Molecular propellers that consist of dehydrobenzo[14]annulene blades

Article abstract of DOI:10.1002/chem.201201061

A new class of propel- ler-shaped compound (4), which consisted of dehydrobenzo[14]annulene ([14]DBA) blades, as well as its naphtho homologues (5 and 6), have been prepared. Although NMR studies of compound 4 did not provide useful information regarding

Full text of DOI:10.1002/chem.201201061

FULL PAPER
DOI: 10.1002/chem.201201061
Molecular Propellers that Consist of Dehydrobenzo[14]annulene Blades
Shunpei Nobusue,[a] Yuichi Mukai,[a] Yo Fukumoto,[a] Rui Umeda,[a, b]
Kazukuni Tahara,[a] Motohiro Sonoda,[a, c] and Yoshito Tobe*[a]
Abstract:
A
new class of propel-
twisted substantially owing to steric re-
pulsion between the neighboring ben-
zene rings. On the contrary, in the case
of compound 6, although the DFT cal-
culations with the B3LYP functional
predicted that the D3-symmetric con-
formation was more stable, calculations
with the M05 and M05-2X functionals
indicated that the C2 conformer was
more favorable because of p–p interac-
tions between the naphthalene units of
a pair of neighboring blades. Indeed,
X-ray analysis of compound 6 showed
that it adopted an approximately C2-
symmetric conformation. Moreover, on
the basis of variable-temperature
1H NMR measurements, we found that
compound 6 adopted a C2 conforma-
tion and the barrier for interconversion
between the C2–C2 conformers was es-
timated to be 16.2 kcalmolꢀ1; however,
no indication of the presence of the D3
isomer was obtained. The relatively
small energy barriers to interconver-
sion, despite the large overlapping of
neighboring blades, was ascribed to the
flexibility of the acetylene linkages,
which could be deformed substantially
in the transition state of the ring-flip.
ler-shaped compound (4), which con-
sisted of dehydrobenzo[14]annulene
([14]DBA) blades, as well as its naph-
tho homologues (5 and 6), have been
prepared. Although NMR studies of
compound 4 did not provide useful in-
formation regarding its conformation
in solution, DFT calculations with dif-
ferent functionals and the 6-31G* basis
set all indicated that the D3-symmetric
structure was energetically more favor-
able than the C2 conformer. From X-
ray crystallographic analysis, it ap-
peared that compound 4 adopted a
propeller-shaped-, approximately D3-
symmetric structure in the solid state,
in which the [14]DBA blades were
Keywords: activation barrier · an-
nulenes · density functional calcula-
tions · fused-ring systems · propel-
ler-shaped compounds
Introduction
phenylene, including hexabenzotriphenylene,[3–7] and decacy-
clenes,[8] which, in principle, can adopt C2- and/or D3 confor-
mations. Pascal and co-workers reported that hexabenzotri-
phenylene (1), a prototypical propeller-shaped compound,
Propeller-shaped polycyclic aromatic compounds, which
consist of a core benzene ring that is fused by three aromat-
ic blades, have been attracting considerable interest in view
of their unique conformations and their relevant dynamic
behaviors.[1] These compounds include suitably substituted
derivatives of triphenylene,[2] extended homologues of tri-
adopted
a strongly twisted D3-symmetric conformation
when it was synthesized by high-temperature vacuum pyrol-
ysis of phenanthrene-9,10-dicarboxylic anhydride.[3] Through
an extensive computational study, they concluded that the
D3 isomer of compound 1 was thermodynamically more
stable than the C2 isomer. They also proposed a rule of
thumb for the C2/D3 dichotomy of sterically overcrowded
triphenylene- and decacyclene derivatives: the D3 conforma-
tion is preferred if the central benzene ring is expected to
be aromatic (i.e., delocalized benzene-like bonds with a
small degree of bond-alternation), whereas the C2 conforma-
tion will be observed if the central ring is nonaromatic (i.e.,
bonds with a high degree of bond-alternation). In other
words, this rule may be understood as follows: if the core
benzene ring is less flexible (i.e., more resistant to deforma-
tion) than the blades, as in the case of compound 1, the
blades will be twisted into a chair-like form with D3 symme-
try, because such a twist will not disturb the overlap be-
tween the p orbitals in the peripheral aromatic rings. Con-
versely, if the blades are more resistant toward deformation
than the core benzene ring, as in the case of substituted tri-
phenylenes, the core benzene ring will be distorted into a
twist boat-like form with C2 symmetry because the local
[a] S. Nobusue, Y. Mukai, Y. Fukumoto, Dr. R. Umeda, Dr. K. Tahara,
Dr. M. Sonoda, Prof. Y. Tobe
Division of Frontier Materials Science
Graduate School of Engineering Science, Osaka University
1-3 Machikaneyama, Toyonaka, Osaka 560-8531 (Japan)
Fax : (+81)6-6850-6225
E-mail: tobe@chem.es.osaka-u.ac.jp
[b] Dr. R. Umeda
Current address:
Department of Chemistry and Materials Engineering
Faculty of Chemistry, Materials and Bioengineering
Kansai University, 3-3-35 Yamate, Suita, Osaka 564-8680 (Japan)
[c] Dr. M. Sonoda
Current address:
Department of Applied Chemistry
Graduate School of Engineering, Osaka Prefecture University
1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201061; including general ex-
perimental methods.
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overlap between the p orbitals of the peripheral benzene
rings will be kept in this form. Pascal’s rule was supported
by several experimental observations. For example, Pꢁrez,
Guitiꢂn, and co-workers reported the isolation of the C2
conformer of compound 1 as a kinetic product of the palla-
dium-catalyzed cyclotrimerization of in-situ-generated 9,10-
didehydrophananthrene and the conversion of the C2 isomer
¼
into the D3 isomer by heating.[4] The barriers (DG ) to the
interconversion between the enantiomeric C2 isomers and
that of C2–D3 isomerization were determined to be 11.7 and
26.2 kcalmolꢀ1, respectively. The kinetically favored forma-
tion of the C2 isomer was also observed in the nickel-cata-
lyzed cyclotrimerization of 9,10-didehydrophenanthrene.[5]
Very recently, the synthesis of the hexa-tert-butyl derivative
of compound 1 by a regioselective FeCl3-mediated Scholl
cyclization was reported;[9] although its dynamic behavior
was not described, it was shown to adopt a D3-symmetric
structure in the crystalline state by X-ray crystallography. In
addition, Mount, Galow, and co-workers revealed that hex-
amethyltriphenylene (2), one of the ideal candidates for ex-
amining Pascal’s C2/D3 dichotomy, underwent a rapid inter-
conversion between two enantiomeric C2 conformers and a
slower C2–D3 interconversion, thus providing strong support
for the rule.[2c]
We have previously reported the synthesis of the nominal-
ly D3h-symmetric, triply fused dehydrobenzo[12]annulene
(3), which consisted of 12-membered dehydroannulene
rings.[10] DFT calculations predicted that it adopted an
almost-planar conformation with a small twist angle of 7.58,
thus indicating that the steric crowding between the blades
of compound 3 was small.[11] However, in the case of higher
homologue
4 (Scheme 1), which contains dehydroben-
zo[14]annulene rings,[12] and its naphthalene homologues (5
and 6), considerable overcrowding in the ground state is ex-
pected because it is not possible to draw a nominal D3h
structure without partial overlapping of the blades. This fact
is more obvious in naphthalene homologues 5 and 6 than in
compound 4 (for the structures of compounds 4–6 in the D3-
and C2 conformations, optimized by DFT calculations, see
the Supporting Information, Figure S1). Therefore, intuitive-
ly, the barriers to their conformational interconversion seem
to be substantial, with those of compounds 5 and 6 being
higher than that of compound 4. If Pascal’s rule is applicable
to compounds 4–6, they would be expected to adopt a D3-
symmetic conformation because the central ring must exhib-
it aromatic character (i.e., a small bond-length alternation)
owing to the relatively weak aromaticity of the dehydroben-
zo[14]annulene system. Because Pascal only argued about
the choice of the C2- and D3 conformations for the triphen-
Scheme 1. Propeller-shaped polycyclic aromatic compounds, including
target compounds 4–6.
that it behaves more flexibly than one may imagine in ex-
tremely overcrowded transition states of conformational
ACHTUNGTRENNUNGylACHTUNGTRENNUNGene- and decacyclene-based propeller-shaped compounds,
the structures of molecular propellers that contain blades
with greater flexibility (owing to a large ring size as well as
weak aromaticity) will provide information on how steric
overcrowding affects the conformational selectivity.
isomACTHGNUTERNNUeG rization, in which this unit causes major steric hin-
drance.[13] However, from the calculated geometries of the
D3-optimized structures (see the Supporting Information,
Figure S1), the racemization of compounds 4–6 through
ring-inversion of the [14]DBA blades seems to be difficult
because the overlap with neighboring blades is so large that
a heavily distorted transition state of the acetylene units is
With regard to the conformational mobility of alkynes, it
ꢀ ꢁ
has been reported by Toyota and co-workers that a C C
ꢀ
C C unit can be substantially deformed from linearity and
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Molecular Propellers with Dehydrobenzo[14]annulene Blades
FULL PAPER
necessary for the slippage of the blades, although the confor-
mational exchanges of compounds 1 and 2 took place with
activation barriers that were smaller than one might imag-
ine.[14] In this respect, it is interesting to see how flexible the
[14]DBA blades are in the conformational interconversion
of compounds 4–6. Thus, we synthesized compounds 4–6
and investigated their confor-
For the preparation of compound 5, key precursor 14 was
prepared by the cross-coupling of iodide 13 with hexaethy-
nylbenzene (12)[17] because larger steric crowding may
hinder the coupling reaction in a direct-substitution route
(Scheme 2b). The Hagihara–Sonogashira coupling reaction
of compound 12, which was derived from its TMS derivative
mational properties by using
computational methods, X-ray
crystallographic analysis, and
variable-temperature
NMR
(VT-NMR) spectroscopy, the
results of which are the topic of
this article.
Results and Discussion
Synthesis of compounds 4–6:
Compounds 4–6 were synthe-
sized by attaching six diethyn
ACHTUNGERTNyNUNG l-
ACHTUNGTRENNUNG
zene ring, followed by intramo-
lecular oxidative coupling, as
shown in Scheme 2. For the
preparation of compound
4
(Scheme 2a), the direct substi-
tution of hexabromobenzene
(7) with TMS-protected o-di-
ethynylbenzene (8)[15] was car-
ried out to give a six-fold-sub-
stitution product (9) in 37%
yield. Removal of the TMS pro-
tecting group gave compound
10, which was subjected without
isolation to subsequent oxida-
tive coupling by using Cu-
ACHTUNGTRENNUNG(OAc)2·H2O in pyridine at
room temperature, thereby
yielding compound 4 as a pale-
yellow solid that quickly turned
dark when the solvent was
evaporated to dryness. The
dark-brown solid was insoluble
in any solvent. Because of its
sensitivity, the exact yield of
compound 4 in the final step
was not determined.[16] Even in
solution, the compound gradu-
ally darkened at room tempera-
ture. Only crystals that con-
tained acetone (see below),
which were used for the X-
ray crystallographic analysis
(Figure 2), could be stored
without showing appreciable
decomposition.
Scheme 2. Synthesis of compounds a) 4, b) 5, and c) 6. TBAF=tetrabutylammonium fluoride, TMS=trime-
thylsilyl.
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Y. Tobe et al.
(11),[18] with compound 13 gave six-fold-substitution product
14 in 11% yield. After removal of the TMS group, oxidative
coupling of compound 15 was conducted at room tempera-
ture in a similar manner to that of compound 10 to give
naphthalene homologue 5 as a pale-yellow solid. Because
compound 5 was even less stable than compound 4, the
yield was not determined exactly.
Table 1. Differences between the calculated energies for the D3- and C2
conformers of compounds 4, 5, and 6.[a]
PM3
DFT:
DFT:
DFT:
B3LYP/6-31G*
M05/6-31G*
M05-2X/6-31G*
4
5
6
0.79
0.84
2.44
1.97
2.00
4.44
1.71
0.54
ꢀ1.08
1.56
ꢀ0.26
ꢀ3.24
[a] DHf8(C2)ꢀDHf8(D3) (PM3) and E(C2)ꢀE(D3) (DFT), in kcalmolꢀ1
.
For the synthesis of compound 6, the preparation of key
precursor 17 was carried out through two pathways:
“route A” and “route B” (Scheme 2c). As a result, whereas
the cross-coupling of iodide 19 with hexaethynylbenzene
(12) only gave compound 17 in 13% yield (route B), the
direct substitution of hexabromobenzene (7) with TMS-pro-
tected o-diethynylnaphthalene (16) gave compound 17 in up
to 51% yield (route A). The intramolecular oxidative cou-
pling of compound 18, which was derived from the removal
of the TMS protecting group of compound 17, was carried
out at 708C, thereby yielding naphthalene homologue 6 in
7% yield. Compound 6 was not obtained when the reaction
was conducted at room temperature. Similar to compound
4, compound 6 was only stable in crystals that contained sol-
vent molecules. The absorption spectra of compounds 4–6,
as well as those of their key precursors (9, 14, and 17, re-
spectively), are shown in the Supporting Information, Fig-
ure S2. Whilst compounds 9, 14, and 17 show broad absorp-
tion bands, compounds 4–6 exhibit absorptions with vibra-
tional structures at longer wavelengths than their corre-
sponding precursors.
Positive values indicate a preference for the D3 conformation. The indi-
vidual energy is given in the Supporting Information, Table S1.
tive energies are summarized in Table 1. Although the
energy differences between the C2- and D3 conformations
are smaller compared to compound 1,[3] all of the computa-
tional results showed a preference for the D3 conformation
for compound 4, in accord with Pascal’s rule. On the other
hand, the DFT calculations with the M05 series of function-
als, which are known to adequately predict noncovalent in-
teractions, such as p–p interactions,[19] tend to show a prefer-
ence for the C2 conformers; more specifically, the calcula-
tions with the M05-2X functional predicted that the C2 con-
former of 1,2-naphtho homologue 6 was significantly more
stable than the D3 conformer. The results from the M05/6-
31G* calculations are always in-between these two ex-
tremes, thus favoring the D3 form for compounds 4 and 5
and the C2 form for compound 6. The reason for the differ-
ence depending on the functional is discussed in the subse-
quent section, in comparison with their X-ray structures.
Calculated relative stabilities of conformers of compounds
4–6: The interconversion between the C2- and D3-symmetric
conformations of compound 4 is shown in Scheme 3; this in-
terconversion represents one of the shortest possible path-
ways, assuming that the barrier for interconversion between
two D3 enantiomers through the synchronous inversion of
three blades is too high. A full map for compound 6, includ-
ing all of conformational exchanges, is shown in the Sup-
porting Information, Scheme S6. Apparently, interconver-
sion between the D3-symmetric enantiomers takes place
through at least two C2-symmetric diastereomers. To esti-
mate the relative stabilities of the C2- and D3 conformers of
compounds 4–6, we performed semiempirical MO calcula-
tions (PM3) and DFT calculations at the B3LYP/6-31G*,
M05/6-31G*, and M05-2X/6-31G* levels of theory. The rela-
Single-crystal- and theoretical structures of compounds 4
and 6: X-ray crystallographic analysis of compound 4 was
carried out for crystals that contained a disordered acetone
molecule because it was stabilized in the acetone-containing
crystals. The parent [14]DBA was also reported to be unsta-
ble and even polymerize in crystals.[20] However, some aryl-
butadiyne derivatives, which are unstable in concentrated
solutions and as amorphous solids, are known to become
substantially more stable in crystals.[21]
The crystal contains a pair of P- and M-enantiomers that
adopt an approximate D3-symmetric conformation with a
propeller-shape, thereby indicating the preference for this
conformation in the crystalline state. The twisting of the
three [14]DBA blades is not equal, most probably owing to
the presence of a disordered acetone molecule that is locat-
Scheme 3. One of the shortest pathways for interconversion between the D3 isomers.
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Molecular Propellers with Dehydrobenzo[14]annulene Blades
FULL PAPER
ed atop ring A (Figure 1). The nonbonded distances be-
tween the closest carbon atoms of the terminal benzene
rings of adjacent blades (C18···C27, C40···C49, and C62···C5
are 3.414, 3.276, and 3.495 ꢃ, respectively) indicate that the
blade that is near the solvent molecule (ring A) is more dis-
torted than the other two because of the steric conflict
(atom labeling is shown in Figure 1). The averaged non-
bonding distance is 3.395 ꢃ, which is almost equal to the
sum of the van der Waals radii of the carbon atoms. The di-
hedral angles between the central benzene ring and the
ꢀ
ꢀ
ꢀ
three peripheral mean planes, C4 C19, C26 C41, and C48
C63, are relatively small (24.58, 23.08, and 29.38, respective-
ly) compared to those of compound 1 (28.2–30.08), thus indi-
cating that the distortion is shared by the triple bonds. The
average bond length of the central ring of compound 4 that
is involved in the [14]DBA ring (endocyclic) is 1.405 ꢃ,
whereas that of the exocyclic bond is 1.428 ꢃ. The alterna-
tion in bond length is less pronounced than that in com-
pound 1 (1.397 versus 1.434 ꢃ). In addition, the central ring
of compound 4 adopts a much-shallower chair conformation,
with dihedral angles of 4.14–7.208, than those of compound
1 (dihedral angles: 14.8–17.68). All of these results indicate
that compound 4 is less distorted than compound 1. The
most-deformed triple bonds are found at the “spoke” bonds,
ꢀ
ꢀ
such as C1 C2 C3, with an average angle of 167.38, al-
though such deformation is typically observed for other
[14]DBA derivatives.[20]
Table 2 lists representative structural parameters for the
D3 conformer of compound 4 that were obtained from X-
ray analysis and from DFT calculations with different func-
tionals. The corresponding parameters for the D3
conformers of compounds 5 and 6 are listed in the
Supporting Information, Table S2. As shown in
Figure 1. Molecular structure of compound 4 with atom-numbering, ther-
mal ellipsoids at 30% probability; solvent molecules are omitted for
clarity (hydrogen atoms are omitted in the top structure for clarity).
Table 2. Comparison of the experimental- and calculated geometries of the D3 isomer
of compound 4.
Table 2, the theoretical results with the M05-2X/6-
31G* functional agreed reasonably well with the X-
ray structure of compound 4 with regard to the
bond lengths and -angles and reproduced the exper-
imental data slightly better than the B3LYP/6-31G*
method. On the other hand, the former results un-
derestimate the twisting of the blades relative to
the core benzene ring and, therefore, overestimate
the close contact between the adjacent blades com-
pared to the experimental data. This trend is in
accord with the fact that the M05 series of function-
X-ray[f]
B3LYP
M05
M05-2X
endocyclic bond length [ꢃ][a]
exocyclic bond length [ꢃ][b]
1.405
1.428
3.395
25.55
167.33
1.416
1.438
3.558
19.52
168.20
1.412
1.432
3.464
18.70
167.91
1.405
1.425
3.323
17.61
168.02
closest nonbonded distance [ꢃ][c]
core–blade dihedral angle [8][d]
distorted triple-bond angle [8][e]
[a] Bond length of the central benzene ring that is endocyclic to the [14]DBA ring.
[b] Bond length of the central benzene ring that is exocyclic to the [14]DBA ring.
[c] Closest nonbonding distances between the closest carbon atoms of the terminal
benzene rings of adjacent blades (i.e., C18···C27, Figure 1). [d] Dihedral angles be-
tween the mean plane of the central benzene ring and that of the diphenylbutadiyne
ꢀ
als reproduce the molecular structures, which in- unit (16 carbon atoms in total; i.e., C4 C19, Figure 1). [e] Bond angles around the
ꢀ
ꢀ
“spoke” acetylene unit (e.g., C1 C2 C3, Figure 1). [f] For the experimental structure,
the averaged values for the three- or six relevant distances/angles are given.
clude noncovalent interactions, better than the
most-frequently used B3LYP method, although
they tend to overestimate the attractive interac-
tions.[19]
Crystals of compound 6 that were suitable for X-ray anal-
ysis were obtained by recrystallization from a mixture of o-
dichlorobenzene and MeCN (Figure 2), although they con-
tained two C6H4Cl2 solvent molecules that were disordered
atop- and below one of the distorted [14]DBA units
(ring D). Similar to the case of compound 4, compound 6
was stabilized in crystals that contained solvent molecules.
Strikingly, in accord with the DFT calculations with the
M05-2X functional, compound 6 adopted an approximately
C2-symmetric conformation in the crystalline state.
With regard to the bond-length alternations in the central
ring, whereas the endocyclic bond lengths are 1.424 and
1.415 ꢃ, the exocyclic bond lengths are 1.411 and 1.427 ꢃ.
That is, the bond lengths do not depend on whether the ring
ꢀ
ꢀ
is endo- or exocyclic; two bonds (C1 C90 and C31 C60,
Figure 2) are relatively short, whereas the other four bonds
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Y. Tobe et al.
are relatively long. Therefore, the bond lengths of the cen-
tral ring reflect the C2 symmetry and do not exhibit alternat-
ing changes. Similar to the case of compound 4, the bond-
length difference is smaller than that of compound 1.[3] Be-
cause compound 6 adopts a C2-symmertic structure in spite
of the aromatic character of the central ring, it may no
longer be obliged to follow Pascal’s rule.
The most-notable feature in the crystal structure of com-
pound 6 is the unique structure of the two [14]DBA units,
which are deformed into a “warped” shape. The third
[14]DBA unit (ring E) adopts an almost-planar dinaphthyl-
butadiyne substructure. The bond angles of the acetylene
“spoke” units (166.6–176.68) are similar to those of com-
pound 4. However, the bond angles of the butadiyne units
(165.0–168.78) are slightly smaller than those of
estimate the noncovalent interactions, the dihedral angles
between the naphthalene units in rings D/F and E/F are un-
derestimated (15.48) compared to the experimental result
(37.48). However, the overlapping geometry between the
naphthalene rings of compound 6 suggests that p–p interac-
tion may contribute to the stability of the C2 conformer. To
estimate the stabilization energy, single-point calculations
were undertaken for two naphthalene molecules that were
located at the same geometry as that in the crystal structure
and its energy was compared with twice that of the single
naphthalene molecules with the same geometry. As a result,
the calculations with the M05-2X functional showed a pref-
erence for the dimeric naphthalene with a stabilization
energy of 2.96 kcalmolꢀ1, whereas the B3LYP and M05
compound 4 (166.0–171.88), although the similar
Table 3. Comparison of the experimental and calculated geometries of the C2 isomers
of compound 6.
degree of distortion has been frequently ob-
served.[22] The dihedral angle between the naphtha-
ꢀ
lene rings of the “warped” [14]DBA unit (i.e., C34
[23]
ꢀ
C43 versus C48 C57, Figure 2) is 39.88.
To the
best of our knowledge, the deformation of
[14]DBAs in structures such as compound 6 has not
been reported previously.
Another remarkable structural feature in the
crystal structure of compound 6 is the small dihe-
dral angles between the naphthalene rings in a pair
of neighboring blades. That is, the two naphthalene
units in blades D and F are almost parallel to each
other, with a small dihedral angle of 6.18 between
their mean planes. Moreover, the distances from
each carbon atom of one naphthalene unit to the
mean plane of the other naphthalene unit are in the
range 3.185–3.553 ꢃ, which is slightly shorter/longer
than the sum of the van der Waals radii, thereby
strongly indicating the presence of p–p stacking in-
teractions. Such an overlapping geometry was not
found in the naphthalene units between the D/E
and E/F pairs, in which the dihedral angle was
37.48.
The theoretical- and experimental (X-ray) struc-
tures of the C2 conformer of compound 6 are com-
pared in Table 3. Most of the structural parameters
agree well, taking into account the presence of two
o-dichlorobenzene molecules per molecule of com-
pound 6 in the crystal. However, the most-notable
difference was found between the theoretical struc-
tures, in particular in the relative geometry of the
two closely located naphthalene rings. As described
above, in the crystal structure, the naphthalene
units in rings D and F were almost parallel to each
other, with a small dihedral angle of 6.18 between
the mean planes. This geometry was best repro-
duced in the calculations with the M05-2X function-
al, which showed a dihedral angle of 13.28; the di-
hedral angles in the B3LYP and M05 structures
were 29.9 and 17.38, respectively. On the other
hand, because the M05-2X functional tends to over-
X-ray
DFT/
B3LYP
DFT/M05 DFT/
M05-2X
endo bond length [ꢃ]
exo bond length [ꢃ]
a=1.424 a=1.417
b=1.415 b=1.419
c=1.411 c=1.446
d=1.427 d=1.442
a=1.413
b=1.415
c=1.438
d=1.435
31.19
a=1.406
b=1.408
c=1.430
d=1.428
28.94
core–blade dihedral angle [8][a]
“spoke” triple-bond angle [8]
30.73
36.52
x1 =171.4 x1 =169.61 x1 =170.85 x1 =171.00
x2 =172.7 x2 =175.94 x2 =174.68 x2 =174.39
y1 =169.4 y1 =170.52 y1 =171.23 y1 =169.97
y2 =175.0 y2 =176.05 y2 =173.86 y2 =172.70
z1 =166.6 z1 =166.34 z1 =166.84 z1 =167.17
z2 =176.6 z2 =176.85 z2 =175.84 z2 =175.02
triple-bond angle in the diyne unit [8] a=168.7 a=166.94 a=168.26 a=168.72
b=167.8 b=169.14 b=168.47 b=168.88
g=168.7 g=168.45 g=168.48 g=169.47
d=165.0 d=167.86 d=168.43 d=167.76
e=170.6 e=169.13 e=169.67 e=169.33
z=170.6 z=171.37 z=171.13 z=171.50
dihedral angle between the naphtha- 1.78,
lene rings of the [14]DBA units [8][b]
39.83
dihedral angle between the peripheral 6.07,
7.71,
6.85,
9.11,
34.86
29.91,
37.78
26.03
17.31,
19.87
22.46
13.18,
15.41
naphthalene rings in adjacent
37.44
[14]DBA units [8][c]
[a] Dihedral angles between the mean plane of the central benzene ring and that of
the dinaphthylbutadiyne unit of ring E (24 carbon atoms in total; that is, C34 C57,
Figure 2). [b] Dihedral angles between the mean planes of the two naphthalene units
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
in rings E and D (i.e., C34 C43 versus C48 C57 and C4 C13 versus C18 C27, respec-
tively, Figure 2). [c] Dihedral angles between the mean planes of the peripheral naph-
ꢀ
ꢀ
ꢀ
thalene rings in adjacent [14]DBA blades (i.e., C4 C13 versus C78 C87 and C18 C27
versus C34 C43, respectively, Figure 2).
ꢀ
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Molecular Propellers with Dehydrobenzo[14]annulene Blades
FULL PAPER
Figure 2. Molecular structure of compound 6 with atom-numbering, ther-
mal ellipsoids at 30% probability; solvent molecules are omitted for
clarity (hydrogen atoms are omitted in the top structure for clarity).
Figure 3. Region of the VT-1H NMR spectra of a) compound
4 and
b) compound 5 with assignment of the peaks (300 MHz, [D8]THF, ꢀ100
to 308C).
methods exhibited repulsive interactions with destabilization
energies of 4.72 and 0.12 kcalmolꢀ1, respectively. Therefore,
we assume that the C2 conformation of compound 6 is more
stable than the D3 isomer, mainly owing to p–p interactions
between the naphthalene rings. Although these results con-
flict with Pascal’s rule, in view of the large flexibility of the
dehydro[14]annulene ring, it is reasonable to assume that
the rule would no longer apply in molecular propellers that
consist of [14]DBA units because the [14]annulene ring can
be readily deformed from planarity without affecting the
overlap of p orbitals in either the central- or peripheral aro-
matic rings. Either the C2- or D3 form will become favored,
depending on subtle changes in the steric/electronic interac-
tions.
1
The H NMR spectra of compounds 4 and 5 at 308C (Fig-
ure 3a, b) exhibited four- and six signals for the aromatic
protons, respectively. Assignment of the signals was carried
out on the basis of the coupling patterns and the calculated
chemical shifts in the gas phase were derived from the
GIAO-HF/6-31G* method for the D3-conformers that were
optimized by DFT calculations at the B3LYP/6-31G* level
of theory (see the Supporting Information, Table S3).[24] We
1
also performed the implicit solvent model of the H NMR
chemical-shift calculations[25] (for the methods and results of
the solvent model, see the Supporting Information). The
chemical shifts of compound 6 were also calculated for the
C2-conformer and were optimized by the M05-2X/6-31G*
level of theory. The calculated chemical shifts by using the
solvent model are in better agreement with the experimen-
tal results than those calculated for the gas phase. For com-
pound 4, although the observed- and calculated chemical
shifts of the solvent model are in reasonable agreement
(Dd=<0.1 ppm), relatively large discrepancies (Dd=0.2–
Conformations- and dynamic behavior of compounds 4–6 in
solution: X-ray crystallographic analysis showed that com-
pounds 4 and 6 adopted D3- and C2-symmetric structures, re-
spectively, in the crystalline state. Next, we investigated the
conformations- and relevant dynamic behavior of com-
pounds 4–6 in solution.
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Y. Tobe et al.
0.3 ppm) are observed for protons Ha and Hb (and Hc for
compound 5), which experience anisotropic effects of the ar-
omatic ring of the adjacent [14]DBA unit. In the case of
[14]DBA itself, the calculated chemical shifts agree better
with those of the experimental shifts, although all of the cal-
culated chemical shifts are uniformly downfield-shifted
(Dd=0.15–0.29 ppm by HF/6-31G* in the gas phase, Dd=
0.05–0.14 ppm by B3LYP/6-31G* in the solvent model calcu-
lation; see the Supporting Information, Figure S3).
isomers. Because compound 5 behaves like compound 4 in
solution (with regard to the VT-NMR- and chiral-chroma-
tography data), we assume that it also adopts the D3 form
with a low kinetic barrier for the C2–D3 or both the C2–D3
and C2–C2 interconversions.
In contrast to the VT-NMR spectra of compounds 4 and
5, which did not give definitive information regarding their
solution-state conformations, the 1H NMR data of com-
pound 6 provided an insight into its conformational behav-
The 13C NMR spectrum of compound 4 at 308C (see the
Supporting Information, Figure S5) shows seven signals for
the aromatic carbon atoms, whilst that of compound 5 shows
eleven signals (see the Experimental Section), which is con-
sistent with a D3-symmetric conformation. However, the ob-
ior. First, the H NMR spectrum at 1508C in [D4]p-dichloro-
1
benzene exhibited a sharp doublet signal for the naphtha-
lene proton (Ha) at d=8.75 ppm, which was resolved from
the other aromatic proton signals (see the Supporting Infor-
mation, Figure S6). As the temperature was lowered to
608C, the signal became significantly broadened. In
[D8]toluene, a broad signal was also observed at 608C,
thereby indicating a slowing of the conformational exchange
(Figure 4). Below 208C, the signal was resolved into three
1
servation of highly-symmetric H- and 13C NMR signals indi-
cates two possibilities regarding their conformational behav-
ior in solution: 1) Both compounds 4 and 5 adopt D3-sym-
metric structures and the ring-inversion into the C2 form is
too slow on the NMR timescale; or 2) the racemization
takes place rapidly on the NMR timescale to exhibit time-
averaged signals of the C2- and D3-symmetric conformations.
In the former case, because the racemization of the D3-con-
formers through the C2 isomer should be slow, P- and M
enantiomers of D3-symmetric compounds 4 and 5 may be
separated by chiral chromatography.[26] To this end, optical
resolution by using a variety of chiral columns (see the Sup-
porting Information) was attempted. However, despite in-
tensive studies, the separation of the enantiomers of neither
compound 4 nor 5 was observed, thus suggesting that the
second possibility was more likely.
To gain an insight into the conformational equilibrium,
we performed VT-NMR experiments. If the rate of ring-in-
version becomes slow enough on the NMR timescale, line-
broadening or the appearance of additional peaks owing to
the less-symmetric C2 conformation is expected. However,
1
resolution of the H NMR signals was not observed for both
compounds 4 and 5, even at ꢀ1008C (Figure 3), although
apparent downfield shifts of some of the signals was ob-
1
served.[27] On increasing the temperature, the H NMR spec-
Figure 4. Region of the VT-1H NMR spectra of compound 6 that shows
the Ha proton (400 MHz, [D8]toluene, ꢀ80 to 808C).
trum of compound 4 did not change (see the Supporting In-
formation, Figure S4). Because the 13C NMR measurements
of compound 5 at low temperatures were hampered by low
solubility, only low-temperature 13C NMR measurements of
compound 4 were performed. Whereas the spectrum at
308C showed sharp, distinct signals, the signals at ꢀ1008C in
[D8]THF exhibited considerable broadening, thereby indi-
cating a slowing of the exchange rate at this temperature.
However, coalescence was not observed (see the Supporting
Information, Figure S5).
Compound 4 crystallizes in the D3 form, in agreement
with the theoretical calculations, which predicted that the D3
conformer was more stable than the C2 form. However, the
VT-NMR data did not provide any information regarding its
conformation in solution and its dynamic behavior. Conse-
quently, based on the theoretical calculations and the results
of the chiral chromatography, we assume that compound 4
adopts the D3 form, which racemizes rapidly through the C2
doublets, which showed the presence of the C2-symmetric
conformation in solution and, at the same time, the absence
of the D3 conformer (as seen in the crystalline state). Nota-
bly, the decoalescence owing to the C2-symmetric conforma-
tion is only observed when all of the interconversion rates
become slow enough on the NMR timescale (see the Sup-
porting Information, Scheme S6) because a time-averaged,
highly-symmetric signal that corresponds to a mixture of in-
terchanging conformations should be observed if only a part
of the interconversion takes place rapidly. This behavior
also suggests that the C2–C2 conversion rate becomes slow
enough below 208C on the NMR timescale and, more im-
portantly, suggests the absence of the D3 conformer of com-
pound 6, as indicated from the equal intensities of the three
doublets. These results are consistent with those that were
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Molecular Propellers with Dehydrobenzo[14]annulene Blades
FULL PAPER
based on the theoretical- and crystallographic studies (see
above) in the sense that the C2 conformer is more stable.
The barrier for the C2–C2 interconversion was estimated to
be 16.2 kcalmolꢀ1 at the coalescence temperature (508C) of
the two signals that were observed at lower fields (Dn=
33.2 Hz). Upon further cooling to ꢀ808C, the two signals at
lower fields started to recombine and the doublet at higher
field became broadened again. This recombination of the
signals can be attributed to an accidental isochrony of two
different types of protons during the downfield shift on low-
ering the temperature.
obtained. On the other hand, from the VT-NMR measure-
ments of compound 6, the barrier for the C2–C2 exchange
was estimated to be 16.2 kcalmolꢀ1. Although these results
are, at first, counterintuitive, in view of the overcrowded
molecular structures, they are in accord with the remarkable
flexibility of acetylene linkages in the rotational isomerism
of sterically crowded acetylene derivatives.[13] These results
are useful for the molecular design of shape-persistent- and
shape-shifting[28] molecules that are based on [14]DBA or on
other related dehydrobenzoannulene units, in particular
when molecular flexibility and dynamic properties are in
view.
Thus, compound 6 exhibited a conformational behavior
that was different to that of hexabenzotriphenylene (1) but
similar to that of hexamethyltriphenylene (2). In the former
case, the D3 isomer was estimated by a theoretical study to
be more stable by about 8 kcalmolꢀ1.[3] The barrier for inter-
conversion between the less-stable C2 isomers was deter-
mined by VT-NMR spectroscopy to be 11.7 kcalmolꢀ1 at
247 K, whereas that for the C2–D3 isomerization was deter-
mined by kinetic study of the C2–D3 conversion to be
Experimental Section
Compound 4: To a suspension of K2CO3 (132 mg, 0.955 mmol) in MeOH
(20 mL) was added a solution of compound 9 (50.1 mg, 39.9 mmol) in
THF (10 mL) under an argon atmosphere. After stirring for 10 h, the
mixture was diluted with water and extracted with Et2O. The extract was
dried over anhydrous MgSO4. Most of the solvent was removed under re-
duced pressure and the residue was dissolved in Et2O (50 mL). This solu-
tion, which contained compound 10, was added dropwise to a solution of
26.2 kcalmolꢀ1.[4b] On the contrary, in the case of triphen
ACHTUNGTRENNUNGene derivative 2, the barrier for the more-stable C2–C2 inter-
ACHTUNGERTNyNUNG l-
conversion was determined by VT-NMR spectroscopy to be
CuACHTNUGTRNEUNG(OAc)2·H2O (398 mg, 1.99 mmol) in pyridine (100 mL) over a period
10.2 kcalmolꢀ1 at 220 K, whereas that for the C2–D3 isom-
of 9 h and the mixture was stirred for a further 3 h. The reaction was
monitored by TLC and by laser-desorption-ionization (LDI) mass spec-
trometry. The reaction mixture was passed through a short column of
silica gel (CHCl3). After removal of the solvents, the products were sepa-
rated by recycling gel-permeation chromatography (GPC) and recrystal-
lized from CHCl3/acetone to afford compound 5 (6.3 mg, 19% yield)[16]
as a pale-yellow solid, which decomposed gradually at RT.[29] 1H NMR
(300 MHz, [D8]THF): d=7.84 (dd, J=7.5, 0.3 Hz, 6H), 7.61 (dd, J=7.5,
1.5 Hz, 6H), 7.38 (ddd, J=7.5, 7.5, 1.5 Hz, 6H), 7.10 ppm (ddd, J=7.5,
7.5, 0.3 Hz, 6H); 13C NMR (75 MHz, [D8]THF): d=134.9, 128.8, 128.6,
128.3, 127.3, 122.7, 101.1, 93.7, 86.7, 80.8 ppm; IR (KBr): n˜ =3058, 2924,
2851, 2214, 2168, 1507, 1470, 1448, 1385, 754, 667, 580 cmꢀ1; MS (FAB):
m/z: 816 [M]+.
ACHTUNGTRENNUNGerization was estimated by computational analysis to be
about 25 kcalmolꢀ1.[2c] The barrier for the more-stable C2–C2
interconversion of compound 6 at 508C was estimated to be
16.2 kcalmolꢀ1. However, the barrier for the C2–D3 isomeri-
zation was not determined experimentally. This relatively
low barrier, despite the large overlapping of the dehydroan-
nulene blades, is ascribed to the flexibility of the acetylene
linkages, which can be deformed substantially in the transi-
tion state of the ring flip.
Compound 5: To a suspension of K2CO3 (110 mg, 0.796 mmol) in MeOH
(40 mL) was added a solution of compound 14 (82.9 mg, 53.3 mmol) in
THF (15 mL) under an argon atmosphere. After stirring for 3 h, the mix-
ture was diluted with water and extracted with CH2Cl2 (20 mL). The ex-
tract was diluted with THF (80 mL). This solution, which contained com-
Conclusions
In the solid state, molecular propeller 4, which consists of
[14]DBA blades, adopts an approximate D3-symmetric struc-
ture, in which the individual [14]DBA ring twists substan-
tially to avoid steric repulsion between the peripheral ben-
zene rings. By contrast, naphthalene homologue 6 crystal-
pound 15, was added dropwise to a solution of CuACTHNUGRTNEUNG(OAc)2·H2O (570 mg,
2.85 mmol) in a degassed (by bubbling with argon for 15 min) mixture of
pyridine (80 mL) and CH3CN (20 mL) over a period of 12 h. The mixture
was stirred for 12 h at RT before and then passed through a short column
of silica gel (CHCl3). After removal of the solvent, the residue was
washed with CH3CN. The residual solid was purified by recycling GPC
and HPLC (ODS column, CH2Cl2/CH3CN, 7:13) to afford compound 5 as
ACHTUNGTRENNUNGlizes in a C2-symmetric structure, in which the acetylene
units are strongly distorted such that two of the blades
adopt an “arched” shape. The crystal structures were com-
pared with those that were derived from the DFT calcula-
tions with different functionals. On the basis of the crystal
structure and the relative energies that were estimated from
the calculations with the M05-2X functional for the C2- and
D3 conformers, together with the stabilization energy that
was estimated for the naphthalene dimer model, we con-
clude that the C2 conformer of compound 6 is stabilized by
p–p interactions between the naphthalene units of a pair of
adjacent blades. With regards to their dynamics in solution,
VT-NMR experiments of compound 4 and 5 suggest that
conformational interconversion take place rapidly on the
NMR timescale, although no quantitative information was
a
pale-yellow solid, which decomposed rapidly at RT.[29] 1H NMR
(300 MHz, [D8]THF): d=8.39 (s, 6H), 8.12 (s, 6H), 7.83 (d, J=8.1 Hz,
6H), 7.34 (dd, J=8.1, 7.8 Hz, 6H), 6.94 (dd, J=8.1, 7.8 Hz, 6H),
6.63 ppm (d, J=8.1 Hz, 6H); 13C NMR (75 MHz, [D8]THF): d=136.6,
133.31, 133.25, 130.0, 129.2, 128.7, 128.4, 128.3, 128.2, 124.9, 120.1, 102.0,
93.4, 87.8, 81.3 ppm; MS (LDI, negative mode): m/z: 1116.9 [M]ꢀ.
Compound 6: To a suspension of K2CO3 (134 mg, 0.970 mmol) in MeOH
(30 mL) was added a solution of compound 17 (99.7 mg, 64.1 mmol) in
THF (50 mL). After stirring at RT for 3.5 h under an argon atmosphere,
an aqueous solution of NH4Cl was added and the reaction mixture was
extracted with Et2O. The extract was washed with brine and diluted with
Et2O (80 mL). This solution, which contained compound 18, was added
dropwise to a solution of CuACHTNUTRGNE(NUG OAc)2·H2O (1.28 g, 6.41 mmol) in a de-
gassed (by bubbling with argon for 15 min) mixture of pyridine (100 mL)
and CH3CN (20 mL) over a period of 18 h at 708C. The mixture was stir-
red for a further 2 h at 708C before it was passed through a short column
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Y. Tobe et al.
of silica gel (CHCl3). After removal of the solvent, the residue was puri-
fied by recycling GPC to afford compound 6 (5.1 mg, 7%) as an orange
solid, which decomposed gradually at RT.[29,30] 1H NMR (400 MHz,
[D4]p-dichlorobenzene, 1508C): d=8.74 (d, J=8.0 Hz, 6H), 7.35 (d, J=
8.4 Hz, 6H), 7.29 (d, J=8.4 Hz, 6H), 7.12 (d, J=8.0 Hz, 6H), 6.84 (dd,
J=7.6, 6.8 Hz, 6H), 6.79 ppm (dd, J=7.6, 6.8 Hz, 6H); MS (LDI, nega-
tive mode): m/z: 1116.7 [M]ꢀ.
-dehydrobenzo[18]annulene core. However, because of the large
core size, these compounds would not exhibit hindered rotation
around the [14]DBA blades. See: T. Takeda, A. G. Fix, M. M. Haley,
Org. Lett. 2010, 12, 3824–3827.
[15] a) C. Huynh, G. Linstrumelle, Tetrahedron 1988, 44, 6337–6344;
b) J. A. John, J. M. Tour, J. Am. Chem. Soc. 1994, 116, 5011–5012.
[16] About one-sixth of the volume of an eluent from GPC analysis that
contained purified compound 4 was evaporated to dryness, the resi-
due was weighed, and its purity was checked by NMR spectroscopy.
From this experiment, the yield of compound 4 was estimated to be
approximately 24% by using CHBr3 as an internal standard.
[17] a) R. Diercks, J. C. Armstrong, R. Boese, K. P. C. Vollhardt, Angew.
Chem. 1986, 98, 270–271; Angew. Chem. Int. Ed. Engl. 1986, 25,
268–269; b) T. X. Neenan, G. M. Whitesides, J. Org. Chem. 1988, 53,
2489–2496; c) M. Sonoda, A. Inaba, K. Itahashi, Y. Tobe, Org. Lett.
2001, 3, 2419–2421.
Crystal data: CCDC-872771 (4) and CCDC-872772 (6) contain the sup-
plementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
[18] Several hexaethynylbenzene derivatives have been prepared by sub-
stitution with compound 12, see: a) S. Ito, H. Inabe, N. Morita, K.
Ohta, T. Kitamura, K. Imafuku, J. Am. Chem. Soc. 2003, 125, 1669–
1680; b) J. Nierle, D. Barth, D. Kuck, Eur. J. Org. Chem. 2004, 867–
872; c) M. Hasegawa, H. Enozawa, Y. Kawabata, M. Iyoda, J. Am.
Chem. Soc. 2007, 129, 3072–3073; d) M. Schmittel, P. Mal, Chem.
Commun. 2008, 960–962.
[19] a) Y. Zhao, N. E. Schultz, D. G. Truhlar, J. Chem. Phys. 2005, 123,
161103; b) Y. Zhao, N. E. Schultz, D. G. Truhlar, J. Chem. Theory
Comput. 2006, 2, 364–382; c) Y. Zhao, D. G. Truhar, J. Chem.
Theory Comput. 2006, 2, 1009–1018; d) S. Toyota, H. Miyahara, M.
Goichi, S. Yamasaki, T. Iwanaga, Bull. Chem. Soc. Jpn. 2009, 82,
931–945.
This work was supported by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Culture, Sports, and Technology, Japan. The
authors thank Professor Yoshio Okamoto (Nagoya University) for the
chiral chromatography experiments, Professors Shinji Toyota (Okayama
University of Science) and Gaku Yamamoto (Kitasato University) for
their valuable suggestions regarding dynamic NMR spectroscopy, and Dr.
Ichiro Hisaki (Osaka University) for his help with the X-ray crystallo-
graphic analysis.
[1] For a review on overcrowded polycyclic aromatic compounds, see:
R. A. Pascal, Jr., Chem. Rev. 2006, 106, 4809–4819.
[2] a) M. B. Hursthouse, V. B. Smith, J. Fluorine Chem. 1977, 10, 145–
155; b) C. S. Frampton, D. D. MacNicol, S. J. Rowan, J. Mol. Struct.
1997, 405, 169–178; c) Y. Wang, A. D. Stretton, M. C. McConnell,
P. A. Wood, S. Parsons, J. B. Henry, A. R. Mount, T. H. Galow, J.
Am. Chem. Soc. 2007, 129, 13193–13200.
[3] L. Barnett, D. M. Ho, K. K. Baldridge, R. A. Pascal, Jr., J. Am.
Chem. Soc. 1999, 121, 727–733.
[4] a) D. PeÇa, D. Pꢁrez, E. Guitiꢂn, L. Castedo, Org. Lett. 1999, 1,
1555–1557; b) D. PeÇa, A. Cobas, D. Pꢁrez, E. Guitiꢂn, L. Castedo,
Org. Lett. 2000, 2, 1629–1632.
[5] M. A. Bennett, M. R. Kopp, E. Wenger, A. C. Willis, J. Organomet.
Chem. 2003, 667, 8–15.
[6] C. Romero, D. PeÇa, D. Pꢁrez, E. Guitiꢂn, Chem. Eur. J. 2006, 12,
5677–5684.
[7] J. M. Alonso, A. E. Dꢄaz-ꢅlvarez, A. Criado, D. Pꢁrez, D. PeÇa, E.
Guitiꢂn, Angew. Chem. 2012, 124, 177–181; Angew. Chem. Int. Ed.
2012, 51, 173–177.
[8] a) K. Zimmermann, R. Goddard, C. Krꢆger, M. W. Haenal, Tetrahe-
dron Lett. 1996, 37, 8371–8374; b) J. K. Gimzewski, C. Joachim,
P. R. Schlittler, V. Langlais, H. Tang, I. Johannsen, Science 1998, 281,
531–533; c) B. Iglesias, D. PeÇa, D. Pꢁrez, E. Guitiꢂn, L. Castedo,
Synlett 2002, 0486–0488.
[20] For a [14]DBA ring, “spoke” bond angles of 168–1728 have been re-
ported; see: a) K. P. Baldwin, A. J. Matzger, D. A. Scheiman, C. A.
Tessier, K. P. C. Vollhardt, W. J. Youngs, Synlett 1995, 1215–1218;
b) H. Hinrichs, A. K. Fischer, P. G. Jones, H. Hopf, M. M. Haley,
Org. Lett. 2005, 7, 3793–3795; c) H. S. Blanchette, S. C. Brand, H.
Naruse, T. J. R. Weakley, M. M. Haley, Tetrahedron 2000, 56, 9581–
9588; d) M. J. O’Connor, R. B. Yelle, L. N. Zakharov, M. M. Haley,
J. Org. Chem. 2008, 73, 4424–4432.
[21] a) W. B. Wan, M. M. Haley, J. Org. Chem. 2001, 66, 3893–3901;
b) K. West, C. Wang, A. S. Batsanov, M. R. Bryce, J. Org. Chem.
2006, 71, 8541–8544.
[22] a) Q. Zhou, P. J. Carroll, T. M. Swager, J. Org. Chem. 1994, 59,
1294–1301; b) I. Hisaki, T. Eda, M. Sonoda, H. Niino, T. Sato, T.
Wakabayashi, Y. Tobe, J. Org. Chem. 2005, 70, 1853–1864.
[23] Another dihedral angle between the mean planes of two almost-
ꢀ
planar butadiyne-bridged naphthalene rings in ring E (i.e., C34 C43
ꢀ
versus C48 C57) was 1.88.
[24] For the chemical-shift calculations we used the HF method because
this had been shown to be the most reliable means for evaluating
the chemical shifts (i.e., magnetic properties) of annulenes. For ex-
ample, see: R. V. Williams, J. R. Armantrout, B. Twamley, R. H.
Mitchell, T. R. Ward, S. Bandyopadhyay, J. Am. Chem. Soc. 2002,
124, 13495–13505.
[9] A. Pradhan, P. Dechambenoit, H. Bock, F. Durola, Angew. Chem.
2011, 123, 12790–12793; Angew. Chem. Int. Ed. 2011, 50, 12582–
12585.
[10] T. Yoshimura, A. Inaba, M. Sonoda, K. Tahara, Y. Tobe, R. V. Wil-
liams, Org. Lett. 2006, 8, 2933–2936.
[25] a) R. Jain, T. Bally, P. R. Rablen, J. Org. Chem. 2009, 74, 4017–
4023; b) T. Bally, P. R. Rablen, J. Org. Chem. 2011, 76, 4818–4830;
c) M. Cossi, G. Scalmani, N. Rega, V. Barone, J. Chem. Phys. 2002,
117, 43–54.
[26] a) Y. Okamoto, T. Ikai, Chem. Soc. Rev. 2008, 37, 2593–2608; b) T.
Ikai, Y. Okamoto, Chem. Rev. 2009, 109, 6077–6101.
[27] The latter behavior can be attributed to the change in the ratio of
the conformational population. As shown in the Supporting Infor-
mation, Table S3, for both compounds 4 and 5, the calculated chemi-
cal shifts of the D3 structure are shifted downfield compared to
those of the C2 structure. Hence, upon lowering the temperature,
the observed downfield shifts are presumably due to the population
of the more-stable D3 conformers.
[11] K. Tahara, T. Yoshimura, M. Sonoda, Y. Tobe, R. V. Williams, J.
Org. Chem. 2007, 72, 1437–1442.
[12] For previous syntheses of [14]DBA and its derivatives, see: a) K. P.
Baldwin, A. J. Matzger, D. A. Scheiman, C. A. Tessier, K. P. C. Voll-
hardt, W. J. Youngs, Synlett 1995, 1216–1218; b) J. A. Marsden, J. J.
Miller, M. M. Haley, Angew. Chem. 2004, 116, 1726–1729; Angew.
Chem. Int. Ed. 2004, 43, 1694–1697.
[13] a) S. Toyota, T. Yamamori, T. Makino, Tetrahedron 2001, 57, 3521–
3528; b) T. Makino, S. Toyota, Bull. Chem. Soc. Jpn. 2005, 78, 917–
928; c) S. Toyota, Chem. Rev. 2010, 110, 5398–5424.
[28] a) A. R. Lippert, A. Naganawa, V. L. Keleshian, J. W. Bode, J. Am.
Chem. Soc. 2010, 132, 15790–15799; b) M. He, J. W. Bode, Proc.
Natl. Acad. Sci. USA 2011, 108, 14752–14756.
[14] Recently, Haley and co-workers reported the synthesis- and optical
properties of trefoil-shaped molecules in which three [14]DBA rings
were fused to a central triphenylene, -dehydrobenzo[12]annulene, or
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Molecular Propellers with Dehydrobenzo[14]annulene Blades
FULL PAPER
[29] Because of the instability of the material, the melting point was not
determined.
of the signals overlapped with the signals of the aromatic solvent
and, hence, were concealed by them.
[30] Compound 6 was barely soluble in any solvent and was only slightly
soluble in aromatic solvents, such as toluene and p-dichlorobenzene,
at elevated temperature. The 13C NMR spectrum of compound 6 is
not reported because the intensities of the signal were low and most
Received: March 29, 2012
Revised: June 10, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
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www.chemeurj.org
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Y. Tobe et al.
Annulenes
S. Nobusue, Y. Mukai, Y. Fukumoto,
R. Umeda, K. Tahara, M. Sonoda,
Y. Tobe* ....................... &&&& — &&&&
Molecular Propellers that Consist of
Dehydrobenzo[14]annulene Blades
The propellerheads: The behavior of
propeller-shaped compounds with
dehydrobenzo[14]annulene rings was
investigated. Interconversion between
the stereoisomers had relatively small
energy barriers owing to the flexibility
of the acetylene linkages.
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!