An Ortho-Tetraphenylene-Based “Gel?nder” Architecture Consisting Exclusively of 52 sp2-Hybridized C Atoms

Article abstract of DOI:10.1002/chem.202101968

A new type of “Gel?nder” molecule based on a ortho-tetraphenylene core is presented. The central para-quaterphenyl backbone is wrapped by a 4,4’-di((Z)-styryl)-1,1’-biphenyl banister, with its aryl rings covalently attached to all four phenyl rings of the backbone. The resulting helical chiral bicyclic architecture consists exclusively of sp²-hybridized carbon atoms. The target structure was assembled by expanding the central ortho-tetraphenylene subunit with the required additional phenyl rings followed by a twofold macrocyclization. The first macrocyclization attempts based on a twofold McMurry coupling were successful but low yielding; the second strategy, profiting from olefin metathesis, provided satisfying yields. Hydrogenation of the olefins resulted in a saturated derivative of similar topology, thereby allowing the interdependence between saturation and physico-chemical properties to be studied. The target structures, including their solid-state structures, were fully characterized. The helical chiral bicycle was synthesized as a racemate and separated into pure enantiomers by HPLC on a chiral stationary phase. Comparison of recorded and simulated chiroptical properties allowed the enantiomers to be assigned.

Full text of DOI:10.1002/chem.202101968

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doi.org/10.1002/chem.202101968
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An Ortho-Tetraphenylene-Based “Geländer” Architecture
Consisting Exclusively of 52 sp2-Hybridized C Atoms
Hervé Dekkiche,[a, b] Juraj Malinčik,[a] Alessandro Prescimone,[a] Daniel Häussinger,[a] and
Marcel Mayor*[a, b, c, d]
In memory of our friend and supporter François Diederich.
Abstract: A new type of “Geländer” molecule based on a
ortho-tetraphenylene core is presented. The central para-
quaterphenyl backbone is wrapped by a 4,4’-di((Z)-styryl)-1,1’-
biphenyl banister, with its aryl rings covalently attached to all
four phenyl rings of the backbone. The resulting helical chiral
bicyclic architecture consists exclusively of sp2-hybridized
carbon atoms. The target structure was assembled by
expanding the central ortho-tetraphenylene subunit with the
required additional phenyl rings followed by a twofold
macrocyclization. The first macrocyclization attempts based
on a twofold McMurry coupling were successful but low
yielding; the second strategy, profiting from olefin metathesis,
provided satisfying yields. Hydrogenation of the olefins
resulted in a saturated derivative of similar topology, thereby
allowing the interdependence between saturation and phys-
ico-chemical properties to be studied. The target structures,
including their solid-state structures, were fully characterized.
The helical chiral bicycle was synthesized as a racemate and
separated into pure enantiomers by HPLC on a chiral
stationary phase. Comparison of recorded and simulated
chiroptical properties allowed the enantiomers to be as-
signed.
Introduction
is found on the molecular level. The spiral spatial arrangement
of biopolymers like DNA and proteins interacts with their
chemical functions, while synthesized spiral molecules are first
aesthetically appealing, but might have interesting chemical
and physical properties due to their helical shape too,[1–3]
resulting in promising applications.[4–8] Of particular interest are
helically arranged π-systems, resulting in chiroptical properties
like circular dichroism (CD) and circularly polarized photo-
luminescence (CPL). Due to their interaction with light, such
structures become interesting elements of opto-electronical
devices, for example as receptors/emitters of polarized light,[9,10]
molecular switches[11] or probes for chiral molecules.[9,12,13]
Recent developments profiting from the spin of the charge
carrier to process information might even expand the applica-
tion potential of conjugated chiral molecules as active elements
for chirality induced spin selectivity (CISS).[14,15]
Spiral objects are ubiquitous in our life. While, in some cases,
the spiral shape is crucial for the object’s function (e.g., springs,
the spiral growth of trees), there are plenty examples with the
helical arrangement as an exclusively pleasing design element
(e.g., ornaments on furniture, spiral columns). A similar dualism
[a] Dr. H. Dekkiche, J. Malinčik, Dr. A. Prescimone, Prof. Dr. D. Häussinger,
Prof. Dr. M. Mayor
Department of Chemistry, University of Basel
St. Johanns-Ring 19, 4056 Basel (Switzerland)
E-mail: marcel.mayor@unibas.ch
[b] Dr. H. Dekkiche, Prof. Dr. M. Mayor
NCCR Molecular Systems Engineering
BPR 1095, Mattenstrasse 24a, 4058 Basel (Switzerland)
[c] Prof. Dr. M. Mayor
The most popular and widely studied helically arranged π-
systems are helicenes (Figure 1a).[16] Their extended spiral π-
system results in intense chiroptical activities with high
extinction coefficients (e.g., Δɛmax of 260 L·molÀ 1 for [6]-
helicene).[17] Recent progresses in this field include the synthesis
of multi-branched-helicenes[18] and helicenes with extended
delocalization,[19–21] which both show unique properties. An
appealing feature of larger helicenes is the in tegrity of their
helical arrangement. For example, already the activation energy
for the enantiomerization of the archetypical [6]-helicene
Institute for Nanotechnology (INT)
Karlsruhe Institute of Technology (KIT)
P. O. Box 3640, 76021 Karlsruhe (Germany)
[d] Prof. Dr. M. Mayor
Lehn Institute of Functional Materials (LIFM)
School of Chemistry
Sun Yat-Sen University (SYSU)
Guangzhou 510275 (P.R. of China)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202101968
This article belongs to a Joint Special Collection dedicated to François
Diederich.
[22,23]
corresponds with about 151.5 kJ·molÀ 1 at 25 C
to a half-
°
live time of ~106 years in the racemization process (page SI-86
in the Supporting Information). Their stability as enantiomer is
not only favorable for enantiopure synthetic strategies, but also
a requirement as active materials in chiroptical devices.
© 2021 The Authors. Chemistry - A European Journal published by Wiley-
VCH GmbH. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which permits use
and distribution in any medium, provided the original work is properly cited,
the use is non-commercial and no modifications or adaptations are made.
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Figure 1. Model compounds with helically arranged subunits. The helically wrapped structures are colored, the molecule’s backbone, which acts as an axle, is
in grey.
An alternative approach towards helical structures are
“Geländer” molecules, developed by Vögtle and co-workers
more than 20 years ago (Figure 1b).[24] Axial chirality emerges
from the interlinking of the phenyl rings of a para-terphenyl,
with the latter as axle of the molecule and the interlinking
structure wrapped helically like the bannister (German: Ge-
arrangement of the four phenyl rings in this central building
block, as the two adjacent biphenyl moieties can arrange in a
right- or left-handed fashion relative to each other. Even so the
racemization barrier of ortho-tetraphenylene is very high with
decomposition prior to inversion reported for the parent
structure at ambient pressure, it can theoretically interconvert
between its P and M forms.[30,31] Recently new synthetic routes
have been reported providing more elaborated structures from
länder) of
a spiral staircase. These pioneering Geländer
structures not only interconvert between enantiomers easily,
the combination of the molecule’s symmetry with the inde-
pendence of both interlinking junctions also allows their meso
forms, having opposed axial chirality along the molecule’s axis.
We developed the concept of Geländer molecules further by
attaching a second phenyl-based oligomer of different step-size
alongside the para-terphenyl axle (Figure 1c).[25–29] While the
improved molecular design avoids meso-forms by propagating
the axial chirality along the oligomers backbone, it came at the
prize of considerably increased synthetic challenges. Further-
more, also these improved Geländer structures enantiomerize
at room temperature and are thus rather model compounds to
investigate the parameters controlling their racemization than
stable enantiopure materials for chiroptical applications.
simple building blocks.[32,33] Double-helical (metallo)[34]
-
oligomers[35] and other interesting materials[32] based on this
subunit have been reported lately.
To combine the structural features of Geländer molecules
with the inherent chirality of ortho-tetraphenylene should be
straightforward. As sketched in Scheme 1, the extension of the
central ortho-tetraphenylene subunit with a pair of additional
ortho-biphenyl units (blue in A, Scheme 1a) already provides
the precursor of the target structure. Decorated with suitable
functional groups (FG in Scheme 1a) at the end of both axis,
double macrocyclization provides the intertwined molecular
target 1.
Herein, we present an alternative strategy to stable «Ge-
länder»-type structures comprising a central ortho-tetrapheny-
lene subunit (Figure 1d). The chirality emerges from the spatial
Scheme 1. Synthetic strategy to the mechanically locked Geländer molecule 1 (for clarity only the M enantiomer is displayed). a) Fundamental structural
considerations. b) Fragmentation into suitable precursors.
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Scheme 2. Synthesis of ortho-tetraphenylene building block 5. a) K2CO3, Pd(PPh3)2Cl2 (cat.), THF/MeOH (4:1), reflux, overnight (argon); b) nBuLi then I2, THF,
°
°
À 78 C to RT, 1.5 h (argon); c) NaHCO3, Pd(OAc)2 (cat.), DMF, 110 C, 1.5 d (air).
Results and Discussion
the substrate. Thus methoxy and methyl decorated precursors
were considered as displayed in Scheme 2.
Synthetic strategy
Starting from commercially available p-tolylboronic acid and
home-made 4-bromo-3-iodoanisole, the intermediate 3 was
obtained in 93% yield following standard Suzuki-Miyaura
coupling reaction conditions. The iodo-derivative 4 could then
be obtained in almost quantitative yield by bromine-lithium
exchange and subsequent quenching with iodine. Treatment of
4 with catalytic amounts of Pd(OAc)2 and a slight excess of
The synthetic strategy has already been mentioned above in
the introduction and is displayed in Scheme 1. The extension of
the structure by two suitably functionalized ortho-biphenyl
subunits (blue in Scheme 1a) brings both ring closing functional
groups (FG in Scheme 1a) in close spatial proximity and thus,
intramolecular ring closing was hypothesized to be favored
compared to intermolecular coupling reactions. An interesting
feature from a structural view-point is that the double macro-
cyclization locks the spatial arrangement of the system
mechanically, further stabilizing the obtained enantiomers. Of
particular interest will be the optical features pointing at the
degree of delocalization of the π-systems in the target structure
1, with a framework consisting exclusively of sp2-hybridized
carbon atoms.
As sketched in the octaphenyl derivative B in Scheme 1b,
aldehydes or olefins were considered as functional group for
the intended macrocyclizations. With the tetra-aldehyde pre-
cursor an intramolecular McMurry coupling[36,37] was considered
as cyclization step, while olefin metathesis[38] was the method of
choice for the tetraolefin precursor. As Wittig chemistry[39,40]
enables to obtain the tetra-olefin precursor from the tetra-
aldehyde precursor, the latter became the main objective of the
synthetic strategy. An ideal ortho-tetraphenylene precursor
would be C. Its methoxy groups should be easily transformable
to leaving groups providing the central building block to attach
the suitably functionalized ortho-biphenyl subunits by Suzuki-
Miyaura coupling chemistry.[41,42]
°
NaHCO3 in DMF at 110 C for more than a day provided the
ortho-tetraphenylenes 5 and 5’ in a yield of about 60% over
both structural isomers. Despite their comparable polarity, both
isomers were successfully separated by column chromatogra-
phy (CC). The desired central building block 5 was isolated in
30% yield, which is in good agreement with the data reported
previously for similar compounds.[43] As both structural isomers
5 and 5’ have very comparable 1H NMR spectra, their differ-
entiation was only possible due to the close proximity of Ha and
Hb in 5’ observed in the NOESY spectrum, which is not the case
in 5 (Figures S2-1 and S2-2). The NOESY spectra further allowed
two sets of three signals to be assigned for the two phenyl
rings of the ortho-tetraphenylene moiety.
The central building block
5 was further developed
(Scheme 3) by removing the methyl protection groups with
boron tribromide to yield 6 exposing two phenolic hydroxy
groups in quantitative yields. The latter was subsequently
transformed into the bis-triflate 7 by treatment with trifluor-
omethanesulfonic anhydride in basic conditions. CC provided 7
in very good 86% yield as white powdery solid. The formation
of 7 was monitored by 19F NMR, where a signal at À 72.91 ppm
Synthesis of building blocks and precursors
For the synthesis of the ortho-tetraphenylene backbone
targeted in this study, a procedure involving the oxidative
coupling of a iodobiphenyl derivative was chosen.[43] Compared
to other routes involving the formation of reactive organo-
metallic species and more sophisticated intermediates,[33] this
catalytic reaction offers the advantage of using simple con-
ditions while providing good yields of the desired product.
Nevertheless, due to the quite harsh reaction conditions
required, exclusively robust functional groups are acceptable in
°
Scheme 3. Synthesis of building-block 8. a) BBr3, CH2Cl2, À 78 C to RT,
overnight; b) Tf2O, pyridine, CH2Cl2, RT, 1.5 h; c) NBS, BPO (cat.), CCl4, reflux,
°
4–5 h; d) DMSO, 100 C, overnight; c’) reaction with 2 equiv. NBS; d’)
hydrolysis, then oxidation.
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indicated the presence of aromatic triflates. To transform both
methyl groups of 7 into aldehydes, we considered first a route
via the benzylic bromide 9, which could be hydrolyzed to the
benzylic alcohol and oxidized to the desired aldehyde 10
(dashed arrows in Scheme 3). However, the controlled mono-
bromination of both methyl groups with a slight excess of N-
bromosuccinimide (NBS) turned out to be challenging and the
apparently almost statistical formation of multi-brominated
species not only handicapped the yield, but also rendered
isolation and purification of 9 laborious. We thus decided to
profit from the compounds tendency for multiple bromination
and treated 7 with a slight excess of NBS (4.2 equiv) to obtain
the tetrabrominated compound 8. Due to the remaining variety
of brominated species, also the isolation of 8 turned out to be
challenging and ineffective. Thus the crude reaction mixture
With both building blocks in hand, the ortho-tetraphenylene
bis-triflate 10 and the biphenyl pinacolborate 13, the assembly
of the octa-aryl precursor 14 was considered (Scheme 4). For
the Suzuki-Miyaura type coupling, reported conditions were
adapted.[45] Thus the bis-triflate 10 was treated with 2.4
equivalents of the pinacolborate 13 in the presence of tetrakis
(triphenylphos-phine)palladium(0) as catalyst and 5 equivalents
of tripotassium phosphate monohydrate as base for several
°
hours at 85 C. After work-up, the tetra aldehyde advanced
precursor 14, already comprising all the carbon atoms of the
target structure, was isolated in very good 86% yield after CC
as white powder. The characteristic feature in the 1H NMR
spectrum of 14 are the two distinct aldehyde-H signals at 10.02
and 9.99 ppm respectively.
°
was kept overnight in 100 C hot DMSO to transform the
dibromo methyl groups into aldehydes,[44] and after work-up
and CC, the dialdehyde 10 was obtained in reasonable 50%
yields over both steps. The course of the reaction was
monitored by 1H NMR spectroscopy, where the raise of the
unshielded aldehyde-H signal at 10.04 ppm at the cost of the
dibromobenzyl-H at 6.64 ppm was indicative for the aldehyde
formation.
Macrocyclization to the conformationally locked structure
The efficient synthetic availability of the tetra aldehyde
precursor 14 enabled the exploration of macrocyclization
conditions. According to our synthetic strategy, a twofold
intramolecular McMurry reaction was considered first
(Scheme 5). Due to lack of in depth experience, the used
reaction conditions were inspired by protocols applied to aryl-
aldehydes[46] and enabled intramolecular cyclizations.[47] Thus
Following our synthetic strategy, a suitably functionalized
biphenyl building block was required to develop the precursor
(14) for the macrocyclization. As displayed in Scheme 4, the
biphenyl scaffold was obtained by exposing 2-(4-iodophenyl)-
1,3-dioxolane and 2-bromophenyl boronic acid to typical
Suzuki-Miyaura coupling conditions. To profit from the superior
reactivity of aryl iodides compared to aryl bromides, the THF/
methanol solvent mixture was chosen to avoid reaction temper-
°
the tetra aldehyde 14 dissolved in THF was added to a 0 C cold
solution of titanium tetrachloride (TiCl4, 4 equiv) and zinc dust
(Zn, 12 equiv) under an argon protection gas atmosphere. After
refluxing the reaction mixture for about 16 h and work up, the
desired doubly cyclized target structure 1 was isolated in 8.5%
yield by CC.
°
atures above 65 C. After work-up and CC, the biphenyl 11 was
Even so the central ortho-tetraphenylene structure hardly
enantiomerizes at room temperature, the twofold intramolecu-
lar ring closing further reinforces mechanically the final
enantiomeric form adopted by the molecule. The presented
reactions sequence provides the mechanically fixed target
structure 1 as racemate, but for clarity only the M enantiomers
are displayed in the following synthetic schemes.
We were excited that in spite of the low yield, the small
amount of 1 could easily be isolated from the remaining
ingredients of the reaction mixture by CC due to its nonpolar
nature. However, in spite of numerous efforts, the yield of the
reaction could not be improved and remained below 10%. To
isolated as colorless oil in 80% yield. The bromine substituent
of 11 was transformed into a pinacol ester of the corresponding
boronic acid by a lithium bromine exchange with n-butyl-
lithium, and subsequent quenching with isopropyl pinacol
borate. The boronic acid derivative 12 was isolated in
quantitative yields by CC. Treatment of 12 in acidic conditions
(para-toluenesulfonic acid (PTSA) in an acetone-water mixture)
provided the aldehyde 13 in 90% isolated yield. In spite of the
sensitivity of the pinacolborate substituent, neither impurities
nor degradation products were detected in the H and 13C NMR
spectra of building block 13.
1
Scheme 4. Synthesis of the biphenyl building-block 13 and its merger with the ortho-tetraphenylene 10 forming the advanced precursor 14. a) K2CO3,
°
Pd(PPh3)2Cl2 (cat.), THF/MeOH (4:1), reflux, 4 h (argon); b) nBuLi then iPrBpin, THF, À 78 C to RT, 1.5 h (argon); c) PTSA, acetone/water (10:1), RT, overnight; d)
K3PO4 ·H2O, Pd(PPh3)4, dioxane/water (5:1), reflux, overnight (argon).
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Scheme 5. Synthesis of the mechanically locked Geländer molecule 1, part I: Intramolecular macrocyclizations by McMurry reactions (for clarity, only the M
°
°
enantiomers are displayed). a) TiCl4 and Zn, THF (high-dilution), 0 C to reflux, overnight, under Ar; b) quenching with NH4Cl(aq.) after 30 min at 0 C; c) heating
overnight.
figure out the origin of the yield limitation in this McMurry
macrocyclization approach, efforts were made to identify the
reaction intermediates. Thus the reaction was performed on a
smaller scale and part of it was quenched with NH4Cl after
tempting to hypothesize that only some of the formed double
titanium diastereoisomers D are able to degrade properly to the
desired bis-olefin 1, while the remaining ones are most likely
decomposing into structures exposing hydroxyl groups consid-
ering their increased polarity compared to the target structure.
While the rational for the limited yield for the double
macrocyclization by McMurry reaction is intellectually satisfying,
it does not improve the acceptability of the yield. Considering
the perfect preorganization of both aldehydes, an efficient ring
closing was expected. A twofold ring-closing metathesis (RCM)
reaction was thus considered as alternative. Therefore, the
tetra-aldehyde 10 was converted first into the tetra-vinyl
derivative 15 in good yield by reaction with meth-
yltriphenylphosphonium bromide using standard Wittig reac-
tion conditions (Scheme 6). The formation of 15 was monitored
°
30 min reacting with the TiCl4/Zn couple at 0 C.
According to the analysis by matrix-assisted laser desorp-
tion-ionization mass spectrometry (MALDI-MS, Figure S2-4), all
the starting material was consumed forming exclusively the
tetra-hydroxy derivative E in Scheme 5 (m/z peak of 743.552
corresponding to [M+H3O]+). Even so a single signal was
recorded in the MALDI-MS analysis, a cluster of several peaks
was detected in the high performance liquid chromatography
(HPLC) analysis of the reaction mixture (Figure S2-3). This group
of peaks of different intensity is expected considering the
variety of possible diastereomers emerging from the spatial
arrangements of the 4 hydroxy groups (in theory there are 10
enantiomeric pairs of diastereomers possible, see Figure S2-5).
After refluxing the intermediate D overnight, the formation of
the target structure 1 together with a variety of other more
polar derivatives is observed. Considering the mechanism of
the McMurry coupling and the variety of diastereoisomers
detected after the quenching of the intermediate D, it is
1
by H NMR, where two sets of vinylic protons at 6.74, 5.76 and
5.28 ppm and 6.65, 5.69 and 5.21 ppm (see Figure S1-17 for the
COSY spectrum) appeared. With the tetra-vinyl precursor 15 in
hands, the RCM was performed with the Hoveyda-Grubbs
second-generation catalyst[48,49] (20 mol%) using high-dilution
conditions in refluxing dichloromethane.[50] The reaction was
monitored by MALDI-MS (Figures S2-6 and S2-7) revealing that
Scheme 6. Synthesis of the mechanically locked Geländer molecule 1, part II: Intramolecular macrocyclizations by RCM and reduction to the saturated
°
derivative 2 (for clarity, only the M enantiomers are displayed). a) MePPh3Br, nBuLi, THF, 0 C to RT, overnight (argon); Hoveyda-Grubbs second-generation.
°
(cat.), CH2Cl2 (high-dilution), reflux, overnight; c) 1,4-cyclohexadiene, Pd/C, dioxane, 90 C (sealed tube), 1.5 h (argon).
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the stepwise transformation of the starting material into the
desired bicyclic 1 was almost complete after 6 h. After 24 h
traces of partially closed intermediate remained constantly and
the reaction was worked-up. To our delight, the racemate of
the mechanically fixed bicyclic Geländer structure 1 was
isolated in very good 91% yield after CC. The considerably
improved availability of 1 even allowed for further derivatiza-
tion. As obvious first example, both double bonds of 1 were
hydrogenated to provide the saturated analogue 2. For this
22) experiments with a 600 MHz NMR-spectrometer enabled
the assignment of all 1H signals of both bicycles 1 and 2
(Figures 2 and S2-10).
Clearly the resonances of the hydrogen atoms of the
interlinking structures differ between both bicycles. While the
hydrogens (H15 and H16 in Figure 2) of the ethen-1,2-diyl linker
of 1 resonate at 6.78 and 6.89 ppm and display the expected
~10 Hz coupling of vicinal cis olefin hydrogens, the H-atoms
(H15-H18 in Figure 2) of the ethan-1,2-diyl linker of 2 are
observed as complex multiplex signals at 2.65 and 3.12 ppm
respectively. Interestingly, the coupling between all four nuclei
combined with their locked arrangement results in a unique
multiplicity pattern. Apart from this expected variation in the
spectroscopic signature, the signals of the oligoaryl scaffold
remain essentially the same between both bicycles. Almost all
aryl hydrogens display minute to small up-field shifts upon the
transition from 1 to 2, with the exception of H11 (highlighted
light blue in Figure 2) which shifts from 6.10 ppm in 1 to
6.17 ppm in 2 (blue arrow in Figure 2). The expected increase in
flexibility by going from 1 to 2 is hardly detectable in the NMR
spectra. The only indications are small increases in the intensity
of weak NOESY signals between hydrogens of neighboring aryl
systems, such as H14 of the biphenyl system and H8 of the
central ortho-tetraphenylene scaffold or H11 and H6.
°
transformation, 1 was treated with 1,4-cyclohexadiene in 90 C
hot dioxane in the presence of catalytic amounts of palladium
on carbon (Pd/C). After work-up and CC the saturated bicyclic
structure 2 was isolated in 84% yield.[51]
All new compounds were fully characterized by 1H and
13C NMR spectroscopy and high resolution mass spectrometry.
The identity of both bicyclic target structures 1 and 2 was
further corroborated by their solid-state structures. Single
crystals suitable for X-ray diffraction analysis were obtained by
slow diffusion of methanol vapor into a CH2Cl2 solution of
racemic 1 and into a chloroform solution of racemic 2.
Furthermore, the racemic mixtures of 1 and 2 were separated
into enantiomers by preparative HPLC with a chiral stationary
phase (Chiralpak IBÀ N, Figures S2-8 and S2-9).
Over all, based on the NMR spectra of both compounds 1
and 2, only minor structural variations were observed upon
hydrogenation of the interlinking olefins. An in-depth analysis
of the spatial rearrangements within the bicyclic scaffold
triggered by this transformation was possible based on their
solid-state structures.
X-ray structures: Both racemates 1 and 2 crystalize in a
monoclinic space group with unit cells comprising 3 and 2 sets
of enantiomeric pairs respectively (see Figures S2-11 and S2-12
for packing views). In spite of the comparable spatial arrange-
ment of their molecular subunits and the fact that both single
crystals were obtained from a similar solvent system, they
Physico-chemical characterization
NMR spectroscopy: The comparison of the physico-chemical
properties of both cyclized target compounds 1 and 2 was of
particular interest. Both bicyclic structures have very compara-
ble geometrical topology, but differ in their composition. While
1 consists exclusively of sp2-hybridized carbon atoms, the
saturated ethylene linkers in 2 consist of sp3 carbons, adding
not only four additional hydrogen atoms, but also some
increased flexibility of the structural scaffold. TOCSY (Figures S1-
14, S1-19, and S1-20) and NOESY (Figures S1-15, S1-21, and S1-
Figure 2. 1H NMR spectra of racemic 1 (top) and racemic 2 (bottom) with assignment of the signals (only the P enantiomers are displayed for the assignment).
The blue arrow shows H11 (cyan) to be the only hydrogen atom experiencing a downfield shift on going from 1 to 2.
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Figure 3. a) Labeling of the aryl rings in the bicyclic oligo-aryl scaffold of 1 and 2 to aid discussion of the inter-aryl torsion angles. Solid-state structures of the
P enantiomers of b) 1 and c) 2 as ORTEP plots with 50% probability. The backbone axes (dot and dash lines) are divided in the upper and lower biphenyl
system of the terphenyl backbone to analyze the deviation from linearity of the molecular backbone.
crystalize in slightly different space groups (C2/c for 1 and P21/c
for 2). To analyze the effects caused by the saturated ethylene
bridge in 2, the torsion angles between the aryl rings and along
the two C atoms of the bridging units were measured. The
labeling of the aryl subunits as α, β, γ, and δ is sketched in
Figure 3a, and the recorded torsion angles are listed in Table 1.
Unsurprisingly, the largest change in torsion angle was
observed for the pair of carbon atoms interlinking the biphenyl
subunit with the central ortho-tetraphenylene subunit, for
which the two-vicinal olefin Z substituents of the scaffold of 1
However, the inter-aryl torsion angles listed in Table 1
display rather small variations between both bicycles 1 and 2.
The structural perturbation caused by the hydrogenation of the
double bond is adsorbed by small adjustments of all inter-aryl
°
°
torsion angles ranging from +0.73 for αβ to À 2.15 for ββ’.
The only substantial variation is observed for γδ, for which
most likely the steric interaction with the hydrogen atoms of
the enthan-1,2-diyl linker are causing an increase of the torsion
°
angle by 15.49 . Interestingly, the vertical axis of 2 (dot and
°
dash blue line in Figure 3c) is by about 5.7 stronger bent
°
were with a torsion angle of 5.63 bent only marginally out of
compared to 1 (dot and dash red line in Figure 3b). At the first
glance this is counterintuitive as the enthan-1,2-diyl linker in 2
is 0.204 Å longer than the ethen-1,2-diyl linker of 1. However,
the difference in length is more than compensated by the
spatial arrangement of the CÀ C bond. While in the case of 1 the
C=C bond is almost parallel to the vertical axis and deviates
the plane. This suggests that in spite of the rigidity emerging
from the fixation in a bicyclic architecture, the oligo-aryl scaffold
provides enough degrees of freedom allowing the arrangement
of the bicycle 1 consisting exclusively of sp2-hybridized carbon
atoms without too much strain. The adaptability of the oligo-
aryl scaffold is further demonstrated by the saturated derivative
2, which comprises two pairs of sp3-hybridized C-atoms as
interlinking units. The torsion angle along the CÀ C single bond
°
only by 3.2 from parallelism, the CÀ C bond of 2 is with a
°
deviation of 34.7 from parallelism much less effective in
bridging the gap between the subunits of the oligo-aryl
scaffold.
°
°
is with 58.74 considerably larger compared to 1 (5.63 ), and
the bond length between both atoms increases from 1.335 Å
for the C=C bond in 1 to 1.539 Å for the CÀ C bond in 2. The
The analyses of the solid-state structures on the whole
corroborates the conclusions already drawn from the NMR
experiments. Namely the transition from 1 to 2 mainly effects
the structure of the bridging C-atom pair and has apart from
that only marginal effects on the arrangement of the oligo-aryl
scaffold.
Electronic properties: Of particular interest were the elec-
tronic properties of both bicycles 1 and 2. Substituting the sp2-
hybridized carbon atom pairs by sp3-hybridized ones removes
the option of electronic communication between the inter-
linked aryl systems by π-conjugation, an effect which should be
reflected in the electronic features of the compounds. The UV–
vis spectra of racemic 1 and 2 are displayed in Figure 4a and b,
respectively (purple solid lines with the corresponding y-axis on
the right side). The UV spectra of both target compounds
consist of broad structureless absorption bands with compara-
ble maxima of their extinction coefficients between 5 and
6.104 L·molÀ 1. Their lowest energy transition only displays a
°
°
50.65 torsion angle resembles the 60 angle between vicinal
hydrogen substituents of the most stable staggered conforma-
tion of ethane and again, it seems that the oligo-aryl scaffold is
able to intercept structural modifications by adapting the
torsion angles between the aryl subunits.
Table 1. Torsion angles measured between the different aryl rings and
along the interlinking pair of C-atoms in the solid-state structures of 1 and
2. In the case of 2, mean torsion angles adopted by the phenyl rings of
both, the upper and lower parts of the molecule are presented.
Tetraphenylene backbone
Other structural features
Torsion angle
αα’
ββ’
Αβ
αγ
γδ
CC
°
°
°
°
°
°
°
°
°
°
°
bicycle 1
bicycle 2
60.10
61.80
68.56
66.41
63.80
64.53
65.52
71.12
42.21
57.70
5.63
50.65
°
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Figure 4. a), b) UV-vis (solid purple line, y-axis on the right side) and CD spectra of both enantiomers (P enantiomer: red line; M enantiomer: blue line) of 1 and
2 respectively. All spectra were recorded at 20 C as 1×10À 5 m solution in chloroform. c), d) Comparison of the simulated (dark red line) and the recorded (red
°
line) CD spectra of the P enantiomers. The brown bars are the calculated first 50 electronic transitions which result in the simulated spectrum (ς=0.25 eV).
small bathochromic shift for 1 compared to 2 with values of
290 and 284 nm respectively. The most apparent difference
between both UV spectra is the bathochromic shift of the
second broad absorption band, which lies at 253 nm in the case
of 1 and is observed as shoulder at about 240 nm for 2. While
the observed bathochromic shifts point at the expected slightly
increased π-electron delocalization in 1, the modesty of the
effect is not surprising as the olefin π-system of the bridging
pair in 1 is almost perpendicular to the neighboring aryl-π-
systems reducing the efficiency of π-conjugation.
values are substantially higher than the ones measured for
previously reported helical chiral Geländer molecules with
terphenyl backbones,[28] or naphthol derivatives (10–
20 L·molÀ 1),[52] but considerably smaller than the ones reported
for more extended systems, such as a twisted cycloparapheny-
lene derivative (~200 L·molÀ 1).[53] While at first glance both
bicyclic helical hydrocarbons 1 and 2 display absorptions of the
same polarity for the same enantiomers, an inversion of the
Cotton bands at 250–290 nm was recorded. The rather modest
Δɛ and gCD values further document the lack of π-conjugation
in the helically wrapped oligo-aryl architectures.
In spite of numerous attempts, the crystallization of pure
enantiomers was not successful for both structures. To assign
the absolute configurations to the separated enantiomers, DFT-
calculations were performed. The molecular structure of interest
was optimized using the B3LYP-D3(BJ)/6-31 g(d) method, profit-
ing from the coordinates of the corresponding enantiomer in
the X-ray structures of the racemate as starting point. The
corresponding CD spectra were obtained by using TD-CAM-
B3LYP/6-31 g(d) and calculating the first 50 singlet excitations
(brown bars in Figure 4c and d). Taking the P enantiomers as
example, the resulting calculated CD spectra are displayed as
dark red lines in Figure 4c and d for P-1 and P-2, respectively.
The simulated spectra fit well with recorded ones of the
enantiomers with longer retention times (red lines in Figure 4c
Pure enantiomers of both bicycles 1 and 2 were obtained
by preparative HPLC with a chiral stationary phase (Chiralpak
IBÀ N, Figures S2-8 and S2-9). All isolated enantiomers were
stable compounds and did not display any racemization
tendency even at elevated temperatures. The CD spectra of the
pairs of enantiomers is displayed in Figure 4a and b for 1 and 2
respectively, with the blue line for the enantiomer with the
shorter retention time in the chiral HPLC analysis for both cases.
In the CD spectra of the enantiopure species, pairs of Cotton
bands of opposite signs were observed at 239, 262, 285, and
305 nm for 1, and at 237, 258, 282, and 300 nm for 2.
Comparable maximal Δɛ values were recorded around 240 nm
for both bicyclic structures with 83 L·molÀ 1 for
1 and
68 L·molÀ 1 for 2, resulting in similar dissymmetry factors (gCD) of
about 1.4·10À 3 at this wavelength (Figure S2-15). These Δɛ
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and d) allowing the assignment of the separated enantiomers.
Namely the M enantiomer as first eluting sample on the chiral
stationary phase (Chiralpak IBÀ N, Figures S2-8 and S2-9)
followed by the P enantiomer for both bicyclic structures 1 and
2. The comparable behavior of both target structures on the
chiral stationary phase was expected after structural analyses by
NMR spectroscopy and X-ray diffraction already displayed rather
moderate structural variations.
In summary, an efficient approach to helical chiral and
enantio-stable model compounds is presented. The modular
synthetic strategy allows further derivatization of the parent
structures, enabling their optimization and integration into
opto-electronic devices as chiroptical building blocks. We are
currently exploring both the scope and limitation of ortho-
tetraphenylene as a chirality-endowing subunit in carbon
architectures.
Additionally, the first molecular orbitals of both compounds
1 and 2 were calculated at the B3LYP/cc-pVTZ level of theory
(Figures S2-16 and S2-17) and the main electronic transitions Experimental Section
were calculated at the CAM-B3LYP/6-31 g(d) level of theory
Synthesis of 1 from 14 (McMurry coupling strategy): A solution of
(Tables S1 and S2). In both cases, the lowest energy absorption
observed in UV-vis (and therefore CD) spectroscopy can easily
be attributed to a combination of the HOMO!LUMO and
HOMO-1!LUMO+1 transitions. Though, as both HOMO and
HOMO-1 and LUMO and LUMO+1 orbitals seems to be
degenerated in the case of 1, they are not in the case of 2, most
probably due to the different symmetry adopted by the
molecule. The variations in the molecular orbitals energies
(À 0.224 eV for the HOMO-1 and À 0.029 eV for the LUMO+1 of
2 compared to À 0.216 and À 0.041 eV respectively for 1) can
also be linked to the rupture of the conjugation through the
linking CÀ C bond, and explain the bathochromic shift observed
in electronic spectroscopy. Furthermore, the electronic transi-
tion analysis reveals that the most intense transition is related
to the HOMO-1!LUMO+3 transition in the case of 1 and
HOMO-2!LUMO+3 in the case of 2.
TiCl4 (1 M in CH2Cl2, 0.72 mL, 0.72 mmol, 4 equiv) was added to a
suspension of Zn dust (140 mg, 2.16 mmol, 12 equiv) in dry THF
°
(25 mL) at 0 C. A solution of the tetra-aldehyde 14 (130 mg,
0.18 mmol) in dry THF (5 mL) was then added dropwise over
10 min. After 20 min, the mixture was heated to reflux and the
reaction was conducted overnight under argon atmosphere. After
cooling, the mixture was poured onto NH4Cl (aq.) (30 mL) and the
organic fraction was diluted with AcOEt (15 mL), extracted, washed
with water (40 mL) and brine (40 mL), dried over Na2SO4 and
evaporated to dryness. The residue was then chromatographed on
silica using PE (2)/CH2Cl2(1) as eluent to provide the desired product
1 as a white solid (10 mg, 0.015 mmol, 8.5%) after evaporation of
the solvents. The two enantiomers were then separated using chiral
HPLC (Chiralpak-IBÀ N column, n-heptane/CH2Cl2 95:5).
1
°
H NMR (600 MHz, CDCl3, 25 C): δ=7.57–7.54 (m, 2H, HPh), 7.45–
7.41 (m, 6H, HPh), 7.38 (dd, J=7.7, 1.8 Hz, 2H, HTP-b), 7.28 (dd, J=7.7,
0.5 Hz, 2H, HTP-b), 7.26–7.24 (m, 2H, HAr), 7.08–7.03 (m, 4H, HTP-a
+
HAr), 6.90–6.86 (m, 4H, HTP-a +Hvinyl), 6.78 (d, J=10.8 Hz, 2H, Hvinyl),
6.50 (d, J=1.7 Hz, 2H, HTP-b), 6.27 (d, J=1.7 Hz, 2H, HTP-a), 6.10 (dd,
J=8.1, 1.9 Hz, 2H, HAr), 6.04 (dd, J=8.1, 1.8 Hz, 2H, HAr). 13C NMR
°
(150 MHz, CDCl3, 25 C): δ=141.51, 141.37, 141.36 (2 C), 140.98,
Conclusion
139.66, 139.40, 139.02, 137.92, 135.97, 133.54 (CH), 132.64 (CH),
130.63 (CH), 130.61 (CH), 130.05 (CH), 129.98 (CH), 129.61 (CH),
129.45 (CH), 128.70 (CH), 128.11 (CH), 127.97 (CH), 127.79 (CH),
127.69 (CH), 127.23 (CH), 127.13 (CH), 126.79 (CH). HR-MS (ESI-TOF),
m/z: calc. for C52H32Ag ([M+Ag]+): 763.1549 ; Found: 763.1538. X-
Ray: Crystals grown from dichloromethane/methanol; size: 0.30×
0.30×0.20 mm. T=150 K, monoclinic, space group C2/c, a=
Profiting from the rigid and stable arrangement of both
biphenyl subunits in ortho-tetraphenylene, conformationally
locked bicyclic “Geländer” architectures were assembled in 11
or 12 synthetic steps. The target structures 1 and 2 are
hydrocarbons of which the first consists exclusively of sp2-
hybridized carbon atoms. Both bicyclic structures 1 and 2 were
synthesized as racemic mixtures and were subsequently
separated into pure enantiomers that do not racemize. The
assembly was based on developing the suitably functionalized
precursors 14 and 15 by functional-group transformations and
CÀ C coupling chemistry, which subsequently underwent a
twofold intramolecular macrocyclization. Initial attempts to
transform the tetra-aldehyde 14 through McMurry coupling
into bicycle 1 were successful, but led also to a variety of side
products. The superior synthetic strategy was a twofold ring-
closing metathesis reaction of the tetra-vinyl precursor 15.
Subsequent hydrogenation of both interlinking double bonds
of 1 provided the ethan-1,2-diyl interlinked bicycle 2. Structural
analyses and comparison of both bicyclic Geländer structures 1
and 2, revealed an oligo-phenyl scaffold with enough degrees
of freedom to absorb the structural differences. Solid-state
structures for both bicycles 1 and 2 were only available for the
racemic mixture. However, a very good fit between simulated
and recorded CD spectra allowed the assignment of the helical
chiral enantiomers.
°
13.7523 (3) Å, b=13.5387 (3) Å, c=18.8886 (4) Å, α=90 , β=
98.660 (2) , γ=90 , V=3476.74 (13) Å , Z=4, θ(max) =56.631 ,
3
°
°
°
N
reflexions =3476, R1 =0.0539 on 3065 reflections with I�2σ(I), wR2 =
°
0.1532 on all data. UV-vis (chloroform, 20 C), λmax (ɛ): 229 (65960),
253 (59290), 290 nm (19320 L·molÀ 1).
Synthesis of 1from 15 (RCM strategy): To a solution of the
tetravinylene 15 (50 mg, 70 μmol) in CH2Cl2 (100 mL) was added
Hoveyda-Grubbs catalyst second-generation (9 mg, 14 μmol,
0.2 equiv). The mixture was then heated to reflux for 24 h. After
evaporation of the solvents, the product was extracted by column
chromatography on silica using PE/CH2Cl2 3:1. After evaporation of
the solvents under reduced pressure, compound 1 (m=42 mg,
64 μmol, 91%) was obtained with similar analytical data as
described for the transformation of product 14.
Synthesis of 2: A solution of compound 1 (26 mg, 40 mmol) in
dioxane (3.5 mL) containing cyclohexa-1,4-diene (0.6 mL) was
bubbled with argon for 20 min in a Schlenk flask. Pd/C (10%,
8.4 mg, 8.0 μmol, 0.2 equiv) was then added, the flask was sealed,
°
and the temperature was raised to 90 C. After 1 h, the mixture was
cooled to room temperature, diluted with AcOEt and filtered over
alumina. After evaporation of the solvents, the residue was purified
by column chromatography using PE/CH2Cl2 2:1 on silica then by
Chem. Eur. J. 2021, 27, 1–11
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9
© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
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doi.org/10.1002/chem.202101968
Chemistry—A European Journal
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chiral HPLC (chiralpak-IBÀ N column, n-heptane/CH2Cl2 90:10) to
afford both enantiomers of 2 in equal amounts as white solids (m=
1
°
22 mg, 33 μmol, 84%). H NMR (600 MHz, CDCl3, 25 C): δ=7.52–
7.49 (m, 2H, HPh), 7.41–7.35 (m, 4H, HPh), 7.34 (dd, J=7.7, 1.8 Hz, 2H,
TP-b), 7.33–7.30 (m, 2H, HPh), 7.22 (d, J=7.7 Hz, 2H, HTP-b), 7.12 (dd,
H
J=7.7, 2.0 Hz, 2H, HAr), 7.04–7.00 (m, 4H, HTP-a +HAr), 6.80 (d, J=
7.8 Hz, 2H, HTP-a), 6.43 (d, J=1.7 Hz, 2H, HTP-b), 6.18–6.15 (m, 4H, HAr
+HTP-a), 5.97 (dd, J=8.0, 1.8 Hz, 2H, HAr), 3.21–3.14 (m, 2H, CH2),
3.10–3.03 (m, 2H, CH2), 2.69–2.60 (m, 4H, CH2). 13C NMR (150 MHz,
°
CDCl3, 25 C): δ=142.14, 141.67, 141.26, 141.23, 141.20, 139.72,
138.91, 138.57, 138.54, 137.92, 130.28 (CH), 130.22 (CH), 129.53
(CH), 129.19 (2CH), 129.16 (CH), 128.11 (CH), 127.93 (CH), 127.50
(CH), 127.48 (2CH), 127.46 (CH), 126.73 (CH), 126.30 (CH), 36.25
(CH2), 36.23 (CH2). HR-MS (ESI-TOF), m/z: calc. for C52H36Ag [M+Ag]+
: 767.1862; found: 767.1867. X-ray: Crystals grown from chloroform/
methanol; size: 0.30 x 0.30 x 0.20 mm. T=150 K, monoclinic, space
group P21/c, a=14.6050 (4) Å, b=15.5266 (3) Å, c=15.7816 (4) Å,
3
°
°
°
α=90 , β=91.980 (2) , γ=90 , V=3576.59 (15) Å , Z=4, θ(max)
°
=56.711 , Nreflexions =7131, R1 =0.0468 on 5549 reflections with I�
°
2σ(I), wR2 =0.1329 on all data. UV-vis (chloroform, 20 C), λmax (ɛ):
229 (51430), 240 (47730), 284 nm (8450 L·molÀ 1).
Deposition Numbers 2087693 (1) and 2087694 (2) contain the
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Conflict of Interest
The authors declare no conflict of interest.
Keywords: bicycles · “Geländer” molecules · helical chirality ·
hydrocarbons · macrocyclization
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Accepted manuscript online: July 13, 2021
Version of record online: ■■■, ■■■■
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FULL PAPER
Circles in a spiral: The synthesis and
physical properties of a helical chiral
bicyclic hydrocarbon are presented.
The design of the Geländer architec-
ture is based on a rigid, chiral, room-
temperature-stable central ortho-tet-
raphenylene subunit, which is further
developed and twice macrocyclized to
provide the target structure. Pure
enantiomers are obtained by chroma-
tography with a chiral stationary
Dr. H. Dekkiche, J. Malinčik, Dr. A. Pres-
cimone, Prof. Dr. D. Häussinger,
Prof. Dr. M. Mayor*
1 – 11
An Ortho-Tetraphenylene-Based
“Geländer” Architecture Consisting
Exclusively of 52 sp2-Hybridized C
Atoms
phase and allow the investigation of
the chiroptical properties. Comparison
between recorded and simulated CD
spectra allows these enantiostable
model compounds to be assigned.