Synthesis, X-ray analysis and photophysical properties of a new N-containing pentacyclic helicene

Article abstract of DOI:10.1016/j.tetlet.2013.07.036

A new helically chiral pentacyclic system containing one pyrrole ring was prepared in a good yield and purity via a three-step sequence involving Heck coupling and classical oxidative photocyclization. X-ray crystal structure analysis indicated that the conformation resembled that of unsubstituted pentahelicene, the idealized symmetry of which is C₂. The optical properties of the pentacyclic helicene were investigated and show interesting behaviour.

Full text of DOI:10.1016/j.tetlet.2013.07.036

Tetrahedron Letters 54 (2013) 5421–5425
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, X-ray analysis and photophysical properties of a new
N-containing pentacyclic helicene
Mourad Ben Braiek a, Faouzi Aloui a, Souad Moussa a, Moncef Tounsi b, Jérome Marrot c,
Béchir Ben Hassine a,
⇑
a Laboratoire de Synthèse Organique Asymétrique et Catalyse Homogène, Faculté des Sciences, Avenue de l’environnement, 5019 Monastir, Tunisia
b Laboratoire des Interfaces et des Matériaux Avancés (LIMA), Faculté des Sciences, Avenue de l’environnement, 5019 Monastir, Tunisia
c Institut Lavoisier de Versailles ILV, UMR CNRS 8180 Université de Versailles Saint-Quentin en Yvelines, 45 Avenue des Etats Unis, 78035 Versailles cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
A new helically chiral pentacyclic system containing one pyrrole ring was prepared in a good yield and
purity via a three-step sequence involving Heck coupling and classical oxidative photocyclization. X-ray
crystal structure analysis indicated that the conformation resembled that of unsubstituted pentahelicene,
the idealized symmetry of which is C2. The optical properties of the pentacyclic helicene were investi-
gated and show interesting behaviour.
Received 14 April 2013
Revised 27 June 2013
Accepted 5 July 2013
Available online 14 July 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Azahelicenes
Carbazole
Fused-rings
Heck coupling
Photocyclization
Helicenes are polycyclic aromatic compounds formed from
ortho-fused benzene or other aromatic rings that adopt a helical
conformation to avoid overlapping of the terminal rings resulting
in helically chiral structures. Their specific backbone, which com-
synthesized by Meisenheimer and Witte in 1903 (Fig. 1).8 In an
independent study, Fuchs and Niszel prepared the racemic pyrrol-
o[6]helicene 2, in a two-step sequence.9 Following the same pre-
parative pathway, Pischel et al.10 reported the synthesis of the
corresponding tetramethyl derivative 3 in a low yield. Resolution
of the latter was achieved only on analytical scale by chiral HPLC.
Shi et al.11 have described the synthesis of carbazole-based
diaza[7]helicene 4, and reported its use as a deep-blue dopant
emitter in an organic light-emitting diode (OLED). Despite devel-
opments in helicene chemistry, it is still desirable to develop meth-
ods for easy access to pyrrolohelicenes via high yielding
procedures for the examination of these compounds as ligands or
bines electron delocalization and non-planarity of the
p-electron
network, makes them very stable towards acids and bases and rel-
atively high temperatures.1 These helically-shaped molecules have
found applications as potentially useful components in chiral dis-
cotic liquid crystalline materials,2 as building blocks for helical
conjugated polymers3 and as rotors.4 In particular, functionalized
helicenes are promising candidates for chiral catalysts5 and
ligands6 in asymmetric syntheses because they have a rigid helical
framework and possess high optical stability.
for broader exploitation. Their delocalized
p-electron system
In recent years, the preparation of heterohelicenes has been
studied extensively in order to exploit the unique properties of
these chiral molecules.7 However, azahelicenes have not been
elaborated sufficiently, and only a few reports have described the
synthesis of nitrogen-containing helicenes, despite their possible
applications in various branches of chemistry. Moreover, there
are reports on only a few examples of azahelicene analogues with
a pyrrole unit (pyrrolohelicenes), and methods with general appli-
cability for the synthesis of such compounds are rare. The first rep-
resentative of this family was pyrrolo[5]helicene 1 which was
allows them to exhibit interesting opto- and photo-electronic
R
NH
Br
N
H13C6
NH
R
R
N
Me
NH
N
C6H13
R
1
2: R = H
4
5
3: R = Me
⇑
Corresponding author. Tel.: +216 73500279; fax: +216 73500278.
E-mail address: bechirbenhassine@yahoo.fr (B. Ben Hassine).
Figure 1. Structures of azahelicenes 1–5.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.036

5422
M. Ben Braiek et al. / Tetrahedron Letters 54 (2013) 5421–5425
Br
properties. Thus, the development of new heterohelicenes and fur-
ther studies of the underlying structure–property relationships
represent an interesting challenge.
Me
N
7
6
4
9
3
10
Thus, due to the small number of known azahelicenes and hav-
ing in hand an alternative synthetic approach to helical com-
pounds,12 we have prepared nitrogen-containing helicene 5.
The new helically chiral pentacyclic system 5, containing one
pyrrole unit, was prepared via a synthetic sequence relying on
Mizoroki–Heck coupling and oxidative photodehydrocyclization
reactions. Our procedure utilizes N-methylcarbazole (6) as the
key building block for the synthesis of the helicene precursor 8,
which is then easily converted into the corresponding nitrogen-
containing pentahelicene 5 by photocyclization. The nitrogen atom
in this helically chiral compound could serve as a hydrogen accep-
tor as well as a metal-chelating agent for chirality recognition.
The synthetic route leading to the target helicene 5 began with
the bromination of N-methylcarbazole (6) using N-bromosuccini-
mide (NBS) in chloroform, which provided monobrominated deriv-
ative 7 in 99% yield (Scheme 1). Compound 7 and 4-bromostyrene
underwent a Mizoroki–Heck coupling reaction using 1% of Her-
mann’s catalyst, sodium acetate as the base and with N,N-dimeth-
ylacetamide (DMA) as the solvent. The desired coupled product 8
was obtained in 75% yield by heating for two days at 140 °C. It
was assumed to have E-stereochemistry at the double bond, based
on a 1H NMR study.13 The Z-isomer was neither isolated nor iden-
tified unambiguously as a minor product in the reaction mixture.
Finally, the resulting diarylethene 8 was subjected to photocyc-
lization in toluene under irradiation with a Hanovia high-pressure
mercury lamp, on a 200 mg scale per run, for about 2 h. The irradi-
ation was carried out in the presence of a stoichiometric amount of
iodine as oxidizing agent and an excess of tetrahydrofuran as a
hydrogen iodide scavenger,14 to give the target aza[5]helicene 5
in 76% yield, after purification by column chromatography
(Scheme 1).15
2
1
13
11
12
9
Figure 2. Chemical structure of the anthracene-like derivative 9.
toluene, ethyl acetate and tetrahydrofuran. The product was highly
stable in air and to light.
Suitable crystals of pentacyclic helicene 5 were obtained as
pale yellow plates by slow evaporation of a dichloromethane
solution at room temperature. X-ray analysis of the helicene
was carried out on a single crystal obtained from the racemic
form as shown in Figure 3. This compound did not undergo spon-
taneous resolution, though its space group was Pbca. Some of the
inner and outer bond lengths are given in Table 1. It was found
that the pyrrole ring of the helicene molecule did not affect
appreciably the outer bond lengths. The outer bonds C(5)–C(6),
C(10)–C(11), C(13)–C(14) and C(16)–C(17) were shortened to
1.34–1.37 Å with respect to the average bond length in benzene
(1.39 Å), whereas the inner bond distances C(1)–C(2), C(2)–C(3),
C(1)–C(21), C(20)–C(21) and C(19)–C(20) were lengthened to
1.40–1.45 Å (Table 1). The distance between the hydrogen atoms
H(3ꢀ ꢀ ꢀH(19) was found to be 2.098 Å and these H atoms point
away from each other.
The torsion angles along the inner helical rim (
C21; 2 = C2–C1–C21–C20; 3 = C1–C21–C20–C19), which varied
u1 = C3–C2–C1–
u
u
from 8.72° to 19.93°, were also a convenient measure of the helic-
ity and were in accord with those observed in pentahelicene (Ta-
ble 2). The terminal inner helical torsion angles (C3–C2–C1–C21
and C1–C21–C20–C19) were not equal and were relatively small
angles of 8.72° and 17.41°, respectively. The distortion of the
molecular structure (46.07°) is defined by the sum of the three
dihedral angles (C3–C2–C1–C21, C2–C1–C21–C20 and C1–C21–
C20–C19). The repulsion of the facing terminal benzene rings leads
to an interplanar angle of 35.69° between the terminal benzene
rings. The angles between the least-squares planes of neighbouring
rings were between 7.39° and 10.74°.
No other isomer was isolated from the reaction mixture, indi-
cating that the ring closure of alkene 8 had occurred from the
opposite side of the tricyclic moiety. Importantly, the anthra-
cene-like derivative 9 (Fig. 2) was not formed during the cycliza-
tion step, as this regioisomer would be expected to display
characteristic signals for both H-6 and H-13 at low field in the 1H
NMR spectrum.16
The helicene obtained was fully characterized by NMR spectros-
copy and by HRMS, and was found to have good solubility in com-
mon organic solvents including dichloromethane, chloroform,
The thermal behaviour of compound 5 was investigated by
differential scanning calorimetry (DSC), with a heating rate of
a nitrogen atmosphere
(Fig. 4). DSC indicated that compound 5 has a melting point of
10 °C/min from 25 to 300 °C, under
Me
N
6
NBS/CHCl3
99%
Me
N
Me
N
[Pd], NaOAc
+
DMA, 140 °C, 48 h
Br
Br
75%
7
Br
Me
8
hν, I2, THF, toluene
N
76%
Br
5
Scheme 1. The synthesis of the helical pentacyclic system 5.

M. Ben Braiek et al. / Tetrahedron Letters 54 (2013) 5421–5425
5423
Figure 4. DSC thermogram of 5 at a heating rate of 10 °C/min.
Figure 3. Crystal structure of the helically chiral pentacyclic system 5: ORTEP
drawing.
Table 1
Selected X-ray crystallographic inner and outer bond lengths (Å) and distances
between non-bonded atoms for compound 5
Inner C–C bond lengths
C19–C20
C20–C21
C20–C1
C1–C2
1.410
1.448
1.422
1.457
1.403
C2–C3
Outer C–C bond lengths
C5–C6
C10–C11
C13–C14
C16–C17
1.371
1.347
1.341
1.367
Distance between non-bonded atoms
Figure 5. The enlarged DSC trace from 105–120 °C indicating the glass transition.
C3–C19
C4–C18
C5–C17
3.140
5.201
7.604
1.5
Dichloromethane
Film
Table 2
Torsion angles of 5 and pentahelicene
1.0
Compound
Torsion anglea (°)
u
1
u
2
u 3
5
8.72
33.5
19.93
8.1
17.41
36.5
Pentahelicene
0.5
0.0
a
u
1 = C3–C2–C1–C21;
u
2 = C2–C1–C21–C20;
u
3 = C1–C21–C20–C19.
179 °C. In addition, helicene 5 also had a high glass transition tem-
perature (Tg) of 113 °C (Fig. 5). These results attest the thermal sta-
bility of the helicene.
250
300
350
λ (nm)
400
450
500
The optical properties of the pentahelicene derivative 5 were
Figure 6. UV/vis absorption spectra of helicene
dichoromethane (red line) and as a thin film (black line).
5
in dilute (1.5 ꢁ 10ꢂ6 M)
investigated using UV/vis absorption studies in
a
dilute
(1.5 ꢁ 10ꢂ6 M) solution in dichloromethane (Fig. 6). The UV/vis
spectra of this compound exhibited a strong absorption in the re-
gion of 250–425 nm. The absorption in the high energy region is
well structured containing five prominent bands at 289, 313,
335, 379 and 399 nm. These absorption bands are associated with
unoccupied molecular orbital (LUMO) energy levels, cyclic voltam-
metry (CV) was applied to the material as a film. Knowledge of the
HOMO and LUMO energy levels is of crucial importance for the
selection of the cathode and anode materials for OLED devices.1d
The use of CV analysis is of good reliability as the electrochemical
processes probed therein are similar to those involved in charge
injection and transport processes in OLED devices.17 Helicene 5
p–
p⁄ and n–p⁄ electronic transitions. Similar absorption behaviour
was found in a thin film.
To investigate the redox behaviour of helicene 5 and to estimate
its highest occupied molecular orbital (HOMO) and lowest

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M. Ben Braiek et al. / Tetrahedron Letters 54 (2013) 5421–5425
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0.0002
0.0001
0.0000
-0.0001
-0.0002
-0.0003
-0.0004
-0.0005
-0.0006
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Figure 7. Cyclic voltammogram for azahelicene 5 film coated onto ITO electrodes in
0.1 M (n-Bu)4NClO4/acetonitrile (scan rate = 50 mV/s).
Table 3
Physical properties of helicene 5
Vonsetꢂox (V) Vonsetꢂred (V) EHOMO (eV) ELUMO (eV) Egꢂel (eV)
Helicene 5 1.08
ꢂ1.13
ꢂ5.18
ꢂ2.97
2.21
was drop-coated onto ITO glass substrate and scanned both posi-
tively and negatively in (n-Bu)4NClO4/acetonitrile. As shown in
the cyclic voltammogram in Figure 7, the onset of oxidation
(Vonset-ox) was found to occur at 1.08 V and the onset of reduction
(Vonset-red) was at ꢂ1.13 V (vs SCE). According to an empirical
method,18 and by assuming that the energy level of the ferro-
cene/ferrocenium couple was 4.8 V below the vacuum level, the
HOMO and LUMO energy levels were calculated as follows:
8. Meisenheimer, J.; Witte, K. Chem. Ber. 1903, 36, 4153–4164.
9. Fuchs, W.; Niszel, F. Chem. Ber. 1927, 60, 209–217.
10. Pischel, I.; Grimme, S.; Kotila, S.; Nieger, M.; Vögtle, F. Tetrahedron: Asymmetry
1996, 7, 109–116.
11. Shi, L.; Liu, Z.; Dong, G.; Duan, L.; Qui, Y.; Jia, J.; Guo, W.; Zhao, D.; Cui, D.; Tao,
X. Chem. Eur. J. 2012, 18, 8092–8099.
12. (a) Ben Braiek, M.; Aloui, F.; Ben Hassine, B. Tetrahedron Lett. 2013, 54, 424–
426; (b) Aloui, F.; Moussa, S.; Ben Hassine, B. Tetrahedron Lett. 2011, 52,
572–575; (c) Aloui, F.; Ben Hassine, B. Tetrahedron Lett. 2009, 50, 4321–
4323; (d) Aloui, F.; El Abed, R.; Ben Hassine, B. Tetrahedron Lett. 2008, 49,
1455–1457.
EHOMO ðionization potentialÞ ¼ ꢂðVonset-ox ꢂ VFOC þ 4:8Þ
ELUMO ðelectron affinityÞ ¼ ꢂðVonset-red ꢂ VFOC þ 4:8Þ
Eg-el ¼ ðELUMO ꢂ EHOMOÞ eV
13. Experimental procedure for diarylethene 8:
A solution of 3-bromo-9-
methylcarbazole (1.5 g, 5.7 mmol) and dry NaOAc (520 mg, 6.3 mmol) in
N,N-dimethylacetamide (10 mL) was placed in a two-necked flask fitted with a
septum, and degassed and purged with argon several times. To this was added
4-bromostyrene (8 mmol, 1 mL) and the mixture was heated to 100 °C. When
this temperature was reached, a solution of Herrmann’s catalyst (54 mg, 1%) in
N,N-dimethylacetamide (2 mL) was added and the mixture was heated at
140 °C for about 48 h. The reaction was quenched by addition of 5% HCl
solution, then stirred for 30 min at room temperature and extracted with
CH2Cl2 (3 ꢁ 40 mL). The combined organic layers were dried over MgSO4 and
evaporated to dryness. After column chromatography with cyclohexane/EtOAc
(75:25), product 8 was obtained in 75% yield (1.56 g) as a light yellow solid.
mp = 191–193 °C; 1H NMR (CDCl3, 300 MHz): d (ppm) = 3.84 (s, 3H, CH3), 7.06
(d, J = 16.2 Hz, 1H, Hvinyl), 7.25–7.32 (m, 2H), 7.37–7.42 (m, 4H), 7.48–7.51 (m,
3H), 7.66 (dd, J1 = 8.4, 1.5 Hz, 1H), 8.14 (d, J = 7.7 Hz, 1H), 8.21 (s, 1H); 13C NMR
(CDCl3, 75 MHz): d (ppm) = 141.40, 140.10, 137.01, 131.77, 130.47, 128.26,
127.72, 126.02, 124.74, 124.59, 123.23, 122.82, 120.59, 120.42, 119.24, 118.79,
108.71, 29.77 (CH3); HRMS (MALDI-TOF) calcd for C21H16BrN [M]+: 361.04661,
found: 361.04579.
where VFOC is the ferrocene half-wave potential (0.7), Vonsetꢂox is the
oxidation onset and Vonsetꢂred is the material reduction onset, all
measured versus (Ag/AgCl). The calculated ELUMO, EHOMO and Eg-el
values are summarized in Table 3.
In summary, we have described a straightforward method for
the preparation of a new nitrogen-containing helicene, starting
from readily available N-methylcarbazole. We accomplished the
synthesis of the helically chiral pentacyclic system 5 in three steps,
with an overall yield of 56%, under mild conditions followed by
simple purification. This helicene may serve as a building block
for supramolecular architectures. Work in this field is currently
in progress.
14. Liu, L.; Yang, B.; Katz, T. J.; Poindexter, M. K. J. Org. Chem. 1991, 56, 3769–
3775.
Acknowledgement
15. Experimental procedure for the photocyclization reaction: To a solution of alkene
8 (0.20 g, 0.55 mmol) in toluene (1.2 L) was added a stoichiometric amount of
I2 and an excess of THF. The solution was irradiated using a falling-film
The authors are grateful to the DGRS (Direction Générale de la
Recherche Scientifique) of the Tunisian Ministry of Higher Educa-
tion for the financial support of this research.
photoreactor and
a high-pressure Hg-vapor lamp (500 W, Hanovia). The
reaction was monitored by TLC. Following completion, solvent was removed
under reduced pressure and the crude residue was purified by column
chromatography on silica gel with cyclohexane/EtOAc (98:2) to give helicene 5
as a pale yellow solid (151 mg, 76%); 1H NMR (CDCl3, 300 MHz): d (ppm) = 4.03
(s, 3H, CH3), 7.33 (ddd, J1 = 8.1, 7.1, 2.7 Hz, 1H), 7.54–7.60 (m, 2H), 7.68 (d,
J = 8.7 Hz, 1H), 7.73 (dd, J1 = 8.4, 2.1 Hz, 1H), 7.75 (d, J = 8.1 Hz, 1H), 7.84 (d,
J = 8.1 Hz, 1H), 7.87 (d, J = 8.4 Hz, 1H), 7.92 (d, J = 8.7 Hz, 1H), 8.79 (d,
References and notes
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d
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129.89, 130.56, 141.19; ESI-MS: m/z = 359 [M]+; HRMS (MALDI-TOF) calcd for
C
21H14BrN [M]+: 359.03096, found: 359.03012.

M. Ben Braiek et al. / Tetrahedron Letters 54 (2013) 5421–5425
5425
Crystallographic data for compound 5 have been deposited with the Cambridge
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