Synthesis and Properties of Partially Saturated Fluorenyl-Derived [n]Helicenes Featuring an Overcrowded Alkene

Article abstract of DOI:10.1002/chem.202100150

The straightforward, multigram-scale synthesis of the partially saturated H₆-fluoreno[n]helicenes (n=5 or 7) featuring a central, overcrowded alkene is described. The key cyclization step was based on an intramolecular McMurry reaction from the corresponding 1,5-diketones. Chiral stationary phase HPLC analysis and isomer separation indicate that each helicenic compound is constituted of three diastereoisomers at room temperature, i. e. the configurationally stable (R,R,P)/(S,S,M) pair of enantiomers and an apparently achiral compound resulting from the rapid interconversion between the (R,S,P) and (S,R,M) enantiomers. The partially saturated H₆-fluoreno[n]helicenes are oxidatively aromatized to give an efficient access to the corresponding fluoreno[n]helicenes. The chiroptical properties (vibrational and electronic circular dichroism) of the chiral, enantiopure compounds have been measured and analyzed by quantum-chemical calculations, confirming their helicoidal nature.

Full text of DOI:10.1002/chem.202100150

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doi.org/10.1002/chem.202100150
Chemistry—A European Journal
Synthesis and Properties of Partially Saturated Fluorenyl-
Derived [n]Helicenes Featuring an Overcrowded Alkene
[a]
[b]
[d]
[c]
[c]
Lenka Pallova, Etienne S. Gauthier, Laura Abella, Marion Jean, Nicolas Vanthuyne,
[
b]
[a]
[d]
[b]
Vincent Dorcet, Laure Vendier, Jochen Autschbach,* Jeanne Crassous,*
[a]
[a]
Stéphanie Bastin,* and Vincent César*
Abstract: The straightforward, multigram-scale synthesis of
rently achiral compound resulting from the rapid interconver-
the partially saturated H -fluoreno[n]helicenes (n=5 or 7)
sion between the (R,S,P) and (S,R,M) enantiomers. The partially
6
featuring a central, overcrowded alkene is described. The key
cyclization step was based on an intramolecular McMurry
reaction from the corresponding 1,5-diketones. Chiral sta-
tionary phase HPLC analysis and isomer separation indicate
that each helicenic compound is constituted of three
diastereoisomers at room temperature, i.e. the configuration-
ally stable (R,R,P)/(S,S,M) pair of enantiomers and an appa-
saturated H -fluoreno[n]helicenes are oxidatively aromatized
6
to give an efficient access to the corresponding fluoreno[n]
helicenes. The chiroptical properties (vibrational and elec-
tronic circular dichroism) of the chiral, enantiopure com-
pounds have been measured and analyzed by quantum-
chemical calculations, confirming their helicoidal nature.
Introduction
from the basic carbo[n]helicenes, formally composed of ortho-
fused benzene rings, several strategies were indeed found
suitable to increase the diversity of helicenic scaffolds by: i)
introducing main-group elements (B, Si, N, P …) leading to the
Helicenes are a special class of inherently chiral molecules that
combine a helical topology and an extended π-conjugated
[1]
[8]
structure. Owing to this unique screw-shaped π-delocalization,
helicenes display enhanced chiroptical properties (Optical
Rotation (OR), Electronic Circular Dichroism (ECD), Circularly
Polarized Luminescence (CPL) …) compared to other common
design of heterohelicenes; ii) varying the size and nature of
the rings within the helix; iii) designing helicenoid or helicene-
[
9]
like systems formed of non-fully aromatic ortho-fused rings; iv)
inclusion of a positive or negative charge within the helical
[1,2]
[10]
chiral molecules.
Thus, they have gained great significance
scaffold, or v) incorporating helicenic ligands into organo-
[11]
over the past decade as key constitutional units in materials for
metallic complexes.
In this context, the development of
[
3,4]
[5]
optoelectronics,
supramolecular chemistry,
chiroptical
efficient, flexible and convenient methods for the synthesis of
helicenic compounds appears of utmost importance, as the
field cannot grow without the advent of more practical, scalable
and cost-efficient synthetic pathways.
[6]
switches or in the design of chiral ligands for asymmetric
[1f,7]
catalysis to name a few.
applications lies the possibility to tune the chiroptical properties
of helicenic compounds by modifying their structure. Starting
At the heart of these blossoming
In a joint research program, we were interested in using the
[5] and [7]-helicene-like fluorenes 1 and 2 as building blocks to
generate novel helicene-NHC-based ligands for organometallic
[
12,13]
[
a] Dr. L. Pallova, Dr. L. Vendier, Dr. S. Bastin, Dr. V. César
LCC-CNRS
Université de Toulouse, CNRS, 31077 Toulouse (France)
E-mail: stephanie.bastin@lcc-toulouse.fr
vincent.cesar@lcc-toulouse.fr
chemistry (Figure 1).
While the dibenzo[c,g]fluorene 1 (dbf)
[14]
has been known for more than half a century,
the [7]
[15,16]
helicene-like fluorene 2 was described in 2016 by Nozaki.
Both compounds are obtained by reduction of the correspond-
ing ketone analogues using hydrazine at elevated temperature.
Moreover, the syntheses of the keto-compounds involve either
a decarboxylative condensation of binaphthyl 2,2’-dicarboxylic
acid under very harsh conditions (300°C) or a transition-metal
[
b] Dr. E. S. Gauthier, Dr. V. Dorcet, Dr. J. Crassous
Institut des Sciences Chimiques de Rennes, UMR 6226
Institut de Physique de Rennes, UMR 6251, Campus de Beaulieu
CNRS-Université de Rennes 1, 35042 Rennes Cedex (France)
E-mail: jeanne.crassous@univ-rennes1.fr
catalyzed, multistep synthesis. We thus devised to develop a
common, direct and efficient synthetic protocol towards the
fluoreno[n]helicenes 1 and 2 from easily available starting
materials and without going through the ketone intermediates.
Our strategy relies on the key intermediates 3 and 4, which
would generate compounds 1 and 2 respectively in a final
dehydrogenative aromatization. Compounds 3 and 4 are thus
partially hydrogenated (or saturated) fluoreno[n]helicenes, and
their most striking feature is the presence of a tetra-substituted,
[
[
c] M. Jean, Dr. N. Vanthuyne
Aix Marseille university, CNRS, Centrale Marseille, Ism2
13397 Marseille (France)
d] Dr. L. Abella, Prof. J. Autschbach
Department of chemistry, University at Buffalo – State University of New
York
Buffalo, NY 14260 (USA)
E-mail: jochena@buffalo.edu
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/chem.202100150
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Figure 1. Structures of helicene-like fluorenes 1 and 2, of their partially
hydrogenated analogues 3 and 4 and of the related acyclic, overcrowded
alkene 5. Notation: Compounds 1 and 3 are composed of the five ortho-
fused rings in black, while compounds 2 and 4 (in blue) possess the five
rings in black and the two additional terminal phenyl rings in blue.
overcrowded C=C bond. As such, they can be considered as the
cyclic analogues of the chiral helical alkene (3R,3’R)-(P,P)-trans-
Scheme 1. Optimized synthetic route to the fluoreno[n]helicenes 1 and 2.
[17]
5
,
which is the archetype of the well-known Feringa’s
[18]
molecular motors.
We report herein the efficient, multi-gram scale synthesis of
compounds 3 and 4, along with the isomer separation,
structural characterization of their different stereoisomers. The
solution behavior and chiroptical properties of the latter have
been examined and analyzed with the help of quantum-
chemical calculations, as well as their conversion into the
aromatic Flu[n]helicenes.
obtained under the same conditions from 9, albeit in a lower
yield of 35%, which can be ascribed to the high steric
congestion around the generated, internal C=C double bond.
1
Analogously to compound 3, the analysis of the H NMR
spectrum of 4 recorded at room temperature led to the
conclusion that 4 might be composed of a diastereoisomeric
mixture (ratio of 1.77/1), whose one diastereoisomer exhibits a
fluxional process in view of the broad signals observed for one
[20]
set of the signals (vide infra).
Noteworthy, both reactions
could be carried out on a multi-gram scale, which is highly
beneficial to the synthetic utility of the strategy.
Results and Discussion
The last aromatization step was performed directly on the
diastereoisomeric mixtures of the partially saturated 3 and 4
without further separation of the latter and consists in an
oxidative dehydrogenation using tritylium tetrafluoroborate as
hydride scavenger. The aromatic helicenic fluorenes 1 and 2
were smoothly isolated in 77% and 76% yields, respectively.
While the dibenzo[c,g]fluorene 1 is not configurationally stable
and is thus achiral, the [7]-helicene-like fluorene 2 was obtained
as a racemic mixture and the M and P enantiomers of 2 were
obtained with ee values of more than 99.5% and 98.5%
respectively by preparative HPLC separation on a chiral sta-
Synthesis and characterization of the partially saturated
fluoreno[n]helicenes 3and 4and of the aromatic fluoreno[n]
helicenes 1and 2 (n=5, 7)
By analogy to the synthesis of the acyclic chiral helical alkene
(3R,3’R)-(P,P)-trans-5, the key final cyclization step leading to 3
and 4 was devised through a McMurry coupling reaction
between the two carbonyl moieties of the corresponding
symmetrical 1,5-diketones 8 and 9 respectively. By optimizing
the direct, base-assisted condensation of formaldehyde with 2
equivalents of α-tetralone 6 or tetrahydrophenanthren-4-one
[19,20]
[15a]
7
,
the synthesis of the 1,5-diketones 8 and 9 was found to
tionary phase, following the conditions reported by Nozaki.
proceed efficiently in good yields and on a multi-gram scale (up
to 18 g) (Scheme 1). Compounds 8 and 9 are each composed of
an equimolar mixture of the racemic (R,R)/(S,S) couple and of
the (R,S) meso compound.
Isomer separation of the stereoisomers of 3and 4, their
structural characterization and their solution behavior
Then, submitting the 1,5-diketone 8 to the McMurry
conditions cleanly furnished the corresponding partially satu-
rated helicene-like compound 3 in an excellent 96% isolated
yield after column chromatography (Scheme 1). While its
formulation was firmly inferred from HRMS and elemental
The H -Flu[n]helicenes 3 and 4 possess two stereogenic carbon
6
atoms at the bridgeheads between the cyclopentenyl and the
neighboring cyclohexenyl rings, and – a priori - a helical
chirality, which led to consider the possible generation of the
three pairs of enantiomers depicted in Scheme 2. The two pairs
1
analysis, the H NMR spectrum of 3 after chromatography
indicated the presence of 2 diastereoisomers at room temper-
ature in a ratio 2.3/1, which was further confirmed by HPLC
of C -symmetric enantiomers (R,R,P)/(S,S,M) and (R,R,M)/(S,S,P)
share the same central chirality and only differ by their helicity,
2
(vide infra). Gratifyingly, the [7]-helicene-like alkene 4 was also
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The DFT-optimized structures for the different stereoisomers
and the calculations of their relative energies revealed that the
À 1
enantiomers (R,R,P)-3 and (S,S,M)-3 are just 0.4 kcal.mol higher
in energy than the achiral meso compound (R,S)-3 (Table 1).
À 1
However, the energy level of 7.8 kcal.mol (relative to (R,S)-3)
of the diastereomeric compounds (R,R,M)-3 and (S,S,P)-3 is too
high to allow the existence to these compounds, and confirms
that only the pair of enantiomers (R,R,P)-3 and (S,S,M)-3 and the
achiral, “meso” compound (R,S)-3 are observed experimentally.
This analysis also revealed that, despite having only 5 ortho-
fused cycles, (R,R,P)-3 and (S,S,M)-3, are configurationally stable
and that the helical chirality is dictated by the central chirality
of the two carbon atoms. Moreover, a conformational search on
the diastereoisomers yielded two conformers for each com-
pound, which differ in the ring puckering of the cyclohexenyl
rings adjacent to the central cyclopentenyl ring (see Supporting
Information for details).
Similar chiral stationary phase HPLC analysis and resolution
were performed on compound 4 and showed that the
diastereoisomeric mixture of 4 is also composed of a pair of
enantiomers as first and third eluted compounds and of an
ECD-non-responsive compound as the second eluted com-
pound. Concerning the pair of enantiomers, their NMR spectra
Scheme 2. General depiction of the possible stereoisomers for compounds 3
and 4 and the respective relationships between them.
and thus should be energetically different. On another hand,
are consistent with a C -symmetry and present in particular a
2
each enantiomer of the (R,S,P)/(S,R,M) couple is C -symmetric.
typical triplet signal at δ=2.17 ppm (J=6.9 Hz) corresponding
1
The HPLC analysis of the crude stereoisomeric mixture of 3
on a chiral stationary phase showed three peaks and the chiral
resolution of 3 by HPLC on a preparative scale enabled the
isolation of the corresponding three diastereoisomers of 3.
While the second eluted compound is achiral, the first and third
eluted compounds correspond to a pair of enantiomers, as
confirmed by their mirror-image spectra of electronic circular
dichroism (ECD) (see Figure 6a), and were isolated in remarkable
to the apical CH protons of the cyclopentenyl ring. Moreover, a
2
first proof of the helical structure was found in the significantly
high field-shifted resonance at δ=6.52 ppm (ddd) of two aryl
protons compared to the other aromatic protons, as they
experience the aromatic ring current anisotropy generated by
the close proximity of the naphthalene moiety in the opposite
part of the molecule. To unequivocally prove the molecular
geometry and the complete configuration of this pair of
enantiomers, single crystals of the first eluted enantiomer
suitable for an X-Ray diffraction analysis were grown (Fig-
>
99.5% optical purities. Noteworthy, the ratio between the
isolated diastereoisomers of 2.4/1 is consistent with the ratio
1
[25]
measured by H NMR on the mixture of 3 before the chiral
ure 2). The crystal was found to be orthorhombic with the
1
resolution (2.3/1). The H NMR spectrum of the achiral
space group P2 2 2 and it corresponds to (R,R,P)-4 stereo-
1
1 1
diastereoisomer features a diastereotopic pattern for the apical
isomer. As it is apparent from Figure 2, the naphthalene moiety
and the proton at the stereogenic carbon atom of the same
one-half of the molecule are orientated on the same side of the
plane defined by C14, C1, C29 and C16. The central double
CH protons of the cyclopentenyl ring, with one resonating at
2
δ=2.35 ppm (dt, J=11.8, 6.1 Hz) and the other one at δ=
1
.22 ppm (pseudo-q, J=11.1 Hz). It is also consistent with a Cs-
symmetry of the molecule. Indeed, considering that (R,S,P)/
S,R,M)-3 is a chiral C -symmetric molecule, of overall apparent
(
1
[a]
Table 1. Computed relative energies of stereoisomers of 3 and 4.
CS symmetry in NMR due to unstable helical configuration, the
achiral diastereoisomer could be firmly assigned to the
enantiomers (R,S,P)/(S,R,M)-3, which are in fast equilibrium at
room temperature. Moreover, decreasing the temperature up to
À 80°C did not allow to freeze this interconversion process and
À 1 [b]
Compound
R,R,P)-3/(S,S,M)-3
Erel (kcal.mol )
(
0.4 (conformer 1)
.2 (conformer 2)
2
(R,R,M)-3/(S,S,P)-3
7.8
(
R,S,P)-3/(S,R,M)-3
0.0 (conformer 1)
the achiral compound was then best described as the meso
2
.6 (conformer 2)
1
compound (R,S)-3. As expected, the H NMR spectrum of the
(
R,R,P)-4/(S,S,M)-4
0.0 (conformer 1)
0.0 (conformer 2)
13.2
2.4 (conformer 1)
3.7 (conformer 2)
16.2 (conformer 3)
isolated pair of enantiomers confirmed their C -symmetry, with
2
(
(
R,R,M)-4/(S,S,P)-4
R,S,P)-4/(S,R,M)-4
a triplet at δ=1.87 ppm (J=7.1 Hz) corresponding to the two
equivalent, apical CH2 protons. At that point, the helical
configuration could not be determined yet and we decided to
[
21]
perform DFT calculations using Gaussian 16,
functional,
the B3LYP
[
a] The different conformers are depicted in the Supporting Information.
[22]
[23]
the def2-SV(P) basis set for atoms,
polarizable continuum model (PCM) for dichloromethane.
and the
[b] The energies are given relative to the more stable stereoisomer and
conformer.
[24]
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The VT NMR behavior of the (R,S,P)-4 and (S,R,M)-4 couple is
illustrated in Figure 3 on the more shielded aromatic protons Ha
1
and H . In the H NMR spectrum recorded at 25°C on a
b
6
00 MHz spectrometer, only one broad signal at δ=6.39 ppm is
observed for the two protons, indicating a fast exchange
process at that temperature. At 0°C, this resonance coalesces,
and two signals at δ=6.16 ppm and 6.58 ppm are detected at –
¼
6
8
0°C. The estimated ΔG of the exchange process associated
À 1
with this coalescence is ca. 52 kJ.mol .
The formulation of this compound as the racemic mixture
between (R,S,P)-4 and (S,R,M)-4 was then firmly confirmed by an
XRD analysis of single crystals grown at room temperature. It
consists of the racemate and crystallizes in the centrosymmetric
space group P21/c. The two enantiomers found in the unit cell
are depicted in Figure 4. Unfortunately, the poor quality of the
crystal prevented any discussion on the metrics and only
proved molecular architecture.
Figure 2. Molecular structure of (R,R,P)-4 determined by XRD analysis
(
1
e. 3l l 5i p2 s( o9 i) d Ås .drawn at 30% probability level). Selected bond length: C1À C29
bond C1À C29 [1.352(9) Å] is only slightly elongated compared
to the literature values of 1.323 Å for cyclopentene
DFT calculations (see Computational Details) confirmed the
experimental results and showed that the (R,R,M)-4/(S,S,P)-4 pair
[26]
derivatives, and is highly similar to the other over-crowded
À 1
C=C bonds reported in related Feringa’s molecular motors
is 13.2 kcal.mol higher in energy than (R,R,P)-4/(S,S,M)-4, which
[17b,27]
2
[1.345–1.350 Å].
The sp -hydridization of C1 and C29
explains why only the latter pair of enantiomers was isolated. It
is worth noting that this is the same trend as in the case of the
5-ring-containing compound 3, where the energy difference is
carbon atoms is evidenced by the sum of the bond angles
around them very close to 360° [ΣC1=360° and ΣC29=359.2°].
À 1
The central, over-crowded C1=C29 bond is only slightly twisted
7.8 kcal.mol . The energy difference is higher in 4 due to the
[
1
dihedral angles: C14À C1À C29À C28=168.65°, C2À C1À C2C16=
presence of the two additional terminal aryl rings. In a
conformational search, two iso-energetic conformers for (R,R,P)-
79.45°, C14-C1-C29À C16=0.06°, C2À C1À C29À C28=À 11.98°],
which is quite similar to the case of the acyclic analogue
(
3R,3’R)-(P,P)-trans-5. Eventually, the helicity (dihedral angle
between the terminal rings) of (R,R,P)-4 is 56.33°, which is quite
[
28]
similar to the values recorded for [6]-carbohelicene (58.5°),
and – more importantly –higher than the value of 42.17°
[15a]
measured for the aromatized Flu[7]helicene 2.
The ECD-non responsive second eluted compound exhibits
1
a symmetrical H NMR spectrum (recorded at 300 MHz) at 25°C
but the presence of some broad signals suggested a fluxional
process. A variable-temperature NMR experiment was thus
carried out. Upon lowering the temperature down to À 80°C, all
signals became duplicated (Figure S1.13), indicating that the
dynamic process was frozen at that temperature and that the
molecule is C -symmetric, as all protons are unequivalent. This
1
led us to assign this compound as the pair of enantiomers
(
R,S,P)-4 and (S,R,M)-4, in which the helical chirality is not
1
configurationally stable at room temperature with a rapid
interconversion of the two helixes at that temperature
Figure 3. Variable temperature H NMR spectra of (R,S,P)/(S,R,M)-4 in the
2 2
region 6.75–6.05 ppm (CD Cl , 600 MHz).
(Scheme 3).
Scheme 3. Interconversion equilibrium between the (R,S,P)-4 and (S,R,M)-4
enantiomers.
Figure 4. Molecular structure of the racemate (R,S,P)-4 and (S,R,M)-4
determined by XRD analysis (ellipsoids drawn at 30% level of probability).
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4
/(S,S,M)-4 were identified, which only differ in the ring
less defined due to the presence of many peaks of moderate
intensity.
The calculated VCD spectra for enantiomers (R,R,P) and
puckering of the cyclohexenyl rings. The average energy (R,S,P)-
4
À 1
/(S,R,M)-4 was calculated at 2.4 kcal.mol above the most
stable (R,R,P)-4/(S,S,M)-4 stereoisomers and three conformers
were identified (Figure S2.2 and S2.3). While the most stable
conformer of (R,S,P)-4/(S,R,M)-4 corresponds to the structure
determined by crystallographic analysis, a second possible
(S,S,M) of 3 and 4 are displayed in Figures 5c and 5f. They are
Boltzmann-averaged for the two conformers found for (R,R,P)
and (S,S,M) of both compounds. Calculations well reproduce the
experiments. Notably, for both 3 and 4, five dominant peaks are
numbered in the VCD spectra, corresponding to the vibrational
normal modes shown in Figures S2.8 and S2.10, respectively.
For enantiomers (R,R,P)-3 and (S,S,M)-3, the type of vibrations
observed from low to high wavenumber are found as follows:
À 1
conformer is only 1.3 kcal.mol higher in energy. The high
À 1
energy of a third conformer at 13.8 kcal.mol excludes its
presence in the experiments.
À 1
bending at 1287 cm (peak 1), combination of bending and
À 1
À 1
VCD spectroscopy
stretching at 1393 cm (peak 2), stretching at 1510 cm (peak
3
À 1
) and bending at 1524 and 1660 cm (peaks 4 and 5,
To get more insights into conformational and stereochemical
features, these helical crowded alkenes were studied by infrared
respectively) (see Figures 5c and S2.7 and S2.8). For enantiomers
(R,R,P)-4 and (S,S,M)-4, the assignment corresponds to bending
[29]
À 1
À 1
and vibrational circular dichroism (VCD) spectroscopy. Indeed,
helical organic molecules are known to display clear strong VCD
signals originating from stretching and bending modes that are
at 1252 cm (peak 1), CÀ C stretching at 1341 cm (peak 2),
À 1
combination of bending and stretching at 1392 cm (peak 3)
and bending at 1431 and 1500 cm
À 1
(peaks 4 and 5,
[30]
generally coupled and that are diagnostic of their absolute
stereochemistry obtained by comparison with theory. The
experimental IR and VCD spectra of enantiopure compounds 2,
respectively) (see Figures 5f and S2.9 and S2.10). Finally,
comparison between experimental and theoretical VCD enabled
to assign the absolute configurations of 3 and 4 (i.e. (R,R,P)-(+)
and (S,S,M)-(À ), see the Supporting Information part for the
specific rotation values).
3
and 4 were measured in CD Cl at concentrations around
2 2
À 1
0
.15 M, in the 1000–2000 cm region. As depicted on Figur-
es 5a,b and d,e both enantiomers of 3 and 4 display similar IR
and mirror-image VCD signatures, with several clear active
bands that have been analyzed by theory. Compared to (R,R,P)/
Analogously to 3 and 4, (M) and (P) enantiomers of helicenic
fluorene 2 display similar experimental IR and mirror-image
VCD spectra (see Figures S2.4 and 2.6). Seven dominant peaks
are numbered in the VCD spectra of fluorene 2 (Figures S2.4
(
S,S,M)-3, the VCD spectra of (R,R,P)/(S,S,M)-4 appear richer and
Figure 5. a) Experimental IR and b) VCD spectra of enantiopure (R,R,P)-3 and (S,S,M)-3. c) Calculated VCD spectra for (R,R,P)-3 and (S,S,M)-3. The numbers
indicate the corresponding peak associated to the vibrational normal modes visualized in Figure S2.8. d) Experimental IR and e) VCD spectra of enantiopure
(R,R,P)-4 and (S,S,M)-4. f) Calculated VCD spectra for (R,R,P)-4 and (S,S,M)-4. The calculated VCD spectra correspond to the Boltzmann average of the two
conformers of (R,R,P)-3 and (R,R,P)-4. The numbers indicate the corresponding peak associated to the vibrational normal modes visualized in Figure S2.10.
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and S2.5). Calculation shows bending vibrations in all the
and (S,S,M)-3 are shown in Figure S2.14. The experimental and
the Boltzmann-averaged computational spectra are in good
agreement. Three distinct spectral bands can be identified,
which are assigned from low to high energy as resulting
predominantly from the following occupied-to-unoccupied
orbital transitions: HOMO to LUMO, HOMO-1 to LUMO, and
HOMO-1 to LUMO+2 (see selected molecular orbitals in
Figures 6e and S2.15). The molecular orbitals (MOs) of (R,R,P)-3
associated with the most intense excitations (Table S2.2) are
provided in Figure S2.15. For (R,R,P)-3, the HOMO and the
LUMO are localized on the π-conjugated system and the two
fragments are involved, while the HOMO-1 is mainly localized
on one of the π-conjugated six-membered rings. We note that
the HOMO-1 and HOMO-2 levels are quasi-degenerate, with a
very small energy difference of 0.067 eV, essentially correspond-
ing to in-phase and out-of-phase (+/À ) linear combinations of
orbitals on either one of the moieties. The broadened calculated
ECD spectra are in very good agreement with the experiments
(Figures 6b and 6d and S2.16). The calculated ECD peaks are
systematically blue shifted by 0.1–0.3 eV, which is not unusual
due to the combination of approximations made in the TD-DFT
calculations. In general, the transitions with large oscillator
strength f also have large rotatory strength R, and the assign-
ments of the ECD bands are of course in line with the
absorption peaks at wavelengths where both types of spectra
have peaks. Additionally, two π!π* electronic transitions
appear active in the ECD spectra, from excitation #7 (HOMO-3
to LUMO) and excitation #23 (HOMO-3 to LUMO+3). The latter
excitation shows the largest R value (415), leading to the
strongest band of the ECD spectrum. Finally, comparison
numbered peaks with the only exception of peak 3 at
À 1
1
342 cm , which is a combination of bending and stretching
vibrations. Here again, the good agreement between calcula-
tion and experiment enabled to assign the (P)-(+) and (M)-(À )
absolute configurations of 2.
UV-Vis and ECD spectroscopy
Helical molecules display characteristic experimental absorption
and electronic circular dichroism (ECD) spectra that can also be
[1]
used to describe their inherent stereochemistry. UV-vis and
ECD spectra of compounds 2, 3 and 4 were therefore measured
À 5
(
in CH Cl at concentrations around 5x10 M) and compared to
2
2
the calculated ones. The UV-vis spectra are reported in
Figures S2.14 and 2.17 while the ECDs are depicted in Figure 6a
and c for 3 and 4, respectively, and in Figure S2.13 for 2. For
instance, the ECD spectrum of (+)-3 displays a strong positive
À 1
À 1
band at 185 nm (Δɛ= +77 M cm ), a strong negative band
at 200 nm (-69) followed by a band of moderate intensity at
2
27 nm (À 26) and two strong positive ones at 241 (+39) and
2
45 (+45) nm, and finally a broad negative band of weak
intensity at 290 nm (À 7). Calculated spectra determined from
time-dependent DFT (TD-DFT) linear response calculations,
[31]
were performed with the PBE0
functional, the def2-SV(P)
basis and solvent effects for dichloromethane. Regarding the
general performance of TD-DFT for chiroptical spectra, see, for
example, references [32a] and [32b]. The calculated UV/Vis
spectra for the two conformers found for enantiomers (R,R,P)-3
Figure 6. Experimental ECD spectra of a) (R,R,P)-3/(S,S,M)-3 and c) (R,R,P)-4/(S,S,M)-4. Calculated ECD spectra of b) (R,R,P)-3/(S,S,M)-3 and d) (R,R,P)-4/(S,S,M)-4. e)
Selected isosurfaces (�0.03 au) of MOs for 3 and 4.
Chem. Eur. J. 2021, 27, 1–10
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between experiment and theory enabled to confirm the (R,R,P)-
(
+) and (S,S,M)-(À ) absolute configurations for 3.
Two dominant spectral regions with distinct intensities can
be identified for the absorption spectra of (R,R,P)-4 and (S,S,M)-
, which are assigned from low to high energy as resulting
4
predominantly from transitions among the following pairs of
occupied and unoccupied MOs: HOMO to LUMO for the low-
intensity low-energy region and HOMO-1 to LUMO+2 and
LUMO+3 for the high-intensity high-energy region (see
Figures S2.17 and S2.18, and Table S2.3). Like in the (R,R,P)-3
system, the highest occupied and lowest unoccupied MOs are
localized over the π-conjugated system. We note that the
LUMO+2 and LUMO+3 with a small energetic splitting are
mainly localized on one moiety, with some contributions from
the other moiety, in an in-phase/out-of-phase sign relationship.
The ECD spectra of (R,R,P)-4 and (S,S,M)-4 are in line with the
absorption spectra, but also showing additional features such
as a weak negative band from the HOMO-1 to LUMO+1
transition (exc. #4 at 292 nm) and an intense positive band for
HOMO-4 and HOMO-2 to LUMO transitions (exc. #7 at 264 nm).
Like in the UV/Vis spectra, the strongest band in the ECD
spectra also appear at 230 nm, caused by excitations #14 and
Scheme 4. Synthesis of enantioenriched Flu[7]helicene 2 form enantiopure
partially hydrogenated helicenes 4.
and in 42% ee. These results led to the conclusion that, while
the reaction is stereospecific, it is not 100% stereoselective as
an erosion of the enantiopurity was observed.
2
2
#
15 with rotatory strengths of À 385 and À 305 esu cm ,
respectively. Similarly to 3, comparison between experiment
and theory enabled to confirm the (R,R,P)-(+) and (S,S,M)-(À )
absolute configurations for 4.
An obvious reason for this loss of enantiopurity would be a
configurational instability of product 2 under the reaction
conditions. As no data was available on this process, we
decided to experimentally quantify the racemization for com-
pound 2. Thus, the kinetic of enantiomerization of (P)-2 in
chlorobenzene at 132°C gave access to the experimental
For helicenic fluorene 2, the calculated UV/Vis spectrum
reveals the appearance of four absorption regions with distinct
intensities (Figure S2.11). The assignment of the absorption
bands from low to high energy is as follows; HOMO to LUMO,
HOMO to LUMO+2, HOMO-2 to LUMO+3 and HOMO-2 to
LUMO+4. The HOMO and LUMO of (P)-2 are mostly localized
on the π-conjugated system of the hexagonal rings close to the
pentagon (Figure S2.12). The calculated broadened ECD spec-
trum of (P)-2 reproduces the experimental spectrum very well
À 1
inversion barrier value of 142.3 kJ.mol . While being a little bit
smaller than the enantiomerization barrier for [6]-carbohelicene
À 1 [33]
(151 kJ.mol ), this barrier showed that fluoreno[7]helicene 2
is configurationally stable at 80°C [t >2 years]. The loss of
1
/2
enantiopurity might be thus ascribed to a configurational
instability of some intermediate generated during the reaction.
A reasonable possibility would be the formation of a 1,3-
cyclopentadiene as the central 5-membered ring by abstraction
of a hydride at one of the stereogenic allylic positions by
tritylium cation followed by loss of a proton on the apical
methylene group. A suprafacial [1,5]-hydrogen shift would then
readily occur on this intermediate leading to a scrambling of
the remaining stereogenic center within the topical ring and to
(Figure S2.13). Excitation #4 at 317 nm has a particularly large
rotatory strength (R=648) and corresponds to a combination of
two helicene π!π* transitions from HOMO-2 to LUMO and
HOMO to LUMO+1. Comparison between experiment and
theory enabled to confirm the (P)-(+) and (M)-(À ) absolute
configurations for 2.
[34]
Stereoselectivity of the oxidative aromatization of 4to the
a loss of the chiral information.
fluoreno[7]helicene 2
In the first part of this work, the fluoreno[7]helicene 2 was Conclusion
obtained as a racemic mixture from the mixture of stereo-
isomers of 4. Having in hand the enantiopure (R,R,P)-4 and its
enantiomer (S,S,M)-4, we were prompted to study the stereo-
selectivity of the oxidative aromatization reaction under the
standard reaction conditions, namely reaction with tritylium
tetrafluoroborate in DCE at 80°C for 3 hours. Starting from the
In this paper we have reported the synthesis of the new,
partially hydrogenated fluoreno[5]-and [7]helicenes H -Flu[n]
6
helicenes by a straightforward McMurry coupling as key
cyclization step. The interplay between the central and helical
chirality types has been established within these cyclic com-
pounds. For each compound, the successful isomer separation
and isolation of the two configurationally stable enantiomers
(R,R,P) and (S,S,M), supplemented by quantum-chemical calcu-
lations, indicate that the helicity is induced by the configuration
enantiopure (R,R,P)-4 (99.5% ee) led to the isolation of (P)-2 in
8
1% yield but with a diminished ee of 46% (Scheme 4). A
similar drop of enantiopurity was observed with the opposite
enantiomer (S,S,M)-4 (99.5% ee), as it furnished (M)-2 in 71%
Chem. Eur. J. 2021, 27, 1–10
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Chemistry—A European Journal
of the two pre-formed stereogenic carbon atoms. Moreover,
due to the presence of two rather-flexible cyclohexenyl rings,
the helical configuration in the (R,S,P)/(S,R,M) couple is not
stable at room temperature leading to the perception of this
couple as the achiral, meso (R,S) compounds at that temper-
ature. As typical helical compounds, the chiral (R,R,P) and (S,S,M)
Juríček, ChemPhotoChem 2019, 3, 180–186; c) H. Isla, J. Crassous, C. R.
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H -Flu[n]helicenes display clear, strong responses in VCD and
6
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Finally, the H -Flu[n]helicenes constitute valuable intermedi-
6
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(
CNRS) and the University of Rennes. This work was supported
by the Agence Nationale de la Recherche (ANR-16-CE07-0019
Hel-NHC” grant). L.A. and J.A. acknowledge the Center for
Computational Research (CCR) at the University at Buffalo for
“
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Manuscript received: January 14, 2021
Accepted manuscript online: March 29, 2021
Version of record online: ■■■, ■■■■
Chem. Eur. J. 2021, 27, 1–10
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FULL PAPER
Dr. L. Pallova, Dr. E. S. Gauthier, Dr. L.
Abella, M. Jean, Dr. N. Vanthuyne, Dr. V.
Dorcet, Dr. L. Vendier, Prof. J. Autsch-
bach*, Dr. J. Crassous*, Dr. S. Bastin*,
Dr. V. César*
1
– 10
Blocking a molecular motor: The
being composed of three diaster-
eoisomers, they serve as valuable in-
termediates for the efficient, multi-
gram scale access to the aromatic
fluoreno[n]helicenes.
Synthesis and Properties of Partially
Saturated Fluorenyl-Derived [n]
Helicenes Featuring an Overcrowded
Alkene
partially saturated H -fluoreno[n]
6
helicenes (see figure), cyclic analogs
of the well-known Feringa’s molecular
motors, are produced via a similar
McMurry cyclization approach. While