Donor-acceptor segregated paracyclophanes composed of naphthobipyrrole and stacked fluoroarenes

Article abstract of DOI:10.1021/ol400882q

The expeditious synthesis of donor-acceptor segregated paracyclophanes has been achieved by a selective S_NAr reaction of hexafluorobenzene with o-dipyrrolylbenzenes and subsequent cyclodehydrogenation. An orthogonally arranged D-A segregated structure was confirmed by X-ray crystallography. The combined results of DFT calculations and absorption spectra revealed the charge transfer (CT) nature from the naphthobipyrrole (donor) to the stacked fluoroarene moiety (acceptor).

Full text of DOI:10.1021/ol400882q

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
DonorꢀAcceptor Segregated
Paracyclophanes Composed of
Naphthobipyrrole and Stacked
Fluoroarenes
Masayoshi Takase,* Ayumi Inabe, Yuki Sugawara, Wataru Fujita, Tohru Nishinaga,*
and Kotohiro Nomura
Department of Chemistry, Graduate School of Science and Engineering,
Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
mtakase@tmu.ac.jp; nishinaga-tohru@tmu.ac.jp
Received March 31, 2013
ABSTRACT
The expeditious synthesis of donorꢀacceptor segregated paracyclophanes has been achieved by a selective SNAr reaction of hexafluorobenzene
with o-dipyrrolylbenzenes and subsequent cyclodehydrogenation. An orthogonally arranged DꢀA segregated structure was confirmed by X-ray
crystallography. The combined results of DFT calculations and absorption spectra revealed the charge transfer (CT) nature from the
naphthobipyrrole (donor) to the stacked fluoroarene moiety (acceptor).
Cyclophanes possessing a rigid and stacked structure
such as [2,2]paracyclophane have attracted a great deal of
attention due to the interest in strongly stacked π-systems.1,2
Such stacked/strained π-systems are regarded as excellent
models for studying not only molecular design and synthesis3
but also optical and electrical properties.4 Recently, π-stacked
structures have been used in the fields of molecular electronics
as (super)conductors and field effect transistors (FET)
and molecular photonics as organic light emitting diodes
(OLED).5 Considering electrical conductors and super-
conductors as well as photovoltaic cells, donorꢀacceptor
segregated structures are a key issue. Such structures have
typically been achieved in the solid state as cocrystals
or mixed supramolecular assemblies, which resulted in
difficulties tuning the crystalline6 and supramolecular
morphologies,7 respectively. On the basis of these findings,
(1) For reviews, see: (a) Cram, D. J.; Cram, J. M. Acc. Chem. Res.
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C.; Vogtle, F. Angew. Chem., Int. Ed. Engl. 1992, 31, 528. (e) de Meijere,
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Cyclophanes; The Royal Society of Chemistry: Cambridge, 1994. (j)
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Bendikov, M.; Chaffins, S.; Perepichka, D. F.; Dautel, O.; Duong, H.;
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Silbey, R.; Bredas, J.-L. Chem. Rev. 2007, 107, 926. (b) Pisula, W.; Feng,
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r
10.1021/ol400882q
XXXX American Chemical Society

we herein report the design and synthesis of a donorꢀ
acceptor segregated paracyclophane and its dimer as an
initial step toward easily processable and well-defined
segregated structures.
The key step, the synthesis of cyclic precursor 4, was
accomplished by the aromatic nucleophilic substitution
(SNAr) reaction of pyrrolyl anions and hexafluorobenzene,12
considering the reported examples of regioselective substitu-
tions at the 1 and 4 positions.9a,13 Utilizing the SNAr reaction
with dipyrrolylbenzene 3c and hexafluorobenzene, the cyclic
structures of both 4c (17%) and 6c (25%) were obtained in
one-pot (Scheme 1),14 which were easily isolated by column
chromatography or recycling gel permeation chromatography
system. Notably, the formation of 6c seemed to be more
favorable than that of 4c (Figure S3, Supporting Information).
Another possible synthetic approach for 4 is a stepwise
synthesis via clipped-type compound 5b. Although a large
excess amount of fluoroarene was necessary to obtain 5b in
moderate yield (73%) because of the rapid SNAr reaction,12
the cyclic structure 4b was obtained in 43% yield from 5b.
The molecular design of donorꢀacceptor segregated
paracyclophane 1 is based on the orthogonally arranged
naphthobipyrroles8 as a donor and the stacked fluoroarenes9
as an acceptor, where the inter NꢀN distances are calculated
˚
to be ca. 3.4 A, which is almost the same as the πꢀπ stacking
distance (Scheme 1). Compared to the numerous examples
of electron-rich cyclophanes and some DꢀA cyclophanes,10
there are only a few examples of electron-deficient cyclo-
phanes composed of stacked or nearly stacked fluoroarenes
are still limited.11
Scheme 1. Synthesis of Paracyclophane 1 and Clipped Structure 2a
Figure 1. Crystal structures of the two independent conformers
of 4a in the unit cell. Thermal ellipsoids are at 50% probability.
Single-crystal structure analysis of 4a showed two inde-
pendent conformers in the unit cell. The fluorobenzene
moieties in the two structures are orthogonally crossed with
CF-π interactions and are pseudostacked (Figure 1).15
Variable-temperature (VT) 19F NMR measurements showed
that the two fluorobenzene moieties can rotate freely even
at ꢀ80 °C (Figure S4, Supporting Information).16,17
a Reagents and conditions: (i) NaH, C6F6 (1 equiv) in DMF and
THF; (ii) NaH, excess C6F6 in DMF and THF; (iii) NaH, 3b in DMF;
(iv) DDQ, Sc(OTf)3 in CH2Cl2.
Subsequent cyclodehydrogenation with 2,3-dichloro-5,6-
18
dicyano-p-benzoquinone (DDQ) and Sc(OTf)3 under di-
(7) (a) Li, W.-S.; Yamamoto, Y.; Fukushima, T.; Saeki, A.; Seki, S.;
Tagawa, S.; Masunaga, H.; Sasaki, S.; Takata, M.; Aida, T. J. Am.
Chem. Soc. 2008, 130, 8886. (b) Yasuda, T.; Shimizu, T.; Liu, F.; Ungar,
G.; Kato, T. J. Am. Chem. Soc. 2011, 133, 13437. (c) Dossel, L. F.;
Kamm, V.; Howard, I. A.; Laquai, F.; Pisula, W.; Feng, X.; Li, C.;
€
Takase, M.; Kudernac, T.; De Feyter, S.; Mullen, K. J. Am. Chem. Soc.
luted conditions (0.5 mM) gave the target cyclophanes 1b
(55%) and 1c (52%) in good yields. The 1H NMR spectrum
of 1c revealed the absence of Ha protons at the inner R
positions of the pyrroles; the other peaks, Hb, Hc, and Hd,
€
2012, 134, 5876.
(8) (a) Nadeau, J. M.; Swager, T. M. Tetrahedron 2004, 60, 7141. (b)
Roznyatovskiy, V. V.;Roznyatovskaya, N. V.; Weyrauch, H.; Pinkwart, K.;
(13) For selective SNAr reaction at 1,4-positions of hexafluoroben-
zene, see: (a) Kim, J.-P.; Lee, W.-Y.; Kang, J.-W.; Kwon, S.-K.; Kim,
J.-J.; Lee, J.-S. Macromolecules 2001, 34, 7817. (b) Deck, P. A.; Maiorana,
C. R. Macromolecules 2001, 34, 9. (c) Woody, K. B.; Bullock, J. E.; Parkin,
S. R.; Watson, M. D. Macromolecules 2007, 40, 4470. (d) Wang, Y.;
Watson, M. D. J. Am. Chem. Soc. 2008, 130, 452. (e) Dutta, T.; Woody,
K. B.; Watson, M. D. J. Am. Chem. Soc. 2008, 130, 452. (f) Dutta, T.;
Woody, K. B.; Parkin, S. R.; Watson, M. D.; Gierschner, J. J. Am. Chem.
Soc. 2009, 131, 17321. (g) Nakamoto, M.; Inagaki, Y.; Nishina, M.;
Sekiguchi, A. J. Am. Chem. Soc. 2009, 131, 3172.
(14) Our preliminary DFT calculations also support the selectivity of
the 1,4-substitutions by pyrrole on hexafluorobenzene. See Figure S9,
Supporting Information.
(15) Single-point DFT calculations for the two geometries indicated that
the “pseudo-stacked structure” was slightly more stable by 0.13 kcal molꢀ1
(B3LYP/6-31G(d,p)).
€
Tubke, J.; Sessler, J. L. J. Org. Chem. 2010, 75, 8355. (c) Roznyatovskiy, V.;
Lynch,V.;Sessler,J.L.Org. Lett. 2010,12, 4424. (d) Sarma, T.; Panda, P. K.;
Anusha, P. T.; Rao, V. Org. Lett. 2011, 13, 188.
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Murofushi, H.; Nakashima, N. Bull. Chem. Soc. Jpn. 2004, 77, 2121.
(10) (a) Kato, S.; Matsumoto, T.; Ideta, K.; Shimasaki, T.; Goto, K.;
Shinmyozu, T. J. Org. Chem. 2006, 71, 4723. (b) Elacqua, E.; Bucar,
D.-K.; Skvortsova, Y.; Baltrusaitis, J.; Geng, M. L.; MacGillivray, L. R.
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Ghiviriga, I.; Dolbier, W. R., Jr. J. Org. Chem. 2009, 74, 6831.
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Smeets, W. J. J.; Speck, A. L.; Meijer, E. W. J. Org. Chem. 1996, 61,
9012. (b) Takase, M.; Enkelmann, V.; Sebastiani, D.; Baumgarten, M.;
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benzene dimer, see: (a) Boden, N.; Davis, P. P. Mol. Phys. 1973, 25, 81.
(b) Steed, J. M.; Dixon, T. A.; Klemperer, W. J. Chem. Phys. 1979, 70,
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RSC Adv. 2012, 2, 3221.
B
Org. Lett., Vol. XX, No. XX, XXXX

transfer (CT) absorption bands at 338 and 314 nm for only
annulated 1c and 2b, respectively. The intramolecular CT
natureof 1 and 2 was supported bymolecular orbital(MO)
calculations at the B3LYP/6-31G(d) level of theory, which
showed that the highest occupied molecular orbital (HOMO)
and lowest unoccupied molecular orbital (LUMO) are sub-
stantially localized at the naphthobipyrrole and fluoroarene
moieties, respectively (Figure 5). The CT band of 1 appeared
at a longer wavelength mainly as a result of the low-lying
LUMO of the stacked fluoroarene. Thus, the transannular
reaction to form naphthobipyrrole units in both 1 and 2 not
only increases the HOMO level but also greatly decreases the
LUMO level with the packing of the two fluoroarenes.
Figure 2. (a) 1H and (b) 19F NMR spectra of 1c and 4c in CDCl3.
Figure 4. Absorption spectra of (a) cyclophane 1c (blue) and 4c
(red) and (b) clipped structure 2b (blue) and 5b (red) in THF
([c] = 5.0 ꢁ 10ꢀ6 M).
Figure 3. Crystal structure of cyclophane 1b: (a) side views and
(b) top view. Thermal ellipsoids are at 50% probability, and
alkyl chains are omitted for clarity.
were shifted downfield, reflecting the expanded conjugation
of the naphthobipyrrole moiety (Figure 2a). The 19F NMR
spectrum of 1c also exhibited a low-field shift associated with
the transannular reaction due to the effect of steric compres-
sion between the two fluorine atoms on the closely packed
fluoroarenes (Figure 2b).19 Similar spectral changes were
observed for the clipped-type compounds going from 5b
to 2b. The tightly stacked fluoroarene structure of 1b with
a DꢀA orthogonal arrangement was umambiguously re-
vealed by X-ray crystallography (Figure 3). The closest CꢀC
˚
distance of the stacked fluoroarene moiety was 3.19 A,
Figure 5. Frontier MO diagrams of (a) 1a, 4a and (b) 2a, 5a
calculated at the B3LYP/6-31G(d) level of theory.
which is smaller than the sum of the van der Waals radii
˚
(3.4 A), and the dihedral angle between D and A was
approximately 70°.
The normalized absorption spectra in THF of 1c, 4c and
2b, 5b are shown in Figure 4, which revealed broad charge-
The redox properties of 1c, 4c and 2b, 5b were investigated
using cyclic voltammetry (CV) in CH2Cl2 (Table 1). All
compounds exhibited irreversible oxidation peaks (Figure
S5, Supporting Information). As predicted from the DFT
calculations, formation of the naphthobipyrrole unit in-
creased the HOMO levels by 0.06 eV (1c) and 0.23 eV (2b)
and decreased the LUMO levels by 0.30 eV (1c) and 0.25 eV
(2b), which were evaluated based on the first oxidation
potentials and the HOMOꢀLUMO gaps estimated from
the absorption spectra, respetively. A much larger decrease
(18) (a) Tsuda, A.; Osuka, A. Science 2001, 293, 79. (b) Nakamura,
Y.; Aratani, N.; Shinokubo, H.; Takagi, A.; Kawai, T.; Matsumoto, T.;
Yoon, Z. S.; Kim, D. Y.; Ahn, T. K.; Kim, D.; Muranaka, A.;
Kobayashi, N.; Osuka, A. J. Am. Chem. Soc. 2006, 128, 4119. (c) Davis,
N. K. S.; Pawlicki, M.; Anderson, H. L. Org. Lett. 2008, 10, 3945.
(19) Similar phenomena have been observed for [n.n]paracyclophanes in
1H NMR spectra; see: (a) Shibahara, M.; Watanabe, M.; Iwanaga, T.;
Matsumoto, T.; Ideta, K.; Shinmyozu, T. J. Org. Chem. 2008, 73, 4433. (b)
Otsubo, T.; Mizogami, S.; Sakata, Y.; Misumi, S. Bull. Chem. Soc. Jpn. 1973,
46, 3831. For the fluorinated C60 on the 19F NMR spectra, see: (c) Kareev,
I. E.; Quinones, G. S.; Kuvychko, I. V.; Khavrel, P. A.; Ioffe, I. N.; Goldt,
I. V.; Lebedkin, S. F.; Seppelt, K.; Strauss, S. H.; Boltalina, O. V. J. Am.
Chem. Soc. 2005, 127, 11497.
(20) The rotation barrier was evaluated to be 11ꢀ12 kcal molꢀ1
(Tc = 0ꢀ25 °C in C2D2Cl4) for the Fa fluorine atoms labeled in the
structure.
Org. Lett., Vol. XX, No. XX, XXXX
C

Table 1. Redox Potentials of 1c, 4c, 2b, 5b, and 7c
Scheme 2. (a) Synthesis and Conformational Equilibrium of
Cyclophane Dimer 7c and (b) the Frontier MOs for the DFT-
Optimized Structure at the B3LYP/6-31G(d) Level of Theory
OX
expc
cald
Epa
(V)a
HOMOb
(eV)
LUMOb
(eV)
Eg
(eV)
Eg
(eV)
1c
4c
0.62, 0.75
0.68
ꢀ5.42
ꢀ5.48
ꢀ5.51
ꢀ5.74
ꢀ5.28
ꢀ2.38
ꢀ2.08
ꢀ2.39
ꢀ2.14
ꢀ2.40
3.04
3.40
3.12
3.60
2.88
3.46
3.87
3.95
4.27
3.35
2b 0.71, 0.87
5b 0.94
7c
0.48,e 0.76
a Determined from the cyclic voltammograms in CH2Cl2 with 0.1 M
n-Bu4NPF6 as the supporting electrolyte, Ag/AgNO3 as the reference
electrode, Pt as the working electrode, Pt wire as the counter electrode,
and a scan rate of 20 mV/s. Potentials are referred to vs Fc/Fcþ (V).
exp
b HOMO = ꢀ(Eox þ 4.8) and LUMO = HOMO þ Eg
.
c Calculated
from the cutoff of the absorption spectra. d Calculated at the B3LYP/6-
31G(d). e Reversible peak.
in the LUMO level was observed in 1c compared to 2b,
which is consistent with the DFT results (Figure 5).
To demonstrate the structural features of the DꢀA
segregated cyclophane, dimer 7c was synthesized with
FeCl3 in 27% yield (Scheme 2). Because the two cyclo-
phane units are considered to rotate about the “new bond”
as depicted in Scheme 2a, VT 19F NMR measurements
were conducted (Figure S7, Supporting Information). The
splitting observed at lower temperatures indicates that
twisted structures with partial stacking between the two
stacked fluoroarene moieties are favorable.20 The DFT
optimized structure also showed a twisted structure with a
dihedral angle for the two naphthobipyrroles of 96°
(Figure S6, Supporting Information). Upon dimerization,
one oxidation peak in the CV became reversible and
appeared at a lower potential (Figure S5, Supporting
Information), and the absorption spectrum showed a long
tail into the lower energy region compared to the cyclo-
phane monomer 1c (Figure S8, Supporting Information,
and Table 1). The extended π-conjugation of the HOMO
and LUMO for the optimized structure (Scheme 2b) led to
the raising and lowering of the HOMO and LUMO levels,
respectively. These results clearly indicate that the integra-
tion of DꢀA moieties in a segregated manner was success-
ful even with “mono-linkage” of the cyclophanes.
first time. On the basis of the orthogonally arranged
electron-rich naphthobipyrroles and electron-deficient
fluoroarenes, intramolecular CT was demonstrated, and
the dimerization of cyclophane 1 revealed the successful
integration of DꢀA segregation. Further studies on synthe-
sizing poly(oligo)meric structures toward polypyrrole will
allow for realization of proccessable and well-defined DꢀA
segregated structures; such investigations are currently
underway.
Acknowledgment. This work was supported by a
Grant-in-Aid for Scientific Research (No. 23750045) from
MEXT, Japan, and a research grant from the Kurata Mem-
orial Hitachi Science and Technology Foundation. We thank
Dr. K. Yamashita (Tokyo Metropolitan University) for help
with the X-ray crystal structure analysis.
Supporting Information Available. Experimental de-
tails and the characterization data for all new compounds;
Figure S3ꢀS8; atomic coordinates of the optimized struc-
tures of 1a, 2a, 4a, 5a and 7a (B3LYP/6-31G(d)); X-ray
data for 1b and 4a (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
In summary, we have demonstrated a facile synthesis of
donorꢀacceptor segregated cyclophane 1 in which the
selective SNAr reaction of hexafluorobenzene enabled the
straightforwardformation of cyclophane structures for the
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX