all-Z-tribenzo[12]-, tetrabenzo[16]- and pentabenzo[20]annulenes

Article abstract of DOI:10.1016/S0040-4039(99)02062-6

A new synthetic strategy of all-Z-[n]benzo[4n]annulenes, which is based on the intramolecular pinacol coupling, followed by the Corey-Winter procedure, has been developed, and three annulenes have been successfully synthesized using this strategy.

Full text of DOI:10.1016/S0040-4039(99)02062-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 359–363
all-Z-Tribenzo[12]-, tetrabenzo[16]- and
pentabenzo[20]annulenes†
Yoshiyuki Kuwatani, Tadahiro Yoshida, Ayako Kusaka and Masahiko Iyoda ∗
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
Received 8 October 1999; revised 25 October 1999; accepted 29 October 1999
Abstract
A new synthetic strategy of all-Z-[n]benzo[4n]annulenes, which is based on the intramolecular pinacol coupling,
followed by the Corey–Winter procedure, has been developed, and three annulenes have been successfully
synthesized using this strategy. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: annulenes; cage compounds; coupling reactions; cyclizations; cyclophanes.
Cylindrical π-conjugated molecules1 attract continuing interest as synthetic challenges,2 unique
structures,3 host/guest interactions,4 and organic materials with interesting electric properties.5 all-Z-
[n]Benzo[4n]annulenes 1–3 are bowl-shaped hydrocarbons with a medium-sized concavity. Furthermore,
Z,Z,Z,Z-pentabenzo-[20]annulene 3 can be regarded as a precursor of the cyclic belt 4 which is a
central aromatic belt of C60 and C70. Although we have previously reported the synthesis of Z,Z,Z-
tribenzo[12]annulene 1 and its metal complexes,6 the synthetic methodology is only limited to 1 in very
low overall yield. Therefore, we developed a new strategy to synthesize all-Z-[n]benzo[4n]annulenes
1–3.
Our new synthetic strategy for 1–3 is based on the intramolecular pinacol coupling of the corres-
ponding linear poly cis-stilbene derivative 5, followed by formation of the final cis-double bond using
reductive dehydroxylation as shown in Scheme 1. The second step can be expected to be achieved by the
Corey–Winter procedure.7
∗
†
Corresponding author. Fax: +00 81 426 77 2525; e-mail: iyoda-masahiko@c.metro-u.ac.jp (M. Iyoda)
Dedicated to Professor Armin de Meijere on the occasion of his 60th birthday.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02062-6
tetl 15990

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Scheme 1. Synthetic route for 1–3
The synthesis of Z,Z,Z-tribenzo[12]annulene 1 was carried out using the sequence outlined in Scheme
2. Recently, we reported the synthesis and structure of Z,Z-tribenzo[12]annulene-1,2-dione 78 which
was prepared by the intramolecular cyclization of the dialdehyde 5 (n=1), followed by Swern oxida-
tion. Although 7 consists of two conformational isomers (7a and 7b), reduction of 7 with NaBH4 in
ethanol–CH2Cl2 afforded the erythro-isomer 8a (80%) with a small amount of the threo-isomer 8b. The
reaction of 8a with TCDI (thiocarbonyldiimidazole) in refluxing toluene led to the thionocarbonate 9
(78%) which was converted into 1 (84%) by the reaction with DMPD (1,3-dimethyl-2-phenyl-1,3,2-
diazaphospholidine) (modified Corey–Winter procedure).9 Z,Z,Z-Tribenzo[12]annulene 1 was synthesi-
zed in 37% overall yield based on 5 (n=1).
Scheme 2. Conditions: (a) NaBH4, EtOH, CH2Cl2, 0°C (80%); (b) TCDI (thiocarbonyldiimidazole), toluene, reflux (78%); (c)
DMPD (1,3-dimethyl-2-phenyl-1,3,2-diazaphospholidine), benzene, reflux (84%)
In analogy with the case of 1, the syntheses of Z,Z,Z,Z-tetrabenzo[16]- and Z,Z,Z,Z,Z-
pentabenzo[20]annulenes 2 and 3 were started from the construction of the corresponding dialdehydes
(Scheme 3). Thus, the Sonogashira coupling of 10 with 2-bromobenzaldehyde in the presence of
Pd(PPh3)4 and CuI in triethylamine at rt, followed by t-butyldimethylsilylation, produced 12 in 92%
overall yield. Reduction of 12 with NaBH4 (93%), followed by partial hydrogenation in the presence of
a Lindlar catalyst (99%) and Dess–Martin oxidation10 (98%) led to the cis-stilbene derivative 13. The
pinacol coupling of 13 with a low-valent vanadium complex gave the threo-diol 14 in 92% yield. The
threo-diol 14 was converted into the erythro-diol by a two-step procedure. Thus, Swern oxidation of 14
led to the diketone which was reduced with NaBH4 under the chelation-controlled conditions to produce
the erythro-diol (89% based on 14). The erythro-diol was treated with thiophosgene in the presence of
4-dimethylaminopyridine to afford the thionocarbonate 15 (94%). The reaction of 15 with DMPD in
refluxing benzene produced the Z,Z,Z-triene-bis(siloxy)ether (83%) which was treated successively with
Bun NF in THF and the Dess–Martin reagent in CH2Cl2 to produce the Z,Z,Z-triene-dialdehyde 16 in
4
89% overall yield.
For the synthesis of the tetraene-dialdehyde 19, the cis-stilbene derivative 13 was converted into the
enyne 17 by a Wittig-type reaction with CBr4 and PPh3 (81%), followed by treatment with LDA in THF
(89%). The Sonogashira coupling of 17 with o-diiodobenzene, followed by desilylation and Dess–Martin
oxidation produced the diendiyne 18 in 84% overall yield. The partial hydrogenation of 18 with the
Lindlar catalyst led to 19 (89%).
The syntheses of the annulenes 2 and 3 starting from the corresponding dialdehydes 16 and 19 were
carried out using a combination of the intramolecular pinacol coupling8 and modified Corey–Winter pro-
cedure (Scheme 3). The reaction of 16 with VCl3(THF)3 (2 equiv.)11 and zinc (2 equiv.) in DMF–CH2Cl2
at rt for 1 h produced a mixture of the threo- and erythro-diols 20a and 20b (49 and 25%, respectively)

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Scheme 3. Conditions: (a) Pd(PPh3)4 (1 mol%), CuI (2 mol%), Et3N, rt; (b) ButMe2SiCl, imidazole, CH2Cl2; (c) NaBH4 (2
equiv.), MeOH, CH2Cl2; (d) H2, Lindar catalyst, benzene, quinoline; (e) Dess–Martin reagent, CH2Cl2; (f) VCl3(THF)3 (1.1
equiv.), Zn (1.1 equiv.), CH2Cl2, rt; (g) (COCl)2 (4 equiv.), DMSO (5 equiv.), Et3N, CH2Cl2, −78°C; (i) Cl2C_S (1.2 equiv.),
4-dimethylaminopyridine (2.4 equiv.), CH2Cl2, 0°C; (j) DMPD, benzene, reflux; (k) Bun NF, THF; (l) CBr4, PPh3, CH2Cl2;
4
(m) LDA, THF, −78°C; (n) o-diiodobenzene, Pd(PPh3)4, CuI, Et3N, ∆
(Scheme 4). The threo-diol 20a was converted into the erythro-diol 20b by successive Swern oxidation
and NaBH4-reduction in 62% overall yield. The reaction of 20b with TCDI in refluxing toluene led to the
thionocarbonate 21 (80%), which was reacted with DMPD in refluxing benzene to produce the desired
all-Z-tetrabenzo[16]annulene 2 in 62% yield.12 In a similar manner, the intramolecular coupling of 19
with VCl3(THF)3 (3.5 equiv.) and zinc (3.5 equiv.) in DMF–CH2Cl2 at rt for 3.5 h gave a mixture of the
threo- and erythro-diols 22a and 22b in 80% yield. In order to line up the stereochemistry of the diol
part, the mixture of 22a and 22b was converted into the corresponding α-diketone by Swern oxidation.
Reduction of the diketone with NaBH4 afforded the erythro-diol 22b in 72% overall yield. The reaction of
22b with TCDI led to the thionocarbonate 23 (74%) which was treated with DMPD in refluxing benzene
to produce the [20]annulene 3 (72%).12
Scheme 4. Conditions: (a) VCl3(THF)3, Zn, DMF, CH2Cl2, rt; (b) (COCl)2, DMSO, Et3N, CH2Cl2; (c) NaBH4, MeOH, CH2Cl2,
0°C; (d) TCDl, 4-toluene, reflux; (e) DMPD, benzene, reflux
Although the [12]annulene 1 has a rigid framework with C3v symmetry, the 1H and 13C NMR spectra of
the [16]annulene 2 show temperature dependence. Thus, the 1H NMR spectrum of 2 in CDCl3 at −50°C
showed slightly complex signals consisting of one AB quartet (δ 6.74 and 6.64, J=12 Hz) and two sets
of AA0BB0 multiplets (δ 7.36, 7.29, 6.52 and 6.45), which suggested that 2 adopted a C2v-symmetric
conformation. However, the spectrum of the same sample gradually broadened along with elevating the
temperature, and the signals coalesced at about 0°C. At 50°C, the spectrum showed a simple pattern
corresponding to C4v-symmetry, and the activation energy for this dynamic process is calculated to be
12.7 kcal mol−1 using the VT 13C NMR measurements. Based on the 1H NMR spectra, the structure of 2
was established as a stacking form 2a at low temperature. The π–π interaction of the benzene rings may

362
prefer the stacking structure 2a, and the steric repulsion between the α-hydrogen of the benzene rings
destabilizes the C4v structure of 2.
1
In contrast to the results obtained from 2, the H and 13C NMR spectra of 3 showed no temperature
dependence. Thus, the C5v-symmetric spectra, which consisted of a set of AA0BB0 multiplets (δ 6.95
1
and 7.02) and a singlet (δ 6.47) in the H NMR and 4 signals at δ 126.5, 129.1, 129.7, and 136.3 in
the 13C NMR, observed at rt, were almost unchanged even at −90°C except for a slight broadening of
the signals. This result suggests that the molecular framework of 3 may be more flexible than that of 2.
Interestingly, the olefinic proton of 3 was observed at a higher field than the corresponding signals of 1
and 2, reflecting the more crowded structure of the five benzene rings.
As shown in Fig. 1, the structure of 2 was determined by X-ray analysis.13 When 2 was recrystallized
from CH2Cl2–hexane, colorless plates and prisms of 2 were obtained. The structure of 2 confirms the
expected stacking form with approximate C2 symmetry. A pair of benzene rings is stacked face-to-face
with a distance of 3.4 Å, whereas the other two are apart from each other to form a hinge-like structure.
The stacked benzene rings are located parallel with a slightly deviated overlap, presumably due to release
of the steric repulsion between the central olefinic hydrogens.
Fig. 1. ORTEP diagram of 2. Selected bond lengths [Å] and angles [°]: C1–C2, 1.34(1); C2–C3, 1.49(1); C3–C4, 1.40(1);
C4–C5, 1.49(1); C5–C6, 1.34(1); C1–C2–C3, 125.5(8); C2–C3–C4, 121.5(9); C2–C1–C16, 117.7(8)
Acknowledgements
Financial support for this study was provided by a Grant-in-Aid for Scientific Research on Priority
Areas from the Ministry of Education, Science and Culture, Japan (06224223).
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1
1
m/z 408 (M+); H NMR (C2D2Cl4, 120°C) δ 6.71 (s, 8H), 6.82 (AA0, 8H), 6.95 (BB0, 8H); H NMR (CDCl3, −55°C) δ
6.44–6.49 (AA0, 4H), 6.51–6.53 (BB0, 4H), 6.45 (s, 4H), 6.76 (s, 4H), 7.28–7.30 (AA0, 4H), 7.36–7.38 (BB0, 4H); 13C NMR
(CDCl3, 60°C) δ 126.5, 128.9, 130.5, 136.5; 13C NMR (CDCl3, −55°C) δ 126.3, 126.4, 127.5, 129.2, 129.7, 130.6, 134.8,
137.1; UV (cyclohexane): λmax (log ε) 227sh (4.31), 274 (4.31) nm. Compound 3: colorless prisms, mp 222.5–223.5°C, MS
m/z 510 (M+); 1H NMR (CDCl3, 25°C) δ 6.47 (s, 10H), 6.93–6.97 (AA0, 10H), 7.01–7.04 (BB0, 10H); 13C NMR (CDCl3)
δ 126.5, 129.1, 129.7, 136.3; UV (cyclohexane): λmax (log ε) 258 (4.63) nm.
13. Crystal data for 2: C32H24, Mw=408.54, orthorhombic, space group Pna21 (# 33), a=21.915(5), b=11.983(4), c=8.732(3) Å,
V=2293(2) Å3, Z=4, Dc=1.183 g cm−3, R=0.043, Rw=0.040, GOF=1.32 for 956 reflections with I>1.50σ(I).