A new synthetic route to tribenzo[a,e,i][12]annulenes

Article abstract of DOI:10.1016/S0040-4039(99)02346-1

The tribenzo[a,e,i][12]annulene 10 and the corresponding twofold annulene 14 were prepared on synthetic routes which first led from suitably functionalised starting compounds (2, 11) to the 1,2-distyryl- and 1,2,3,4- tetrastyrylbenzene derivatives 9 and 13 with two and four formyl groups, respectively. The cyclisation to 10 and 14 was achieved by applying the McMurry reaction. The extended ring system of compound 14 represents a discotic mesogen. The generation of liquid crystals was obtained with eight hexyloxy chains on the periphery. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02346-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1535–1538
A new synthetic route to tribenzo[a,e,i][12]annulenes
Herbert Meier ∗ and Michael Fetten
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, D-55099 Mainz, Germany
Received 11 November 1999; accepted 14 December 1999
Abstract
The tribenzo[a,e,i][12]annulene 10 and the corresponding twofold annulene 14 were prepared on synthetic
routes which first led from suitably functionalised starting compounds (2, 11) to the 1,2-distyryl- and 1,2,3,4-
tetrastyrylbenzene derivatives 9 and 13 with two and four formyl groups, respectively. The cyclisation to 10 and 14
was achieved by applying the McMurry reaction. The extended ring system of compound 14 represents a discotic
mesogen. The generation of liquid crystals was obtained with eight hexyloxy chains on the periphery. © 2000
Elsevier Science Ltd. All rights reserved.
Keywords: annulenes; cyclisation; liquid crystals.
Areno-condensed annulenes represent discotic mesogens which are suitable for the generation
of liquid crystals in very broad temperature intervals.1 Due to their interesting photophysical and
photochemical properties they attract considerable attention in materials science.2 (5E,11E,17E)-
Tribenzo[a,e,i]cyclododecene (tribenzo[a,e,i][12]annulene) 1 was first prepared by H. A. Staab and
co-workers.3 This approach, as well as an alternative route4 to the parent compound, proved not to
be appropriate for the synthesis of derivatives with long flexible alkoxy chains;5 therefore we had to
conceive a novel route for the formation of such ring systems which represent the first substituted
tribenzo[a,e,i][12]annulenes.
3,4-Dihexyloxybenzaldehyde (2) yielded with bromine in acetic acid compound 3 in which the formyl
group was protected as acetal 4 by the reaction with trimethoxy orthoformate in the presence of Dowex
50 W-X8® (Scheme 1). Bouveault formylation led to the aldehyde 5; additionally 4 was transformed
to the phosphonate 6. The latter process consisted of three steps, namely reduction of the acetal to the
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02346-1
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corresponding primary alcohol, substitution of the hydroxy group by bromine and an Arbusov reaction
with triethyl phosphite. The overall yield amounted to 73%. A twofold Wittig–Horner reaction of 6
and the dialdehyde 7, obtained by a quantitative deprotection of 5, gave the diolefin 8 as a mixture of
stereoisomers. The equilibration with iodine in boiling p-xylene resulted in a 3:1 ratio of (E,E)- and (E,Z)-
configuration. The twofold Bouveault reaction 8→9 yielded the starting compound for the McMurry
cyclisation to the desired annulene 10. The recrystallised dialdehyde 9 contained selectively the (E,E)-
isomer; moreover, 10 was obtained in a pure (E,E,E)-configuration. Concerning the parent system 1,
apart from the (E,E,E)-isomer the (Z,Z,Z)-isomer6,7 as well as the (E,Z,Z)-isomer7 are stable compounds,
whereas the (E,E,Z)-isomer3,4 was only postulated as an intermediate.
Scheme 1.
The enantiotopic protons of the OCH2 groups in 10 prove the fast inversion of the central 12-membered
ring.8 Thus, the disk-like shape of 10 resembles strongly the hexahexyloxytricycloquinazoline which
forms liquid crystals.9 Nevertheless, annulene 10 shows a sharp melting point at 108°C. The DSC
measurement reveals onset temperatures of 107.5 and 95.5°C for the phase transition solid/liquid in
the heating and the cooling curve, respectively. Probably the interaction of neighbouring ring systems is
too low to establish a mesophase.
Consequently, we synthesised an enlarged ring system 14 with two [12]annulene units. Starting
from tetrakis(bromomethyl)benzene 11, the tetraphosphonate 12 was prepared by an Arbusov reaction
(Scheme 2). The Wittig–Horner olefination with 12 and 5 yielded after deprotection the tetraaldehyde
13 which was subjected to a McMurry reaction. The twofold cyclisation product 14 was obtained as
first fraction in the column chromatography (SiO2/CH2Cl2).10 The DSC (second heating curve) shows at
165°C a transition of the crystalline state to an LC phase. The mosaic textures observed in the polarisation

1537
microscope indicate a columnar phase. The clearing point cannot be determined because decomposition
reactions start above 280°C.
Scheme 2.
1
All compounds were characterised by H NMR, 13C NMR and mass spectra and gave satisfactory
elemental analyses. The characteristic data of the di- and tetrastyrylbenzenes11 8, 9 and 13, and the
[12]annulenes12 10 and 14 are listed below.
Acknowledgements
We are grateful to the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie for
financial support.
References
1. (a) Müller, K.; Meier, H.; Bouas-Laurent, H.; Desvergne, J. P. J. Org. Chem. 1996, 61, 5474; (b) Kretzschmann, H.; Müller,
K.; Kolshorn, H.; Schollmeyer, D.; Meier, H. Chem. Ber. 1994, 127, 1735; (c) Meier, H.; Kretzschmann, H.; Lang. M. J.
Photochem. Photobiol., A 1994, 80, 393; (d) Meier, H.; Kretzschmann, H.; Kolshorn, H. J. Org. Chem. 1992, 57, 6847; (e)
See also: Yu, R.; Yakimansky, A.; Voigt-Martin, I. G.; Fetten, M.; Schnorpfeil, C.; Schollmeyer, D.; Meier, H. J. Chem. Soc.,
Perkin Trans. 2 1999, 1881.
2. (a) Meier, H.; Stalmach, U.; Fetten, M.; Seus, P.; Lehmann, M.; Schnorpfeil, C. J. Inf. Recording 1998, 24, 47; (b) Meier,
H.; Müller, K.; Fetten, M. J. Inf. Recording 1996, 22, 421; (c) Meier, H.; Müller, K. Angew. Chem. 1995, 107, 1598; Angew.
Chem., Int. Ed. Engl. 1995, 34, 1437.
3. Staab, H. A.; Graf, F.; Doerner, K.; Nissen, A. Chem. Ber. 1971, 104, 1159.
4. Tausch, M. W.; Elian, M.; Bucur, A.; Cioranescu, E. Chem. Ber. 1977, 110, 1744.
5. Fetten, M. Dissertation, Mainz, 1998.
6. Mohler, D. L.; Vollhardt, K. P. C.; Wolff, S. Angew. Chem. 1990, 102, 1200; Angew. Chem., Int. Ed. Engl. 1990, 29, 1151.
7. Iyoda, M.; Kuwatani, Y.; Yamauchi, T.; Oda, M. J. Chem. Soc., Chem. Commun. 1988, 65.
8. The three inner hydrogen atoms in 1 and 10 prevent a totally planar structure.3
9. (a) Kumar, S.; Wachtel, E. J.; Keinan, E. J. Org. Chem. 1993, 58, 3821; (b) Keinan, E.; Kumar, S.; Singh, S. P.; Ghirlando,
R.; Wachtel, E. J. Liq. Cryst. 1992, 11, 157.
10. A ketone C90H126O9 and higher oligomers are the major products in the later fractions.

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1
11. Di- and tetrastyrylbenzenes: compound 8: colourless crystals, mp 110°C; H NMR (CDCl3) : δ 0.89 (m, 18H, CH3),
1.31 (m, 24H, CH2), 1.46 (m, 12H, CH2), 1.81 (m, 12H, CH2), 3.97 (m, 8H, OCH2), 4.07 (t, 4H, OCH2), 7.02, s,
3
2H/7.06, s, 2H/7.12, s, 2H (aromat. CH), 7.10/7.17 (AB, J=15.9 Hz, 4H, olefin. H); 13C NMR (CDCl3): δ 14.0 (CH3),
22.6/25.6/29.2/31.6 (CH2), 69.5/69.8 (OCH2), 112.0/112.4/117.6 (aromat. CH), 114.9 (CqBr), 127.5/128.7 (olefin. CH),
129.3/129.8 (Cq), 148.8/149.3/149.6 (CqO); compound 9: yellow crystals, mp 113°C; 1H NMR (CDCl3): δ 0.88 (m, 18H,
CH3), 1.32 (m, 24H, CH2), 1.46 (m, 12H, CH2), 1.81 (m, 12H, CH2), 4.04 (m, 12H, OCH2), 7.00, s, 2H/7.08, s, 2H/7.33,
3
s, 2H (aromat. CH), 7.17/7.61 (AB, J=15.9 Hz, 4H, olefin. H), 10.24 (s, 2H, CHO); 13C NMR (CDCl3): δ 14.0 (CH3),
22.6/22.6/25.6/25.7/29.0/29.2/31.5/31.6 (CH2), 69.2/69.3 (OCH2), 110.6/111.6/113.1 (aromat. CH), 125.3/130.7 (olefin.
CH), 126.3/129.0/135.7 (Cq), 148.7/149.6/154.0 (CqO), 190.2 (CHO); compound 13: yellow crystals, which decompose
1
above 200°C; H NMR (CDCl3): δ 0.89 (m, 24H, CH3), 1.33 (m, 32H, CH2), 1.47 (m, 16H, CH2), 1.83 (m, 16H, CH2),
4.05 (t, 8H, OCH2), 4.09 (t, 4H, OCH2), 7.07 (s, 4H, aromat. CH), 7.23/7.85 (AB, 3J=15.8 Hz, 8H, olefin. H), 7.34 (s, 4H,
aromat. H), 7.81 (s, 2H, aromat. H), 10.25 (s, 4H, CHO); 13C NMR (CDCl3): δ 14.0 (CH3), 22.6/25.6/29.0/31.5 (CH2),
69.2 (OCH2) 110.7/113.6/126.0 (aromat. CH), 126.5/135.1/135.7 (Cq), 127.8/130.3 (olefin. CH), 149.0/154.0 (CqO), 190.3
(CHO).
12. [12]Annulenes: compound 10: light beige crystals, mp 107.5°C (DSC); 1H NMR (CDCl3): δ 0.90 (t, 18H, CH3), 1.35 (m,
24H, CH2), 1.48 (m, 12H, CH2), 1.83 (m, 12H, CH2) 4.04 (t, 12H, OCH2), 6.88 (s, 6H, aromat. H), 7.02 (s, 6H, olefin.
H); 13C NMR (CDCl3): δ 14.0 (CH3), 22.6/25.8/29.4/31.6 (CH2), 69.6 (OCH2), 113.9 (aromat. CH), 130.9 (Cq), 132.6
(olefin. CH), 148.4 (CqO); compound 14: light yellow solid which is transformed at 165°C (DSC) to the LC phase; 1H NMR
(CDCl3): δ 0.91 (m, 24H, CH3), 1.36 (m, 32H, CH2), 1.49 (m, 16H, CH2), 1.84 (m, 16H, CH2), 4.04 (t, 16H, OCH2), 6.90
(s, 4H, aromat. H), 6.91 (s, 4H, aromat. H), 7.03 (s, 4H, olefin. H), 7.07/7.13 (AB, 3J=16.5 Hz, 8H, olefin. H), 7.37 (s, 2H,
aromat. H); 13C NMR (CDCl3): δ 14.1 (CH3), 22.7/25.8/29.3/31.6 (CH2), 69.4 (OCH2), 113.3/113.3/128.2 (aromat. CH),
130.5/130.8/136.7 (Cq), 132.3/132.7/133.7 (olefin. CH), 148.3/148.4 (CqO).