Construction of helical structures with planar chiral [2.2]paracyclophane: fusing helical and planar chiralities

Article abstract of DOI:10.1039/d1cc03320d

Optically active conjugated molecules bearing one-handed helical structures, in which two [4]helicenes or two phenanthrenes are stacked at terminal benzene moieties, were prepared using enantiopure [2.2]paracyclophane as a chiral scaffold. The obtained molecules exhibited excellent chiroptical properties such as circularly polarized luminescence with high disymmetry factors. Planar chiral pseudo-meta-disubstituted [2.2]paracyclophane was also optically resolved on the sub-gram-scale.

Full text of DOI:10.1039/d1cc03320d

ChemComm
View Article Online
View Journal
COMMUNICATION
Construction of helical structures with planar
chiral [2.2]paracyclophane: fusing helical and
Cite this: DOI: 10.1039/d1cc03320d
planar chiralities†
Received 23rd June 2021,
Accepted 16th August 2021
Motoki Tsuchiya, Hazuki Maeda, Ryo Inoue
and Yasuhiro Morisaki
*
DOI: 10.1039/d1cc03320d
rsc.li/chemcomm
11,12
Optically active conjugated molecules bearing one-handed helical (PL) quantum efficiencies (F ) and high glum values.
Conse-
PL
structures, in which two [4]helicenes or two phenanthrenes are quently, we proposed fusing helical chirality with planar chirality
stacked at terminal benzene moieties, were prepared using enan- in a single molecule. The syntheses of helicenes in which one
tiopure [2.2]paracyclophane as a chiral scaffold. The obtained helicene is stacked with a benzene ring in a [2.2]paracyclophane-
13,14
molecules exhibited excellent chiroptical properties such as circu- based molecule have been reported.
However, to the best of
larly polarized luminescence with high disymmetry factors. Planar our knowledge, the preparation of [2.2]paracyclophane-based heli-
chiral pseudo-meta-disubstituted [2.2]paracyclophane was also cal compounds, in which two helicenes are stacked in a single
optically resolved on the sub-gram-scale.
molecule has not been reported. In this study, we synthesized a
new type of one-handed helical molecule, in which two [4]
Helical structures are familiar to us and are frequently observed helicenes are stacked at the terminal benzene units using planar
in our daily lives. Helical molecular structures are common chiral [2.2]paracyclophane. Such a double-decked one-handed-
second-order structures in organic chemistry, and helicenes helicene is a non-classified double helicene, according to the recent
1h
composed of fused aromatic rings are representative helical review article. In addition, enantiopure phenanthrene-stacked
1
compounds. Helicenes are chiral compounds devoid of stereo- molecules (chiral phenanthrenophanes) were also prepared; inci-
genic centers, with helical chirality represented by P (plus) and dentally, the synthesis of racemic phenanthrenophane has already
15
M (minus) descriptors instead of R and S. A wide variety of one- been reported by Hopf et al. Pseudo-meta-disubstituted [2.2]para-
handed helicenes have been prepared, and their optical and cyclophane was selected as the chiral building block and a sub-
1
chiroptical properties have been revealed. Katz et al. reported gram-scale method for its optical resolution was also developed.
in 2001 that heterocyclic helicenes emit circularly polarized Herein, we report the optical resolution of pseudo-meta-
2
luminescence (CPL), and Venkataraman et al. reported in 2003 disubstituted [2.2]paracyclophane, and the synthesis and chiropti-
3
CPL emissive triarylamine helicenes; since then, various cal properties of the above-mentioned helical molecules in detail.
helicene-based CPL-emitters with high dissymmetry factors
Chromatographic optical resolution of pseudo-meta-
lum values) have been produced. For example, in 2012 Tanaka disubstituted [2.2]paracyclophane has already achieved by L u¨ tzen
et al. constructed one-handed helical structures through et al. in 2014, and several cyclophanes have been sufficiently
4
(g
16
Rh-catalyzed enantioselective [2 + 2 + 2] cyclization; the corres- separated using chiral columns. We used the diastereomer
ponding helical molecules emitted CPLs with high |glum| values method to optically resolve pseudo-meta-disubstituted [2.2]para-
À2
5
of the order of 10 (up to 0.032).
cyclophane, as shown in Scheme 1. One bromo group in racemic
6,7
[
2.2]Paracyclophane
containing substituent(s) is a planar pseudo-meta-dibromo[2.2]paracyclophane (rac-1) was converted to
8
9
chiral molecule, depending on the substitution positions. We the corresponding phenol to give rac-2 in 91% yield. (1S,4R)-
1
0
developed methods for optically resolving planar chiral [2.2] Camphanoyl chloride was reacted with rac-2, and ensuing
paracyclophane and prepared various optically active p-stacked (R ,1S,4R)- and (S ,1S,4R)-3 diastereomers were separated by SiO
molecules with outstanding CPL profiles, high photoluminescence column chromatography, with (R ,1S,4R)- and (S ,1S,4R)-3
obtained in isolated yields of 24% (diastereomeric ratio (dr)
99.5%) and 26% (dr 499.5%), respectively (Fig. S1 in the ESI†).
p
p
2
p
p
Department of Applied Chemistry for Environment, School of Biological and
Environmental Sciences, Kwansei Gakuin University. 2-1 Gakuen, Sanda, Hyogo
4
Their absolute configurations were confirmed by X-ray crystal-
lography, as shown in Fig. S2 (ESI†). Recently, pseudo-meta-type
6
†
2
69-1337, Japan. E-mail: ymo@kwansei.ac.jp
Electronic supplementary information (ESI) available. CCDC 2084825–2084828,
087318 and 2087319. For ESI and crystallographic data in CIF or other electronic
[2.2]paracyclophanes have been used to synthesize p-stacked
12b,17
format see DOI: 10.1039/d1cc03320d
molecules;
therefore, this sub-gram-scale optical resolution
This journal is © The Royal Society of Chemistry 2021
Chem. Commun.

View Article Online
Communication
ChemComm
Scheme 1 (i) n-BuLi in Et
2
O at 0 1C for 1 h; (ii) B(OMe)
in Et O at r.t. for 1 h; (iv) (1S,4R)-camphanoyl
chloride in pyridine at 0 1C for 24 h; (v) KOH in EtOH and CH Cl at r.t. for
3 h; (vi) Tf O in pyridine and CH Cl at 0 1C for 2 h; (vii) 2-vinyl-
naphthalene, Pd(OAc) , and S-Phos in DMF and Et
, and S-Phos in DMF and Et
3 2
in Et O at r.t. for
1
h; (iii) NaOH and H
O
2 2
2
2
2
1
2
2
2
Fig. 1 Results of X-ray crystallography for (R
p
)-7 and (R
p
)-8. The thermal
2
3
N, reflux for 2 d;
N, reflux for 2 d;
ellipsoids were shown at the 30% probability level, and the hydrogen
atoms were omitted for clarity.
(
(
viii) styrene, Pd(OAc)
ix) I
2
3
2
2
in toluene and THF, hn (l = 365 nm), at r.t. for 7 h; (x) I in toluene
and THF, hn (l = 365 nm), at r.t. for 2 h.
the [4]helicene units is controlled by the (R)-planar chirality,
that forms the (P)-[4]helicene and endows the entire molecule
method meets the increasing demand for the enantiomers to with the (P)-helical structure. Two phenanthrene units are also
produce various optically active through-space conjugation systems. twisted in phenanthrenophane (R )-8, with helicity in the
The chiral auxiliary on each diastereomer was removed with phenanthrene moieties appearing to form the (P)-[3]helicenes-
KOH, and the phenol (S )-2 was reacted with Tf
O (Tf = trifluor- stacked structure and to produce (P)-phenanthrenophane. Aver-
p
p
2
omethanesulfonyl) to afford the 4-bromo-15-trifluoromethansul- age torsion angles of 24.31 and 15.01 were determined using
fonyl[2.2]paracyclophane; for example, (S )-4 was obtained in 92% selected carbon atoms in the [4]helicene and [3]helicene units,
p
18
yield from (S ,1S,4R)-3. Mizoroki–Heck cross-coupling of (S )-4 respectively.
p
p
with 2-vinylnaphthalene and styrene afforded the corresponding
)-5 (39%) and (S )-6 (36%), respectively. Single crystals of (S
)-5 gated; the corresponding UV-vis absorption, circular dichroism
and (S
)-6 suitable for X-ray crystallography were obtained from (CD), photoluminescence (PL), and CPL spectra in CHCl
toluene/MeOH and CHCl
confirmed their molecular structures as shown in Fig. S3 and S5 while corresponding data for diarylethene-stacked 5 and 6 are
ESI†), respectively. Finally, oxidative photocyclization (LED shown in Fig. S7 and S8 (ESI†), respectively. The UV-vis absorp-
Their optical and chiroptical properties were next investi-
(S
p
p
p
p
3
À5
/MeOH solutions, respectively, which (1.0 Â 10 M), including optical data, are shown in Fig. 2,
3
(
l = 365 nm) in the presence of I2 under air afforded target tion spectra of 7 and 8 both exhibit absorption bands in the
molecules (R )-7 and (R )-8 in yields of 18% and 13%, respectively. ultraviolet region (l = 250–350 nm) with sufficient molar
p
p
4
À1
À1
Racemic phenanthrenophane (rac-8) was previously synthesized by absorption coefficients (e B5 Â 10 M cm ). Clear Cotton
Hopf et al. through the reaction of pseudo-meta-divinyl[2.2] effects are observed in the absorption regions of their CD
paracyclophane with benzyne in a much higher yield than those spectra, with relatively large molar ellipticities ([y]) of the order
15
5
2
À1
p p
obtained in the present method. However, our route allowed us of 10 deg cm dmol . The (R )- and (S )-isomers exhibit
to investigate the chiroptical properties of the corresponding positive and negative Cotton effects, respectively, and are
enantiopure diarylethene-stacked molecules 5 and 6, as shown mirror images in the long-wavelength region. These spectra
in Fig. S7 and S8 (ESI†), respectively.
are similar to those of helicenes, and exhibit Cotton effects that
Single crystals of (R )-7 and (R )-8 (obtained from toluene/ are consistent with those of (P)- and (M)-helicenes.
1
9
p
p
MeOH and toluene solutions, respectively) were subjected to
X-ray crystallography, the results of which are shown in Fig. 1, to gain insight into the chiroptical absorption properties of
Fig. S4, and S6 (ESI†). Two [4]helicene moieties in (R
)-7 formed (R )-7 and (R )-8, using structures of ground states optimized at
right-handed spirals due to the steric hindrance of the planar the MN15/6-31G(d) level of theory. The calculated ECD spectra
We used time-dependent density functional theory (TD-DFT)
p
p
p
chiral [2.2]paracyclophane bridges. Hence, the (P)-helicity of of (R
)-7 and (R )-8 are shown in Fig. S11 and S12 (ESI†),
p
p
Chem. Commun.
This journal is © The Royal Society of Chemistry 2021

View Article Online
ChemComm
Communication
À3
À3
3
.5 Â 10 and 2.7 Â 10 , respectively. Several enantiomeri-
cally stable [n]carbohelicenes (n = 3 and 4) have been prepared
by utilizing steric hindrance;
1
b,1d,1f,20
however, there have been
no reports on their CPL properties. The absolute glum values of
helicenes-stacked molecules 7 and 8 (Fig. 2) are higher than
those of the corresponding arylene-vinylene-stacked molecules
5
and 6 (ESI,† Fig. S7 and S8, respectively). The (R
p
)- and
(S )-isomers of the helicene-stacked molecules show positive
p
and negative signs, respectively, that corresponded to the signs
of the Cotton effects.
The S1 structures were optimized using the state-specific
complete active space self-consistent field (SS-CASSCF) (16e,
1
4o) method, and their CPL properties were investigated using
configuration interaction single and double (CIS(D)) calcula-
tions. The molecular orbitals of (R )-7 and (R )-8 are shown in
p
p
Fig. S10A and S10B (ESI†), respectively; their HOMOs and
LUMOs are localized on the monomeric p-electron systems,
while HOMOÀ2/LUMO+2 and HOMOÀ1/LUMO+1 are deloca-
lized over the entire (R )-7 and (R )-8 molecules, respectively.
p
p
The major S1 electronic transitions were determined to be
LUMO+2/HOMO (27%) and LUMO/HOMOÀ2 (25%) for (R )-7
p
and LUMO+1/HOMO (33%) and LUMO/HOMOÀ1 (29%) for
p
(R )-8, which indicate that they emit between monomeric and
dimeric p-electron systems. The transition electronic dipole
moment (l), transition magnetic dipole moment (m), and the
angle between l and m (y) of the S
1 p
transitions for (R )-7 and
(
Rp)-8 are shown in Fig. 3. The glum,calc values were theoretically
2
2 21
determined using glum,calc = 4|l||m| cos y/(|l| + |m| ).
Although the glum,calc values of both (R )-7 and (R )-8 are over-
p
p
À2
À3
estimated (1.8 Â 10 for (R
p
)-7 and 7.3 Â 10 for (R
p
)-8), their
orders and positive signs are consistent with the experimental
results. The transition electronic dipole moment l lies across
the [2.2]paracyclophane skeleton in (R )-8, with y determined to
p
be 19.21, whereas it lies along the [2.2]paracyclophane skeleton
in (R )-7, with y determined to be 69.21, suggesting that the
p
angle y is correlated with the glum value.
Fig. 2 (A) UV-Vis absorption, CD, PL, and CPL spectra of (R
p
)- and (S
p
)-7
In summary, we developed a method for separating the
(1.0 Â 10 M); excited at 308 nm for PL and 300 nm for CPL. diastereomers of pseudo-meta-disubstituted [2.2]paracyclo-
À5
in CHCl
3
p p 3
(B) UV-Vis absorption, CD, PL, and CPL spectra of (R )- and (S )-8 in CHCl
(
1.0 Â 10À5 M); excited at 276 nm for PL and CPL.
phane derivatives. The optically active [2.2]paracyclophane
was converted into new types of one-handed double helicenes
based on its planar chirality, in which polycyclic aromatic
respectively, and clearly reproduce the experimental spectra hydrocarbons ([4]helicene and phenanthrene) are stacked at
well (Fig. 2). The molecular orbitals of (R )-7 and (R )-8 are
p
p
shown in Fig. S10A and S10B (ESI†), respectively. Monomeric
p-electron systems are stacked without effective orbital hybri-
dization, resulting in narrow energy gaps between HOMO and
HOMOÀ1 and LUMO and LUMO+1. The major electronic S
transitions were determined to be HOMO/LUMO+2 ((R )-7) and
HOMO/LUMO ((R )-8), both with positive rotatory strengths.
The small oscillator strengths (f = 0.0072 for (R )-7 and 0.0036
for (R )-8) are attributable to symmetry-forbidden transitions;
that is, HOMO/LUMO+2 of (R )-7 and HOMO/LUMO of (R )-8
have the same C -antisymmetry.
1
p
p
p
p
p
p
2
Compounds 7 and 8 were found to emit with low FPL values
of 0.03 and 0.05, respectively, and emit intense CPL when
Fig. 3 Transition electronic dipole moment (l) and transition magnetic
dipole moment (m) of (A) (R )-7 and (B) (S )-8 in the S states, and their CPL
p
p
1
photoexcited in their PL regions, with high |glum| values of parameters (CIS(D)/6-31G//SS-CASSCF(16e,14o)/6-31G).
This journal is © The Royal Society of Chemistry 2021
Chem. Commun.

View Article Online
Communication
ChemComm
the terminal benzene rings. (P)-Helicity was imparted onto the
8 (a) D. J. Cram and N. L. Allinger, J. Am. Chem. Soc., 1955, 77,
6
289–6294; (b) V. Rozenberg, E. Sergeeva and H. Hopf, in Modern
[4]helicene and phenanthrene units to form a (P)-[4]helicene
Cyclophane Chemistry, ed. R. Gleiter and H. Hopf, Wiley-VCH,
Weinheim, Germany, 2004, pp. 435–462; (c) G. J. Rowlands, Org.
Biomol. Chem., 2008, 6, 1527–1534; (d) S. E. Gibson and J. D. Knight,
Org. Biomol. Chem., 2003, 1, 1256–1269; (e) J. Paradies, Synthesis,
and a (P)-[3]helicene from the (R )-[2.2]paracyclophane scaffold.
p
These one-handed helical molecules emit CPL with high glum
values, with that of the [4]helicene-stacked system higher than
that of the [3]helicene-stacked system, which is consistent with
theoretical calculations. The simulation shows that their elec-
tronic dipole moments in the excited states become shorter
2
011, 3749–3766; ( f ) M.-L. Delcourt, S. Felder, S. Turcaud,
C. H. Pollok, C. Merten, L. Micouin and E. Benedetti, J. Org. Chem.,
2019, 84, 5369–5382.
9
(a) N. V. Vorontsova, V. I. Rozenberg, E. V. Sergeeva, E. V. Vorontsov,
Z. A. Starikova, K. A. Lyssenko and H. Hopf, Chem. – Eur. J., 2008, 14,
2
2
with an increase of n in [n]helicene, as shown in Fig. 3,
leading to the enhanced chiroptical properties. Further
studies will focus on the synthesis of [n]helicene-stacked mole-
cules (n Z 5).
4
600–4617; (b) O. R. P. David, Tetrahedron, 2012, 68, 8977–8993;
(c) Z. Hassan, E. Spuling, D. M. Knoll and S. Br ¨a se, Angew. Chem., Int.
Ed., 2020, 59, 2156–2170.
1
0 (a) For pseudo-ortho-[2.2]paracyclophane:Y. Morisaki, R. Hifumi,
L. Lin, K. Inoshita and Y. Chujo, Chem. Lett., 2012, 41, 990–992;
(b) for 4,7,12,15-tetrasubstituted [2.2]paracyclophane:Y. Morisaki,
M. Gon, T. Sasamori, N. Tokitoh and Y. Chujo, J. Am. Chem. Soc.,
The authors are grateful to Professor Kazuo Tanaka and Dr
Masayuki Gon (Graduate School of Engineering, Kyoto Univer-
sity) for CD and CPL spectroscopy. The authors are also grateful
to Professor Takuji Hatakeyama (School of Science, Kwansei
Gakuin University) for PL lifetime measurements. The financial
support by Grant-in-Aid for Scientific Research (B)
2
014, 136, 3350–3353; (c) for bis-(para)-pseudo-ortho-[2.2]paracyclo-
phane:Y. Morisaki, R. Sawada, M. Gon and Y. Chujo, Chem. – Asian
J., 2016, 11, 2524–2527; (d) for bis-(para)-pseudo-meta-[2.2]paracy-
clophane:R. Sawada, M. Gon, J. Nakamura, Y. Morisaki and
Y. Chujo, Chirality, 2018, 30, 1109–1114.
1
1
1 Y. Morisaki and Y. Chujo, Bull. Chem. Soc. Jpn., 2019, 92, 265–274.
(
No. 19H02792) from MEXT is acknowledged (Y. M.). This work 12 Recent examples of CPL-active molecules based on planar chiral
[
2.2]paracyclophane, see:(a) S. Ishioka, M. Hasegawa, N. Hara,
H. Sasaki, Y. Imai and Y. Mazaki, Chem. Lett., 2019, 48, 640–643;
b) M.-Y. Zhang, Z.-Y. Li, B. Lu, Y. Wang, Y.-D. Ma and C.-H. Zhao,
was partly supported by the Izumi Science and Technology
Foundation (Y. M.).
(
Org. Chem., 2018, 20, 6868–6871; (c) E. Benedetti, M.-L. Delcourt,
B. Gatin-Fraudet, S. Turcaud and L. Micouin, RSC Adv., 2017, 7,
50472–50476.
Conflicts of interest
3 (a) J. Tribout, R. H. Martin, M. Doyle and H. Wynberg, Tetrahedron
Lett., 1972, 28, 2839–2842; (b) M. Nakazaki, K. Yamamoto and
M. Maeda, Chem. Lett., 1980, 1553–1554; (c) M. Nakazaki,
K. Yamamoto and M. Maeda, J. Org. Chem., 1981, 46, 1985–1987;
(d) H. Hopf, J. Hucker and L. Ernst, Phys. Org. Chem., 2007, 81,
There are no conflicts to declare.
Notes and references
947–969; (e) H. Hopf, J. Hucker and L. Ernst, Eur. J. Org. Chem., 2007,
1
(a) R. H. Martin, Angew. Chem., Int. Ed. Engl., 1974, 13, 649–660;
1891–1904; ( f ) A. E. Mourad, J. Hucker and H. Hopf, Z. Naturforsch.,
B: J. Chem. Sci., 2014, 42, 1142–1146.
(
(
(
b) S. Yun and C.-F. Chen, Chem. Rev., 2012, 112, 1463–1535;
c) M. Gingras, Chem. Soc. Rev., 2013, 42, 968–1006; 14 A. A. Aly and A. B. Brown, Tetrahedron, 2009, 65, 8055–8089.
d) M. Gingras, G. Felix and R. Peresutti, Chem. Soc. Rev., 2013, 42, 15 A. A. Aly, H. Hopf and L. Ernst, Eur. J. Org. Chem., 2000, 3021–3029.
1
007–1050; (e) M. Gingras, Chem. Soc. Rev., 2013, 42, 1051–1095; 16 G. Meyer-Eppler, R. Sure, A. Schneider, G. Schnakenburg, S. Grimme
(
f ) C.-F. Chen and S. Yun, ed. Helicene Chemistry, Springer, Berlin
and A. L u¨ tzen, J. Org. Chem., 2014, 79, 6679–6687.
Heidelberg, 2017; (g) C. Li, Y. Yang and Q. Miao, Chem. – Asian J., 17 (a) Y. Tanaka, T. Ozawa, A. Inagaki and M. Akita, Dalton Trans.,
2
018, 13, 884–894; (h) T. Mori, Chem. Rev., 2021, 121, 2373–2412.
2007, 928–933; (b) C.-Y. Yu and C.-C. Hsu, Polymer, 2018, 137, 30–37;
(c) H. Maeda, M. Kameda, T. Hatakeyama and Y. Morisaki, Polymers,
2018, 10, 1140.
2
3
4
K. E. S. Phillips, T. J. Katz, S. Jockusch, A. J. Lovinger and N. J. Turro,
J. Am. Chem. Soc., 2001, 123, 11899–11907.
J. E. Field, G. Muller, J. P. Riehl and D. Venkataraman, J. Am. Chem. 18 (a) T. Mizoroki, K. Mori and A. Ozaki, Bull. Chem. Soc. Jpn., 1971,
Soc., 2003, 125, 11808–11809.
44, 581; (b) R. F. Heck and J. P. Nolley, Jr., J. Org. Chem., 1972, 37,
2320–2322.
(a) J. Crassous, in Circularly Polarized Luminescence of Isolated Small
Organic Molecules, ed. T. Mori, Springer, Singapore, 2020, pp. 53–98; 19 For example, see: (a) Y. Nakai, T. Mori and Y. Inoue, J. Phys. Chem. A,
(
2
b) W.-L. Zhao, M. Li, H.-Y. Lu and C.-F. Chen, Chem. Commun.,
019, 55, 13793–13803.
(a) Y. Sawada, S. Furumi, A. Takai, M. Takeuchi, K. Noguchi and
2012, 116, 7372–7385; (b) H. Tanaka, M. Ikenosako, Y. Kato,
M. Fujiki, Y. Inoue and T. Mori, Commun. Chem., 2018, 1, 38; and
also see ref. 1f, 1h, 13b and 13c.
5
6
K. Tanaka, J. Am. Chem. Soc., 2012, 134, 4080–4083; (b) K. Tanaka, 20 (a) For first chiral [3]helicene: M. S. Newman and A. S. Hussey, J. Am.
Y. Kimura and K. Murayama, Bull. Chem. Soc. Jpn., 2015, 88,
75–385.
(a) F. V ¨o gtle, Cyclophane Chemistry: Synthesis, Structures and Reac-
Chem. Soc., 1947, 69, 978–979; (b) for first chiral [4]helicene:
M. S. Newman and W. B. Wheatley, J. Am. Chem. Soc., 1948, 70,
1913–1916.
3
tions, John Wiley & Sons, Chichester, 1993; (b) R. Gleiter and 21 (a) N. Berova, K. Nakanishi and R. W. Woody, ed. Circular Dichroism,
H. Hopf, ed. Modern Cyclophane Chemistry, Wiley-VCH, Weinheim,
004; (c) H. Hopf, Angew. Chem., Int. Ed., 2008, 47, 9808–9812;
d) Z. Hassan, E. Spuling, D. M. Knoll, J. Lahann and S. Br ¨a se,
Chem. Soc. Rev., 2018, 47, 6947–6963.
(a) C. J. Brown and A. C. Farthing, Nature, 1949, 164, 915–916;
Wiley-VCH, Toronto, 2nd ed., 2000; (b) J. P. Riehl and
F. S. Richardson, Chem. Rev., 1986, 86, 1–16; (c) J. P. Riehl and
F. Muller, in Comprehensive Chiroptical Spectroscopy, ed. N. Berova,
P. L. Polavarapu, K. Nakanishi and R. W. Woody, Wiley and Sons,
New York, 2012, vol. 1, pp 65–90.
2
(
7
(
b) D. J. Cram and H. Steinberg, J. Am. Chem. Soc., 1951, 73, 22 Calculated transition dipole moments of [n]carbohelicenes
5
691–5704.
(n = 3–10) in the ground state were reported; see reference 19a.
Chem. Commun.
This journal is © The Royal Society of Chemistry 2021