M. Bushey, A. Hwang, P. Stephens
Jul 31, 2001
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Journal of the American Chemical Society
Abstract
The study described below considers whether substituents in the 2,4,6-positions can force 1,3,5-triamides into conformations favorable for intermolecular hydrogen bonding. 6 Suprisingly, there areno examples of benzene rings with secondary amides at the 1,3,5-positions with any substituents other than hydrogen at the remaining positions. 7 Described below are syntheses of the first members of this new class of molecules ( 1a-d) and studies showing that they self-assemble into columns. Their physical properties show that 1d forms a liquid crystalline phase with its columns perpendicular to the surface and that 1b forms a highly ordered phase whose columns are parallel to the surface. Shown in Figure 1 is the lowest energy 8 dimer of a benzene ring that is alternatingly substituted with methoxyls and methylamides. To relieve steric congestion, the amides twist out of the aromatic plane by ca. 45 ° allowing three intermolecular hydrogen bonds, whileπ-surfaces are “in-registration”, stacked 3.8 Å apart. Key to the synthesis of this class of molecules 9 (Scheme 1) was the discovery that 1,3,5-tribromo-2,4,6-tridodecyloxylbenzene ( 2) undergoes a triple lithium/halogen exchange at -78 °C.10 After quenching with methyl chloroformate, 4 is produced in an unoptimized 30% yield on a 4-g scale. The target structures 1a-d are then synthesized in three steps: saponification, conversion to 5, and reaction with primary amines (75 -81% yield). The synthesis is both expeditious and flexible. As shown in Table 1, 1a-d undergo an initial thermal transition between 47 and 98 °C. At higher temperatures, both 1a and1c form isotropic liquids. In contrast, 1b (at 176-232 °C) and1d (at 85-200 °C) form another phase before becoming isotropic. Upon cooling the isotropic liquids, 1b and 1d undergo phase transitions ( 1b, 3 kJ/mol, and1d, 8.6 kJ/mol) with enthalpies similar to those observed for discotic liquid crystals (ca. 1 -20 kJ/mol).1,3-5 Indicative of hydrogen bonds forming in the mesophases, 5f the N-H stretching frequency of 1b shifts from 3295 (at 200°C) to 3361 cm-1 (at 250°C), and for1d it shifts from 3282 (at 135°C) to 3374 cm-1 (at 220°C). Displayed in Figure 2 is the diffraction pattern of synchrotron radiation (λ ) 1.151 Å) by1b at 200°C. The diffractogram is dominated by a single sharp peak at low angle, diagnostic of columnar assemblies. 11 Remarkably, diffraction peaks up to fifthorder are seen that can be indexed to a hexagonal lattice. The diffuse reflection at ca. 4.5 Å arises from the fluidlike packing of side chains. 11 The lateral core-to-core separation 12 is 21 Åsin ‡ Columbia University. § State University of New York at Stony Brook. (1) (a) Guillon, D.Struct. Bonding1999, 95, 41-82. (b) Chandrasekhar, S.; Ranganath, G. S. Rep. Prog. Phys. 1990, 53, 57-84. (2) (a) Van de Craats, A. M.; Warman, J. M.; Fechtenkotter, A.; Brand, J. D.; Harbison, M. A.; Mullen, K.AdV. Mater. 1999, 11, 1469-1472. (b) Chandrasekhar, S.; Prasad, S. K. Contemp. Phys. 1999, 40, 237-245. (c) Boden, N.; Bushby, R. J.; Clements, J.; Movaghar, B. J. Mater. Chem. 1999, 9, 2081-2086. (3) (a)Metallomesogens ; Serrano, J. L., Ed.; VCH: New York, 1996. (b) Simon, J.; Bassoul, P. InPhthalocyanines: Properties and Applications ; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989; Vol. 2, Chapter 6. (c) Serrette, A. G.; Lai, C. K.; Swager, T. M. Chem. Mater.1994, 6, 225268. (4) (a) Bengs, H.; Ebert, M.; Karthaus, O.; Kohne, B.; Praefcke, K.; Ringsdorf, H.; Wendorff, J. H.; Wuestefeld, R. AdV. Mater.1990, 2, 141-4. (b) Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkovsky, E. B.; Grubbs, R. H.Angew. Chem., Int. Ed. Engl. 1999, 38, 2741-2745. (5) (a) Paleos, C. M.; Tsiourvas, D. Angew. Chem., Int. Ed. Engl. 1995, 34, 1696-711. (b) Brienne, M.-J.; Gabard, J.; Lehn, J.-M.; Stibor, I. J. Chem. Soc., Chem. Commun. 1989, 1868. (c) Goldmann, D.; Dietel, R.; Janietz, D.; Schmidt, C.; Wendorff, J. H. Liq. Cryst.1998, 24, 407-411. (d) Ungar, G.; Abramic, D.; Percec, V.; Heck, J. A. Liq. Cryst.1996, 21, 73-86. (e) Percec, V.; Ahn, C.-H.; Bera, T. K.; Ungar, G.; Yeardley, D. J. P. Chem.-Eur. J. 1999, 5, 1070-1083. (f) Matsunaga, Y.; Miyajima, N.; Nakayasu, Y.; Sakai, S.; Yonenaga, M. Bull. Chem. Soc. Jpn. 1988, 61, 207-10. (g) Brunsveld, L.; Zhang, H.; Glasbeek, M.; Vekemans, J. A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 6175-6182 and references therein. (h) Malthete, J.; Levelut, A. M.; Liebert, L.AdV. Mater.1992, 4, 37-41. (i) Pucci, D.; Veber, M.; Malthete, J.Liq. Cryst.1996, 21, 153-155. (6) Benzenes with meta-disposed secondary amides form columnar liquid crystals (refs 5f -i). (7) No associative properties were reported for examples with primary amides: (a) Wallenfels, K.; Witzler, F.; Friedrich, K. Tetrahedron1967, 23, 1845-55. (b) Kolotuchin, S. V.; Thiessen, P. A.; Fenlon, E. E.; Wilson, S. R.; Loweth, C. J.; Zimmerman, S. C.hem.-Eur. J.1999, 5, 2537-2547. (8) MacroModel v.7.0 (Amber*): Mohamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.; Hendrickson, T.; Still, W. C. J. Comput. Chem. 1990, 11, 440-67. (9) Experimental details are in the Supporting Information. (10) With methyls ethers this reaction was inoperably low yielding: Engman, L.; Hellberg, J. S. E. J. Organomet. Chem. 1985, 296, 357-66. (11) For diffraction from discotics: (a) Levelut, A. M. J Chim. Phys. Phys.Chim. Biol.1983, 80, 149-61. (b) The citations in refs 1 and 3 -5. (12) Given byd100/cos30°. Figure 1. Energy minimized dimeric model. Methyls on the ether oxygens were included in the minimization and removed to clarify the view.