摘要:

RO RO OR OR OR OH RO RO OR OR OR O O RO RO OR OR OR OR RO RO OR OR OR OAc RO AcO 1a R = Bu b R = C5H11 2a,b 3a,b 4a,b i ii iii Novel unsymmetrical triphenylene discotic liquid crystals first synthesis of 1,2,3,6,7,10,11-heptaalkoxytriphenylenes Sandeep Kumar*† and M. Manickam Centre for Liquid Crystal Research PO Box 1329 Jalahalli Bangalore-560 013 India Oxidation of 2-hydroxy-3,6,7,10,11-pentaalkoxytriphenylene yields the pentaalkoxytriphenylene-1,2-quinone; reductive acetylation of this o-quinone furnishes the diacetate that can be directly alkylated to 1,2,3,6,7,10,11-heptaalkoxytriphenylene derivatives showing columnar Dh phases. Mesophases formed by discotic liquid crystals (LCs) are now well-recognised to be suitable for many device applications.1,2 Most of the discogens reported to date consist of a flat or nearly flat rigid core surrounded by a number of aliphatic side chains having binary trigonal tetragonal or hexagonal symmetries.Symmetrically substituted hexethers of triphenylene are the most widely synthesised and studied discotic mesogens. The potential uses of these materials as one-dimensional conductors 2,3 photoconductors4 and light emitting diodes5 are attracting considerable attention. Several research groups are currently working on the synthesis of symmetrical unsymmetrical and functionalized triphenylene discotic liquid crystals.6–14 Triphenylene derivatives show mesomorphism only when the six peripheral positions are substituted with aliphatic chains. Whereas the synthesis of symmetrical hexalkoxytriphenylenes by oxidative trimerisation of 1,2-dialkoxybenzenes is quite easy the synthesis of well-defined unsymmetrically substituted derivatives is more complicated.Breaking of symmetry in a hexaalkoxytriphenylene can be achieved by different ways e.g. by changing the chain length of some of the side chains by changing the nature of one or more side chain or by using lower or higher degrees of substitution. Tinh et al. reported the synthesis of several dissymmetrical hexasubstituted triphenylenes by incorporating different alkyl chains into the periphery using a statistical approach and found that introduction of dissymmetric side-chains does not affect the nature of the Dh phase but does result in the reduction of the mesophase stability.15 An easy route to unsymmetrically-substituted triphenylenes was recently reported using the so-called biphenyl route.6e,7e A well-defined synthesis of unsymmetrical and low degree substituted triphenylenes has also been reported recently using organometallic chemistry.10 When one out of the six ether side chains in hexaalkoxytriphenylenes was replaced by an ester group the stability of the mesophase was enhanced significantly.16 A plastic columnar discotic phase is reported in this type of unsymmetric triphenylene derivatives.17 In an another approach to the preparation of unsymmetrical triphenylenes and to the induction of molecular dipole colour etc. nitration and halogenation of hexaalkoxytriphenylenes has been investigated. 6a–d,9 While the nitro or halogen group in these triphenylene derivatives could not be replaced by an alkylthio group by nucleophilic aromatic substitution,9 reduction of the nitro group followed by acylation with various acid chlorides yielded ‘seven tail’ triphenylene discotics.6a However because of the presence of the amide group the clearing temperature of these derivatives are rather high.We have very recently reported a highly improved synthesis of symmetrical unsymmetrical and mono-functionalized triphenylene derivatives using MoCl5.8c We have also reported the synthesis of various functionalized triphenylene,7f,8d mixed tail triphenylene,7b low symmetry fluorescent triphenylene7a and core functionalized triphenylene discotic LCs.8b,c All the triphenylene derivatives hitherto known have six or less alkoxy chains. While triphenylene derivatives with six alkoxy chains are mesomorphic triphenylenes having less than six alkoxy chains are nonmesomorphic but can be made mesomorphic by putting other substituents onto the periphery.7a,b To the best of our knowledge there are no examples of triphenylene discotic species having more than six alkoxy chains. Here we report on a novel approach to the synthesis of triphenylene-based discotic liquid crystals containing seven alkoxy chains in the periphery. The synthesis of these novel heptaalkoxytriphenylenes is outlined in Scheme 1. During the nitration 2-hydroxy-3,6,7,10,11-pentabutoxytriphenylene we always found a black product in addition to the nitrated and some unreacted starting material.8e If the reaction is not monitored carefully this black material becomes the major product. We suspected that the formation of this black material was due to ring oxidation to an o-quinone.Oxidation of 2-hydroxy-3,6,7,10,11-pentabutoxytriphenylene 1a with other known oxidising agents such as chromium trioxide and cerium(iv) ammonium nitrate yielded the same product. The structure of this compound was assigned as 3,6,7,10,11-petabutyloxytriphenylene- 1,2-dione 2a on the bases of its 1H NMR and mass spectral data. Reductive acetylation of this o-quinone resulted in the formation of diacetate 3a. The diacetate was directly alkylated8a with alkyl halide to heptabutoxytriphenylene 4a in very high yield. Heptapentoxytriphenylene was prepared in the same manner. The 1H NMR data of the products were found to be in perfect agreement with the structure.‡ Scheme 1 Reagents and conditions i CAN MeCN room temp. 2 min 94%; ii Ac2O Zn NEt3 reflux 0.5 h 92%; iii DMSO KOH RBr 60 °C 1 h 95% Chem.Commun. 1998 1427 Both heptaalkoxytriphenylenes 4a and 4b are mesogenic. While compound 4a melted at 65.7 °C and clears at 70.1 °C compound 4b exhibited a very broad mesophase from room temperature to 65 °C. The mesophase–isotropic transition temperatures of both heptaalkoxytriphenylenes are very low compared to their hexasubstituted analogues (ca. 145 °C for hexabutoxy- and 122 °C for hexapentoxytriphenylene). This could be due to the presence of the extra alkoxy chain and the steric hindrance caused by this chain. This methodology provides an easy high-yielding process for the preparation of various unsymmetrical low clearing temperature broad mesophase triphenylene discotics. It can also be extended to other discotic cores.The potential of this new method is currently under investigation for the synthesis of various hepta- octa- nona- and per-alkoxytriphenylene derivatives. We are very grateful to Professor S. Chandrasekhar for many helpful discussions. The authors also gratefully acknowledge the technical assistance of Mr Sanjay K. Varshney. Notes and References † E-mail uclcr@giasbg01.vsnl.net.in ‡ Selected data for 2a m/z (FAB) 620.2 (M+ + 2 H); dH(CDCl3) 8.91 (s 1 H) 7.64 (s 1 H) 7.61 (s 1 H) 7.35 (s 1 H) 6.98 (s 1 H) 4.20 (m 10 H) 1.89 (m 10 H) 1.55 (m 10 H) and 1.05 (m 15 H). For 2b dH(CDCl3) 8.98 (s 1 H) 7.72 (s 1 H) 7.68 (s 1 H) 7.45 (s 1 H) 7.08 (s 1 H) 4.20 (m 10 H) 1.95 (m 10 H) 1.48 (m 20 H) and 0.98 (m 15 H). 3a dH(CDCl3) 8.48 (s 1 H) 7.84 (s 1 H) 7.83 (s 1 H) 7.81 (s 1 H) 7.78 (s 1 H) 4.19 (m 10 H) 2.44 (s 3 H) 2.38 (s 3 H) 1.92 (m 10 H) 1.60 (m 10 H) and 1.03 (m 15 H).For 3b dH(CDCl3) 8.48 (s 1 H) 7.84 (s 1 H) 7.83 (s 1 H) 7.81 (s 1 H) 7.78 (s 1 H) 4.18 (m 10 H) 2.44 (s 3 H) 2.38 (s 3 H) 1.88 (m 10 H) 1.47 (m 20 H) and 0.97 (m 15 H). For 4a dH(CDCl3) 9.22 (s 1 H) 7.83 (s 2 H) 7.81 (s 1 H) 7.66 (s 1 H) 4.20 (m 12 H) 4.01 (t J 7.1 2 H) 1.88 (m 14 H) 1.56 (m 14 H) and 0.99 (m 21 H). For 4b dH(CDCl3) 9.21 (s 1 H) 7.83 (s 2 H) 7.81 (s 1 H) 7.66 (s 1 H) 4.20 (m 12 H) 4.01 (t J 7.1 2 H) 1.83 (m 14 H) 1.49 (m 28 H) and 1.02 (m 21 H). 1 S. Chandrasekhar and S. Kumar Science Spectra 1997 8 66. 2 N. Boden R. Bissell J. Clements and B. Movaghar Liq. Crystal. Today 1996 6 1. 3 N. Boden R. J. Bushby J. Clements M. V. Jesudason P. F. Knowles and G. Williams Chem. Phys. Lett. 1988 152 94; N.Boden R. J. Bushby and J. Clements J. Chem. Phys. 1993 98 5920; N. Boden R. J. Bushby A. N. Cammidge J. Clements and R. Luo Mol. Cryst. Liq. Cryst. 1995 261 251; E. O. Arikainen N. Boden R. J. Bushby J. Clements B. Movaghar and A. Wood J. Mater. Chem. 1995 5 2161. 4 D. Adams F. Closs T. Frey D. Funhoff D. Haarer H. Ringsdorf P. Schuhmacher and K. Siemensmeyer Phys. Rev. Lett. 1993 70 457; D. Adam F. Closs T. Frey D. Funhoff D. Haarer H. Ringsdorf P. Schuhmacher and K. Siemensmeyer Ber. Bunsenges. Phys. Chem. 1993 97 1366; D. Adam P. Schuhmacher J. Simmerer L. H�aussling W. Paulus K. Siemensmeyer K. H. Etzbach H. Ringsdorf and D. Haarer Adv. Mater. 1995 7 276; J. Simmerer B. Glusen W. Paulus A. Kettner P. Schuhmacher D. Adam K. H. Etzbach K. Siemensmeyer J. H. Wendorff H.Ringsdorf and D. Haarer Adv. Mater. 1996 8 815. 5 I. H. Stapff V. Stumpflen J. H. Wendorff D. B. Spohn and D. Mobius Liq. Cryst. 1997 23 613. 6 (a) N. Boden R. J. Bushby A. N. Cammidge S. Duckworth and G. Headdoc J. Mater. Chem. 1997 7 601; (b) N. Boden R. J. Bushby and A. N. Cammidge Mol. Cryst. Liq. Cryst. 1995 260 307; (c) N. Boden R. J. Bushby and A. N. Cammidge Liq. Cryst. 1995 18 673; (d) N. Boden R. J. Bushby A. N. Cammidge and G. Headdock J. Mater. Chem. 1995 5 2275; (e) N. Boden R. J. Bushby A. N. Cammidge and G. Headdock Synthesis 1995 31; (f) N. Boden R. J. Bushby and A. N. Cammidge J. Chem. Soc. Chem. Commun. 1994 465; (g) N. Boden R. C. Borner R. J. Bushby A. N. Cammidge and M. V. Jesudason Liq. Cryst. 1993 15 851. 7 (a) J. A. Rego S. Kumar and H. Ringsdorf Chem. Mater.1996 8 1402; (b) J. A. Rego S. Kumar I. J. Dmochowski and H. Ringsdorf Chem. Commun. 1996 1031; (c) P. Henderson S. Kumar J. A. Rego H. Ringsdorf and P. Schuhmacher J. Chem. Soc. Chem. Commun. 1995 1059; (d) F. Closs L. H�aussling P. Henderson H. Ringsdorf and P. Schuhmacher J. Chem. Soc. Perkin Trans. 1 1995 829; (e) P. Henderson H. Ringsdorf and P. Schuhmacher Liq. Cryst. 1995 18 191; (f) S. Kumar P. Schuhmacher P. Henderson J. Rego and H. Ringsdorf Mol. Cryst. Liq. Cryst. 1996 288 211. 8 (a) S. Kumar Mol. Cryst. Liq. Cryst. 1996 289 247; (b) S. Kumar and M. Manickam Mol. Cryst. Liq. Cryst. 1998 309 291; (c) S. Kumar and M. Manickam Chem. Commun. 1997 1615; (d) S. Kumar and M. Manickam Synthesis 1998 in the press; (e) S. Kumar M. Manickam V. S. K. Balagurusamy and H. Schonherr unpublished work.9 K. Praefcke A. Eckert and D. Blunk Liq. Cryst. 1997 22 113. 10 R. C. Borner and R. F. W. Jackson J. Chem. Soc. Chem. Commun. 1994 845. 11 J. W. Goodby M. Hird K. J. Toyne and T. Watson J. Chem. Soc. Chem. Commun. 1994 1701. 12 H. Naarmann M. Hanack and R. Mattmer Synthesis 1994 477. 13 F. C. Krebs N. C. Schiodt W. Batsberg and K. Bechgaard Synthesis 1997 1285; K. Bechgaard and V. D. Parker J. Am. Chem. Soc. 1972 94 4749. 14 V. Le Berre L. Angely N. Simonet-Gueguen and J. Simonet J. Chem. Soc. Chem. Commun. 1987 984; V. Le Berre J. Simonet and P. Batail J. Electroanal. Chem. 1984 169 325; J. Chapuzet and J. Simonet Tetrahedron 1991 47 791. 15 N. H. Tinh M. C. Bernaud G. Sigaud and C. Destrade Mol. Cryst. Liq. Cryst. 1981 65 307. 16 M. Werth S. U. Valklerien and H. W. Spiess Liq. Cryst. 1991 10 759. 17 B. Glausen A. Kettner and J. H. Wendorff Mol. Cryst. Liq. Cryst. 1997 303 115. Received in Cambridge UK 9th April 1998; 8/02698I 1428 Chem. Commun.

ISSN:1359-7345    

DOI:10.1039/a802698j

出版商:RSC    

年代:1998

数据来源: RSC