All-Z-hexabenzo[24]annulene with a triangular benzene cluster substructure

Article abstract of DOI:10.1016/j.tetlet.2003.10.163

All-Z-hexabenzo[24]annulene was synthesized via a poly-cis-stilbene intermediate. The single crystal of the annulene has a chiral C ₃-symmetry with a central benzene trimer substructure. Although the compound is highly flexible in solution, the C₃-symmetric structure is stabilized by three concurrent CH/π interactions.

Full text of DOI:10.1016/j.tetlet.2003.10.163

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 359–362
All-Z-hexabenzo[24]annulene with a triangular benzene cluster
substructure
Yoshiyuki Kuwatani,* Jun-ichi Igarashi and Masahiko Iyoda*
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
Received 19 September 2003; revised 20 October 2003; accepted 24 October 2003
Abstract—All-Z-hexabenzo[24]annulene was synthesized via a poly-cis-stilbene intermediate. The single crystal of the annulene has
a chiral C3-symmetry with a central benzene trimer substructure. Although the compound is highly flexible in solution, the
C3-symmetric structure is stabilized by three concurrent CH/p interactions.
Ó 2003 Elsevier Ltd. All rights reserved.
Weak intermolecular interactions have been recognized
to play an important role in supramolecular chemistry.1;2
Aromatic interaction is one of these, and this important
interaction frequently governs the geometry and stability
of supramolecular entities. The interactions between two
benzene rings have been thoroughly investigated3;4 in
terms of both the geometry5 and the stabilizing energy.6;7
Based on theoretical calculations, the T-shaped interac-
tion can be stabilized with comparable energy to a
stacked arrangement.4 On the other hand, interactions
among more than two arene rings are usually difficult to
understand because many other factors can affect a
practical system. Gas phase studies on benzene aggre-
gates provide interesting information on multiple inter-
actions among some benzene nuclei.8 Stable benzene
trimers with a large binding energy have been observed,
and theoretical calculations have predicted a C3-sym-
metric cyclic structure.9 We found a closely similar
structure with three benzene nuclei in the title com-
pound, which should have the same CH/p stabilization
as the benzene trimer.
We have studied a series of all-Z-[n]benzo[4n]annulenes
1, which represent a partial structure of cylindrical
p-systems 2.10 These compounds (1a–c) have electron rich
cavities, which have been utilized for the incorporation of
metal cations.11 Increase of the ring size of 1 makes the
structure flexible, so that metal complexes may be formed
by induced fit. To investigate the structure and guest-
accommodating properties of the larger homologue, 3
has been synthesized. We report here a unique structure
of 3 with a triangular benzene trimer substructure both in
crystal and solution (Chart 1).
The title compound was synthesized by a similar strategy
to that used for the synthesis of 1, which was based on
the intramolecular pinacol coupling of the corresponding
linear oligomeric cis-stilbene dialdehyde 10, followed by
n-2
n-2
1 a: n = 3
b: n = 4
c: n = 5
benzene trimer
2
3
Chart 1.
Keywords: p-Conjugated system; Annulene; CH/p Interaction; Host molecule; X-ray crystal structure.
* Corresponding authors. Tel.: +81-426-77-1111; fax: +81-426-77-2525; e-mail: kuwatani@comp.metro-u.ac.jp; iyoda-masahiko@c.metro-u.ac.jp
0040-4039/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.163

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Y. Kuwatani et al. / Tetrahedron Letters 45 (2004) 359–362
Scheme 1. Synthesis of 3. Reagents and conditions: (a) 2-bromobenzaldehyde, Pd(PPh3)4, CuI, NEt3, reflux, 10 min; (b) NaBH4, EtOH; (c) LindlarÕs
catalyst, H2, quinoline, benzene; (d) Dess–Martin periodinane, CH2Cl2; (e) VCl3(thf)3, Zn, DMF, CH2Cl2; (f) (COCl)2, DMSO, NEt3, CH2Cl2;
(g) NaBH4, EtOH, 0 °C; (h) thiophosgene, DMAP, CH2Cl2, 0 °C; (i) 1,3-dimethyl-2-phenyl-1,3,2-diazaphospholidine, benzene, reflux; (j) nBu4NF,
THF; (k) Dess–Martin periodinane, CH2Cl2; (l) VCl3(thf)3, Zn, DMF, CH2Cl2; (m) thiocarbonyldiimidazole, toluene, reflux; (n) DMPD, benzene,
reflux.
the formation of the final cis-double bond using reduc-
tive dehydroxylation10 (Scheme 1). The poly-cis-stilbene
dialdehyde 10 was prepared from 4. Sonogashira cou-
pling of the alkyne 4 and subsequent reductions afford
the alcohol 5, which was readily oxidized to the corre-
sponding aldehyde 6 (73% from 4). This aldehyde 6 was
dimerized by the low valent vanadium complex gener-
ated in situ to afford the threo-pinacol 7 (87% with 7% of
8), which was selectively converted to erythro-pinacol 8
by a Swern oxidation and NaBH4 reduction sequence
(two steps, 8: 82% and 7: 12%). The central double bond
was introduced by Corey–Winter reaction of the corre-
sponding thionocarbonate with 1,3-dimethyl-2-phenyl-
1,3,2-diazaphospholidine (DMPD) in refluxing benzene
to afford 9 (83% from 8). The oligo-stilbene 9 was con-
verted to the dialdehyde 10 (93% from 9) by conventional
functional group transformations. The cyclization of 10
and the construction of the final double bond completed
the synthesis of 3. Intramolecular cyclization of 10 by the
low valent vanadium complex proceeded smoothly to
afford the erythro-pinacol 11 in 70% yield with the for-
mation of the threo-isomer (14%). The predominant
formation of erythro-pinacol is unusual for this reac-
tion,12 probably because of the preference for a transition
state leading to this isomer. All-Z-hexabenzo[24]annul-
ene 3 was obtained by the Corey–Winter reaction of the
corresponding thionocarbonate derived from 11 using
DMPD in refluxing benzene (two steps, 55%).13
Figure 1. ORTEP drawing of 3: (a) top view and (b) side view.
the others were assembled inside, with dihedral angles
ca. 60° to each other. This structure is analogous to
Ôflattened coneÕ conformation in C3-symmetric calix[6]-
arene derivatives,15 but has a more compressed struc-
ture. The intramolecular assembly of the inner benzene
rings is attributed to the three concurrent CH/p inter-
actions among these benzene rings. The averaged dis-
tance between the hydrogen atom and the center of the
The product 3 is a stable crystalline compound. The
single crystal X-ray diffraction of 3 was measured by
using a hexagonal column crystal obtained from
CH2Cl2–hexane.14 As shown in Figure 1, the molecular
structure of 3 was found to be a C3-conformation in
which three benzene rings were projected outward and
ꢀ
arene nuclei is only 2.65 A (the corresponding Cꢀ ꢀ ꢀAr
ꢀ
distance is 3.44 A) and the averaged centroid–centroid
ꢀ
distance of these arene rings is 4.70 A.

Y. Kuwatani et al. / Tetrahedron Letters 45 (2004) 359–362
361
Figure 2. 1H NMR spectra of 3: (a) Spectrum at )90 °C in CD2Cl2 with the signal assignment. The numbering scheme was shown above.
(b) Spectrum at 100 °C in (CDCl2)2.
The molecular structure of 3 in solution was also studied
1
by temperature dependent H NMR spectra. The spec-
The optimized molecular structure by B3LYP/6-31G*
level calculation is also represented by the folded C3-
conformation with close contact between three benzene
rings with slightly longer distances (Ar–Ar: 5.13 A;
ꢀ
H–Ar: 3.01 A). The expansion in the structure obtained
trum at ambient temperature showed broad aromatic
signals centered at d 6.75 and 6.44 and an olefinic singlet
at d 6.66 in 1,1,2,2-tetrachloroethane-d2. The aromatic
signals changed to sharp AA0BB0 multiplets at d 6.72 and
6.47 when the temperature was elevated, as shown in
Figure 2b. This type of spectral change is ascribed to the
slow conformational change of the molecule. The spec-
trum in CD2Cl2 at room temperature showed the same
signals as that in (CDCl2)2 and broadened when the
temperature was lowered. Sharp separated signals
appeared below )70 °C, as shown in Figure 2a. All
signals were assigned by the experimentally observed
coupling connectivity and by comparison with the the-
oretically calculated shift values.16 The spectrum at
)90 °C is composed of signals from two types of
symmetrical disubstituted benzene rings and two sets of
cis-double bonds, and shows good accordance with the
C3-symmetric structure observed in the crystal. The
characteristic signal here is observed at a chemical shift
of d 4.4, which is assigned to the o-aryl proton (H1)
shielded by a neighboring arene ring. Such a large higher-
field shift suggests considerable proximity of the proton
to the neighboring arene p-cloud, as is found in the
crystal structure. The distance between the proton (H1)
ꢀ
by the DFT calculation is probably due to underesti-
mation of the interaction energy between arene rings.
Interestingly, the arrangement of the inside three ben-
zene rings in 3 is essentially identical to the arrangement
of the benzene trimer. The structure of 3 obtained from
crystallographic analysis, and the structure of the ben-
zene trimer optimized by MP2/6-31G* level MO calcu-
lation, are shown in Figure 3. Both structures have a
C3-symmetric arrangement and the structural parame-
ters between two benzene rings in 3 are very close to
those of the trimer. The Ar–Ar, C–Ar, and H–Ar dis-
ꢀ
tances in the trimer are 4.70, 3.40, and 2.47 A, respec-
tively. The binding energy of the benzene trimer has
been reported to be 6.2 kcal/mol, as determined by a gas
phase experiment.8 This large binding energy could be
attributable to the three concurrent cyclic CH/p inter-
actions. The structural similarity suggests that the same
magnitude of attractive interaction takes place in the
C3-conformation of 3.
The annulene 3 forms a silver complex on treatment
with AgClO4 in CD2Cl2 or CDCl3. The 1H NMR
spectra of 3ꢀAgClO4 showed lower field shifts for the
aromatic and olefinic protons, reflecting the formation
ꢀ
and the arene ring can be estimated as 2.50–2.70 A based
on the chemical shift,18 which is consistent with the X-ray
analysis.

362
Y. Kuwatani et al. / Tetrahedron Letters 45 (2004) 359–362
9. (a) Gonzalez, C.; Lim, E. C. J. Phys. Chem. A 2001, 105,
1904–1908; (b) Engkvist, O.; Hobza, P.; Selzle, H. L.;
Schlag, E. W. J. Chem. Phys. 1999, 110, 5758–5762.
10. (a) Kuwatani, Y.; Yoshida, T.; Kusaka, A.; Oda, M.;
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Tetrahedron 2001, 57, 3567–3576; (b) Kuwatani, Y.;
Yoshida, T.; Kusaka, A.; Iyoda, M. Tetrahedron Lett.
2000, 41, 359–363.
11. (a) Kuwatani, Y.; Yoshida, T.; Hara, K.; Yoshida, M.;
Matsuyama, H.; Iyoda, M. Org. Lett. 2000, 2, 4017–4020;
(b) Yoshida, T.; Kuwatani, Y.; Hara, K.; Yoshida, M.;
Matsuyama, H.; Iyoda, M.; Nagase, S. Tetrahedron Lett.
2001, 42, 53–56.
12. In the case of strained-ring formation, a few examples of
erythro-selective pinacol coupling by the vanadium com-
plex have been reported: (a) Raw, A. S.; Pedersen, S. F.
J. Org. Chem. 1991, 56, 830–833; (b) Iyoda, M.; Fuchi-
gami, K.; Kusaka, A.; Yoshida, T.; Yoshida, M.; Ma-
tsuyama, H.; Kuwatani, Y. Chem. Lett. 2000, 860–861; (c)
Iyoda, M.; Horino, T.; Takahashi, F.; Hasegawa, M.;
Yoshida, M.; Kuwatani, Y. Tetrahedron Lett. 2001, 42,
6883–6886.
Figure 3. Relative geometry of three benzene rings in the benzene
trimer optimized by MP2/6-31G* level MO calculation and the
molecular structure of 3 obtained by the X-ray crystallographic anal-
ysis.
of the aromatic and olefinic complexes in equilibrium.
Since 3 possesses no inner cavity to incorporate a silver
cation, the complexation of 3 with AgClO4 may take
place outside the annulene ring or inside by induced-fit.
Further studies on the properties of 3 as a cylindrical
p-system are now being carried out.
13. Spectroscopic data for 3: mp ca. 340 °C (decomp.); EI-MS
(m=z) 612 (Mþ); 1H NMR (500 MHz, CDCl3, 60 °C)
d 6.75–6.72 (12H, m), 6.67 (12H, s), 6.46 (12H, br m); 13C
NMR (125 MHz, CDCl3, 60 °C) d 136.7, 130.7, 129.9,
126.1; IR (KBr) 3057, 3007, 2957, 2361, 2341, 1466, 1154,
773 cmÀ1; UV (cyclohexane) kmax (log e) 285 (4.71) nm;
Anal. Calcd C, 94.08; H, 5.92. Found C, 93.68; H, 6.05.
14. Crystal data for 3: C48H36, Mw ¼ 612:81, T ¼ À150 °C,
orthorhombic, space group Pna21 (#33), a ¼ 22:621ð4Þ,
Acknowledgements
3
ꢀ
ꢀ
b ¼ 13:043ð5Þ, c ¼ 11:422ð3Þ A, V ¼ 3370ð1Þ A , Z ¼ 4,
Dc ¼ 1:208 g/cm3, R ¼ 0:110, Rw ¼ 0:183, GOF ¼ 1.73 for
2497 reflections with I > 1:0 rðIÞ. Crystallographic data
(excluding structure factors) for the structures in this
paper, have been deposited with the Cambridge Crystal-
lographic Data Centre as supplementary publication
numbers CCDC 219642. Copies of the data can be
obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (Fax: +44(0)-
1223-336033).
This work has been supported in part by CREST of
Japan Science and Technology Corporation (JST) and a
Grant-in-Aid for Scientific and Research no. 14540501
from the Ministry of Education, Science, Sports and
Culture.
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