Dibenzo[a,e]pentalenophanes: Bending a Non-Alternant Hydrocarbon

Article abstract of DOI:10.1002/chem.201800322

In cyclophanes, an aromatic moiety is incorporated into a (strained) cyclic structure. Of particular interest as model systems for bent carbon nanostructures are those containing polycyclic aromatic hydrocarbons. Dibenzo[a,e]pentalene (DBP) is a non-alternant polycyclic hydrocarbon with small band gap and tunable optoelectronic properties. However, changing these properties by bending of the DBP structure has yet to be investigated. Herein, we report the synthesis, optoelectronic, and structural properties of (2,7)dibenzo[a,e]pentalenophanes with four different bridge sizes and bending angles of the DBP unit, accompanied by (TD)DFT calculations. The last, strain-inducing dehydration reaction was accomplished by using Burgess’ reagent. The HOMO and LUMO levels and the magnetic shielding of protons pointing inside the cyclophane cavity grew stepwise with increasing ring strain. Single-crystal X-ray structures of the smallest three derivatives revealed a near semi-circle and a bend angle of the DBP unit of almost 88° for the smallest derivative. We demonstrated the synthetic versatility of our approach by varying the substituents at the DBP unit, allowing for further tuning of optoelectronic properties. The synthetic strategy presented herein may pave the way for the synthesis of conjugated DBP nanorings.

Full text of DOI:10.1002/chem.201800322

A Journal of
Accepted Article
Title: Dibenzo[a,e]pentalenophanes - Bending a Non-Alternant
Hydrocarbon
Authors: Mathias Hermann, Daniel Wassy, Daniel Kratzert, and Birgit
Esser
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Dibenzo[a,e]pentalenophanes – Bending a Non-Alternant
Hydrocarbon
Mathias Hermann,[a] Daniel Wassy,[a] Daniel Kratzert[b] and Birgit Esser*,[a]
Abstract: In cyclophanes an aromatic moiety is incorporated into a
(strained) cyclic structure. Of particular interest as model systems for
bent carbon nanostructures are those containing polycyclic aromatic
usually planar π-system on the optoelectronic and structural
properties and the aromaticity of the PAH. For this, a cyclophane
design as shown in Scheme 1 is most suitable. Here, the PAH is
held in a bent conformation through a saturated chain, attached
to the PAH at the outermost positions.
hydrocarbons. Dibenzo[a,e]pentalene (DBP) is
a non-alternant
polycyclic hydrocarbon with small bandgap and tunable
optoelectronic properties. However, changing these properties by
bending of the DBP structure have yet to be investigated. Herein, we
report the synthesis, optoelectronic and structural properties of
(2,7)dibenzo[a,e]pentalenophanes with four different bridge sizes and
bending angles of the DBP unit, accompanied by (TD)DFT
calculations. The last, strain-inducing dehydration reaction was
accomplished using the Burgess reagent. The HOMO and LUMO
levels and the magnetic shielding of protons pointing inside the
cyclophane cavity grew stepwise with increasing ring strain. Single
crystal X-ray structures of the smallest three derivatives revealed a
near semi-circle and a bend angle of the DBP unit of almost 88° for
the smallest derivative. We demonstrated the synthetic versatility of
our approach by varying the substituents at the DBP unit, allowing for
a further tuning of optoelectronic properties. The synthetic strategy
presented herein may pave the way for the synthesis of conjugated
DBP nanorings.
PAH
Generation of the
aromatic system
PAH-
precursor
low strain energy
high strain energy
Scheme 1. Synthetic strategy towards PAH-cyclophanes.
In the synthesis of such cyclophanes, one of the biggest
challenges is to introduce strain through bending of the
preferentially planar π-system. Several strategies have been
used, of which the most successful ones leading to highly strained
PAH-cyclophanes involve the synthesis of a bridged PAH-
precursor.[5] This relatively unstrained species is then converted
to the PAH, usually in the last synthetic step, whereby the
obtained aromatic stabilization energy can counterbalance the
strain energy (Scheme 1). This concept was successfully applied
in the synthesis of (2,7)pyrenophanes with different chain
lengths.[6–9] Here, a valence isomerization/dehydrogenation was
used to generate the aromatic pyrene core. Using the same
strategy, (2,11)teropyrenophanes containing ten annelated six-
membered rings were later synthesized.[12,13] The same concept,
a transformation of a benzene precursor to a phenyl ring, was also
the key step in some of the synthetic routes leading to
[n]cycloparaphenylenes[19–21] and similar systems.[22] We herein
present the synthesis, optoelectronic and structural properties of
(2,7)dibenzo[a,e]pentalenophanes 1[n] (DBP-phanes, Figure 1)
with four different bridge sizes, accompanied by (TD)DFT
Introduction
The field of cyclophane chemistry has fascinated chemists for
more than 100 years.[1,2] Cyclophanes are rings, built up of
mancude-ring systems[3] (or assemblies thereof) alternating with
(un)saturated chains.[4] Of the large number of reported
cyclophanes, however, only few contain polycyclic aromatic
hydrocarbons (PAHs) consisting of at least four annelated rings.[5]
Examples of such incorporated PAHs are pyrene,[6–9]
corannulene,[10,11]
acenes,
teropyrene[12,13]
and
hexabenzocoronene.[14–17] These cyclophanes are of particular
interest as model systems for bent carbon nanostructures such
as carbon nanotubes, in some cases representing subunits
thereof.[18] They allow for studying the effect of bending of the
calculations.
Mes
Mes
DBP
O
O
[a]
[b]
Institute for Organic Chemistry, University of Freiburg, Albertstr.
21, 79104 Freiburg, Germany
E-mail: besser@oc.uni-freiburg.de
Institute for Inorganic and Analytical Chemistry, University of
Freiburg, Albertstr. 21, 79104 Freiburg, Germany
(CH2)n
1[n] (n = 8–10, 12)
Figure 1. (2,7)Dibenzo[a,e]pentalenophanes 1[n], synthesized in this study,
and dibenzo[a,e]pentalene (DBP).
Supporting Information, including additional synthetic procedures,
additional CVs, NMR spectra, energies and Cartesian coordinates
of calculated structures, results of (TD)DFT calculations, and
single-crystal X-ray diffraction data can be found under http://....
CCDC 1565441, 1554556, 1583862, 1479037, 1579148, 1585200,
1585195 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via
Investigating the optoelectronic properties of these cyclophanes,
containing bent dibenzo[a,e]pentalenes (DBPs), allowed us to
evaluate the effect of bending on the properties of the DBP unit.
DBP is a small-bandgap organic semiconductor, which, due to its
www.ccdc.cam.ac.uk/structures.
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ambipolar electrochemical character, is attractive for applications
in organic field-effect transistors. The small bandgap is caused by
the non-alternant nature of its π-system due to the presence of
five-membered rings,[23] which leads to a shift in orbital energies,
i.e. an increase in HOMO and a decrease in LUMO energy,
compared to an alternant π-system of the same size.
Despite its structural similarity to pentalene, a highly reactive
species with antiaromatic character,[24] DBP is stabilized by two
Clar sextets[25] in the fused six-membered rings and shows only
little antiaromaticity.[26] Bending of the DBP unit could alter its
optoelectronic properties, its aromaticity and stability.
Furthermore, we reasoned that a successful synthetic strategy to
DBP-phanes would pave the way for the synthesis of conjugated
nanorings containing DBP units.[27]
While many new strategies for the synthesis of DBPs have been
reported in recent years[28–33] they all generate a planar DBP from
a planar starting material. In our synthetic strategy to DBP-phanes
1[n] we made use of the original synthetic route by Brand,[34]
which proceeds via a diphensuccindandione as an inherently bent
precursor. This is converted to a DBP derivative through the
addition of a Grignard reagent followed by a two-fold elimination
of water, as shown in Scheme 2. We reasoned that the natural
bending angle in the diketone as DBP precursor would allow for
facile formation of the macrocycle, and that the achieved
conjugation energy in the DBP moiety would be sufficiently high
to make the last synthetic dehydration step energetically
favorable. In dibromo derivative 2, the bending angle (angle
between the planes of the phenyl rings) amounts to 107.2º, which
optimized this procedure such that 2 can now be accessed in
gram-scale quantities without the use of column chromatography
(see SI for details). A ring closing Suzuki-Miyaura cross-coupling
of diketone 2 with the alkyl chain-linked bis-functionalized boronic
acid pinacol esters 3[n] allowed accessing five different cyclic
diketones 4[n] (Scheme 3). The yields in this cyclization reaction
ranged from 3% for 4[6] to 54% for 4[12], reflecting the higher
strain for the smaller bridges. After optimization of the reaction
conditions, using Buchwald’s SPhos ligand[36,37] with Pd(OAc)2
under high dilution conditions (0.76 mM) provided the best yields
for this cyclization reaction. When the concentration was doubled
to 1.5 mM, for instance, under otherwise nearly identical
conditions, the yields dropped by over ten percentage points, in
the case of 4[12] from 54% to 40%. Byproducts were the cyclic
dimers and trimers, which could be separated from the desired
product via column chromatography. The model compound 5 was
also synthesized via a Suzuki-Miyaura cross-coupling reaction
and was obtained in 92% yield after recrystallization (Scheme 3).
O
O
(CH2)n
(CH2)n
Bpin
Bpin
3[n]
O
Pd(OAc)2, SPhos
K3PO4
O
O
Toluene/H2O
100 °C, 24 h
O
n = 6 (4[6])
3%
8 (4[8])
9 (4[9])
10 (4[10])
12 (4[12])
21%
30%
43%
54%
4[n]
2
is sufficiently small to provide
precursor.
a non-strained macrocyclic
MeO
Bpin
O
PdCl2(dppf)
K3PO4
MeO
OMe
Toluene/H2O
100 °C, 4 h
92%
O
5
Scheme 3. Synthesis of cyclic diketones 4[n] and model compound 5 via
palladium-catalyzed Suzuki-Miyaura cross-coupling reactions.
Layering of a dichloromethane solution with acetonitrile provided
single crystals suitable for X-ray diffraction analysis of cyclic
diketones 4[8] and 4[10]. The structure of 4[8] revealed two
independent molecules in the asymmetric unit with one molecule
shown in Figure 2a. The other molecule contained a number of
disordered atoms, reflecting the flexibility of the alkyl chain linker
in 4[8] (see SI for further views). Cyclic diketone 4[10] exhibits a
high molecular symmetry with a C2 rotational axis through the
center of the molecule, while 4[8] has no symmetry elements and
therefore an unsymmetrical structure. The larger space in the
cavity of 4[10] can accommodate a (highly disordered) CH2Cl2
solvent molecule. Measuring the angle between the two planes
spanned through carbon atoms 1-2-3 and 6-7-8 (see Scheme 3
Scheme 2. Synthetic strategy towards DBP-phanes and molecular structure of
diketone 2 in the solid state. Displacement ellipsoids are shown at 50%
probability; hydrogen atoms are omitted for clarity.
To the best of our knowledge, apart from corannulene, no other
non-alternant hydrocarbon has been incorporated into
cyclophane structure.[5]
a
Results and Discussion
for
numbering)
provided
the
bend
angle
in
the
diphensuccindandione moiety. The bend angle in 4[8] amounts to
63.9–69.1° (considering both independent molecules and the
disorder) and in 4[10] to 75.5° (see SI for further description).
The synthesis of DBP-phanes 1[n] started from dibromo-
substituted diphensuccindandione 2, which can be prepared in
three steps from ethyl 2-(4-bromophenyl)acetate.[35] We further
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These angles are significantly smaller than that in the “free”
diketone 2 of 107.2° (see Scheme 3).
NMR spectra showed that the rotation of the mesityl groups in
diols 6[n] was hindered. In 6[12], for instance, the ortho-methyl
groups of the mesityl substituents reached coalescence at
A comparison of the 1H NMR spectra (see SI) of cyclic diketones
4[n] showed a clear trend, where the protons in the aromatic
region were shifted up-field by up to Δδ = 0.27 ppm with
decreasing bridge size. This indicates an increasing shielding
effect with shorter bridge length. The bridgehead protons in the
diketone structure, on the other hand, experienced no or only a
small up-field shift of Δδ ≤ 0.005 ppm.
1
ambient temperature (300 K). Recording a H NMR spectrum at
343 K led to peak sharpening, while cooling to 254 K effected
peak separation into two signals (see SI for VT NMR experiments).
The last step in the synthesis of DBP-phanes 1[n] was the twofold
elimination of water from diols 6[n]. The non-strained DBP 7 could
be obtained in 99% yield by refluxing a mixture of diol 8 with a
catalytic amount of p-TsOH·H2O in toluene (Scheme 5). However,
this route proved unsuitable for cyclic diols 6[n]. While conducting
experiments with the largest member 6[12] we discovered an
unusual behavior when using an acid or acid anhydride. Instead
of the expected bright red DBPophane, a light-yellow solid was
isolated after heating 6[12] with trifluoroacetic anhydride in
1
chloroform. The H NMR spectrum suggested an unsymmetrical
structure, which we initially ascribed to the formation of the mono-
eliminated product. However, 2D NMR experiments (HSQC,
HMBC) revealed this product to be ring-opened 9[12] (Scheme 5).
Varying the solvent and/or acid led to no improvement, the DBP
1[12] remained a minor side product.
Figure 2. Molecular structures of cyclic diketones 4[8] (molecule A) (a) and
4[10] (b) in the solid state with CH2Cl2 incorporated in the center of the ring
(displacement ellipsoids are shown at 50% probability; hydrogen atoms are
omitted for clarity).
In the course of synthesizing p-terphenylophanes Merner and
coworkers
successfully
used
methyl
N-
(triethylammoniumsulfonyl)carbamate (Burgess’ reagent[40]) as a
mild dehydration agent in order to avoid strain-induced
rearrangement reactions.[41,42] After optimization of the reaction
conditions, we were able to obtain cyclic DBPs 1[n] in yields
ranging from 65% to 84% by using eight equivalents of Burgess
reagent in 1,4-dioxane at reflux temperature.
As substituent R for the DBP units in DBP-phanes 1[n] we chose
mesityl groups, since we reasoned that they would provide steric
protection in case of an increased reactivity of the DBP core.
Using the cerium trichloride-mediated[38] addition of mesityl
magnesium bromide to cyclic diketones 4[n] provided diols 6[n]
in excellent yields as well as model compound 8 (Scheme 4). This
procedure has proved the method of choice to add carbon
nucleophiles to the keto groups in diphensuccindandiones[35,39]
and allows for the introduction of various aryl moieties (see also
below). Applying this methodology yielded diols 6[n] in yields
between 89% and 94%, which remained unaffected by the bridge
size. Due to the small yield achieved for 4[6] in the
macrocyclization step (Scheme 3), this compound was not carried
forward in this synthetic step.
b
p-TsOH (cat.)
MeO
8
Toluene
reflux, 3 h
99%
OMe
a
c
d/e
7
(CH2)12
Mes
O
(CF3CO2)O
6[12]
O
(CH2)n
CHCl3, reflux, 1 h
OH
O
MesMgBr
O
99%
Mes
CeCl3
O
4[n]
9[12]
THF, 0 °C, 4 h
n = 8 (6[8])
89%
92%
94%
92%
HO
9 (6[9])
10 (6[10])
12 (6[12])
(CH2)n
O
O
O
(racemic)
6[n]
b
O
S
Et3N
N
OMe
O
6[n]
Mes OH
1,4-Dioxane, 100 °C, 24 h
a
MesMgBr
CeCl3
c
e
MeO
d
n = 8 (1[8])
9 (1[9])
75%
84%
65%*
72%
5
OMe
THF, 0 °C, 4 h
10 (1[10])
12 (1[12])
HO Mes
82%
1[n]
8
Scheme 5. Water elimination to yield model compound 7 and DBP-phanes 1[n]
and unexpected formation of 9[12].
Scheme 4. CeCl3-mediated Grignard addition to yield cyclic diols 6[n] and
model compound 8.
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Figure 3. Comparison of the 1H NMR spectra of DBP-phanes 1[n] and model compound 7 in CDCl3 (300 MHz).
The built-up ring strain seemed to have no effect on the yields of
this double elimination reaction. Other solvents such as toluene
provided smaller yields—especially with the more strained
derivatives 1[8] and 1[9]—and in the case of DMF a complex
mixture was obtained containing only little DBPophane product.
Using only a stoichiometric amount of Burgess reagent led to an
incomplete conversion to the desired DBP.
character of the aromatic rings of the DBP unit at this distance,
which overruled the, if present, minor deshielding effect of the
central pentalene unit.
Crystals of 1[8]–1[10] suitable for X-ray diffraction could be
obtained by layering of
a dichloromethane solution with
acetonitrile (1[8]) or methanol (1[9] and 1[10]). 1[8] and 1[9]
contained one independent molecule in the asymmetric unit
whereas in 1[10] two independent molecules were present. Both
molecules deviated from each other regarding the geometrical
parameters (molecule A is shown in Figure 4c, molecule B can be
found in the SI). As shown in Figure 4a–c, the DBP moiety in all
three DBP-phanes 1[8]–1[10] is highly distorted from planarity.
The bending of an aromatic system can be quantified using the
so-called bend angles θ, defined by Bodwell and coworkers for
pyrenophanes (Figure 4d).[13,47] Here, the bend angle θDBP
describes the distortion of the DBP moiety from planarity. With
increasing bridge size, the bend angle θDBP decreased in DBP-
phanes 1[n] (Table 1) with values of 87.6° for 1[8], 85° for 1[9],
and 84.4°/72.0° for molecule A/B in 1[10]. The decrease in these
values, however, does not follow a clear pattern. In particular, the
deviation between the two independent molecules of 1[10] is
significant. For comparison we performed DFT calculations using
the composite scheme PBEh3c[48] to obtain structural data for all
four synthesized DBP-phanes 1[n] (Table 1). The calculated θDBP
values showed a steady increase in bend angle θDBP from 1[12]
(57.4°) to 1[8] (91.9°). The deviation from experimental (X-ray)
values can be explained by packing effects in the solid state
compared to the calculations, which were performed in the gas
phase. In addition, these results show that the DBPophane
framework possesses a degree of flexibility, which allows for
different conformations to be energetically viable. For
(2,7)pyrenophanes, Bodwell and coworkers found bend angles
for the central pyrene unit between 34.6° and 109.2°, depending
on the cyclophane size.[5] As a measure for the total bending of
the π-system, including the aryl ether groups, the distance d
A comparison of the NMR spectra of DBP-phanes 1[n] showed
that bending of the DBP unit had the strongest effect on one of
the ortho-methyl groups on the mesityl substituents (signals e in
Figure 3, see Scheme 5 for assignment). While the aromatic
protons of the DBP unit experienced only a small up-field shift of
up to Δδ = 0.17 ppm for signal b, the methyl group signal e
dramatically shifted to higher field by up to Δδ = 1.42 ppm. In the
smallest DBPophane 1[8] the methyl group protons b resonated
at 0.91 ppm, hence experiencing a strong shielding effect.
NOESY experiments confirmed that signal e belongs to the
methyl groups pointing towards the center of the cyclophane (see
SI for NMR experiments). Due to their diamagnetic ring current,
the phenyl rings in the DBP unit exhibited a magnetic anisotropic
effect. This led to a magnetic shielding of nuclei above or below
the ring center, which, as Klod and Kleinpeter demonstrated,
amounts to 1 ppm at a distance of 4 Å for benzene.[43] In
antiaromatic pentalene, on the other hand, the paramagnetic ring
current leads to a deshielding above and below the molecular
plane, which is significantly smaller in extend (0.1 ppm at 2–
4 Å).[43] The central pentalene unit in DBP shows only little
antiaromaticity (NICS(0)iso 11.0 compared to 24.7 for pentalene
(GIAO B3LYP[44,45]/6-31G*[46])), and hence an even smaller
deshielding effect is expected. In 1[8], as found in the solid-state
structure and from DFT calculations, the CH3 group e was located
4.2 Å away from the center of the nearest phenyl ring of the DBP
unit and 3.4 Å from the five-membered ring of the pentalene unit.
Relative to model compound 7 protons e experienced a shielding
effect of 1.42 ppm. This can be explained by the strong shielding
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between the two aryl carbon atoms at the ether bridge was used
(Figure 4d and Table 1). With decreasing bridge size this distance
To allow for a comparison of bond lengths, the molecular structure
of model compound 7 in the crystal was also determined by X-ray
became shorter (12.30 Å in molecule B of 1[10] to 10.47 Å in 1[8]). diffraction (see SI for details). 7 showed a high alternation in bond
Here, the agreement with the calculated values is better, which
decreased from 13.8 Å for 1[12] to 10.3 Å for 1[8]. The aryl ether
rings in 1[n] also adopt a slightly bent conformation, which can be
quantified using the angle θPhenyl, which lay between 5.5° and
16.4° and did not vary much with the bridge size (see SI). Hence,
the DBP unit in DBP-phanes 1[n] experienced the strongest
distortion from planarity. For all bridge sizes n, the aryl ether units
were twisted relative to the DBP unit with dihedral angles between
33° and 46° (see SI for details).
lengths in the pentalene unit, ranging from 1.352–1.487 Å,
described well by the valence structure depicted in Scheme 5.
The bond lengths of 1[9] lay in the same region with 1.355–
1.487 Å. Because of the low resolution of the X-ray diffraction data
caused by molecular disorder of 1[8] and 1[10], exact bond
lengths cannot be discussed, however, they can be evaluated in
their relative lengths. The bond lengths in the pentalene unit of
1[8] and 1[10] showed a similar trend with alternating short (C1-
C2 and C9-C10) and long bonds (C1-C16 and C8-C9), indicating
that a valence structure with two Clar sextets in the six-membered
rings well describes DBP-phanes 1[n].
In order to calculate strain energies for bent DBP-phanes 1[n], all
structures were optimized using PBEh-3c, including the not
synthesized smallest derivative 1[6]. Then, the alkyl bridges were
removed from the optimized geometries and replaced with
hydrogen atoms to obtain structures 10[n] (see SI for details).
Single-point
energy
calculations
(PW6B95[49]-D3[50]/def2-
QZVP[51]) and comparison with an optimized structure of identical
constitution (11) allowed assessing the strain energies (see SI for
details). As anticipated, the strain energy grew with increasing
distortion of the DBP unit from planarity and reached a value of
33.8 kcal mol−1 for 1[8] and 46.2 kcal mol−1 for the smallest and
not synthesized derivative 1[6]. This strain energy for 1[8] is low
in comparison to [5.5.6]ncyclacenes,[27] containing DBP units, and
to [n]cycloparaphenylenes. Here, even for n = 12 a strain energy
of 48 kcal mol−1 was calculated,[52] although the semi-circle,
comparable to the structure of 1[8], consisted of six phenyl rings.
Table 2. Calculated strain energies of DBP-phanes 1[n].[a]
1[6]
1[8]
1[9]
1[10]
1[12]
Strain energy / kcal mol−1
46.2
33.8
27.6
25.1
17.1
[a] PW6B95-D3/def2-QZVP (single points) on PBEh-3c-optimized (cut-out)
geometries.
Electrochemical measurements allowed assessing the influence
of its bending on the redox properties of the DBP unit. Cyclic
voltammograms showed an amphoteric behavior in all DBP-
phanes 1[n] and model compound 7 (Figure 5, more CV
measurements can be found in the SI). In 7 two reversible
oxidations were found, while for cyclic derivatives 1[n] only the
first oxidation wave was reversible. All compounds showed one
reduction wave. The reversibility of both oxidation and reduction
slightly decreased with higher ring strain, culminating in a quasi-
reversible electrochemical behavior for 1[8], where the re-
oxidation and re-reduction (anodic peak potential and cathodic
peak potential) were less pronounced.
Figure 4. Molecular structures of DBP-phanes 1[8] (a), 1[9] (b) and 1[10]
(molecule A) (c) in the solid state (displacement ellipsoids at 50% probability,
hydrogen atoms, solvent molecules and disordered atoms are omitted for
clarity) and definition of bend angles (d); e) top view of 1[8] with atom numbering.
Table 1. Geometrical parameters of DBP-phanes 1[n].
1[8]
1[9]
1[10]
1[12]
-
θDBP / ° (X-ray)
θDBP / ° (calc)[c]
d / Å (X-ray)
87.6
91.9
10.47
10.25
85.0
80.4
11.18
11.57
84.4[a]; 72.0[b]
74.2
57.4
-
11.56[a]; 12.30[b]
12.19
d / Å (calc)[c]
13.84
[a] Molecule A; [b] molecule B; [c] calculated using PBEh-3c.
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asterisk (7* and 1[n]*). The longest wavelength absorption bands,
assigned to the HOMO à LUMO transitions, had maxima at
503 nm for DBP-phanes 1[n] and 490 nm for 7 (Figure 6 inset).
The low extinction coefficients of this transition can be explained
by its forbidden nature for planar and centrosymmetric DBP
derivatives, where both orbitals are of au symmetry (Laporte’s
rule).[35,39,55] Even though the symmetry is broken in cyclic DBP-
phanes 1[n], the calculated oscillator strengths for the
HOMO à LUMO transitions lay below 0.016 (see SI). The second
absorption bands with maxima from 434–436 nm could be
ascribed to the HOMO−1 à LUMO transitions, which agreed well
with the calculated values of 400–420 nm. The absorptions with
maxima from 409–412 nm and from 388–391 nm corresponded
to multiple transitions from/to lower or higher lying orbitals (see SI
for further details on the assignment of the electronic transitions).
The main absorption peaks with maxima from 322–339 nm
reflected the HOMO à LUMO+1 transitions. These bands
showed a clear trend in the series of 1[n], namely a small
bathochromic shift with increasing ring strain. The optical band
gaps were determined from the onsets of the longest wavelength
absorption peaks and lay lower than the electrochemically
obtained values (Table 3). Here, no trend concerning the bridge
size was observed.
Figure 5. Cyclic voltammograms of 1[n] and 7 (1 mM in CH2Cl2, 0.1 M
n-Bu4NPF6, scan rate 0.1 V s−1 (0.2 V s−1 for 1[10]), glassy carbon electrode).
Determining the onset of the first reduction/oxidation peak vs. the
ferrocene/ferrocenium couple (Fc/Fc+) provided HOMO and
LUMO energy levels (assuming an ionization energy of 4.8 eV for
ferrocene)[53] (Table 3). With increasing strain, the HOMO and
LUMO of DBP-phanes 1[n] rose in energy in steps of about
0.02 eV. Compared to planar model compound 7, the HOMO-
LUMO energy gap in 1[n] was smaller by about 0.06 eV, and the
HOMO and LUMO energy in 7 was slightly lower than in the
largest congener 1[12].
Table 3. Electrochemical data for DBP-phanes 1[n], 12[12] and model
compounds 7 and 13.
E1/2,Red
V
/
E1/2,Ox
V
/
ELUMO
/
EHOMO
/
Eg,CV
eV
/
Eg,opt /
eV[a]
eV[a]
eV[b]
0.66,
1.01
Figure 6. Absorption spectra of 7 and 1[n] in CH2Cl2 solution.
7
−1.95
−2.93
−5.39
2.46
2.06
1[12]
1[10]
1[9]
−1.97
−1.98
−1.99
−2.02
0.59
0.57
0.55
0.52
−2.91
−2.89
−2.87
−2.85
−5.32
−5.29
−5.27
−5.25
2.41
2.40
2.40
2.40
1.99
1.90
1.89
1.90
None of the DBP-phanes 1[n] nor model compound 7 showed
fluorescence. This is known for DBP derivatives[35,39] and for the
structurally related indeno[1,2-b]fluorenes,
where transient
absorption spectroscopy measurements showed that the excited
state lifetimes were shorter than the timescale, at which
fluorescence is usually observed.[56]
Selected calculated molecular orbitals (HOMO and LUMO) of 7*
and 1[8]* as representative examples are shown in Figure 7. The
HOMO is spread out over the whole aromatic system—including
the aryl ether moieties—while the LUMO is mainly located on the
DBP core. These findings are in agreement with previously
reported literature.[57]
1[8]
−1.79,
−1.45
0.85,
1.06
13
−3.42
−3.42
−5.58
−5.49
2.16
2.07
1.86
1.81
−1.81,
−1.46
12[12]
0.77
[a] From the onsets of the reduction/oxidation peaks; [b] optical band gap from
the onset of the longest wavelength absorption band.
The absorption spectra of 7 and 1[n] (Figure 6) showed the
characteristic absorption bands of the DBP unit, which were
assigned to electronic transitions using TDDFT (B3LYP-D3/def2-
TZVP[54]). To speed up calculations the mesityl groups were
replaced by methyl groups, denoted in the following with an
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bis(trifluoromethyl)phenyl-substituted DBP 13. In contrast to the
mesityl-substituted derivatives, no dynamic NMR behavior was
observed for 12[12] due to the lack of a di-ortho-substitution
pattern.
Figure 7. Frontier molecular orbitals of 7* and 1[8]* (B3LYP-D3/def2-TZVP).
In order to investigate the aromaticity in the DBP moiety in DBP-
phanes 1[n], we calculated NICS[58,59] (nucleus independent
chemical shift) values using B3LYP/6-31G* (Table 4). All
NICS(0)iso values were positive in the centers of the five-
membered rings and negative for the six-membered rings,
indicating aromatic character for the latter and slightly
antiaromatic character for the former. With increasing distortion of
the DBP moiety from planarity both NICS(0)iso values decreased
from 8.9 for model compound 7* to 6.8 for 1[8]* in the five-
membered rings and from −4.5 to −5.5 in the six-membered rings.
In the smallest DBPophane 1[6]*, these values changed more
drastically to 5.9 and −6.3 for the five- and six-membered ring,
respectively, due to the strong bending of the DBP unit (see SI).
This trend shows that with increasing bend the aromatic character
in the six-membered rings became stronger, represented by the
valence structure in Scheme 5, while the double bonds in the
central, five-membered rings became more isolated.
Scheme 6. Synthesis of DBPophane 12[12] and model compound 13 with
electron-withdrawing substituents.
In the CV (Figure 8), 13 showed two reversible oxidation and
reduction processes with half-wave potentials listed in Table 3.
The
3,5-bis(trifluoromethyl)phenyl-substituents
facilitated
reduction by ΔE = +0.50 V relative to 7, but exhibited a smaller
effect on the oxidation (ΔE = +0.19 V). We have observed the
same trend in other DBP derivatives, where substituents in the
5,10-positions have a stronger influence on the LUMO energy and
2,7-substituents on the HOMO energy.[35,39] In DBPophane 12[12],
reduction was likewise facilitated by ΔE = +0.51 V compared to
mesityl-substituted 1[12], while the oxidation was shifted to higher
potential by only ΔE = +0.18 V. The second oxidation process
was irreversible as had been the case for 1[12]. While the LUMO
energy level was not affected by bending the DBP unit compared
to planar 13, the HOMO energy rose by 0.09 eV, thus decreasing
the electrochemically determined band-gap to 2.07 eV. The
absorption spectra of the 3,5-bis(trifluoromethyl)phenyl-
Table 4. NICS(0)iso values for DBP-phanes 1[n]* and 7*.[a]
5-membered ring
6-membered ring
7*
8.9
8.2
7.5
7.0
6.8
5.9
−4.5
−5.0
−5.1
−5.4
−5.5
−6.3
1[12]*
1[10]*
1[9]*
1[8]*
1[6]*
substituted DBPs 12[12] and 13 (Figure 9) showed
a
bathochromic shift of all peaks compared to the mesityl-
substituted derivatives 1[n] and 7 with the main absorption peak
at 336 nm for 13 and 347 nm for 12[12].
[a] B3LYP/6-31G* on PBEh-3c-optimized geometries.
Finally, in order to probe the versatility of our synthetic approach,
we synthesized DBPophane 12[12] carrying electron-withdrawing
3,5-bis(trifluoromethyl)phenyl-substituents as well as the
corresponding model compound 13 (Scheme 6). By using the
established cerium trichloride-mediated conditions, diol 14[12]
was obtained in 95% yield from 4[12] (see SI). The twofold water
elimination using Burgess’ reagent provided DBPophane 12[12]
as
a yellowish-brown-colored solid in 29% yield, already
suggesting a lower band gap compared to red-colored, mesityl-
substituted DBP-phanes 1[n]. The same conditions previously
applied
for
model
compound
7
furnished
3,5-
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Materials and Methods. Chemicals were purchased from ABCR, Acros-
Organics, Alfa-Aesar, Bernd-Kraft, ChemPur, Roth, Sigma-Aldrich or TCI
and used directly without further purification unless otherwise noted.
Moisture- or oxygen-sensitive reactions were carried out in dried
glassware, heated under vacuum (10−3 mbar), using standard Schlenk
techniques in a dry argon atmosphere (Argon 5.0 from Sauerstoffwerk
Friedrichshafen). Anhydrous solvents (THF, toluene, CH2Cl2) were
obtained from an M. BRAUN solvent purification system (MB-SPS-800) and
stored over 3 Å molecular sieves for a minimum duration of 48 h before
use. Other anhydrous solvents were obtained by drying over activated
molecular sieves (3 Å) for several days.[60] Ethyl acetate and cyclohexane
for flash chromatography were purchased in technical grade and purified
by distillation using a rotary evaporator. Other solvents were purchased
and used in analytical or HPLC grade. Analytical thin layer
chromatography was carried out using silica gel-coated aluminum plates
with a fluorescence indicator (Merck 60 F254). Detection was carried out by
using short wave UV light (λmax = 254 nm) and/or with a KMnO4 staining
solution (3.0 g KMnO4, 20 g K2CO3, 5.0 mL 5.0 wt% NaOH, 300 mL H2O).
Flash column chromatography was carried out using silica gel 60, grain
size 40–63 µm (230–400 mesh) from Machery-Nagel. In some cases an
automated flash chromatography system (Grace Reveleris X2) was
employed using prepacked columns (silica gel, 25 g to 80 g, 30 µm grain
size) from Interchim (Interchim Puriflash Silica HP 30 µm flash column).
NMR spectra were recorded at 300 K, unless otherwise noted, on the
following spectrometers: Bruker Avance III HD [300.1 MHz (1H),
282.4 MHz (19F)], Bruker Avance II [400.1 MHz (1H), 100.6 MHz (13C),
376.5 MHz (19F)] and Bruker Avance III HD [500.3 MHz (1H), 125.8 MHz
Figure 8. Cyclic voltammograms of 13 and 12[12] in CH2Cl2 solution (1 mM in
CH2Cl2, 0.1 M n-Bu4NPF6, scan rate 0.1 V s−1, glassy carbon electrode; due to
the low solubility of 13, the concentration was < 1 mM).
(
13C), 470.8 MHz (19F)]. Chemical shifts are reported in parts per million
(ppm, δ scale) relative to the signal of tetramethylsilane (δ = 0.00 ppm).
1H NMR spectra are referenced to tetramethylsilane as an internal
standard or the residual solvent signal of the respective solvent: CDCl3:
δ = 7.26 ppm; CD2Cl2: δ = 5.32 ppm, [D6]DMSO: δ = 2.50 ppm, C6D :
6
δ = 7.16 ppm. 13C NMR spectra are referenced to the following signals:
CDCl3: δ = 77.16 ppm; CD2Cl2: δ = 53.84 ppm [D6]DMSO: δ = 39.52 ppm,
C6D : δ = 128.06 ppm.[61] 19F NMR spectra are referenced to
Figure 9. Absorption spectra of 13 and 12[12] in CH2Cl2 solution. Inset:
Magnification of the longest wavelength absorption bands.
6
tetramethylsilane following the IUPAC recommendations.[62] Analysis
followed first order, and the following abbreviations for multiplets were
used: singlet (s), broad singlet (br. s), doublet (d), triplet (t), quartet (q),
septet (sept), multiplet (m) and combinations thereof i.e. doublet of
doublets (dd). Coupling constants (J) are given in Hertz [Hz]. High
resolution mass spectra were measured on a Thermo Fisher Scientific inc.
Exactive or LCQ Advantage via electron spray ionization (ESI) or
atmospheric pressure chemical ionization (APCI) with an orbitrap analyzer.
UV/Vis absorption spectra were measured on a Tidas I diode array
spectrometer from J&M Analytik AG and on a Perkin Elmer Lambda 950.
Conclusions
We herein presented the first report on dibenzo[a,e]pentalenes
(DBPs), whose π-system is bent out of planarity through
incorporation in a cyclophane structure. The smallest out of four
different DBP-phanes, 1[8], showed a large bend angle of 87.6°
correlated with a strain energy of 33.8 kcal mol−1, displaying
significant distortion of the DBP unit from planarity. The
optoelectronic properties of DBP-phanes 1[n], investigated
through cyclic voltammetry, UV/Vis absorption spectroscopy and
TDDFT calculations, showed only a small dependence on the
bend angle of the DBP unit, as did the (anti)aromatic character of
the DBP unit, assessed through NICS-calculations. Adding
electron-withdrawing substituents to the DBP unit in cyclophane
12[12], on the other hand, significantly stabilized of the LUMO
energy and lowered the bandgap. The versatile synthetic strategy
presented herein may, in the future, be applied in the synthesis of
conjugated nanorings containing DBP units.
Melting points were measured using
Gerätetechnik.
a MPM-HV2 from Schorpp-
Synthetic Manipulations.
2,7-Bis(4-methoxyphenyl)-4b,9b-dihydroindeno[2,1-a]indene-5,10-
dione (5). Diketone (300 mg, 0.765 mmol), 2-(4-methoxyphenyl)-
2
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (391 mg, 1.67 mmol, 2.18 eq.)
and anh. K3PO4 (1.30 g, 6.12 mmol, 8.00 eq.) were dissolved in a mixture
of toluene (31 mL) and water (3 mL). The mixture was purged with argon
for 1 h, then PdCl2(dppf) (28 mg, 38 µmol, 5.0 mol%) was added in one
portion, and the mixture was refluxed at 100 °C for 4 h. The mixture was
transferred into a separatory funnel, and the aq. layer was extracted with
CH2Cl2 (3 × 50 mL). The combined organic layers were dried over Na2SO4,
and volatiles were removed under reduced pressure. Recrystallization
from i-PrOH yielded diketone 5 as a brown solid (313 mg, 0.701 mmol,
92%). Rf 0.12 (cyclohexane/EtOAc: 6/1); m.p: 195 °C; 1H NMR (400 MHz,
CDCl3): δ 8.00–7.95 (m, 2H), 7.90–7.86 (m, 4H), 7.53–7.47 (m, 4H), 7.00–
6.94 (m, 4H), 4.46 (s, 2H), 3.84 (s, 6H); 13C NMR (101 MHz, CDCl3): δ
Experimental Section
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3
201.8, 159.9, 148.2, 142.4, 135.6, 134.7, 132.2, 128.4, 126.9, 122.4, 114.6,
55.5, 52.9; HRMS (pos. APCI): m/z calcd for C30H23O4 447.1591 [M+H]+,
found 447.1589.
121.8 (sept, JCF = 4.1 Hz), 114.4, 83.9, 64.2, 55.5; 19F NMR (471 MHz,
CDCl3): δ −62.59; HRMS (pos. ESI): m/z calcd for C46H30O4F12Na
897.1845 [M+Na]+, found 897.1835; HRMS (neg. ESI): m/z calcd for
C46H29O4F12 873.1880 [M−H]−, found 873.1876.
5,10-Dimesityl-2,7-bis(4-methoxyphenyl)-4b,5,9b,10-
tetrahydroindeno[2,1-a]indene-5,10-diol (8). In an oven-dried Schlenk
tube anhydrous CeCl3 (171 mg, 0.694 mmol, 3.1 eq.) was dried at 135 °C
and 1∙10−3 mbar for 3.5 h. The tube was purged with argon, cooled to rt,
and anh. THF (2.3 mL) was added. The resulting white suspension was
stirred at rt overnight. The mixture was cooled to 0 °C, and a solution of
mesityl magnesium bromide (1.08 M in THF, 0.62 mL, 0.67 mmol, 3.0 eq.)
was added dropwise. The grey suspension was stirred at 0 °C for 1.5 h.
Diketone 5 (100 mg, 0.224 mmol) was added in one portion, and the
resulting orange mixture was stirred at 0 °C for 4 h. The reaction mixture
was quenched with 10% aq. HOAc (4 mL), warmed to rt and extracted with
CH2Cl2 (2 × 15 mL). The organic layer was washed with sat. aq. NaHCO3,
dried over Na2SO4, and the solvent was removed under reduced pressure.
Column chromatography (SiO2, cyclohexane/EtOAc 20/1 to 10/1) yielded
5,10-Bis(3,5-bis(trifluoromethyl)phenyl)-2,7-bis(4-
methoxyphenyl)indeno[2,1-a]indene (13). Diol 15 (51.4 mg, 58.8 µmol)
and p-TsOH∙H2O (7.0 mg, 37 µmol, 0.63 eq.) were refluxed in toluene
(6.0 mL) for 30 min. The reaction mixture was loaded onto a short silica
gel column and eluted with CH2Cl2. The red to yellow-colored eluent was
collected, and the solvent was removed under reduced pressure to yield
DBP 13 as a yellow-brown-colored solid (49 mg, 58 µmol, 99%). Rf 0.61
(cyclohexane/CH2Cl2: 1/1); m.p. 334 °C; 1H NMR (500 MHz, CD2Cl2): δ
8.20 (s, 4H), 8.04 (s, 2H), 7.47–7.43 (m, 4H), 7.15–7.08 (m, 6H), 6.96–
6.92 (m, 4H), 3.82 (s, 6H); 13C NMR (126 MHz, CD2Cl2): δ* 160.1, 150.1,
145.6, 142.1, 138.1, 136.2, 132.9, 132.8, 132.5, 132.3, 129.2, 127.9, 126.6,
122.7, 121.0, 114.6, 55.7; 19F NMR (282 MHz, CDCl3) δ −62.85; HRMS
(pos. ESI): m/z calcd for C46H26O2F12 838.1736 [M]+•, found 838.1724.
*Despite a high number of scans (61440) not all signals (especially C-F
splitting patterns) are visible due to a low solubility.
diol
8 as a colorless solid (126 mg, 0.183 mmol, 82%). Rf 0.22
(cyclohexane/EtOAc: 10/1); m.p. 224 °C (decomposition); 1H NMR
(500 MHz, CDCl3): δ 7.49 (dd, J = 7.8, 1.7 Hz, 2H), 7.48–7.44 (m, 4H),
7.28 (d, J = 7.9 Hz, 2H), 7.19 (d, J = 1.7 Hz, 2H), 6.95 (s, 2H), 6.92–6.88
(m, 4H), 6.82 (s, 2H), 4.62 (s, 2H), 3.81 (s, 6H), 3.09 (s, 2H), 2.64 (s, 6H),
2.31 (s, 6H), 1.77 (s, 6H); 13C NMR (126 MHz, CDCl3) δ 159.2, 153.6,
141.4, 138.0, 137.9, 137.5, 136.1, 135.9, 133.6, 132.5, 131.8, 128.2, 126.9,
126.5, 121.7, 114.1, 87.9, 62.1, 55.4, 25.7, 23.8, 20.6; HRMS (pos. ESI):
m/z calcd for C48H45O3 669.3363 [M−OH]+, found 669.3359; HRMS (neg.
ESI): m/z calcd for C48H45O4 685.3323 [M−H]−, found 685.3329.
General procedure A for the synthesis of cyclic ketones 4[n]. In a 2 L
three-necked flask equipped with a reflux condenser and a Schlenk line
connection, diketone 2 (400 mg, 1.02 mmol, 1.0 eq.), diborylated bisether
3[n] (1.02 mmol, 1.0 eq.) and K3PO4 (1.73 g, 8.16 mmol, 8.0 eq.) were
dissolved in a mixture of toluene (1.22 L) and H2O (122 mL). Under
vigorous stirring the mixture was purged with argon for 2 h. Then a solution
of Pd(OAc)2 (11.5 mg, 51.2 µmol, 5.0 mol%) and SPhos (42.0 mg,
102 µmol, 10 mol%) in degassed toluene (2 mL) was added, and the flask
was immersed in a preheated oil bath. The mixture was stirred at 100 °C
for 24 h. After cooling to rt the mixture was filtered over a pad of celite and
concentrated under reduced pressure. The remaining mixture was
extracted with CH2Cl2 (2 × 50 mL), dried over Na2SO4, and volatiles were
removed under reduced pressure. Automated flash column
chromatography (SiO2, cyclohexane/CH2Cl2 10/1 + EtOAc 1% to 18%)
yielded cyclic ketones 4[n].
5,10-Dimesityl-2,7-bis(4-methoxyphenyl)indeno[2,1-a]indene (7). Diol
8 (50.6 mg, 73.7 µmol) and p-TsOH∙H2O (2.9 mg, 15 µmol, 0.2 eq.) were
refluxed in toluene (7.5 mL) for 2 h. The solvent was removed under
reduced pressure. Column chromatography (SiO2, cyclohexane/CH2Cl2
1/0 to 1/1) yielded DBP 7 as a red solid (47.6 mg, 73.1 µmol, 99%). Rf 0.38
(cyclohexane/CH2Cl2: 1/1); m.p. 337 °C; 1H NMR (500 MHz, CDCl3): δ
7.37–7.33 (m, 4H), 7.00 (s, 4H), 6.94 (dd, J = 7.6, 1.7 Hz, 2H), 6.89–6.85
(m, 4H), 6.70 (d, J = 1.8 Hz, 2H), 6.62 (d, J = 7.6 Hz, 2H), 3.79 (s, 6H),
2.38 (s, 6H), 2.33 (s, 12H); 13C NMR (126 MHz, CDCl3): δ 159.2, 151.5,
144.9, 140.8, 139.4, 137.5, 136.5, 133.7, 133.4, 129.9, 128.4, 127.8, 125.4,
122.3, 120.8, 114.1, 55.4, 21.3, 20.4; HRMS (pos. APCI): m/z calcd for
C48H43O2 651.3258 [M+H]+, found 651.3258.
18,23-dioxo-1,8-dioxa[8]paracyclo[0](2,7)(4b,9b-dihydroindeno[2,1-
a]indene-5,10-dione)o[0]paracyclophane (4[6]). Following general
procedure A using diketone 2 (400 mg, 1.02 mmol, 1.0 eq.), diborylated
bisether 3[6] (533 mg, 1.02 mmol, 1.0 eq.), K3PO4 (1.73 g, 8.16 mmol,
8.0 eq.), Pd(OAc)2 (11.5 mg, 51.2 µmol, 5.0 mol%) and SPhos (41.9 mg,
102 µmol, 10 mol%) in a mixture of toluene (1.22 L) and H2O (122 mL),
4[6] was obtained as a colorless solid (16.5 mg, 33.0 µmol, 3%). Rf 0.17
(cyclohexane/EtOAc: 6/1); Rf 0.15 (cyclohexane/CH2Cl2/EtOAc: 10/1/1);
1H NMR (400 MHz, CDCl3): δ 7.68 (dd, J = 8.3, 1.8 Hz, 2H), 7.63 (d, J =
8.2 Hz, 2H), 7.46 (dd, J = 1.8, 0.7 Hz, 2H), 7.25–7.21 (m, 4H), 6.81–6.77
(m, 4H), 4.52 (s, 2H), 4.14–4.00 (m, 4H), 1.52–1.32 (m, 4H), 1.29–1.10 (m,
4H); 13C NMR (101 MHz, CDCl3): δ 203.3, 157.6, 147.5, 142.3, 134.4,
133.0, 131.9, 128.3, 127.6, 123.4, 116.7, 68.2, 53.6, 27.9, 25.4; HRMS
(pos. ESI): m/z calcd for C34H29O4 501.2060 [M+H]+, found 501.2061.
5,10-Bis(3,5-bis(trifluoromethyl)phenyl)-2,7-bis(4-methoxyphenyl)-
4b,5,9b,10-tetrahydroindeno[2,1-a]indene-5,10-diol (15). In an oven-
dried Schlenk tube anhydrous CeCl3 (174 mg, 0.706 mmol, 3.1 eq.) was
dried at 135 °C and 1∙10−3 mbar for 2 h. The tube was purged with argon,
cooled to rt, and anhydrous THF (2.3 mL) was added. The white
suspension was stirred at rt overnight. The mixture was cooled to 0 °C,
and
a solution of 3,5-bis(trifluoromethyl)phenylmagnesium bromide
(0.87 M in THF, 0.77 mL, 0.67 mmol, 3.0 eq.) was added dropwise. The
resulting brown suspension was stirred at 0 °C for 1.5 h. Diketone 5
(100 mg, 0.224 mmol) was added in one portion, and the resulting mixture
was stirred at 0 °C for 4 h. The reaction mixture was quenched with 10%
aq. HOAc (4 mL), warmed to rt and extracted with CH2Cl2 (2 × 20 mL). The
organic layer was washed with sat. aq. NaHCO3, dried over Na2SO4, and
the solvent was removed under reduced pressure. Column
chromatography (SiO2, cyclohexane/EtOAc 20/1 to 10/1) yielded diol 15
as an off-white solid (155 mg, 0.177 mmol, 79%). Rf 0.22
(cyclohexane/EtOAc: 10/1); m.p. 187 °C; 1H NMR (500 MHz, CDCl3): δ
8.06 (s, 4H), 7.95–7.94 (m, 2H), 7.61 (dd, J = 7.9, 1.6 Hz, 2H), 7.44–7.41
(m, 4H), 7.15 (d, J = 1.6 Hz, 2H), 7.12 (d, J = 7.9 Hz, 2H), 6.94–6.91 (m,
4H), 4.40 (s, 2H), 3.82 (s, 6H), 3.28 (s, 2H); 13C NMR (126 MHz, CDCl3):
δ 159.6, 149.5, 148.2, 143.0, 137.7, 132.9, 132.1 (q, 2JCF = 33.4 Hz), 128.0,
20,25-dioxo-1,10-dioxa[10]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
a]indeno[0]paracyclophane (4[8]). Following general procedure A using
diketone 2 (400 mg, 1.02 mmol, 1.0 eq.), diborylated bisether 3[8] (561 mg,
1.02 mmol, 1.0 eq.), K3PO4 (1.73 g, 8.16 mmol, 8.0 eq.), Pd(OAc)2
(11.5 mg, 51.2 µmol, 5.0 mol%) and SPhos (42.0 mg, 102 µmol, 10 mol%)
in a mixture of toluene (1.22 L) and H2O (122 mL), 4[8] was obtained as a
colorless solid (114 mg, 0.216 mmol, 21%). Rf 0.23 (cyclohexane/EtOAc:
6/1); Rf 0.14 (cyclohexane/CH2Cl2/EtOAc: 10/1/1); m.p. 242 °C; 1H NMR
(500 MHz, CDCl3): δ 7.79 (dd, J = 8.2, 1.9 Hz, 2H), 7.71 (d, J = 8.2 Hz,
2H), 7.58 (d, J = 1.9 Hz, 2H), 7.42–7.38 (m, 4H), 6.87–6.83 (m, 4H), 4.52
(s, 2H), 4.15–4.02 (m, 4H), 1.52–1.40 (m, 2H), 1.40–1.28 (m, 2H), 1.22–
0.99 (m, 8H); 13C NMR (126 MHz, CDCl3): δ 203.1, 157.5, 147.8, 142.0,
3
1
128.4, 126.7 (q, JCF = 3.7 Hz), 126.0, 123.5 (q, JCF = 272.8 Hz), 123.4,
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134.0, 132.9, 131.5, 128.2, 127.2, 123.5, 117.0, 68.3, 53.3, 29.7, 27.8,
25.6; HRMS (pos. APCI): m/z calcd for C36H33O4 529.2373 [M+H]+, found
529.2372.
20,25-dimesityl-20,25-dihydroxy-1,10-
dioxa[10]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
a]indeno[0]paracyclophane (6[8]). Following general procedure B using
diketone 4[8] (100 mg, 189 µmol, 1.0 eq.), anhydrous CeCl3 (145 mg,
588 µmol, 3.1 eq.), mesityl magnesium bromide (0.90 M in THF, 0.63 mL,
57 µmol, 3.0 eq.) and THF (3.8 mL), 6[8] was obtained as a white solid
(129 mg, 168 µmol, 89%). Rf 0.21 (cyclohexane/EtOAc: 10/1); m.p.
236 °C (decomposition); 1H NMR (500 MHz, CDCl3, 243 K): δ 7.89 (d, J =
8.1 Hz, 2H), 7.65 (dd, J = 8.1, 1.7 Hz, 2H), 7.53 (d, J = 1.7 Hz, 2H), 7.50–
7.46 (m, 4H), 7.03 (s, 2H), 6.90–6.85 (m, 4H), 6.61 (s, 2H), 4.37 (s, 2H),
4.22–4.08 (m, 4H), 2.86 (s, 6H), 2.51 (s, 2H), 2.22 (s, 6H), 1.68–1.55 (m,
2H), 1.42 (s, 6H), 1.38–1.15 (m, 6H), 1.12–0.90 (m, 4H); 13C NMR
(126 MHz, CDCl3, 243 K): δ 156.1, 150.3, 139.2, 139.2, 138.3, 136.9,
136.7, 135.0, 132.5, 132.0, 131.5, 128.2, 127.4, 126.4, 123.5, 116.0, 87.0,
67.4, 58.9, 29.4, 27.0, 25.1, 23.9, 23.0, 20.6; HRMS (pos. APCI): m/z calcd
for C54H60NO4 786.4517 [M+NH4]+, found 786.4515; m/z calcd for
C54H55O3 751.4146 [M−OH]+, found 751.4144.
21,26-dioxo-1,11-dioxa[11]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
a]indeno[0]paracyclophane (4[9]). Following general procedure A using
diketone 2 (400 mg, 1.02 mmol, 1.0 eq.), diborylated bisether 3[9] (576 mg,
1.02 mmol, 1.0 eq.), K3PO4 (1.73 g, 8.16 mmol, 8.0 eq.), Pd(OAc)2
(11.6 mg, 51.7 µmol, 5.0 mol%) and SPhos (42.0 mg, 102 µmol, 10 mol%)
in a mixture of toluene (1.22 L) and H2O (122 mL), 4[9] was obtained as
an
off-white
solid
(164 mg,
0.302 mmol,
30%).
Rf
0.21
(cyclohexane/EtOAc: 6/1); m.p. 233 °C; 1H NMR (400 MHz, CDCl3): δ
7.78 (dd, J = 8.2, 1.9 Hz, 2H), 7.73 (dd, J = 8.2, 0.6 Hz, 2H), 7.65 (dd, J =
1.9, 0.7 Hz, 2H), 7.44–7.39 (m, 4H), 6.89–6.84 (m, 4H), 4.52 (s, 2H), 4.19–
4.04 (m, 4H), 1.55–1.44 (m, 4H), 1.20–0.93 (m, 10H); 13C NMR (101 MHz,
CDCl3): δ 202.9, 158.5, 147.7, 141.8, 134.0, 133.1, 131.1, 128.0, 127.0,
122.8, 116.4, 68.5, 53.2, 30.7, 29.8, 29.3, 26.5; HRMS (pos. APCI): m/z
calcd for C37H35O4 543.2530 [M+H]+ found 543.2527.
21,26-dimesityl-21,26-dihydroxy-1,11-
dioxa[11]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
22,27-dioxo-1,12-dioxa[12]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
a]indeno[0]paracyclophane (4[10]). Following general procedure A
using diketone 2 (400 mg, 1.02 mmol, 1.0 eq.), diborylated bisether 3[10]
(590 mg, 1.02 mmol, 1.0 eq.), K3PO4 (1.73 g, 8.16 mmol, 8.0 eq.),
Pd(OAc)2 (11.5 mg, 51.2 µmol, 5.0 mol%) and SPhos (42.1 mg, 103 µmol,
10 mol%) in a mixture of toluene (1.22 L) and H2O (122 mL), 4[10] was
obtained as an off-white solid (247 mg, 0.444 mmol, 43%). Rf 0.28
(cyclohexane/EtOAc: 6/1); Rf 0.18(cyclohexane/CH2Cl2/EtOAc: 10/1/1);
m.p. 222 °C; 1H NMR (400 MHz, CDCl3): δ 7.80 (dd, J = 8.2, 1.9 Hz, 2H),
7.76 (dd, J = 8.2, 0.6 Hz, 2H), 7.67 (dd, J = 1.9, 0.7 Hz, 2H), 7.47–7.39 (m,
4H), 6.93–6.84 (m, 4H), 4.52 (s, 2H), 4.14–4.04 (m, 4H), 1.51–1.38 (m,
4H), 1.22–0.98 (m, 12H); 13C NMR (101 MHz, CDCl3): δ 202.9, 158.1,
148.1, 142.1, 134.2, 133.3, 131.6, 128.1, 127.1, 123.1, 117.3, 68.6, 53.3,
30.3, 30.2, 28.4, 25.7; HRMS (pos. APCI): m/z calcd for C38H37O4
557.2686 [M+H]+, found 557.2688.
a]indeno[0]paracyclophane (6[9]). Following general procedure B using
diketone 4[9] (119 mg, 220 µmol, 1.0 eq.), anhydrous CeCl3 (169 mg,
686 µmol, 3.1 eq.), mesityl magnesium bromide (0.90 M in THF, 0.74 mL,
67 µmol, 3.0 eq.) and THF (2.2 mL), 6[9] was obtained as a white solid
(159 mg, 203 µmol, 92%). Rf 0.08 (cyclohexane/EtOAc: 20/1); m.p.
253 °C (decomposition); 1H NMR (500 MHz, CDCl3, 243 K): δ 7.89 (d, J =
8.6 Hz, 2H), 7.60–7.56 (m, 4H), 7.49–7.44 (m, 4H), 7.02 (br. s, 2H), 6.90–
6.86 (m, 4H), 6.59 (br. s, 2H), 4.35 (s, 2H), 4.21–4.10 (m, 4H), 2.85 (s, 6H),
2.52 (s, 2H), 2.20 (s, 6H), 1.60–1.44 (m, 4H), 1.40 (s, 6H), 1.21–1.08 (m,
8H), 1.02–0.91 (m, 2H); 13C NMR (126 MHz, CDCl3, 243 K): δ 157.4, 150.6,
139.3, 139.2, 138.3, 136.9, 136.8, 135.0, 132.5, 131.8, 131.5, 127.9, 127.3,
127.0, 122.9, 115.5, 87.0, 67.9, 58.8, 30.4, 29.3, 28.6, 26.3, 23.9, 23.0,
20.6; HRMS (pos. ESI): m/z calcd for C55H58O4 782.4330 [M]●+, found
782.4329; m/z calcd for C55H56O3 764.4224 [M−OH2]●+, found 764.4224.
22,27-dimesityl-22,27-dihydroxy-1,12-
dioxa[12]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
24,29-dioxo-1,14-dioxa[14]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
a]indeno[0]paracyclophane (4[12]). Following general procedure A
using diketone 2 (400 mg, 1.02 mmol, 1.0 eq.), diborylated bisether 3[12]
(619 mg, 1.02 mmol, 1.0 eq.),K3PO4 (1.73 g, 8.16 mmol, 8.0 eq.),
Pd(OAc)2 (11.5 mg, 51.2 µmol, 5.0 mol%) and SPhos (41.9 mg, 102 µmol,
10 mol%) in a mixture of toluene (1.22 L) and H2O (122 mL), 4[12] was
obtained as an off-white solid (321 mg, 0.549 mmol, 54%). Rf 0.20
(cyclohexane/CH2Cl2/EtOAc: 10/1/1); m.p. 202 °C; 1H NMR (400 MHz,
CDCl3): 7.82–7.80 (m, 4H), 7.73–7.72 (m, 2H), 7.47–7.41 (m, 4H), 6.93–
6.88 (m, 4H), 4.52 (s, 2H), 4.12 (t, J = 6.7 Hz, 4H), 1.61–1.47 (m, 4H),
1.38–0.96 (m, 16H); 13C NMR (101 MHz, CDCl3): δ 202.7, 158.4, 148.1,
142.1, 134.2, 133.5, 131.5, 128.0, 126.9, 122.8, 116.9, 68.4, 53.2, 30.6,
30.2, 30.2, 28.6, 25.8; HRMS (pos. APCI): m/z calcd for C40H41O4
585.2999 [M+H]+, found 585.2994.
a]indeno[0]paracyclophane (6[10]). Following general procedure B
using diketone 4[10] (140 mg, 251 µmol, 1.0 eq.), anhydrous CeCl3
(193 mg, 783 µmol, 3.1 eq.), mesityl magnesium bromide (0.90 M in THF,
0.84 mL, 0.76 mmol, 3.0 eq.) and THF (5.0 mL), 6[10] was obtained as a
white solid (188 mg, 236 µmol, 94%). Rf 0.24 (cyclohexane/EtOAc: 10/1);
Rf 0.10 (cyclohexane/EtOAc: 20/1); m.p. 242 °C (decomposition); 1H NMR
(500 MHz, [D6]DMSO, 373 K): δ 7.94 (d, J = 8.1 Hz, 2H), 7.69 (d, J = 1.8
Hz, 2H), 7.56–7.52 (m, 4H), 7.50 (dd, J = 8.1, 1.8 Hz, 2H), 6.94–6.90 (m,
4H), 6.71 (s, 4H), 5.34 (s, 2H), 4.14–4.03 (m, 4H), 3.97 (s, 2H), 2.13 (s,
6H), 2.10 (br. s, 12H), 1.48–1.40 (m, 4H), 1.21–1.09 (m, 4H), 1.09–0.96
(m, 8H); 13C NMR (126 MHz, [D6]DMSO, 373 K): δ 156.7, 150.6, 139.7,
139.5, 136.7, 135.7, 134.6, 131.9, 130.9, 127.9, 126.5, 124.8, 121.6, 116.6,
85.4, 67.7, 58.7, 29.0, 28.8, 27.5, 24.6, 22.1, 19.3; HRMS (pos. ESI): m/z
calcd for C56H60O4Na 819.4384 [M+Na]+, found 819.4382, m/z calcd for
C56H59O3 779.4459 [M−OH]+, found 779.4457.
General procedure B for the synthesis of cyclic diols 6[n] and 14[n].
In an oven-dried Schlenk tube anhydrous CeCl3 (3.1 eq.) was dried at
135 °C at 1∙10−3 mbar for 2 h. The tube was purged with argon, cooled to
rt, and anh. THF was added. The resulting white suspension was stirred
at rt overnight. The mixture was cooled to 0 °C, and a solution of mesityl
magnesium bromide or 3,5-bis(trifluoromethyl)phenyl magnesium bromide
(~0.90 M in THF, 3.0 eq.) was added dropwise. The grey suspension was
stirred at 0 °C for 1.5 h. Diketone 4[n] (1.0 eq.) was added in one portion,
and the resulting yellow mixture was stirred at 0 °C for 4 h. The reaction
mixture was quenched with 10% aq. HOAc, warmed to rt and extracted
with CH2Cl2 (2 times). The organic layer was washed with sat. aq. NaHCO3,
dried over Na2SO4, and the solvent was removed under reduced pressure.
Automated flash column chromatography (SiO2, cyclohexane/EtOAc 20/1
to 10/1) yielded diol 6[n].
24,29-dimesityl-24,29-dihydroxy-1,14-
dioxa[14]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
a]indeno[0]paracyclophane (6[12]). Following general procedure B
using diketone 4[12] (234 mg, 0.400 mmol, 1.0 eq.), anhydrous CeCl3
(306 mg, 1.24 mmol, 3.1 eq.), mesityl magnesium bromide (0.90 M in THF,
1.30 mL, 1.17 mmol, 2.9 eq.) and THF (4.0 mL), 6[12] was obtained as a
white solid (305 mg, 0.370 mmol, 92%). Rf 0.27 (cyclohexane/EtOAc:
10/1); Rf 0.13 (cyclohexane/EtOAc: 20/1); m.p. 253 °C (decomposition);
1H NMR (500 MHz, CDCl3, 254 K): δ 7.93 (d, J = 8.1 Hz, 2H), 7.66 (d, J =
1.8 Hz, 2H), 7.58 (dd, J = 8.1, 1.8 Hz, 2H), 7.51–7.44 (m, 4H), 7.02 (s, 2H),
6.95–6.88 (m, 4H), 6.59 (s, 2H), 4.34 (s, 2H), 4.20–4.06 (m, 4H), 2.86 (s,
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10.1002/chem.201800322
Chemistry - A European Journal
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6H), 2.53 (s, 2H), 2.20 (s, 6H), 1.62–1.49 (m, 4H), 1.38 (s, 6H), 1.31–0.95
(m, 16H); 13C NMR (126 MHz, CDCl3, 254 K): δ 157.5, 150.8, 139.7, 139.7,
138.4, 137.4, 137.0, 134.9, 132.6, 132.2, 131.6, 127.7, 127.7, 127.1, 123.3,
116.3, 86.8, 67.9, 59.3, 30.4, 30.0, 29.9, 28.1, 25.4, 24.0, 22.8, 20.6;
HRMS (pos. ESI): m/z calcd for C58H64O4Na 847.4697 [M+Na]+, found
847.4695, m/z calcd for C58H63O3 807.4772 [M−OH]+, found 807.4767.
22,27-dimesityl-1,12-dioxa[12]paracyclo[0](2,7)indeno[2,1-
a]indeno[0]paracyclophane (1[10]). Following general procedure C
using diol 6[10] (39.2 mg, 49.2 µmol, 1.0 eq.), Burgess’ reagent (94.0 mg,
395 µmol, 8.0 eq.) and 1,4-dioxane (5.0 mL), the mixture was stirred for
48 h at 100 °C. The mixture was then loaded onto a short column of silica
gel and eluted with cyclohexane/acetone 1/0 to 20/1. The red-colored band
was collected and the volatiles removed under reduced pressure. To ease
the separation process the mono-eliminated species was reacted to the
ring-opened rearrangement product (cf. Scheme 5). Therefore the crude
product was dissolved in CHCl3 (10 mL), trifluoroacetic anhydride (0.1 mL)
was added, and the mixture was refluxed for 2 h. The reaction mixture was
washed with sat. aq. NaHCO3 (10 mL) and dried over Na2SO4. Flash
column chromatography (silica gel, cyclohexane/acetone: 100/1) yielded
24,29-di(3,5-bis(trifluoromethyl)phenyl)-24,29-dihydroxy-1,14-
dioxa[14]paracyclo[0](2,7)4b,9b-dihydroindeno[2,1-
a]indeno[0]paracyclophane (14[12]). Following general procedure B
using diketone 4[12] (59.8 mg, 0.102 µmmol, 1.0 eq.), anhydrous CeCl3
(79.0 mg, 321 µmol, 3.1 eq.), 3,5-bis(trifluoromethyl)phenyl magnesium
bromide (0.87 M in THF, 0.36 mL, 0.31 mmol, 3.0 eq.) and THF (2.1 mL),
14[12] was obtained as a slightly yellow solid (98 mg, 97 µmol, 95%). Rf
0.14 (cyclohexane/EtOAc: 20/1); m.p. 151 °C (decomposition); 1H NMR
(500 MHz, CDCl3): δ 7.91 (d, J = 8.1 Hz, 2H), 7.78 (s, 2H), 7.73 (s, 4H),
7.71 (dd, J = 8.2, 1.9 Hz, 2H), 7.52–7.45 (m, 4H), 7.44 (d, J = 1.7 Hz, 2H),
6.95–6.88 (m, 4H), 4.19–4.06 (m, 4H), 4.05 (s, 2H), 2.70 (s, 2H), 1.62–
1.52 (m, 4H), 1.28–1.20 (m, 4H), 1.15–1.01 (m, 12H); 13C NMR (126 MHz,
1[10] as
a
red solid (24.3 mg, 31.9 µmol, 65%). Rf 0.49
(cyclohexane/EtOAc: 20/1); m.p. 320 °C; 1H NMR (300 MHz, CDCl3): δ
7.33–7.27 (m, 4H), 7.08 (dd, J = 8.0, 1.8 Hz, 2H), 7.06–7.03 (m, 2H), 6.89–
6.82 (m, 6H), 6.61 (d, J = 1.8 Hz, 2H), 6.52 (d, J = 8.0 Hz, 2H), 4.16–3.97
(m, 4H), 2.50 (s, 6H), 2.38 (s, 6H), 1.29 (s, 6H), 1.37–1.09 (m, 4H), 1.01–
0.71 (m, 12H); 13C NMR (101 MHz, CDCl3) δ 156.0, 149.4, 142.3, 139.5,
139.1, 136.2, 136.1, 133.8, 133.5, 132.0, 129.1, 127.4, 127.2, 126.3, 123.5,
122.1, 120.0, 118.2, 70.0, 30.3, 30.1, 28.5, 26.2, 20.2, 19.3, 16.8; HRMS
(pos. ESI): m/z calcd for C56H56O2 760.4275 [M]●+, found 760.4272.
2
CDCl3): δ 158.4, 149.1, 146.7, 141.8, 136.6, 131.9 (q, JCF = 32.6 Hz),
3
1
131.8, 128.0, 127.9, 127.7, 125.7 (q, JCF = 3.9 Hz), 123.0 (q, JCF
=
272.8 Hz), 121.7 (sept, 3JCF = 4.2 Hz), 121.6, 116.6, 85.5, 68.3, 58.6, 30.6,
30.2, 30.1, 28.6, 25.8; 19F NMR (471 MHz, CDCl3): δ −62.70; HRMS (neg.
ESI): m/z calcd for C56H47O4F12 1011.3288 [M−H]−, found 1011.3267.
24,29-dimesityl-1,14-dioxa[14]paracyclo[0](2,7)indeno[2,1-
a]indeno[0]paracyclophane (1[12]). Following general procedure C
using diol 6[12] (42.9 mg, 52.0 µmol, 1.0 eq.), Burgess’ reagent (99.1 mg,
416 µmol, 8.0 eq.) and 1,4-dioxane (5.0 mL), the mixture was stirred for
24 h at 100 °C. The mixture was then transferred into a separatory funnel,
diluted with CH2Cl2, and H2O (5 mL) and sat. aq. NaHCO3 (10 mL) solution
were added. The aq. layer was extracted with CH2Cl2 (2 × 20 mL), the
combined organic layers were dried over Na2SO4 and volatiles were
removed under reduced pressure. To ease the separation process, the
mono-eliminated species was reacted to the ring-opened rearrangement
product (cf. Scheme 5). Therefore the crude product was dissolved in
CHCl3 (10 mL), trifluoroacetic anhydride (0.1 mL) was added and the
mixture was refluxed for 1 h. The reaction mixture was washed with sat.
aq. NaHCO3 (10 mL) and dried over Na2SO4. Flash column
chromatography (silica gel, cyclohexane/acetone: 100/1) yielded 1[12] as
a red solid (29.7 mg, 37.6 µmol, 72%). Rf 0.50 (cyclohexane/EtOAc: 20/1);
m.p. 319 °C; 1H NMR (300 MHz, CDCl3): δ 7.38–7.31 (m, 4H), 7.08 (dd,
J = 7.9, 1.7 Hz, 3H), 7.07–7.04 (m, 2H), 6.92–6.85 (m, 6H), 6.65 (d, J =
1.7 Hz, 2H), 6.52 (d, J = 7.9 Hz, 2H), 4.14–4.04 (m, 4H), 2.51 (s, 6H), 2.39
(s, 6H), 1.54 (s, 6H), 1.38–1.21 (m, 4H), 0.93–0.69 (m, 16H); 13C NMR
(101 MHz, CDCl3): δ 157.3, 150.7, 143.8, 140.1, 140.0, 137.3, 137.2,
135.2, 134.6, 133.1, 130.2, 128.4, 128.3, 127.3, 124.5, 122.9, 120.8, 119.9,
71.4, 31.5, 31.2, 31.1, 29.5, 27.3, 21.3, 20.5, 18.6; HRMS (pos. ESI): m/z
calcd for C58H60O2 788.4588 [M]●+, found 788.4591.
General procedure C for the synthesis of DBP-phanes 1[n] and 12[12].
Diol 6[n] (1.0 eq.) or 14[12] and Burgess’ reagent[40] (8 eq.) were placed
in a dried Schlenk tube, and the tube was evacuated and backfilled with
argon (3 times). Anh. 1,4-dioxane was added, the tube was sealed, and
the mixture was stirred at 100 °C for 24 h. The mixture was transferred into
a separatory funnel, diluted with CH2Cl2 and H2O, and sat. aq. NaHCO3
solution were added. The aq. layer was extracted with CH2Cl2 (2 times),
the combined organic layers were dried over Na2SO4, and volatiles were
removed under reduced pressure. The crude product was purified by flash
column chromatography (silica gel, cyclohexane/acetone: 100/1) to yield
DBP-phanes 1[n] and 12[12].
20,25-dimesityl-1,10-dioxa[10]paracyclo[0](2,7)indeno[2,1-
a]indeno[0]paracyclophane (1[8]). Following general procedure C using
diol 6[8] (42.9 mg, 55.7 µmol, 1.0 eq.), Burgess’ reagent (106 mg,
445 µmol, 8.0 eq.) and 1,4-dioxane (5.5 mL), 1[8] was obtained as a red
solid (31.2 mg, 42.6 µmol, 76%). Rf 0.44 (cyclohexane/EtOAc: 20/1); m.p.
345 °C; 1H NMR (500 MHz, CDCl3): δ 7.24–7.20 (m, 4H), 7.09 (dd, J = 8.1,
1.8 Hz, 2H), 7.04 (s, 2H), 6.86–6.77 (m, 6H), 6.53 (d, J = 1.8 Hz, 2H), 6.49
(d, J = 8.1 Hz, 2H), 4.18–3.96 (m, 4H), 2.50 (s, 6H), 2.38 (s, 6H), 1.27–
1.17 (m, 4H), 0.91 (s, 6H), 0.99–0.74 (m, 8H); 13C NMR (126 MHz, CDCl3):
δ 156.9, 150.1, 142.9, 141.2, 140.7, 137.4, 137.3, 134.8, 134.6, 133.2,
130.4, 128.7, 128.3, 127.7, 124.6, 123.8, 122.2, 118.4, 70.3, 31.2, 29.8,
27.6, 21.4, 20.5, 17.1; HRMS (pos. APCI): m/z calcd for C54H53O2
733.4040 [M+H]+, found 733.4033.
24,29-di(3,5-bis(trifluoromethyl)phenyl)-1,14-
dioxa[14]paracyclo[0](2,7)indeno[2,1-a]indeno[0]paracyclophane
12[12]). Following general procedure C using diol 14[12] (38.1 mg,
37.6 µmol, 1.0 eq.), Burgess’ reagent (72.0 mg, 302 µmol, 8.0 eq.) and
1,4-dioxane (3.9 mL), the mixture was stirred for 24 h at 100 °C. The
reaction mixture was loaded onto a short column of silica gel and eluted
with cyclohexane/acetone 1/0 to 20/1. The ocher-colored band was
collected, and the volatiles were removed under reduced pressure. To
ease the separation process, the mono-eliminated species was reacted to
the ring-opened rearrangement product (cf. Scheme 5). Therefore the
crude product was dissolved in CHCl3 (10 mL), trifluoroacetic anhydride
(0.08 mL) was added and the mixture was refluxed for 2.5 h. The reaction
mixture was washed with sat. aq. NaHCO3 (10 mL) and dried over Na2SO4.
Flash column chromatography (silica gel, cyclohexane/acetone: 100/1)
yielded 12[12] as a black solid (10.7 mg, 11.0 µmol, 29%). Rf 0.28
21,26-dimesityl-1,11-dioxa[11]paracyclo[0](2,7)indeno[2,1-
a]indeno[0]paracyclophane (1[9]). Following general procedure C using
diol 6[9] (51.7 mg, 66.0 µmol, 1.0 eq.), Burgess’ reagent (127 mg,
533 µmol, 8.0 eq.) and 1,4-dioxane (6.5 mL), 1[9] was obtained as a red
solid (41.2 mg, 55.2 µmol, 84%). Rf 0.45 (cyclohexane/EtOAc: 20/1); m.p.
331 °C; 1H NMR (400 MHz, CDCl3): δ 7.28–7.24 (m, 4H), 7.03 (dd, J = 8.0,
1.8 Hz, 4H), 6.86–6.82 (m, 6H), 6.61 (dd, J = 1.8, 0.4 Hz, 2H), 6.50 (dd,
J = 8.1, 0.4 Hz, 2H), 4.12–3.98 (m, 4H), 2.50 (s, 6H), 2.38 (s, 6H), 1.22–
1.11 (m, 10H), 0.96–0.85 (m, 6H), 0.81–0.73 (m, 4H); 13C NMR (101 MHz,
CDCl3): δ 156.9, 150.5, 143.2, 141.0, 140.6, 137.4, 137.3, 135.1, 134.8,
133.2, 130.3, 128.6, 128.4, 127.7, 125.3, 123.5, 121.2, 119.5, 71.3, 32.5,
30.9, 29.7, 26.9, 21.4, 20.5, 17.6; HRMS (pos. APCI): m/z calcd for
C55H55O2 747.4197 [M+H]+, found 747.4189.
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(cyclohexane/acetone: 100/1); m.p. 264 °C; 1H NMR (500 MHz, CDCl3): δ
8.10 (s, 4H), 8.00 (s, 2H), 7.38–7.33 (m, 4H), 7.17 (dd, J = 8.1, 1.7 Hz, 2H),
7.04 (d, J = 1.7 Hz, 2H), 7.00 (d, J = 8.1 Hz, 2H), 6.93–6.89 (m, 4H), 4.11
(t, J = 7.3 Hz, 4H), 1.37–1.20 (m, 4H), 1.12–0.83 (m, 16H); 13C NMR
(126 MHz, CDCl3): δ 157.6, 149.9, 144.6, 142.6, 138.3, 136.1, 133.3,
132.6 (q, JCF = 33.7 Hz), 131.2, 128.7 (q, JCF = 3.7 Hz), 127.8, 125.7,
123.3 (q, JCF = 272.8 Hz), 122.8 (sept, JCF = 3.7 Hz), 121.4, 119.0, 70.3,
31.6, 31.2, 31.1, 28.7, 26.5; 19F NMR (471 MHz, CDCl3): δ −62.91; HRMS
(pos. ESI): m/z calcd for C56H44F12O2 976.3144 [M]●+, found 976.3153.
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M.H. thanks J. Selau for preparative assistance. Generous
support by the German Research Foundation (Emmy Noether
grant ES 361/2-1 and grant No. INST 40/467-1 FUGG) and the
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Entry for the Table of Contents
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Bent dibenzo[a,e]pentalenes (DBPs)
were synthesized through incorporation
into a cyclic structure. The last, strain-
inducing dehydration reaction was
accomplished using the Burgess
reagent. Single crystal X-ray structures
revealed a near semi-circle and bend
angle of the DBP unit of almost 88° for
the smallest derivative. The synthetic
strategy presented herein may pave the
way for the synthesis of conjugated
DBP nanorings.
Mathias Hermann,[a] Daniel Wassy,[a]
Daniel Kratzert[b] and Birgit Esser*,[a]
Page No. – Page No.
Dibenzo[a,e]pentalenophanes –
Bending a Non-Alternant
Hydrocarbon
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