our first attempt to obtain PyMC6 was a direct route via
Yamamoto coupling of monomer 2 under high-dilution
conditions, which was not successful. Our alternative syn-
thetic strategy was based on a chemoselective Suzuki cross-
coupling reaction to get 3,3’’-dibromo-7,7’,7’’-tri-tert-butyl-
1,1’:3’,1’’-terpyrene (5) followed by Yamamoto coupling
(Scheme 1). To this end, dibromo pyrene derivative 2 was
treated with an excess of tert-butyl lithium, which led to
a halogen–metal exchange. The resulting carbanion was
[1c]
phenylenes were reported by our group.
Macrocycles
containing polycyclic aromatic hydrocarbons (PAHs) can be
considered as monomeric and well-defined precursors to
holey-nanographenes, which play an essential role in appli-
cations, such as supercapacitors, electrode materials, and
[
3]
energy storage. Thus, the synthesis of specific graphene
cutouts with holes is of importance to study the influence of
[4]
the holes on the electronic structure. Pyrene, the smallest
peri-condensed aromatic hydrocarbon, has not been utilized
to make a homo-macrocycle so far. Pyrene is of substantial
interest in many applications owing to its optoelectronic
properties. Despite this, only a few examples of oligomers and
quenched with I to obtain 1,3-diiodo-7-tert-butylpyrene (3)
2
in 62% yield.
Several attempts to selectively obtain the boronate
derivative 2-[3-bromo-7-(tert-butyl)pyren-1-yl]-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (4) have been made. First, lith-
iation using nBuLi and tert-butyl lithium were investigated
but without success. The careful preparation of 4 was based on
a Pd-catalyzed borylation reaction in dioxane. The borylation
of compound 2 with 0.5 equivalents of bis(pinacolato)diboron
[5]
polymers containing pyrene have been studied. Mainly, 1,1’-
bipyrenyl, 2,2’-bipyrenyl, linear 1,6-disubstituted oligopyr-
[6]
enes, and [4]cyclo-2,7-pyrenylene have been investigated.
Recently we reported a polymer completely made up of
pyrene units with highest degree of polymerization known for
pyrene-containing polymers, poly-7-tert-butyl-1,3-pyrenylene
was performed using a [Pd(dppf)Cl ] catalyst and KOAc as
2
[
7]
for application in blue light-emitting diodes. Herein we
present the synthesis of the first pyrene-based macrocycle,
namely cyclohexa-1,3-pyrenylene (PyMC6).
base in dioxane under reflux. However, the reaction yield of 4
was very low (< 15%). A range of solvents, temperatures, and
equivalents of bis(pinacolato)diboron were tested for opti-
mization. The best results were achieved using one equivalent
of bis(pinacolato)diboron and KOAc in anhydrous dioxane at
708C for 12 h. Using these conditions, dibromo pyrene 2 could
be selectively converted into 4 in 62% yield (Scheme 1).
The Suzuki cross-coupling reaction of the bis(iodo)
compound 3 with two equivalents of the 1-bromo-3-boronic
ester derivative 4 was performed at room temperature to
provide the dibromo terpyrene analogue 5. A chemoselective
reaction was expected owing to the much higher reactivity of
the iodide compared to that of the bromide. However, the
reaction did not proceed at room temperature even for longer
reaction times. In particular cases with such high steric
hindrance or in the case of large aromatic systems, ambient
conditions are not enough to promote the reaction. With the
purpose of enhancing the coupling reaction between com-
pounds 3 and 4, the temperature was progressively increased.
The synthesis of the pyrene macrocycle required appro-
priate coupling methods. The conceptual basis of this work
was the 1,3- or 4,10-disubstitution on dihalo pyrene which we
[7a,8]
demonstrated recently.
In that approach, the coupling
reaction of pyrenes through 4,10-positions favors the forma-
tion of linear oligomers and polymers rather than leading to
[
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[
*] D. Lorbach, Dr. A. Keerthi, Dr. T. M. Figueira-Duarte,
Prof. Dr. M. Baumgarten, Dr. M. Wagner, Prof. Dr. K. Müllen
Max Planck Institute for Polymer Research
Ackermannweg 10
5
5128 Mainz (Germany)
E-mail: muellen@mpip-mainz.mpg.de
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[
] These authors contributed equally to this work.
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 418 –421