Synthesis, structures, and optical properties of aza[4]helicenes

Article abstract of DOI:10.1002/ejoc.201300002

A series of aza[4]helicenes have been conveniently synthesized in yields of 71-92 % by a one-pot procedure involving the Pictet-Spengler reaction. The structures of the aza[4]helicenes were characterized by ¹H and ¹³C NMR spectroscop

Full text of DOI:10.1002/ejoc.201300002

FULL PAPER
DOI: 10.1002/ejoc.201300002
Synthesis, Structures, and Optical Properties of Aza[4]helicenes
Yuan-Hui Zheng,[a,b] Hai-Yan Lu,*[b] Meng Li,[a,b] and Chuan-Feng Chen*[a]
Keywords: Helical structures / Nitrogen heterocycles / Self-assembly / Fluorescence
A series of aza[4]helicenes have been conveniently synthe-
sized in yields of 71–92% by a one-pot procedure involving
the Pictet–Spengler reaction. The structures of the aza[4]-
helicenes were characterized by 1H and 13C NMR spec-
troscopy, mass spectrometry, and X-ray crystallography.
Moreover, it was found that the aza[4]helicenes self-as-
semble into 2D layers or herringbone-like structures in the
solid state. In addition, the optical properties were investi-
gated by UV and fluorescence spectroscopic methods.
Introduction
helicenes have been reported, which has also limited their
practical applications.[18]
Helicenes[1] are a class of molecules with many intriguing
features such as extended aromaticity, chirality, and the
ability to self-assemble into columnar solid-state architec-
tures as well as to behave as organic conductors. Since New-
man and Lednicer reported the synthesis and resolution of
hexahelicene in 1956,[2] a great deal of work has been car-
ried out on helicenes and heterohelicenes with the backbone
composed of carbon atoms only and selected heteroatoms,
respectively.[1,3] So far, various synthetic strategies for the
synthesis of carbo- and heterohelicenes have been devel-
oped, including photocyclization reactions,[4] Diels–Alder
cycloadditions,[5] cyclotrimerizations of acetylenes,[6] car-
benoid couplings,[7] electrophilic aromatic cyclizations,[8]
radical cyclizations,[9] olefin metathesis,[10] cycloisomeriza-
tions,[11] and Friedel–Crafts-type cyclizations.[12] However,
limited substrate scope, multistep reactions, air-sensitive or-
ganometallic reagents, or metal catalysis have usually re-
stricted the usefulness and generality of these strategies for
affording helicenes or heterohelicenes. Thus, developing
new methodologies for their synthesis remains very impor-
tant in this area of research.
The Pictet–Spengler reaction[19] remains one of the most
widely used methods for the synthesis of isoquinolines and
β-carbolines by C–C and C–N bond formation.[20] Recently,
Kundu and co-workers employed the reaction to afford
various nitrogen-substituted heteroaromatic molecules.[21]
Moreover, Youn and Bihn successfully synthesized a series
of 6-substituted phenanthridines by the Pictet–Spengler re-
action.[22] Herein we report the synthesis of aza[4]helicenes
by a one-pot Pictet–Spengler reaction. In this way, a series
of aza[4]helicenes with different functional groups have
been conveniently obtained in good yields, and their struc-
tures were characterized by 1H and 13C NMR spectroscopy,
MS, and X-ray diffraction. In addition, the optical proper-
ties of the azahelicenes were investigated by UV and fluo-
rescence spectroscopic methods.
Results and Discussion
Synthesis of Aza[4]helicenes
We first prepared the arylamine 4, the key precursor for
the Pictet–Spengler reaction.[23] As shown in Scheme 1, the
Suzuki coupling reaction between naphthylboronic acid (1)
Recently, owing to their interesting properties, azahelic-
enes have attracted considerable attention in the develop-
ment of asymmetric catalysis,[13] chemosensors,[14] dye ma-
terials,[15] and supramolecular assemblies.[16] In addition,
examples of the synthesis and applications of [4]helicenes
have been described.[17] However, few studies on aza[4]-
[a] Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Molecular Recognition and Function, Institute
of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
Fax: +86-10-62554449
E-mail: cchen@iccas.ac.cn
Homepage: http://cchen.iccas.ac.cn
[b] University of Chinese Academy of Sciences,
Beijing 100049, China
E-mail: haiyanlu@ucas.ac.cn
Homepage: http://people.gucas.ac.cn/~haiyanlu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300002.
Scheme 1. Synthesis of arylamine 4.
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and 1-bromo-2-nitrobenzene (2) gave compound 3 in 95% 2% trifluoroacetic acid (TFA) and the mixture was heated
yield, which was then reduced by H2 in the presence of Pd/ at reflux in a round-bottomed flask for even 72 h, aza[4]-
C to produce the arylamine 4 in 98% yield.
helicene 5c was only obtained in Ͻ5% yield. However,
With compound 4 in hand, we first tested the Pictet– when the mixture was heated with stirring at 140 °C in a
Spengler reaction between 4 and the commercially available sealed tube for 48 h, the target product was isolated in 56%
4-methoxybenzaldehyde (Scheme 2). It was found that when yield. We next investigated the effect of solvent on the con-
both of the reactants were dissolved in toluene containing densation reaction (Table 1) and found that under the same
Table 1. Effect of solvent on the synthesis of aza[4]helicene 5c.
Entry
Solvent
Yield [%]
1
2
3
4
5
6
toluene
chlorobenzene
xylene
N,N-dimethylformamide
acetonitrile
TFA
56
50
40
35
10
92
Scheme 2. Synthesis of aza[4]helicenes 5.
Table 2. Synthesis of aza[4]helicenes 5 by the Pictet–Spengler reaction.[a]
[a] Performed under the optimized experimental conditions: 4 (1 equiv.), aldehyde (2 equiv.), TFA (2 mL), 140 °C, 48 h in an autoclave.
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Synthesis, Structures, and Optical Properties of Aza[4]helicenes
reaction conditions with chlorobenzene, xylene, N,N-di-
methylformamide, or acetonitrile as solvent, product 5c was
isolated in lower yields than in toluene. However, when
TFA was used as solvent, aza[4]helicene 5c was obtained in
92% yield (Table 1, entry 6).
The Pictet–Spengler reactions between amine 4 and other
commercially available aryl aldehydes were then carried out
under the optimized reaction conditions and the results are
summarized in Table 2. It was found that both electron-rich
and -deficient aryl aldehydes gave the corresponding aza[4]-
helicenes in good yields. In addition, the positions of the
substituents in the aryl aldehyde showed no obvious effects
on the reaction (Table 2, entries 3–5). The Pictet–Spengler
reactions of thiophene-2- and pyridine-2-carbaldehyde with
4 also worked well in TFA in an autoclave over 48 h, giving
the aza[4]helicenes 5k and 5l (Table 2, entries 11 and 12) in
yields of 84 and 80%, respectively. The aza[4]helicenes 5m
and 5n (Table 2, entries 13 and 14) were also obtained in
good yields by the reaction of 4 with paraformaldehyde or
propanal, respectively, under the same conditions. In ad-
dition, we also obtained aza[4]helicene 5o in 98% yield by
the demethylation of compound 5e with boron tribromide
in dichloromethane (Scheme 3). The structures of the new
1
aza[4]helicenes were all characterized by H and 13C NMR
spectroscopy, MS, and elementary analyses.
Figure 1. Top views of the crystal structures of (a) 5a and (b) 5b.
(c) View of the noncovalent interactions (dashed lines) between ad-
jacent molecules of 5a. (d) Crystal packing of 5a viewed along the
c axis. (e) Crystal packing of 5b viewed along the c axis. Hydrogen
atoms not involved in interactions have been omitted for clarity.
Scheme 3. Synthesis of aza[4]helicene 5o.
virtue of CH···N hydrogen bonding and CH···π interactions
between adjacent molecules, molecule 5b also self-assembles
into a 2D layer structure (Figure 1, e).
For the azahelicenes 5c–e with a methoxyphenyl group
(Figure 2, a–c), the corresponding distortion angles are
30.3, 38.5, and 35.8°, respectively, which are all smaller than
Crystal Structures of Aza[4]helicenes
To further investigate the structures of the aza[4]helicenes
and their self-assembling behavior in the solid state, we ob-
tained single crystals of 5a–5f, 5h, and 5o suitable for X-ray
diffraction analysis by vapor diffusion of petroleum ether
into their solutions in ethyl acetate. As shown in Figure 1
(a), the distortion[24] of the molecular structure of 5a, which
is defined by the sum of the two torsion angles of C5–C6–
C7–C8 and C6–C7–C8–C9, is 40.0°, which is consistent
with a helical structure. Moreover, we noted a dihedral an-
gle of 46.1° between the aromatic rings A and B. We also
found that by virtue of CH···N hydrogen bonding between
the nitrogen atom of one azahelicene molecule and an aro-
matic proton of an adjacent molecule, or vice versa, with a
distance of 2.59 Å (a), and a pair of CH···π interactions
between an aromatic proton and an adjacent aromatic ring
of the azahelicene, with a distance of 2.85 (b) or 2.86 Å
(c), respectively, molecule 5a self-assembles into a 2D layer
structure (Figure 1, c,d).
Similar to the case of 5a, molecule 5b also shows a helical
structure with a distortion of 39.2° and a dihedral angle
between the aromatic rings A and B of 48.8°. Moreover, by drogen atoms have been omitted for clarity.
Figure 2. Crystal structures of (a) 5c, (b) 5d, (c) 5e, and (d) 5o. Hy-
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Y.-H. Zheng, H.-Y. Lu, M. Li, C.-F. Chen
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that of 5a, but they are also consistent with helical struc-
tures. The dihedral angles between the substituted benzene
ring and the pyridine ring of the azahelicene skeleton are
63.0 (5c), 51.5 (5d), and 57.5° (5e), respectively, which are
larger than that of 5a. Moreover, we also obtained the crys-
tal structure of 5o, the demethylated product of 5e. As
shown in Figure 2 (d), molecule 5o also shows a helical
structure, but the distortion of 34.6° is smaller than that of
its precursor 5e. In addition, the dihedral angle between the
two adjacent aromatic rings (67.4°) is greater than that of
5e and no intramolecular OH···N hydrogen bonding was
found.
The self-assembling behavior of 5c in the solid state was
also investigated. As shown in Figure 3, it was found that
by virtue of a pair of CH···O hydrogen bonds (a, 2.59 Å)
and a pair of CH···N hydrogen bonds (b, 2.66 Å) between
the aromatic protons of molecule A and the oxygen and
nitrogen atoms of molecule B, and the aromatic protons of
molecule B and the oxygen and nitrogen atoms of the adja-
cent molecule C, molecule 5c self-assembles into a wave-like
layer structure (Figure 3b). The layers can further pack into
a 3D microporous architecture through a pair of CH···π
interactions (c) between two adjacent helicene molecules
with a distance of 2.87 Å. Similarly, by virtue of multiple
CH···O and CH···N hydrogen bonds and CH···π and π···π
interactions between adjacent molecules, 5d and 5e also
self-assemble into 2D layer or wave-like layer structures
Figure 3. (a) View of the noncovalent interactions (dashed lines)
between adjacent molecules of 5c. (b) Crystal packing of 5c. Hy-
drogen atoms not involved in the interaction have been omitted for
clarity.
Figure 4. (a) Packing of 5d. (b) Packing of 5e. (c) Packing of 5o.
Hydrogen atoms have been omitted for clarity.
Figure 5. Crystal structures of (a) 5f and (b) 5h. Packing of (c) 5f
and (d) 5h. Hydrogen atoms have been omitted for clarity.
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Synthesis, Structures, and Optical Properties of Aza[4]helicenes
(Figure 4, a,b). The azahelicene 5o was found to adopt a might be due to the lone-pair electrons on the nitrogen
2D herringbone-like layer structure (Figure 4, c) and these causing fluorescence quenching.
layers can further pack into a 3D microporous architecture
in the solid state.
Conclusions
In the case of azahelicenes 5f and 5h, the distortion of
the helical structure of 5f containing a p-nitrophenyl moiety
(38.6°) is clearly larger than that of 5h (27.7°) with an o-
nitrophenyl group. However, the dihedral angle of 5f (47.2°)
between the substituted benzene ring and the pyridine ring
of the azahelicene skeleton is significantly smaller than that
of 5h (69.4°) in which the o-nitro group exerts steric strain
(Figure 5, a,b). In addition, it was found that azahelicene
5f can also pack into a wave-like layer structure (Figure 5,
c) through multiple CH···O hydrogen bonds and π···π inter-
actions between adjacent molecules. However, for azahel-
icene 5h, a microporous network structure (Figure 5, d) is
formed instead of a layer-by-layer structure through mul-
tiple noncovalent interactions, including CH···N and
CH···O hydrogen bonds and CH···π and π···π interactions
between adjacent molecules. This is in contrast to 5h, which
might be due to the different dihedral angles of azahelicenes
5f and 5h.
We have shown a simple and convenient one-pot pro-
cedure for the synthesis of a series of aza[4]helicenes by
using the Pictet–Spengler reaction. The structures of the
aza[4]helicenes were characterized by 1H and 13C NMR
spectroscopy, MS, and X-ray diffraction analyses. It was
found that the aza[4]helicenes self-assemble into 2D layer
or herringbone-like structures in the solid state. The optical
properties of the aza[4]helicenes were also investigated. We
believe that this method could also be used for the synthesis
of other azahelicenes, including those with more rings, and
that these new azahelicenes could find widespread applica-
tions in materials science and supramolecular chemistry.
Experimental Section
General: Melting points were taken with an electrothermal melting
point apparatus and are uncorrected. 1H and 13C NMR spectra
were recorded with a Bruker AV 300 NMR spectrometer. EI mass
spectra were obtained with an FTICR mass spectrometer. Elemen-
tal analyses were carried out with an Elemental FLASH EA1112
instrument. Materials obtained commercially were used without
further purification.
Optical Properties of the Aza[4]helicenes
The optical properties of the aza[4]helicenes were also
studied by UV and fluorescent spectroscopy, and the results
are summarized in Table 3. The UV/Vis spectra of aza[4]-
helicenes 5a–o in CH2Cl2 exhibit absorption bands with
maxima at 274–307 nm, with the azahelicenes with electron-
withdrawing groups appearing at longer wavelengths than
those with electron-donating groups. However, the posi-
tions of the substituents had no obvious effect on the maxi-
mum of the absorption peak (entries 3–5). In the fluores-
cent spectra, the emission maxima for the helicene skeleton
were observed at about 400 nm, and the substituent groups
and their positions had no obvious effect on the emission,
probably due to the indirect connection of the substituent
General Procedure for the Synthesis of Aza[4]helicenes: An aldehyde
(2 equiv., 0.20 mmol) was added to a solution of arylaniline 4
(1 equiv., 0.10 mmol) in TFA (2 mL). The resulting mixture, in a
5 mL tube, was placed in an autoclave and then heated with stirring
at 140 °C for 48 h. The reaction mixture was diluted with ethyl
acetate, neutralized with sat. NaHCO3, extracted with ethyl acetate
(3ϫ 10 mL), and washed with brine. The organic layer was dried
with MgSO4 and concentrated in vacuo to give a residue, which
was purified by column chromatography on silica gel with ethyl
acetate and petroleum ether (1:1–1:10) as eluent to provide the cor-
responding product.
The spectral data for compounds 5a, 5b, and 5m are consistent
groups to the helicene skeleton. In addition, the fluores- with the data reported in the literature.[22,25]
cence intensities of the aza[4]helicenes were weak, which
Compound 5a: Rf = 0.50 (ethyl acetate/petroleum ether, 1:10), yield
90%, yellow solid; m.p. 136–138 °C. H NMR (300 MHz, CDCl3):
1
δ = 9.28–9.01 (m, 2 H), 8.37 (d, J = 7.1 Hz,1 H), 8.13–7.93 (m, 2
H), 7.93–7.68 (m, 7 H), 7.55 (s, 3 H) ppm. MS (EI): m/z = 305
[M]+.
Table 3. Optical properties of aza[4]helicenes 5 in CH2Cl2.[a]
Entry
Aza[4]helicene
λabs [nm]
λem [nm]
Compound 5b: Rf = 0.65 (ethyl acetate/petroleum ether, 1:5), yield
89%, yellow solid; m.p. 128–129 °C. H NMR (300 MHz, CDCl3):
δ = 8.98 (d, J = 8.0 Hz, 1 H), 8.85 (d, J = 8.3 Hz, 1 H), 8.23 (d, J
= 8.0 Hz, 1 H), 7.84 (d, J = 8.7 Hz, 2 H), 7.72–7.58 (m, 2 H), 7.56–
7.44 (m, 5 H), 7.22 (d, J = 7.5 Hz, 2 H), 2.34 (s, 3 H) ppm. MS
(EI): m/z = 319 [M]+.
1
2
3
4
5
6
7
8
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
284
281
283
286
278
306
307
307
281
287
300
281
274
275
413
414
403
411
406
398
397
400
414
410
410
422
402
400
1
Compound 5c: Rf = 0.40 (ethyl acetate/petroleum ether, 1:5), yield
1
92%, yellow solid; m.p. 189–191 °C. H NMR (300 MHz, CDCl3):
9
δ = 9.20–9.15 (m, 1 H), 9.04 (d, J = 8.4 Hz, 1 H), 8.35 (d, J =
7.2 Hz, 1 H), 8.06–8.02 (m, 2 H), 7.88 (d, J = 8.9 Hz, 1 H), 7.83–
7.67 (m, 6 H), 7.10 (d, J = 8.6 Hz, 2 H), 3.92 (s, 3 H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 160.1, 160.0, 145.9, 134.8, 132.8,
132.6, 131.6, 130.9, 130.2, 129.0, 128.9, 128.5, 128.4, 128.9, 127.7,
127.0, 126.8, 126.4, 125.1, 124.0, 123.9, 113.9, 55.5 ppm. MS (EI):
10
11
12
13
14
[a] [5] = 1.0ϫ10–5 m.
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m/z = 335 [M]+. C24H17NO (335.40): calcd. C 85.94, H 5.11, N
4.18; found C 85.90, H 5.13, N 4.19.
C23H14BrN (384.27): calcd. C 71.89, H 3.67, N 3.65; found C 71.92,
H 3.66, N 3.64.
Compound 5d: Rf = 0.32 (ethyl acetate/petroleum ether, 1:10), yield
90%, yellow solid; m.p. 135–137 °C. H NMR (300 MHz, CDCl3):
Compound 5j: Rf = 0.70 (ethyl acetate/petroleum ether, 1:5), yield
73%, white solid; m.p. 177–179 °C. H NMR (300 MHz, [D6]ace-
1
1
δ = 9.17 (d, J = 8.0 Hz, 1 H), 9.05 (d, J = 8.2 Hz, 1 H), 8.37 (d, J tone): δ = 9.28–9.21 (m, 1 H), 9.20–9.11 (m, 1 H), 8.29 (dd, J =
= 8.0 Hz, 1 H), 8.16–7.94 (m, 2 H), 7.92–7.66 (m, 5 H), 7.47 (t, J 8.0, 1.5 Hz, 1 H), 8.20 (dd, J = 6.9, 2.5 Hz, 1 H), 8.08 (d, J =
= 7.9 Hz, 1 H), 7.26 (d, J = 13.4 Hz, 2 H), 7.08 (d, J = 7.9 Hz, 1 8.9 Hz, 1 H), 7.99 (dd, J = 13.5, 8.4 Hz, 5 H), 7.93–7.83 (m, 4
H), 3.88 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 160.2,
159.7, 145.7, 141.4, 134.75, 132.7, 130.3, 129.5, 129.0, 128.6, 128.6,
128.5, 128.0, 127.8, 127.0, 126.8, 126.6, 125.0, 124.1, 123.8, 122.6,
115.3, 114.7, 55.4 ppm. MS (EI): m/z = 335 [M]+. C24H17NO
(335.40): calcd. C 85.94, H 5.11, N 4.18; found C 85.96, H 5.10, N
4.19.
H) ppm. 13C NMR (75 MHz, [D6]acetone): δ = 159.4, 155.1, 146.6,
145.1, 135.7, 133.2, 132.0, 131.8, 131.2, 129.7, 129.7, 129.6, 129.4,
129.3, 129.2, 128.2, 128.1, 127.9, 126.1, 126.1, 125.0, 124.8,
124.2 ppm. MS (EI): m/z = 373 [M]+. C24H14F3N (373.38): calcd.
C 77.20, H 3.78, N 3.75; found C 77.23, H 3.76, N 3.77.
Compound 5k: Rf = 0.66 (ethyl acetate/petroleum ether, 1:10), yield
84%, pale-yellow solid; m.p. 110–113 °C. 1H NMR (300 MHz,
[D6]acetone): δ = 9.17 (s, 1 H), 9.05 (s, 1 H), 8.50 (d, J = 6.7 Hz, 1
Compound 5e: Rf = 0.33 (ethyl acetate/petroleum ether, 1:5), yield
1
85%, yellow solid; m.p. 136–139 °C. H NMR (300 MHz, CDCl3):
δ = 9.21 (d, J = 7.9 Hz, 1 H), 9.09 (d, J = 8.0 Hz, 1 H), 8.38 (d, J H), 8.30–8.05 (m, 3 H), 7.90–7.67 (m, 6 H), 7.31 (s, 1 H) ppm. 13C
= 7.9 Hz, 1 H), 8.02 (d, J = 6.8 Hz, 1 H), 7.88–7.62 (m, 6 H), 7.51
(t, J = 7.5 Hz, 2 H), 7.17 (t, J = 7.4 Hz, 1 H), 7.08 (d, J = 8.1 Hz,
1 H), 3.67 (s, 3 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 158.9,
157.3, 146.0, 134.8, 131.7, 131.1, 130.4, 130.2, 129.4, 129.1, 128.6,
128.6, 128.2, 127.8, 127.7, 127.1, 126.6, 126.5, 125.2, 125.0, 124.3,
121.1, 111.1, 55.6 ppm. MS (EI): m/z = 335 [M]+. C24H17NO
(335.40): calcd. C 85.94, H 5.11, N 4.18; found C 85.93, H 5.12, N
4.17.
NMR (75 MHz, [D6]acetone): δ = 153.6, 146.6, 144.1, 135.7, 133.6,
130.8, 130.7, 129.7, 129.6, 129.4, 129.3, 129.2, 129.1, 128.4, 128.1,
127.9, 127.8, 124.69, 124.5, 123.9 ppm. MS (EI): m/z = 311 [M]+.
C21H13NS (311.40): calcd. C 81.00, H 4.21, N 4.50; found C 80.99,
H 4.22, N 4.51.
Compound 5l: Rf = 0.50 (ethyl acetate/petroleum ether, 1:1), yield
80%, pale-yellow solid; m.p. 146–148 °C. 1H NMR (300 MHz,
CDCl3): δ = 9.17 (d, J = 5.2 Hz, 1 H), 9.08 (d, J = 8.2 Hz, 1 H),
8.83 (s, 1 H), 8.38 (d, J = 8.1 Hz, 1 H), 8.27 (d, J = 8.9 Hz, 1 H),
8.07–7.88 (m, 4 H), 7.80 (dd, J = 18.1, 9.9 Hz, 4 H), 7.53–7.43 (m,
1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 158.6, 157.7, 148.9,
145.5, 137.1, 134.8, 133.0, 130.4, 129.01, 128.6, 128.5, 128.5, 128.1,
128.0, 127.1, 127.0, 126.7, 125.5, 124.7, 124.5, 123.6, 123.3 ppm.
MS (EI): m/z = 306 [M]+. C22H14N2 (306.37): calcd. C 86.25, H
4.61, N 9.14; found C 86.29, H 4.60, N 9.13.
Compound 5f: Rf = 0.26 (ethyl acetate/petroleum ether, 1:5), yield
83%, pale-yellow solid; m.p. 185–187 °C. 1H NMR (300 MHz,
CDCl3): δ = 9.20 (d, J = 8.2 Hz, 1 H), 9.09 (d, J = 7.9 Hz, 1 H),
8.28 (ddd, J = 7.7, 4.8, 1.1 Hz, 2 H), 8.09–7.97 (m, 1 H), 7.87–7.62
(m, 8 H), 7.52 (d, J = 8.8 Hz, 1 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 157.5, 148.7, 145.6, 135.6, 134.8, 133.6, 132.3, 132.2,
130.2, 129.7, 129.1, 128.8, 128.7, 128.6, 128.6, 128.1, 127.2, 127.1,
127.1, 124.9, 124.3, 124.0, 123.2 ppm. MS (EI): m/z = 350 [M]+.
C23H14N2O2 (350.38): calcd. C 78.84, H 4.03, N 8.00; found C
78.83, H 4.02, N 8.02.
Compound 5m: Rf = 0.20 (ethyl acetate/petroleum ether, 1:10), yield
1
85%, yellow solid; m.p. 189–191 °C. H NMR (300 MHz, CDCl3):
δ = 9.36 (s, 1 H), 9.20 (d, J = 9.1 Hz, 1 H), 9.11 (d, J = 8.2 Hz, 1
H), 8.39–8.23 (m, 1 H), 8.08 (dd, J = 6.3, 3.0 Hz, 1 H), 8.02 (d, J
= 8.5 Hz, 1 H), 7.95 (d, J = 8.5 Hz, 1 H), 7.87–7.70 (m, 4 H) ppm.
Compound 5g: Rf = 0.20 (ethyl acetate/petroleum ether, 1:5), yield
78%, pale-yellow solid; m.p. 173–175 °C. 1H NMR (300 MHz,
CDCl3): δ = 9.22–9.17 (m, 1 H), 9.09 (d, J = 8.2 Hz, 1 H), 8.67 (d, MS (EI): m/z = 229 [M]+.
J = 1.7 Hz, 1 H), 8.48–8.30 (m, 2 H), 8.15–8.01 (m, 2 H), 7.95–
Compound 5n: Rf = 0.70 (ethyl acetate/petroleum ether, 1:10), yield
7.66 (m, 7 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 157.5, 148.4,
145.6, 141.8, 136.2, 134.8, 133.1, 130.3, 129.5, 129.0, 128.8, 128.7,
128.5, 128.3, 127.2, 127.2, 127.1, 125.3, 124.3, 123.8, 123.2 ppm.
MS (EI): m/z = 350 [M]+. C23H14N2O2 (350.38): calcd. C 78.84, H
4.03, N 8.00; found C 78.80, H 4.04, N 8.01.
82%, pale-yellow solid; m.p. 78–79 °C. 1H NMR (300 MHz,
CDCl3): δ = 9.19–9.07 (m, 1 H), 8.96 (d, J = 8.3 Hz, 1 H), 8.24 (d,
J = 8.2 Hz, 1 H), 8.15 (d, J = 8.9 Hz, 1 H), 8.04 (dd, J = 6.5,
2.9 Hz, 1 H), 7.97 (d, J = 8.8 Hz, 1 H), 7.79–7.68 (m, 3 H), 7.68–
7.61 (m, 1 H), 3.48 (q, J = 7.5 Hz, 2 H), 1.54 (t, J = 7.5 Hz, 3
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 162.3, 145.7, 134.6,
132.0, 129.5, 129.3, 128.6, 128.5, 128.2, 128.2, 127.7, 127.1, 126.7,
125.9, 123.9, 123.7, 122.6, 29.8, 13.9 ppm. MS (EI): m/z = 257
[M]+. C19H15N (257.33): calcd. C 88.68, H 5.88, N 5.44; found C
88.65, H 5.87, N 5.45.
Compound 5h: Rf = 0.24 (ethyl acetate/petroleum ether, 1:5), yield
74%, pale-yellow solid; m.p. 163–165 °C. 1H NMR (300 MHz,
CDCl3): δ = 9.21–9.13 (m, 1 H), 9.05 (d, J = 8.3 Hz, 1 H), 8.34
(dd, J = 8.1, 1.2 Hz, 1 H), 8.10–8.02 (m, 1 H), 7.89 (q, J = 8.9 Hz,
2 H), 7.84–7.69 (m, 6 H), 7.66–7.60 (m, 2 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 159.1, 145.7, 139.0, 134.7, 132.8, 131.8,
131.7, 130.3, 129.0, 128.6, 128.6, 128.1, 128.0, 127.1, 127.0, 126.8,
124.5, 124.1, 123.5, 123.2 ppm. MS (EI): m/z = 350 [M]+.
C23H14N2O2 (350.38): calcd. C 78.84, H 4.03, N 8.00; found C
78.91, H 4.04, N 7.92.
Compound 5o: Boron bromide in dichloromethane (2.0 mL, 1 m)
was added to a solution of 5e (34 mg, 0.1 mmol) in dry dichloro-
methane (10 mL) in an ice bath. After stirring the reaction mixture
at room temp. for 1 h, it was hydrolyzed with hydrochloric acid
(0.1 m, 3 mL). The aqueous layer was extracted twice with dichloro-
methane (2ϫ 10 mL) and the organic layers were combined,
washed with water, dried with anhydrous Na2SO4, and evaporated
under reduced pressure. The crude product was purified by column
Compound 5i: Rf = 0.40 (ethyl acetate/petroleum ether, 1:5), yield
1
71%, white solid; m.p. 121–123 °C. H NMR (300 MHz, CDCl3):
δ = 9.20–9.14 (m, 1 H), 9.06 (d, J = 8.3 Hz, 1 H), 8.35 (d, J =
8.1 Hz, 1 H), 8.08–8.02 (m, 1 H), 7.95–7.86 (m, 2 H), 7.85–7.69 (m, chromatography over silica gel (eluent: 1:5 ethyl acetate/petroleum
6 H), 7.63 (d, J = 8.4 Hz, 2 H) ppm. 13C NMR (75 MHz, CDCl3): ether) to afford 33 mg of 5o. Rf = 0.70 (ethyl acetate/petroleum
δ = 167.7, 159.1, 145.7, 139.0, 134.7, 132.8, 132.3, 131.8, 131.7,
ether, 1:5), yield 98%, yellow solid; m.p. 201–203 °C. 1H NMR
130.9, 130.3, 129.0, 128.9, 128.6, 128.1, 128.0, 127.1, 126.9, 126.7,
(300 MHz, CDCl3): δ = 9.13 (dd, J = 6.0, 3.5 Hz, 1 H), 9.03 (d, J
124.5, 124.1, 123.5, 123.2 ppm. MS (EI): m/z = 383 [M]+. = 8.3 Hz, 1 H), 8.32 (d, J = 8.9 Hz, 1 H), 8.23 (dd, J = 8.1, 1.1 Hz,
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Eur. J. Org. Chem. 2013, 3059–3066

Synthesis, Structures, and Optical Properties of Aza[4]helicenes
1 H), 8.07 (dd, J = 6.1, 3.3 Hz, 1 H), 7.93 (d, J = 8.9 Hz, 1 H),
7.84–7.64 (m, 5 H), 7.41 (dd, J = 11.2, 4.2 Hz, 1 H), 7.27–7.21 (m,
2 H), 7.02 (t, J = 7.5 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 158.2, 158.0, 143.5, 134.9, 134.4, 132.4, 131.1, 128.9, 128.8,
128.8, 128.4, 127.6, 127.2, 127.0, 126.9, 124.9, 123.8, 123.1, 121.6,
118.9, 118.2 ppm. MS (ESI): m/z = 321 [M]+. C23H15NO (321.38):
calcd. C 85.96, H 4.70, N 4.36; found C 88.90, H 4.72, N 4.38.
b = 13.347(3), c = 18.983(4) Å, V = 3375.6(12) Å3, Dc = 1.379 Mg/
m3, Z = 8, 17109 reflections collected, 3855 independent [R(int) =
0.0656], giving R1 = 0.1043 for observed unique reflections
[F2 Ͼ2σ(F2)] and wR2 = 0.2094 for all data.
Compound 5o: C23H15NO, M 321.36, crystal dimensions 0.45ϫ
0.21ϫ 0.05 mm3, monoclinic, space group P2(1)/c, a = 11.245(3),
b = 13.306(3), c = 11.013(2) Å, V = 1605.2(8) Å3, Dc = 1.330 Mg/
m3, Z = 4, 10713 reflections collected, 3641 independent [R(int) =
0.0507], giving R1 = 0.0818 for observed unique reflections
[F2 Ͼ2σ(F2)] and wR2 = 0.1544 for all data.
X-ray Crystallographic Study: X-ray crystallographic data were col-
lected by using a Rigaku RAXIS-RAPID Imaging Plate dif-
fractometer with graphite-monochromated Mo-Kα (λ = 0.71073 Å)
radiation. The data were corrected for Lorentzian, polarization,
and absorption effects. Structures were solved by direct methods
and refined by full-matrix least-squares on F2 with anisotropic dis-
placement parameters for all non-hydrogen atoms. Hydrogen atoms
were placed in calculated positions and refined by using a riding
model.
Supporting Information (see footnote on the first page of this arti-
1
cle): H and 13C NMR spectra of aza[4]helicenes 5a–o and crystal
data of aza[4]helicenes 5a–f, 5h, and 5o.
Acknowledgments
CCDC-915879 (for 5a), -915880 (for 5b), -915874 (for 5c), -915875
(for 5d), -915876 (for 5e), -915877 (for 5f), -915878 (for 5h), and
-915986 (for 5o) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
The authors thank the National Natural Science Foundation of
China (NSFC) (grant numbers 21072220 and 21272264) and the
National Basic Research Program (grant number 2011CB932501)
for financial support.
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Compound 5a: C23H15N, M 305.36, crystal dimensions 0.80ϫ
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b = 7.317(4), c = 20.485(11) Å, V = 1527.4(14) Å3, Dc = 1.328 Mg/
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b = 7.289(15), c = 21.401(4) Å, V = 1663.2(6) Å3, Dc = 1.276 Mg/
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Eur. J. Org. Chem. 2013, 3059–3066
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Y.-H. Zheng, H.-Y. Lu, M. Li, C.-F. Chen
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Received: January 1, 2013
Published Online: April 5, 2013
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Eur. J. Org. Chem. 2013, 3059–3066