Communications
doi.org/10.1002/ejoc.202100190
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A Three-Step Synthesis of 4H-Cyclopenta[def]phenanthrene
from Pyrene
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Figure 1C[3f]), or alternate with other specific aromatic moi-
4H-Cyclopenta[def]phenanthrene (CPP) is a valuable building
block in the production of photoactive polymers, which find
eties to tune the HOMO–LUMO levels (Figure 1B).[4c] It can
sometimes be fortuitous to cap the polymer with aromatic
moieties, to suppress the broad emission bands that origi-
nate from excimer formation (Figure 1C).[6] Synthesis of these
constructs usually starts from the parent CPP molecule 1,
which is first doubly alkylated on the C4 position to increase
solubility, and then hydrogenated to the 8,9-dihydro com-
pound, to allow site selective dibromination. Back-oxidation
then yields a general building block which is then further
diversified, i. e. by conversion to or coupling with boronic
acids in Suzuki-type cross coupling reactions, or via a
Yamamoto type homo-coupling (Scheme 1A).
One important factor that hampers the large-scale produc-
tion of CPP based polymers is the high cost associated with
commercial samples of 1. In addition, reported synthetic
procedures to obtain 1 are either lengthy, poor yielding, or
require the use of hazardous materials, making them unamend-
able for large scale synthesis (Scheme 1B).[7] In this paper, we
report on an economic, three-steps procedure to obtain 1 on a
gram scale, starting from pyrene, with an overall yield of 10%
(Scheme 1C).
The oxidation of pyrene 2 to the 4,5-dione 3 using a
NaIO4/RuCl3 system has been reported several times in
literature, including a detailed description on a 15 gram scale
by Walsh et al.[8] One of the major drawbacks associated with
this reaction is the formation of a dark-colored, intractable
suspension which is difficult to filter off, seriously hampering
the work-up. However, we found that, when the NaIO4 was
carefully added as an aqueous solution, rather than as a solid,
to a solution of 2 and RuCl3 in DCM:THF, and the addition
was stopped when the reaction starts to warm up, a much
finer suspension could be obtained. This can then be filtered
off using a standard glass fritted filter, significantly expedit-
ing the work-up procedure, without negatively affecting the
yield of 3 obtained from the filtrate (~50%) (Figure 2A). The
product can be purified by column chromatography over
silica gel using neat DCM as eluent, which can be recovered
and reused to reduce the total amount of solvent needed.[8]
The crucial step in our synthesis is the direct conversion
of 3 to 4. We initially tried to accomplish this conversion by
use in a wide range of organic electronic applications. Of
particular importance is their use in the development of blue-
colored, organic light-emitting diodes (OLEDs), which remains a
challenge in the field. Unfortunately, commercial sources and
synthetic procedures known in the literature are unable to
provide enough CPP for large scale implementation. Herein, we
report on the development of a novel, gram-scale synthesis of
CPP in three steps, starting from pyrene. The key steps in our
methodology are the ring contraction of pyrene-4,5-dione to
oxoCPP in a single step, as well as the direct reduction of
oxoCPP to CPP. Apart from the small number of synthetic steps,
our methodology benefits from the use of relatively non-
hazardous reagents, together with optimized purification
procedures, making CPP accessible in useful quantities.
Organic light-emitting diodes (OLEDs) show significant
advantages over the now commonplace inorganic ones,
including simpler manufacturing, lower power consumption
and better durability, to name just a few.[1] Of the three
primary colors, blue colored OLEDs have proven to be
especially elusive, making them the focus of much research.[2]
Thin film, multi-layer devices using low molecular weight
polymers incorporating the 4H-cyclopenta[def]phenanthrene
(CPP) motive hold promise in this regard, generally showing
stable emissions and good color purity.[3] Other electro-
luminescent and photovoltaic devices,[4] including dye-sensi-
tized solar cells,[5] also benefit from the low band gap of CPP-
based polymers, which is responsible for its superior
qualities.[4e] Some typical examples of polymeric constructs
incorporating the CPP motive are shown in Figure 1. They
can consist entirely of repeating CPP units (Figure 1A,[3b]
[a] A. van der Ham, Prof. Dr. H. S. Overkleeft, Dr. D. V. Filippov,
Dr. G. F. Schneider
Leiden Institute of Chemistry, Leiden University,
Einsteinweg 55, 2333 CC Leiden, The Netherlands
E-mail: filippov@lic.leidenuniv.nl
Supporting information for this article is available on the WWW under
°
pyrolysis of 3 at 245–260 C in an evacuated tube in the
presence of lead(II) oxide.[9] This provided the desired
product 4 as a yellow sublimate but only in poor yields (2–
8%). When the reaction was performed on a one-gram scale,
trace amounts of phenanthrene-4,5-lactone 5 were also
obtained as ascertained by comparison with an authentic
© 2021 The Authors. European Journal of Organic Chemistry published by
Wiley-VCH GmbH. This is an open access article under the terms of the
Creative Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original work is
properly cited, the use is non-commercial and no modifications or adap-
tations are made.
Eur. J. Org. Chem. 2021, 2013–2017
2013
© 2021 The Authors. European Journal of Organic Chemistry
published by Wiley-VCH GmbH