A facile and versatile approach to double N-heterohelicenes: Tandem oxidative C-N couplings of N-heteroacenes via cruciform Dimers

Article abstract of DOI:10.1002/anie.201500684

Novel double N-hetero[5]helicenes that are composed of two nitrogen-substituted heteropentacenes are synthesized by tandem oxidative C-N couplings via the cruciform heteropentacene dimers. The developed method is very facile and enables the synthesis of a double helicene in only two steps from commercially available naphthalene derivatives. These double N-hetero[5]helicenes have larger torsion angles in the fjord regions than typical [5]helicenes, and optical/electrochemical measurements revealed a significant increase in the electronic communication between the two heteropentacene moieties of the double helicenes compared with their cruciform dimers. The optical resolution of one of the double helicenes was successfully carried out, and their stability towards racemization was remarkably higher than those of typical [5]helicenes. The synthetic strategy proposed in this paper should be versatile and widely applicable to the preparation of double helicenes from other N-containing π-conjugated planar molecules.

Full text of DOI:10.1002/anie.201500684

Angewandte
Chemie
DOI: 10.1002/anie.201500684
Helicenes
A Facile and Versatile Approach to Double N-Heterohelicenes:
À
Tandem Oxidative C N Couplings of N-Heteroacenes via Cruciform
Dimers**
Daisuke Sakamaki,* Daisuke Kumano, Eiji Yashima, and Shu Seki*
Abstract: Novel double N-hetero[5]helicenes that are com-
posed of two nitrogen-substituted heteropentacenes are synthe-
shortest synthesis of double N-hetero[5]helicenes by a direct
oxidative coupling of N-substituted pentacenes via their
cruciform dimers to demonstrate the usefulness of these
compounds as building blocks for novel three-dimensional
(3D) p-systems (Scheme 1).
À
sized by tandem oxidative C N couplings via the cruciform
heteropentacene dimers. The developed method is very facile
and enables the synthesis of a double helicene in only two steps
from commercially available naphthalene derivatives. These
double N-hetero[5]helicenes have larger torsion angles in the
fjord regions than typical [5]helicenes, and optical/electro-
chemical measurements revealed a significant increase in the
electronic communication between the two heteropentacene
moieties of the double helicenes compared with their cruciform
dimers. The optical resolution of one of the double helicenes
was successfully carried out, and their stability towards
racemization was remarkably higher than those of typical
[5]helicenes. The synthetic strategy proposed in this paper
should be versatile and widely applicable to the preparation of
double helicenes from other N-containing p-conjugated planar
molecules.
Scheme 1. Synthesis of double heterohelicenes via cruciform hetero-
acene dimers by tandem oxidative couplings.
A
zaacenes, the nitrogen-substituted analogues of acenes,
The oxidative coupling is one of the most straightforward
strategies to connect p-systems.[5] Recently, we found that
a 6,13-dihydro-6,13-diazapentacene (DP) with an alkyl sub-
stituent on one of the nitrogen atoms (5a and 5b) could be
easily oxidatively dimerized in the presence of 2,3-dichloro-
5,6-dicyano-1,4-benzoquinone (DDQ; Scheme 2a). X-ray
single-crystal analysis revealed the cruciform structure of
these dimers with a single bond between the N13 and C12’
positions of each monomer, and the two diazapentacene
moieties were found to be almost perpendicular to each other
(Supporting Information, Figure S1). It is well-known that DP
is converted into 6,13-diazapentacene by chemical oxida-
tion,[1] and this reaction could be understood as the formation
of a pyrazine moiety resulting from the removal of two
electrons and two protons. On the other hand, 5a and 5b have
only one NH group, and therefore, the formation of a pyrazine
moiety is impossible; instead, oxidation results in the
dimerization between the N13 and the C12’ positions, which
have a large spin density in the radical cation state (Fig-
ure S2). The oxidative dimerization of 13H-dibenzo-
[b,i]phenoxazine (6), which is regarded as the nitrogen- and
oxygen-substituted analogue of pentacene and contains only
one NH group, also gave a cruciform dimer, namely
compound 4 (Scheme 2a). In contrast to the previously
reported oxidative dimerizations of phenoxazines, which
provided mixtures of two structural isomers,[6] the dimeriza-
tions of 5a, 5b, and 6 proceeded with high regioselectivity,
have attracted much attention in the field of organic
electronics as promising candidates for charge-transport
materials because of their facile syntheses and higher
chemical stability compared with larger acenes, such as
pentacene.[1] Over the past decade, several research groups
have reported the synthesis of novel azaacene derivatives by
means of chemical modification of azaacene skeletons, for
example, by introducing functional groups[2,3] and/or extend-
ing the p-plane.[4] However, there have been few attempts to
construct complicated structures by combining azaacene
molecules, in spite of their potential for three-dimensional
p-conjugated molecular bricolages. Herein, we report the
[*] Dr. D. Sakamaki, Prof. Dr. S. Seki
Department of Applied Chemistry, Graduate School of Engineering
Osaka University
Suita, Osaka 565-0871 (Japan)
E-mail: d.sakamaki@chem.eng.osaka-u.ac.jp
seki@chem.eng.osaka-u.ac.jp
D. Kumano, Prof. Dr. E. Yashima
Department of Applied Chemistry, Graduate School of Engineering
Nagoya University
Nagoya, Aichi, 464-8603 (Japan)
[**] We deeply thank Dr. Nobuko Kanehisa (Osaka University) for her
help with single-crystal X-ray analysis. This work was partly
supported by a Grant-in-Aid for Young Scientists (B) from the Japan
Society for the Promotion of Science (JSPS; 26810023). D.S. thanks
the JSPS for a Research Fellowship for Young Scientists.
À
giving only the N13 C12’ connected dimers because the fused
benzene rings prevent reactions at the para positions with
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500684.
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À
N N bonds. In our case, on the other
hand, the two heteroacenes are con-
À
nected by two C N bonds, and there-
fore, our double helicenes are thought
to have higher chemical stability. Fur-
thermore, because our strategy does
not require any premodifications of
the starting heteroacenes, such as the
introduction of reactive substituents,
it should be widely applicable to the
construction of double helicenes from
other planar p-conjugated monomers
with an NH group, such as phenothia-
zine and carbazole derivatives.[10]
Fortunately, the structures of 1b
and 2 were confirmed by single-crystal
X-ray crystallography (Figure 1),
which revealed their double helicene
structure consisting of two N-heter-
o[5]helicene moieties with the same
helicity in a molecule. The two enan-
tiomers (the P,P’ and M,M’ isomers)
were found to be present in the unit
cell in a ratio of 2:2. The torsion
Scheme 2. Synthesis of a) cruciform dimers and b) double N-hetero[5]helicenes.
respect to the nitrogen atoms, which are the carbon atoms
with the largest spin density in the radical cation state.
An important feature of these cruciform dimers is the
presence of an intact NH group, which enables further
chemical modifications. It is easily predicted that the oxida-
tion of these cruciforms will result in ring-fusion reactions
angles, which are defined as the dihedral angles between three
successive purple bonds in Figure 1, were determined to be
54.78 and 51.88 for 1b and 50.18 and 41.48 for 2; these values
are significantly larger than the dihedral angle of [5]helicene
(ca. 308).[11] We could not observe the formation of the achiral
P,M’ isomer. These results were explained by DFT calcula-
tions.[12] The achiral isomers of 1b and 2 were calculated to be
unstable with respect to the chiral isomers by 10.1 and
8.6 kcalmolÀ1, respectively, probably because of the large
strain energy that results from the highly bent structure of the
heteropentacene moieties in the central hexagons (Figures S3
and S4). All of the double helicenes are well soluble in
affording
double
N-hetero[5]helicene
structures
(Scheme 1).[7–9] First, we tried to further oxidize the cruciform
dimers with DDQ, but the ring-fused products were not
obtained. After screening some oxidation conditions, we
found that a combination of DDQ and Sc(OTf)3 resulted in
the ring-fusion reaction (Scheme 2b).[5] Moreover, we suc-
ceeded to reduce the number of
synthetic steps required for the for-
mation of 1a and 2 by developing
a method for the one-pot tandem
oxidation of 5a and 6. To the best of
our knowledge, these are the shortest
syntheses of double helicenes to date,
as only two and three steps are
required to obtain 1a and 2 from
commercially available 2,3-disubsti-
tuted naphthalene derivatives, respec-
tively. Rajca et al. have reported the
synthesis of double N-hetero[5]heli-
cenes by three oxidative homocou-
À
À
plings (one C C and two N N cou-
plings) between two planarized meta-
phenylenediamine
derivatives.[8]
However, because the helical struc-
ture was maintained by the weak
À
N N single bonds, Rajcaꢀs double
helicene could be reverted back to
its non-helical precursor by chemical
reduction and the dissociation of two
Figure 1. X-ray crystal structures of a) (P,P’)-1b and b) (P,P’)-2. Hydrogen atoms are omitted for
clarity. Thermal ellipsoids are set at 50% probability. The torsion angles are defined as the
dihedral angles between three successive purple bonds.
2
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Table 1: Oxidation potentials (V vs. Fc0/Fc+) of the double N-hetero[5]-
helicenes 1a, 1b, and 2 and the cruciform dimers 3a and 3b in CH2Cl2
(0.1m nBu4NBF4), determined by differential pulse voltammetry (DPV)
measurements and simulations.[14]
common organic solvents, such as toluene, dichloromethane,
chloroform, and THF, which reflects their distorted molecular
structures, whereas the cruciform dimers 3b and 4 with short
or no side chains showed low solubilities.
The absorption spectra of 1a, 3a, and 6,13-dihexyl-6,13-
dihydro-6,13-diazapentacene[3] (7) are shown in Figure 2a.
The spectral shape for cruciform dimer 3a was very similar to
E1
E2
E3
E4
E2ÀE1
1a
3a
1b
3b
2
À0.103
0.044
0.475
0.165
0.509
0.158
0.717
0.802
0.190
0.820
0.220
0.578
0.121
0.574
0.101
0.565
0.725
0.696
À0.065
0.057
0.152
results also suggest that the ring fusion of two heteroacene
moieties greatly enhances the electronic communication
between the two azaacene units. The forth oxidations of 1a
and 1b were prohibited probably because of the increased
Coulomb repulsion in their highly oxidized states resulting
+
+
from the ring fusion. The absorption spectra of 1aC and 2C
showed that the generated radical spin was fully delocalized
over the whole molecular skeleton (see the Supporting
Information).
We succeeded in the optical resolution of double helicene
2 by using an HPLC column with a chiral stationary phase.
The circular dichroism (CD) spectra of the fractions that
eluted more rapidly and more slowly were mirror images of
each other (Figure 3a), and it was confirmed that these two
fractions consisted of enantiomers with opposite helicity. The
CD spectra of (P,P’)-2 and (M,M’)-2 that were simulated by
time-dependent density functional theory (TD-DFT) calcu-
lations were in good agreement with the observed spectra,
and it is strongly suggested that the faster eluting enantiomer
has P,P’ helicity and the more slowly eluting enantiomer M,M’
helicity (Figure 3b). The CD spectrum of enantiopure 2
remained unchanged after heating in toluene at 1008C for
13 hours (Figure S20), showing that double hetero[5]helicene
2 has a much higher stability towards racemization than
[5]helicene, which racemizes with a half-life of 62.7 minutes at
578C.[15]
Figure 2. Absorption spectra of a) 1a, 3a, and 7 and of b) 2, 4, and 6
in CH2Cl2.
that of 7, and therefore, the two DP units in 3a basically
maintain their monomeric electronic properties because of
their perpendicular arrangement. On the other hand, the
absorption of DP double helicene 1a was globally broadened
and red-shifted. The same trend was observed in the spectra
of 2, 4, and 6, suggesting an increase in the p-conjugation
between the two heteroacene moieties in the double heli-
cenes. The ring fusion to the double helicenes also affected
the emission properties (see the Supporting Information).
Electrochemical measurements revealed that the double
helicenes show three (1a and 1b) and two (2) couples of
reversible one-electron oxidation processes (Table 1; for the
voltammograms see Figure S10–S14). Interestingly, for the
double helicenes, the difference between the first and the
second oxidation potential DE (E2ÀE1), which relates to the
magnitude of the electronic interaction between two redox
sites, is much larger than for their cruciform precursors. For
example, the DE values of the cruciform DP dimers (3a:
0.121 V; 3b: 0.101 V) increased substantially to 0.578 V (1a)
and 0.574 V (1b) in the double N-heterohelicenes.[13] These
In conclusion, we have successful developed a facile and
straightforward approach for the preparation of novel double
N-hetero[5]helicenes that consist of two fused heteropenta-
cenes by employing tandem oxidative coupling reactions of
heteropentacenes that proceed via the cruciform dimers. X-
ray single-crystal analysis confirmed the structures of the
double helicenes and the cruciform intermediates.[16] The
absorption and emission spectra, electrochemical measure-
ments, and DFT calculations revealed an increase in p-
conjugation between the two heteroacene moieties in the
double helicenes compared to the cruciform precursors. The
optical resolution of 2 was accomplished by preparative
HPLC on a chiral stationary phase, and the helical structures
were shown to be stable reflecting their rigid molecular
structure. The synthetic strategy towards double helicenes
that we propose in this paper is thought to be highly general
and applicable to various heteroacenes. The physical proper-
ties of the heteroacenes, such as their charge transport and
circular polarized luminescence (CPL) properties, will be
discussed in detail in our future work, and further investiga-
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Received: January 24, 2015
Published online: && &&, &&&&
Keywords: electrochemistry · electron transfer · helicenes ·
.
heteroacenes · oxidation
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4
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Angew. Chem. Int. Ed. 2015, 54, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Helicenes
Double N-hetero[5]helicenes that are
composed of two nitrogen-substituted
heteropentacenes were synthesized by
D. Sakamaki,* D. Kumano, E. Yashima,
À
S. Seki*
&&&& — &&&&
tandem oxidative C N couplings via the
cruciform dimers in only two steps from
commercially available naphthalene
derivatives. These compounds are
remarkably stable towards racemization,
and the two heteroacenes moieties were
shown to be strongly electronically cou-
pled.
A Facile and Versatile Approach to Double
N-Heterohelicenes: Tandem Oxidative
À
C N Couplings of N-Heteroacenes via
Cruciform Dimers
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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