Angewandte
Communications
Chemie
Substituted Hexaarylbenzenes
A Strategy towards the Multigram Synthesis of Uncommon
Hexaarylbenzenes
Dominik Lungerich, David Reger, Helen Hçlzel, RenØ Riedel, Max M. J. C. Martin,
Abstract: A novel rational synthetic pathway—the “function-
alization of para-nitroaniline” (FpNA)—provides substituted
hexaarylbenzenes (HABs) with uncommon symmetries that
bear up to five different substituents, fully avoiding regioiso-
meric product distributions during the reactions. 4-Nitroaniline
is functionalized by a cascade of electrophilic halogenations,
Sandmeyer brominations, and Suzuki cross-coupling reac-
tions, leading to 26 substitution geometries, of which 18
structures are not available by the current established tech-
niques. Furthermore, we demonstrate that this method is
applicable to the bulk production of such systems on a multi-
gram scale. Regarding optoelectronic properties, we demon-
strate how highly functionalized HABs can show strong
luminescent behavior, making these molecules very attractive
to organic electronic devices.
regioisomers are obtained.[7] As recently as 2015, a spectacular
route consisting of a variety of palladium-catalyzed arylation
reactions led to the first HAB bearing six different substitu-
ents.[8] Despite the great achievement, this eight-step method-
ology suffers from low overall yields, is not applicable for bulk
synthesis, and the final statistical Diels–Alder reaction lead-
ing to two regioisomers. Nevertheless, engineering the
arylated periphery of benzene results in compounds display-
ing properties of fundamental importance, for example an
outstanding blue emission in organic light-emitting diodes
(OLEDs)[9] and detection of H2S in living cells[10] and of
explosives like picric acid and TNT.[11] Importantly, toroidal
p-delocalization in HABs, for a long time underestimated,
can be fine-tuned by the nature of aryl substituents, strongly
suggesting that HABs are candidates for organic electron-
ics.[12] However, HABs with substitution patterns such as
those in compounds 1–4 have hardly been investigated. In this
context, the facile access to larger amounts of these materials
is of utmost importance.
I
nvestigations starting in the early 2000s have brought
hexaarylbenzenes and their oxidized derivatives—hexa-peri-
hexabenzocoronenes (HBCs)—tremendous attention in
materials chemistry, for example in nano-electronics and
nonlinear optics.[1,2] The sixfold symmetry proved to be a key
factor in templating nano-ring structures.[3] These compounds
with star-like substitution patterns represent extraordinary
building blocks in large light-harvesting architectures,[4] and
they proved to be versatile backbones for catalytic scaf-
folds.[5,6] The literature provides a plethora of further
examples in which HABs and HBCs have been introduced
into highly sophisticated molecules or molecular assemblies.
Interestingly, until quite recently, only HABs and HBCs with
a rather small variety of symmetries were available—the most
prominent being the fully symmetrical A6 HAB, the mono-
substituted A5B, and the ortho and para disubstituted HABs
with A4B2 and (A2B)2 patterns (compare structures #1, #2, #3,
and #5 in Figure 2). Regrettably, the controlled formation of
HABs with specific substitution patterns from unsymmetrical
starting materials via standard techniques, such as the
[2+2+2] cyclotrimerization of tolans and the Diels–Alder
reactions of perarylated cyclopentadienones and acetylenes,
is limited. Very often, instead of only one product, mixtures of
Here, we present our own wet-chemical approach, which
is quite different to the established methods. Importantly, we
wanted this procedure to be simple and applicable towards
the preparation of bulk material, without involving highly
sophisticated techniques or purification difficulties due to the
formation of regioisomers.
Our method, the functionalization of para-nitroaniline
(FpNA), utilizes 5 as a cheap and commercially available
starting material, which leads selectively to 26 substitution
patterns (Figure 2). Depending on the desired structure, the
first step involves the conversion of 5 to bromo species 6, 7, or
8 (see Figure 1), which can be conducted on a multiple-
hundred-gram scale.[13–15] In this way the pattern of the
upper—“northern” hemisphere is determined. As described
in Scheme 1, an A3 hemisphere is generated by the threefold
Suzuki cross-coupling reaction of 8 with three equivalents of
arylboronic acid in a microwave reactor,[16] or under conven-
tional conditions.[17] An ABA hemisphere is derived from 7 by
coupling two equivalents of aryl moieties A, followed by
halogenation of the mid position and a final coupling reaction
with aryl moiety B. Similarly, an ABC hemisphere requires,
after the first cross-coupling reaction of 6 with aryl moiety A,
an aromatic halogenation ortho to the amine functionality,
followed by the same procedure as described for the ABA
hemisphere. After the completion of this “northern” hemi-
sphere, the design of the “southern” hemisphere can be
addressed. Thus, triarylnitrobenzene is activated by reductive
conversion of the nitro moiety to the amine by using either
SnCl2 or H2 and Pd/C. Halogenating agents can be either
iodine monochloride or bromine, yielding diodo- and dibro-
[*] D. Lungerich, D. Reger, H. Hçlzel, R. Riedel, M. M. J. C. Martin,
Dr. F. Hampel, Prof. Dr. N. Jux
Department Chemie und Pharmazie &
Interdisciplinary Center for Molecular Materials (ICMM)
Friedrich-Alexander-Universität Erlangen-Nürnberg
Henkestrasse 42, 91054, Erlangen (Germany)
E-mail: norbert.jux@fau.de
Supporting information and the ORCID identification number(s) for
5602
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 5602 –5605