Selective Synthesis of Perfluoroalkylated Corannulenes and Investigation of their Structural, Dynamic and Electrochemical Behavior

Article abstract of DOI:10.1002/chem.201801021

Herein we report general methods allowing the synthesis of various perfluoroalkylated corannulenes with a specific substitution pattern. Variable temperature NMR spectroscopic investigations revealed dynamic behavior which was analyzed by line shape analysis. The activation parameters of these dynamic processes were determined. For a tetrasubstituted compound it was possible to observe through space scalar coupling. The packing motifs were elucidated by X-ray crystallography, showing that the substitution pattern as well as the size of substituents strongly influence intermolecular π-stacking. The reduction potentials of the perfluoroalkylated compounds were determined by cyclic voltammetry.

Full text of DOI:10.1002/chem.201801021

A Journal of
Accepted Article
Title: Selective Synthesis of Perfluoroalkylated Corannulenes and
Investigation of their Structural, Dynamic and Electrochemical
Behavior
Authors: Axel Haupt, Lisa-Marie Keller, Maximilian Kutter, and Dieter
Lentz
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To be cited as: Chem. Eur. J. 10.1002/chem.201801021
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Selective Synthesis of Perfluoroalkylated Corannulenes and
Investigation of their Structural, Dynamic and Electrochemical
Behavior
Axel Haupt,[a] Lisa-Marie Keller[a], Maximilian Kutter[a] and Dieter Lentz*[a]
Abstract: Herein we report general methods allowing the synthesis
of various perfluoroalkylated corannulenes with
a
specific
substitution pattern. Variable temperature NMR spectroscopic
investigations revealed dynamic behavior which was analyzed by
line shape analysis. The activation parameters of these dynamic
processes were determined. For a tetrasubstituted compound it was
possible to observe through space scalar coupling. The packing
motifs were elucidated by X-ray crystallography, showing that the
substitution pattern as well as the size of substituents strongly
influence intermolecular -stacking. The reduction potentials of the
perfluoroalkylated compounds were determined by cyclic
voltammetry.
Introduction
The introduction of fluorine or perfluoroalkyl groups into a
molecule strongly influences its properties.[1-3] This fact has led
to numerous applications of fluorinated molecules and materials
during the last decades. As demonstrated by us and others
before, the electron affinity of the bowl shaped polycyclic
aromatic hydrocarbon corannulene increases with the number of
perfluoroalkyl groups introduced.[4] By now there have been
several synthetic approaches towards the selective preparation
of this type of compound. However, only 1,2-bis(trifluoromethyl)
corannulene can be prepared on a larger scale using hexa-
fluorobutyne as building block.[4c-e] For example, the C5-
Scheme 1. Previously reported synthesis of 1,3,5,7,9-pentakis-
(trifluoromethyl) corannulene
pinacolboronic ester.[4a, 4b, 5]
1 by gas phase reaction and coupling of
perfluoroalkyl groups.[8] However, there exist only very few
examples for the introduction of two of these substituents,
especially in neighboring positions. Copper coupling of
perfluorinated cycloalkenes yields substituted benzenes or
cyclooctatetraenes.[9] Another approach is the coupling of aryl
bromides with copper bronze and perfluoroalkyl iodides which is
rather limited to the long chained, liquid RFI.[10]
Furthermore, the supramolecular chemistry and the crystal
packing of corannulene can be changed by introducing
trifluoromethyl substituents.[4] In its solid-state X-ray structure,
the orientation of unsubstituted corannulene molecules is
dominated mainly by -interactions resulting in intermolecular
edge to bowl contacts.[11] This significantly changes, when CF3
groups are introduced. Here one can observe solid state
structures that are dominated by -interactions resulting in
one dimensional, columnar stacking of the corannulene
moieties.[4]
Scott could already show more than 25 years ago by NMR
experiments, that corannulene is not a rigid molecule, but can
invert its bowl shape quite readily even at low temperatures.[12]
Some correlation of substituents attached to corannulene and
the observed inversion barrier of the bowl was published some
years later by Siegel.[13] Further, Mislow and coworkers could
show, that various alkylbenzenes, e. g. hexakis(isopropyl)
benzene, can be analyzed concerning their conformational
behavior.[14] The rotational barriers of neighboring alkyl groups
are high enough to make dynamic processes visible by variable
symmetric 1,3,5,7,9-pentakis-(trifluoromethyl) corannulene
1
was first prepared by the group of Boltalina using an unselective
gas phase reaction on mg scale, that yields complicated product
mixtures and hence requires subsequent purification by HPLC
(Scheme 1).[4a, 4b] Recently, Siegel and coworkers published an
optimized synthetic protocol for this compound starting from the
respective pinacolboronic ester (Scheme 1).[5] Despite its
selectivity, this method still requires some extra steps to prepare
the starting material using precious metal catalysts.[6] For this
reason, one could think about synthetic protocols using readily
available starting materials, e. g. aryl halides. The kilogram scale
synthesis of corannulene published by Siegel some years ago
affords such a halogenated compound as synthetic intermediate,
which was already described earlier by the group of Sygula.[7]
Countless different methods are described in literature
concerning the monosubstitution of aromatic moieties with
[a]
A. Haupt, M. Kutter, Prof. Dr. D. Lentz
Freie Universität Berlin, Fachbereich Biologie, Chemie, Pharmazie
Institut für Chemie und Biochemie
Fabeckstraße 34-36, D-14195 Berlin
E-mail: dieter.lentz@fu-berlin.de
Supporting information for this article is given via a link at the end of
the document.
temperature NMR spectroscopy. This also allows
a
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quantification of the rotational barriers.[14a] Finally, the presence
of energetically favored conformers could be proven by X-ray
crystallography.[14c] Such investigations have not been described
for perfluoroalkylated compounds by now. Despite its first
synthesis more than 50 years ago, hexakis-(pentafluoroethyl)
benzene has never been characterized by variable temperature
NMR or X-ray crystallography.[15] Other examples contain the
Scheme 2. Synthesis of monoperfluoroalkylated corannulenes.
crystallographic
analysis
of
ortho-perfluoroalkylated
polyaromatics.[10] However, to the best of our knowledge, there
were no investigations concerning the conformational analysis of
such type of compound based on variable temperature NMR.
Results and Discussion
Synthesis of mono perfluoroalkylated corannulenes.
The substitution of monoiodocorannulene by a perfluorooctyl
group described by the group of Siegel is a low yield reaction.[5]
Thus we looked for an alternative synthesis which introduces the
perfluoroalkyl group by a Diels-Alder cycloaddition reaction
during the construction of the respective fluoranthene derivative
5a-d (Scheme 2). The cyclopentadienone precursor 3 was
reacted with mono perfluoroalkylated acetylenes 4a-d. In case of
trifluoromethyl derivative 5d this was already described earlier in
Scheme 3. Preparation of double perfluoroalkylated corannulenes.
starting materials for the synthesis of tetrasubstituted
corannulenes with a 1,2,4,6-substitution pattern (see Scheme 5).
The derivatives 7a-c could be obtained in acceptable yields, but
the necessity to perform several reaction steps for each
compound was not too attractive for further investigations.
literature.[16] The dienophiles 4a-c could be prepared by a
slightly optimized literature procedure.[17,
The subsequent
18]
cyclization and bromination steps follow in general the
procedure that is also used for the synthesis of unsubstituted
corannulene. This protocol has the advantage that the
intermediate tribromocorannulene derivates 6 can be used as
Synthesis of double perfluoroalkylated corannulenes.
To introduce two trifluoromethyl groups in 1,2-position of
corannulene hexafluorobutyne can be used as dienophile.[4c-e]
The limited access to other bis(perfluoroalkyl) acetylenes
inspired us to use diiodoacetylene
8 as dienophile and
subsequently replace the iodo substituents of diiodofluoranthene
9.[19] This could be achieved using a method adapted from the
general perfluoroalkylation of aryl iodides by Mikami and
coworkers (Scheme 3).[20] Due to the steric congestion of the
disubstituted compounds 10a-b (see also Fig. 1) and to
suppress the amount of monosubstituted side products 5a and
5e an excess of zinc reagent and copper iodide is needed.[21]
Using these conditions the perfluoroalkylated fluoranthene
derivatives can be obtained in acceptable yields. The mono- and
disubstituted compounds can be separated by column
chromatography on silica. The yields of the conversion of the
fluoranthene derivatives 10a-b into the respective corannulenes
11a-b are rather low but no attempts have been made to
optimize the procedures for the cyclization and debromination
reactions.
Synthesis of fourfold perfluoroalkylated corannulenes.
1,2,5,6-Tetrabromocorannulene 2 is a synthetic intermediate in
Siegel’s large scale synthesis of corannulene.[7a] Thus, one can
obtain multigram amounts of this compound in hand, having an
ideal starting material for further functionalization. Usually,
perfluoroalkylation reactions start from aryl iodides or even more
activated arene compounds. The use of aryl bromides is only
very scarcely described.[8, 22] This fact can be a major obstacle
when a fourfold substitution is desired.
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Although aryliodides were mainly used as starting materials in
Mikami’s perfluoroalkylation method we were able to substitute
compound
Tetrakis(pentafluoroethyl) corannulene 12 along with the
trisubstituted side products 1,2,6-tris(pentafluoroethyl)
2
by
pentafluoroethyl
groups.
1,2,5,6-
corannulene 13a and 1,2,5-tris(pentafluoroethyl) corannulene
13b can be obtained in acceptable yield (Scheme 4) in a
microwave reactor. In
a typical experiment, we treated
tetrabromocorannulene 2 with varying amounts of copper iodide
and Zn(C2F5)2dmpu2. The ratio of desired fourfold substituted
product to the trisubstituted side products could be influenced by
varying the amount of copper iodide. Whereas the reduction
from a molar 2:1 ratio of copper iodide to zinc reagent towards a
1:1 ratio significantly raised the yield of 12, a further decrease of
copper salt did not improve the outcome of the reaction.
Investigation of different polar solvents showed, that dmpu is the
best solvent for this type of reaction. Further we clearly could
see, that the reactions run significantly better when performed
under microwave irradiation instead of heating in an oil bath.
The crude product mixture was analyzed by 19F NMR
spectroscopy to determine the product ratio. For full analysis,
the mixture could be separated using reversed phase HPLC on
silica with fluorinated tags.
Against our expectations we were unable to prepare 1,2,5,6-
tetrakis(trifluoromethyl) corannulene 14 analogously by reacting
tetrabromoarene 2 with copper iodide and the respective zinc
reagent (Scheme 4). Surprisingly, we could not even observe
the formation of any minor trifluoromethylated product. However,
when we used Hartwig’s reagent (phen)CuCF3,[23] we could
substitute all the bromo substituents by trifluoromethyl groups in
32% yield (Scheme 4). Compound 14 was purified by HPLC on
unmodified silica using n-hexane as eluent. Due to the small
differences in chemical shift values of the trifluoromethyl groups
one can observe a high order 19F NMR spectrum of product 14
which looks very complicated (Fig. 1, top). Additionally, scalar
coupling (J(FA-FB) = 17 Hz) between the fluorine nuclei of the
CF3 groups and additional splitting due to coupling to adjacent
protons can be observed. By simulating the spectrum with
gNMR, we were able to identify the chemical shifts as well as
the respective coupling constants (Fig. 1, bottom).[24] The
observation of scalar coupling between the different CF3-groups
is only explicable, when through space coupling is considered.
Figure 1. Recorded 19F NMR spectrum of 1,2,5,6-tetrakis-(trifluoromethyl)
corannulene 14 (top) and simulated resonances (mirrored, bottom). The
obtained parameters are the following: (FA) = -50.65 ppm, (FB) = -50.54 ppm,
J(FA-FB) = 17 Hz, J(FA-HA) = 2 Hz, J(FB-HB) = 1 Hz.
This phenomenon had been reported the first time on a protein
with fluorinated tryptophan residues.[25] Further, it was possible
to observe the same effect in smaller fluorinated molecules.[26] It
could be shown previously, that the coupling constants strongly
depend on the distance of the fluorine atoms.[27] If the nuclei are
situated close together, they will interact more intensely, hence
yielding larger coupling constants. The observed value for 14 of
17 Hz is in the lower range of the literature (~10 – 110 Hz). This
is a consequece, as neighboring CF3 groups are close enough
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Scheme 4. Pentafluoroethylation and trifluoromethylation of 1,2,5,6-tetrabromocorannulene.
together to interact with each other, but they are too small to
allow larger scalar coupling.
As mentioned above intermediates 16 and 6d can be used to
prepare
selectively
(mixed)
fourfold
perfluoroalkylated
corannulenes with different substitution patterns. Radical
bromination of the benzylic positions of the known
bis(trifluoromethyl)fluoranthene 15 and cyclization of the
intermediate compound furnishes 16.[4c-e] The two bromo
substituents in 1- and 6-position could be replaced by
perfluoroalkyl groups applying the copper/zinc system again
(Scheme 5). The products 1,6-bis(nonafluorobutyl)-3,4-
bis(trifluoromethyl) corannulene 17a and the pentafluoroethyl
derivative 17b were isolated and fully characterized. Another
derivative with a 1,2,4,6 substitution pattern could be generated
starting from known mono trifluoromethylated fluoranthene 5d.[16]
Upon radical bromination of the methyl groups and subsequent
cyclization one obtains the unsymmetrical, tribrominated
corannulene 6d which then can be perfluoroalkylated.
Surprisingly, the conditions that could be applied for substitution
of compounds 2 and 16 were unsuccessful for the synthesis of
compound 18a. The target product could only be isolated in very
poor yield. This changed, when we trifluoromethylated 6d with
Hartwig’s reagent to get product 18b in satisfying yield. In
conclusion we could demonstrate, that in principle various
substitution patterns and substituents are accessible in a
systematic manner by choosing appropriate synthetic routes and
starting materials.
Scheme 5. Preparation of fourfold perfluoroalkylated corannulenes with
different substitution pattern.
X-ray crystal structure analysis.
The solid-state structures of the fluoranthene derivatives 10a-b
and the corannulenes 11b, 12, 13a and 17b were elucidated by
X-ray diffraction. Analysis revealed very strained structures for
the fluoranthene derivatives 10a and 10b (Figure 2). In the
molecular structure of both compounds the perfluoroalkylated
six-membered ring is twisted by 21° out of the naphthalene
plane. Furthermore, in compounds 10 the C4F9 and C2F5 groups
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point into opposite directions of the aromatic ring due to steric
repulsion. Interestingly, the fluorine atoms of the CF2 and the
CF3 groups of 10b are in almost eclipsed position demonstrating
the extreme congestion. All these structural properties underline
the steric congestion in this type of compound. Looking at the
intermolecular interactions reveals numerous F-F-contacts
between the fluoroalkyl substituents which are about 0.25 Å
shorter than the sum of their van der Waals radii. This results in
a
layered structure of fluorinated substituents and the
nonfluorinated moieties of the fluoranthene molecules.[28] In
addition, some H-F-contacts can be observed. Interestingly,
there are no C-C-contacts for either of the derivatives 10. This
clearly shows, that the solid-state packing is dominated by the
fluorinated substituents and not by the aromatic moieties.
Another phenomenon can be found in the case of derivative 10a
which has a dichloromethane molecule incorporated in its solid-
state structure. This structure shows contacts between fluorine
atoms of the fluoranthene derivative and the chlorine atoms of
the solvent that are about 0.1 to 0.2 Å shorter than their sum of
van der Waals radii. One can consider this as some type of
halogen bonding which is a quite recent topic.[29]
The disubstituted corannulene 11b could be crystallized as two
polymorphs that differ in packing of the bowls (Figure 3). In both
phases, the packing is dominated by -interactions resulting in
one-dimensional columnar stacks along the crystallographic a-
axis. However, the shortest contacts in the structures can be
found between hydrogen and fluorine atoms as well as between
fluorine and fluorine substituents of the fluoroalkyl groups. There
exist a few C-C contacts that are a little shorter than the sum of
their van der Waals radii. Of course, the latter strongly depends
on the choice of the crystallographic or equilibrium radii,
respectively.[30] In the asymmetric unit of the monoclinic structure
one can find four molecules. In the stack two neighboring
corannulenes are rotated by roughly 180° against each other
Figure 3. Solid-state cell packing of 1,2-bis(pentafluoroethyl) corannulene 11b
as monoclinic (top) and triclinic polymorph (bottom). Ellipsoids are at
probability level of 50%.
a
resulting in an ABAB stacking motif. In contrast, the asymmetric
unit of the triclinic phase consists of three molecules. The twist
angles of adjacent molecules in the ABCABC type stacks are
roughly 90° between A and B as well as B and C, whereas the
twist angle from C to A is about 180°. In both of the polymorphs
the distance between two neighboring bowls is about 3.8 Å and
the bowl depth of the molecules is comparable and ranges
around 0.8 Å.
The trisubstituted side product 13a could also be investigated by
X-ray crystallography, revealing cell packing that is still
dominated by --interactions (Fig. 4). The analysis of the data
set was achieved by refining it with disordered dichloromethane
molecules that are incorporated in the crystals. Refinement by
Figure 2. Molecular solid-state structures of perfluoroalkylated fluoranthene
derivatives 10a (left) and 10b (right). Ellipsoids are at a probability level of
50%.
Figure 4. Solid-state cell packing of 1,2,6-tris-(pentafluoroethyl) corannulene
13a. Solvent molecules (dichloromethane) are omitted for clarity. Ellipsoids are
at a probability level of 50%.
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squeezing the solvent out of the structure resulted in too faulty
parameters of the final data set. In the structure, one can clearly
observe one dimensional stacks along the crystallographic a-
axis, which are slightly slipped. The stacking motif again follows
an ABAB type, with a twist of 90° between A and B and 270°
between B and A, respectively. The distance between two
neighbouring bowls is already slightly elongated, compared to
11b, being now very close to 4.0 Å. In contrast, the bowl depth is
decreased to 0.7 Å.
opposite directions of the bowl. In the case of derivative 12, the
molecule even adapts C1 symmetry. This means, that molecular
symmetry is lowered as much as possible in the solid state.
Despite efforts to investigate various crystallization conditions
and solvents, we were not able to crystallize a polymorph of 12
with higher molecular symmetry.
Investigation of dynamic behavior by variable temperature
19F NMR spectroscopy.
Increasing the number of substituents to four, changes the
stacking behavior. In both compounds 12 and 17b the
asymmetric unit is composed of two molecules forming a
stacked pair. However, the pairs are in contact (3.5 Å) on the
concave and convex side of both molecules, respectively, due to
the crystallographic inversion centre (Figure 5). The shortest
contact distance between two carbon atoms of the convex sides
is around 3.1 Å, being about 0.3 Å shorter than the sum of their
van der Waals radii. Interestingly the other molecule in the
asymmetric unit of 12 does not exhibit C-C contacts at all. In
total one can find only 20 contacts from which only five have a
distance that is about 0.1 Å shorter than the sum of their van der
Waals radii. The larger sterical congestion in comparison to
mono- and disubstituted derivatives makes the bowls become
even shallower with a depth of 0.6 to 0.7 Å. In all compounds
with RF groups larger than CF3, the ortho substituents point into
As Scott already could show more than 25 years ago,
corannulene is a fluxional molecule.[12] The process of the bowl-
to-bowl inversion can easily be investigated by measuring
variable temperature NMR spectra of monosubstituted
compounds bearing diastereotopic nuclei. Slowing down the
fluxional behavior of the polyaromatic scaffold will give rise to an
AB-type system instead of a singlet. This also holds for
perfluoroalkylated derivatives 7, out of which 7a and 7c were
investigated by line shape analysis using gNMR. Due to limited
simulation capacity of the software, the spin systems had to be
simplified as only consisting of the relevant CF2-groups directly
attached to the polyaromatic moiety. Further, an automatic line
shape analysis could not be run, so that the rate constants of the
CF2-exchange had to be iterated manually. The obtained rate
constants k were plotted as ln k vs. 1/T getting the activation
parameters H‡ and S‡ from the slope and the intersection with
the y-axis using the linearized Eyring equation.[31] Due to the
error in k, the fitted lines might be tilted, which would especially
result in faulty values for the activation entropy S‡. However,
the graphical analysis of the Eyring plot yields at least a
qualitative estimation for the observed energy barriers. For both
monosubstituted derivatives 7a and 7c comparable results can
be found (Tab. 1). The enthalpy of activation has a value of
about 40 kJ/mol. In contrast, the contribution of the entropy of
activation is only small. This is consistent with the consideration,
that bowl-to-bowl inversion does not afford a highly ordered
transition state. Hence, the free enthalpy of activation G‡ has
only a small temperature dependence (Fig. 6). The values of the
Figure 6. Cutout of CF2-resonances in variable temperature 19F NMR spectra
of bis(pentafluoroethyl) compounds 10b ( ~ -90 ppm, left) and 11b ( = -85 -
-100 ppm, right). The measurements were performed at -50, -20, 10 and 60 °C
(10b, from bottom to top) and -30, 0, 30 and 100 °C (11b) in toluene-d8.
Figure 5. Solid-state cell packing of 1,2,5,6-tetrakis(pentafluoroethyl)
corannulene 12 (top) and 1,6-bis(pentafluoroethyl)-3,4-bis(trifluoromethyl)
corannulene 17b (bottom). Ellipsoids are at a probability level of 50%.
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were not able to observe the low temperature limiting spectrum
of this phenomenon. Thus, we could only analyze the
synchronous rotation of the neighbored RF-groups for
compounds 10a, 10b, and 11b (Tab. 1). For this process, the
enthalpy of activation is comparable to the bowl-to-bowl
inversion. In contrast, the entropic contribution is much larger in
this case as the transition state geometry for the concerted
rotation has to be highly ordered. Hence, the free enthalpy of
activation for this transformation is quite temperature dependent
(Fig. 7).
Table 1. Activation parameters H‡ and S‡ obtained by graphical analysis.
Errors are based on standard deviations of slope and intersection of the
fitted lines.
7a
7c
10a
H‡ [kJ/mol]
S‡ [J/(mol*K)]
40.2 ± 0.5
-1.3 ± 2.5
37.8 ± 1.1
-11.3 ± 5.0
46.3 ± 0.8
-37.1 ± 2.6
10b
11b
13a
H‡ [kJ/mol]
S‡ [J/(mol*K)]
32.6 ± 0.6
-61.2 ± 2.6
38.7 ± 1.8
-48.7 ± 5.9
26.0 ± 1.6
-90.3 ± 5.9
Adding a third perfluoroalkyl substituent makes the situation
more complicated. Again, it is convenient to think about possible
conformers first. Ignoring the ones with neighbored RF-groups
on the same side of the bowl yields four pairs of enantiomers
(Fig. 8). There are three different processes to interconvert
these conformers into each other. The easiest transformation is
the rotation of the single C2F5-group (red arrows) which can be
considered as barrierless under the measurement conditions.
Further, the two processes described before for mono- and
disubstituted corannulenes hold. Bowl-to-bowl inversion (grey
arrows) isomerizes the neighboring pentafluoroethyl substituents
whereas concerted rotation (blue arrows) is liable for the
exchange of diastereotopic 19F-nuclei again. The analysis of the
low temperature NMR-spectra of compounds 13a and 13b did
not allow any insight into the energetics of the dynamic
processes of these compounds. The reason is, that both
transformations seem to be significantly decelerated below room
temperature. Furthermore, we could not obtain the low
temperature limiting spectra of 13a and 13b.[32] Thus, it was not
possible to assign any set of signals to one of the probable
conformers. Due to this, we could only gain qualitative insight
into the activation barrier of the concerted rotation of compound
13a (Tab. 1). In this case, the enthalpy of activation is quite low
with about 26 kJ/mol, which is reasonable as for this compound
the bowl depth is decreased, too (Fig. 4). As expectable, the
entropy of activation has a large contribution to the free
activation enthalpy. In total, the barrier has a G‡ value
comparable to the aforementioned compounds, being around
10 kcal/mol. But in this case, the temperature dependence is
quite large (Fig. 7). Again, this is reasonable due to highly
ordered transition state structures which are likely to be present.
The situation gets even more complicated for compound 12.
Keeping in mind that the preferred orientation of neighbouring
C2F5 groups is on opposite sides of the bowls and that only
synchronous movement of those is possible, three conformers
can be described (Figure 9b). As can be seen from Figure 9a,
the molecule is not rigid at higher temperatures. At room
free enthalpy of activation of 7a and 7c are in line with the result
obtained by Scott of about 10 kcal/mol.[12]
Interestingly, one can also observe dynamic behavior in the
case of ortho-disubstituted perfluoroalkyl fluoranthenes 10 and
-corannulenes 11 (Fig. 6). This is consistent with the molecular
solid-state structures shown in Fig. 2 and 3. Due to steric
congestion conformations resulting in CS-symmetric molecules
are energetically disfavored. Thus, the fluoranthenes 10 and
corannulenes 11 both are C1-symmetric as has been
investigated by single crystal X-ray diffraction (Fig. 2 and 3). For
the explanation of the fluxional behavior, one has to consider
two different processes. A twisting motion of the fluoranthene
framework and a bowl-to-bowl inversion of the corannulene
moiety result in C2-symmetric transition states and
interconversion of the RF-groups into each other. Nevertheless,
no higher symmetry than C2 can be found, keeping the fluorine
nuclei of the CF2-groups diastereotopic. However, concerted
rotation of the neighbored perfluoroalkyl substituents makes
them chemically equivalent. Further, it helps to think about
possible stereoisomers in order to understand the NMR-spectra.
Keeping the conformers with the lowest energy in mind, one can
neglect the CS-symmetric molecules. This yields only two
possible ways to arrange the RF-groups. The resulting isomers
form a pair of enantiomers. When the system is cooled below
-50 °C, further broadening of the signals is observed. This can
be explained by deceleration of the bowl-inversion process that
makes the perfluoroalkyl substituents inequivalent. However, we
temperature,
the
19F
NMR
spectrum
of
1,2,5,6-
tetrakis(perfluoroethyl) corannulene 12 shows four signals. Two
different CF3 groups can be observed at -79.0 and -79.5 ppm,
further, different types of fluorine atoms of the CF2 groups can
be found at -87.4 and -100.2 ppm. Upon heating to 100 °C, the
fluorine atoms of the difluoromethylene units become equivalent,
still showing a very broad signal. When the compound is cooled
to -70 °C, additional splitting is observed. Interestingly, both of
the fluorine atoms of the CF2 units exhibit a different fine
structure. Further, the number and integrals of the signals clearly
Figure 7. Free activation enthalpies G‡ in dependence of the temperature T,
based on the parameters from Tab.1.
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point to the presence of different conformers at low temperature.
For example, the C1 symmetric compound has four inequivalent
C2F5 groups. Those can be identified in the region from -98 to
-103 ppm where different doublet of quartet resonances are
found. The doublets (2JFF ~ 280 Hz) result from geminal coupling
to the other CF2 fluorine atom, whereas the quartets (≥4JFF
~ 30 Hz) rise from through space coupling with the CF3 group of
the neighbouring pentafluoroethyl substituent, which could be
proven by low temperature 19F COSY NMR.[33] In contrast, the
other CF2 fluorine atom has to point away from the CF3 groups
and exhibits only geminal doublet splitting, which underlines,
that quartet splitting can only result from coupling through space.
The integrals of the aforementioned species are clearly
underlining, that the C1 symmetric molecule is the conformer of
lowest energy. This is also in line with the conformation of the
obtained solid-state crystal structure (Fig. 5). As mentioned
above, scalar coupling through space had been described
several times in literature before. The observed through space
coupling constant for 12 of about 30 Hz is in the medium range
of literature (~10 – 110 Hz). This is consequent, as neighboring
C2F5 groups are in proximity and can interact with each other,
but rotation of CF3 groups is still possible. Otherwise, the
situation in the low temperature 19F NMR spectrum would be
even more complicated.
For comparison, we also investigated the dynamic behavior of
non-fluorinated tetraethyl corannulene 19, which could be
prepared following a procedure described for methylation of
tetrabromocorannulene.[7b] As expected, in variable temperature
Figure 8. Possible conformers of 13a. Molecules with neighboring substituents
on the same side are neglected. The blue and red spheres represent C2F5
groups pointing to the convex and concave side of the bowl, respectively.
Figure 9. Figure 9. a) Variable temperature 19F NMR of 1,2,5,6-tetrakis(pentafluoroethyl) corannulene 12 in toluene-d8 at 100 °C (top), 20 °C (middle) and -70 °C
(bottom). The intensity of the right part of the spectrum is enlarged by factor four for clarification. The asterisk marks an unknown impurity. b) Schematic drawing
of different CS-symmetric (top, middle) and C1-symmetric (bottom) conformers of 12. The blue and red spheres represent C2F5 groups pointing to the convex and
concave side of the bowl, respectively.
1H NMR, no dynamic processes can be observed. This is
reasonable, as the van der Waals radius of a hydrogen atom is
considerably smaller than that of a fluorine atom (110 vs.
146 pm).[34] Thus, two ortho substituted ethyl groups can freely
rotate at -70 °C, whereas rotation is strongly hindered in case of
the fluorinated derivative. To observe the same effect for the
ethyl derivative, NMR measurements at temperatures close to or
even below -100 °C would be necessary, which is then a
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problem in terms of appropriate NMR solvents. For a better
comparison, one would have to investigate the respective
isopropyl compound, or derivatives with even larger branched
alkyl substituents as Mislow and coworkers have studied earlier
for benzene derivatives.[14] The problem of these derivatives is
their synthetic accessibility. The introduction of these alkyl
groups with a Ni(dppp)Cl2 catalyst and alkyl alanes does not
satisfyingly afford the alkylated corannulenes maybe due to
competitive -hydride elimination.
perfluoroalkylated compounds 17a and 17b which only bear one
ortho-disubstitution. The unsymmetrically trifluoromethylated
derivative 18b ranges between the symmetrically substituted
compounds, but still the differences are very small. Compared to
pentasubstituted compound 1, which was first investigated in
CH3CN by Boltalina and coworkers and recently remeasured by
Siegel in THF for better comparability, the reduction potentials of
4b, 5]
12 and 14 can be considered being equal.[4a,
The small
difference in potentials which is caused due to the effect of chain
length of the perfluoro substituents can rather be neglected. The
electron-withdrawing abilities of longer perfluoroalkyl chains
might be slightly larger, however, the reduction potentials for
fourfold trifluoromethylated and pentafluoroethylated derivatives
14 and 12 are identical within the error of the measurement.
Electrochemical investigations by cyclic voltammetry.
It has already been shown before, that the introduction of a
single trifluoromethyl group into corannulene shifts its reduction
potential by roughly 0.2 V to more anodic potential. The effect of
several of these substituents depends on their substitution
pattern, but in general can be considered as being almost linear
in respect to the number of electron-withdrawing substituents.
The same trend can be observed for the derivatives that we
synthesized and investigated electrochemically by cyclic
voltammetry. Whereas the single substituted derivatives 7 show
only irreversible reductions at strong negative potentials, as was
also observed by Siegel recently, the higher substituted
corannulenes can be reduced easier (Fig. 10).[5] This can clearly
be observed, when the disubstituted compound 11b is
investigated. In this case, the reduction potential is shifted by
about 0.5 V, compared to unsubstituted corannulene. This is in
line with earlier results for bis-(trifluoromethyl) corannulene.[4e]
Adding one more substituent yields a shift in reduction potentials
of about 0.8 V for derivatives 13a and 13b. Compared to an
earlier synthesized, analogous compound of 13a, but with
trifluoromethyl substituents these values are again the same.[4e]
Moving to fourfold substituted perfluoroalkyl corannulenes
further increases the shifts of reduction potentials to about 0.9 to
1.0 V. As already shown in earlier studies, the substitution
pattern has an influence. Here, the double ortho-disubstituted
compounds 12 and 14 are shifted by about 0.1 V towards anodic
potentials compared to the other symmetrically fourfold
Conclusions
In conclusion, we demonstrated that corannulenes with a well-
defined substitution pattern are accessible by introduction of
perfluoroalkyl
groups
into
different
substrates.
The
intermolecular packing is strongly influenced by the number of
substituents and their size. Dynamic behavior of the substituted
compounds and the corresponding activation parameters could
be investigated by line shape analysis of variable temperature
19F-NMR spectra. Graphical evaluation revealed a free energy
barrier for the bowl-to-bowl inversion of about 10 kcal/mol, which
is in line with the literature. Further, for ortho-disubstituted
compounds another dynamic process could be discovered.
Concerted rotation has a comparable activation barrier but is
much more temperature dependent due to larger contribution of
the entropy of activation. Finally, the dynamic processes of a
tetrasubstituted derivative could be investigated by 19F NMR
spectroscopy, revealing through space coupling. The resulting
coupling constants of about 30 Hz underline the rather strong
steric
congestion
of
ortho-disubstituted
perfluoroalkyl
corannulenes. Electrochemical investigation of the perfluoroalkyl
derivatives is in line with the prediction and demonstrates that
the reduction potential is not only influenced by the number of
functional groups but also the substitution pattern. As expected,
the fourfold perfluoroalkylated compounds have the least
negative reduction potentials. To our delight, their substitution
pattern results in a shift in reduction potentials which is almost
as large as for symmetrical pentakis-trifluoromethyl corannulene.
Acknowledgements
Financial support by DFG and GRK 1582/2 “Fluorine as a Key
Element” is gratefully acknowledged. The authors would like to
thank Simon Steinhauer for experimental help with variable
temperature NMR and line shape analysis.
Figure 10. Relative reduction potentials of synthesized corannulene
derivatives x. Shift values are referenced to ferrocene/ferrocenium (left part,
orange bars) and the first reduction of corannulene to its monoanion (right part,
blue bars). Corannulene (Cor) is also shown for comparison.
Keywords: perfluoroalkylation • X-ray crystallography • variable
temperature NMR • cyclic voltammetry • corannulene
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(A)
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Perfluoroalkylated Corannulenes:
Selective introduction of multiple
fluoroalkyl groups gives rise to
corannulenes with interesting
structural, electrochemical and
dynamic properties.
Axel Haupt, Lisa-Marie Keller,
Maximilian Kutter, Dieter Lentz*
1 – 10
Selective Synthesis of
Perfluoroalkylated Corannulenes and
Investigation of their Structural,
Dynamic and Electrochemical
Behavior
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