New liquid crystalline tolanes from (-)-menthone

New Liquid Crystalline Tolanes from (-)-Menthone

Fernando Ely, Adailton J. Bortoluzzi, Hugo Gallardo,

Department of Chemistry

Universidade Federal de Santa Catarina, CEP 88040-900, Florianópolis, SC, Brazil

and Aloir A. Merlo

Institute of Chemistry

Universidade Federal do Rio Grande do Sul, CEP 91510-970, Porto Alegre, RS, Brazil

Received on 16 November, 2001

Synthetic manipulations were carried out with the aim of obtaining an advanced linear fragment for production of thermotropic liquid crystals. The chiral fragment synthesised was introduced in a tolanbenzoate mesogenic core by coupling of Sonogashira. The final chiral tolans showed smectic A and C and nematic liquid crystalline phases. The crystallographic structure of lactone from (-) menthone is presented.

I Introduction

The design and synthesis of new chiral liquid crystals has experienced a great growth, especially of ferroelectric liquid crystals (FLCs). This is due to practical applications of FLCs as high resolution displays and in photonical technologies for storage and reproduction of information based on non-linear optical (NLO) effects [1]. The correlation between molecular chirality and physical properties has been well studied and some models proposed [2]. Conventionally, natural abundant materials like amino acids or commercially available alcohols and esters such as (S)-2-methylbutan-1-ol, (S)-2-methyloctan-1-ol and (S)-ethyl lactate are used to synthesize chiral liquid crystals [3]. (-)-Menthone derivatives have been used only as chiral dopants, to induce ferroelectricity in smectic C (S_C) liquid crystals or more recently, in photochromic polymers [4]. In all cases the original cyclic structure of (-)-menthone was maintained. In the present work we describe a efficient synthetic approach to transform the (-)-menthone to an appropriate chiral structure which can provide high stereo- and regio-control for liquid crystals synthesis. To verify the efficiency of the chiral chain, we present the synthesis of new liquid crystalline tolanes (diphenylacetylenes) as a continuation of our most recent study [5].

II Experimental

II.1 Tecniques

¹H-NMR and ¹³C-NMR spectra were obtained in CDCl₃ with 270 MHz (Bruker HX-270) spectrometer using TMS as internal standard. IR Spectra were recorded in KBr discs, nujol dispersions or films with a Perkin-Elmer model 283 spectrometer. Elemental analysis was done using a Perkin Elmer 2400CHN. Optical rotations were measured on a Perkin-Elmer 341 polarimeter at the sodium D line. GC analysis was conducted in a Varian 2 equipped with a FID detector using H₂ as carrier gas; capillary column used b-DEX 120 (30m x 0.25mm). X-ray data were collected in a difratometer Nonius CAD-4; refinement: full-matrix least-squares on F², GooF (F²), programs: solution ® SHELXS97; refinement ® SHELXL97; molecular drawing ® ZORTEP. The thermal transitions and the mesomorphic textures were determined using a Olympus BX 50 polarising microscope in conjunction with a Mettler Toledo FP-90 heating stage and Perkin Elmer DSC-2, using indium as a standard calibration.

The synthetic methodology used was very convenient, it is depicted in scheme 1.

Baeyer-Villiger oxidation afforded the correspondent lactone (1) [6]. Three oxidation systems were tested on a preparative scale (ca. 10 g of ketone) and the results obtained are summarized in Table 1. This is the most important step in the synthesis because the stereo- and regio-control must be guaranteed. NMR experiments (¹H, DEPT, COSY ¹H-¹H and HETCOR), chiral capillary GC and X-ray crystal analysis were used for controlling the structure of Baeyer-Villiger oxidation product 1 (Fig. 1 and Table 2).

The chiral precursors (4a and 4b) were coupled with a tolanebenzoate mesogenic core to investigate its efficiency in the production of liquid crystals. Scheme 2 illustrates the synthesis of the mesogenic core and the target molecules.

Methyl 4-hydroxybenzoate was alkylated with n-decylbromide by a traditional method. After hydrolysis the resultant acid was esterificated with 4-ethynylphenol [7] furnishing the alkyne 5. Palladium catalysed cross-coupling between 5 and 4a or 4b led to the desired compounds 6a and 6b. Selected data for final compounds are displayed in the Table 3.

III Results and Discussion

Conventional chiral terminal groups are almost always linear and possess contiguous stereogenic centers. The unique branching points are only the chiral centers. From this point of view the compound 3 is not a conventional chiral tail such as 2-methyloctanol or aminoacids are. In the compound 3 the asymmetric centers are spaced by two atoms of carbon and additionally is present a methyl group at the end of the hydrocarbon chain. However, important factors such as a straight chain and the presence of good linking carboxylate group remain. Methyl branched systems generally show lower clearing points than their linear analogs [8]; presumably because the methyl group sticks out at an angle from the molecular rotational cylinder, and thereby disrupts efficient packing by increasing the separation between neighboring molecules. However, as the methyl branch is moved further away from the core of the mesogen, the clearing point depression is reduced. This demonstrates the greater degree of conformational freedom further away from the core since the effect of branching is less marked [9]. Thus we believe that the methyl group, placed at the last carbon of the main chain in 3, does not significantly take effect on the mesomorphic behavior. Also the hydroxyl group may behave in the same way or even help molecular packing by establishing intermolecular hydrogen bonds.

The mesomorphic behavior of the final tolanes was investigated by optical microscopy and the thermal properties are compiled in Table 4. These compounds exhibited chiral nematic (N*), smectic A (S_A) and ferroelectric (S_C*) liquid crystalline phases.

The triple bond between two aromatic rings is interesting, because it contributes to the polarizability, linearity and rigidity of the final molecules. These factors are important and determine many physical properties, such as thermal stability and viscosity in liquid crystals. The presence of the nitro group decreased the melting point expressively (compare 113.2 ^°C for compound 6a and 78.6 ^°C for compund 6b) and also reduced the thermal stability (compare 164.4 ^°C for compound 6a and 128.1^°C for compound 6b). Such behaviour is due the greater van der Waals volume of the nitro group and its effect on molecular packing.

IV Conclusions

(-)-Mentone has been transformed into a nearly linear moiety through several chemical reactions including the Baeyer-Villiger oxidation. Two new chiral tolanes were synthesised, one non-laterally substituted 6a and the other nitro-laterally substituted 6b, using the chiral compound from (-)-mentone. Both compounds showed a N^* mesophase and 6b shows also an enantiotropic S_A phase. But undoubtedly, the more exciting result is the S_C* ferroelectric mesophase observed for compound 6a, because it proves that compound 3 is efficient for producing a S_C* phase.

Acknowledgements

This work was supported by the programs PADCT-III and PRONEX and the agencies CNPq and FAPERGS. We thank Ms Valéria Belli Riatto for the donating reagents and Prof. Ted Ray Taylor for the helpfull discussions.

References

[1] Macromolecular Host-Guest Complexes: Optical and Optoelectronic Properties and Applications, D. M. Walba, J. A. Rego, N. A. Clark, and R. Shao, Materials Research Soc. 1992; (b) M. Schadt, Liq. Cryst. 14(1), 73, (1993).

[2] (a) T. Sierra, J. L. Serrano, M. B. Ros, A. Ezcurra, and J. Zubia, J. Am. Chem. Soc. 114, 7645, 1992 and references cited therein; (b) J. W. Goobby, I. Nishiyama, A. J. Slaney, C. J. Booth, and K. J. Toyne. Liq. Cryst. 14(1), 37, (1993); (c) C. J. Booth, D. A. Dunmur, J. W. Goodby, and K. J. Toyne. Liq. Cryst. 20(6), 815, (1996).

[3] (a) J. Schacht, P. Zugenmaier, and F. Horh. Liq. Cryst. 26(4), 525, (1999); (b) H. Gallardo, and A. A. Merlo. Synth.Comm., 23(15), 2159, (1993); (c) H. T. Nguyen, J. C. Rouillon, A. Babeau, J. P. Macerou, M. Cotrait, and H. Allouchi. Liq. Cryst. 26(7), 1007, (1999).

[4] (a) A. R. Imamaliev, M. G. Nadzhafova, and D. F. Aliev, Russ. J. Phys. Chem. 67(10), 2023, (1993); (b) L. A. Kutulya, L. D. Patsenker, V. V. Vashchenko, V. P. Kuznetsov, V. I. Kulishov, Y. N. Surov, and V.V. Kravets, Russ. Chem. Bull. 44(7), 1200, (1995); (c) A. Y. Bobrovsky, N.I. Boiko, and V.P. Shibaev, Liq. Cryst. 26(12), 1749, (1999).

[5] A. A. Merlo, J. E. Braun, U. B. Vasconcelos, F. Ely, and H. Gallardo, Liq. Cryst. 27(5), 657, (2000).

[6] S.S.C. Koch and Chamberlin, R. Synth. Comm. 5, 829, (1989).

[7] This compound decomposes with light or heating and was synthesised as described by E. T. Sabourin and A. Onopchenko, J. Org. Chem. 48, 5135, (1983).

[8] G.W. Gray and S.M. Kelly. Mol. Cryst. Liq. Cryst. 104, 335, (1984).

[9] Handbook of Liquid Crystals Research, Collings, P.J. and Patel, J.S., (Oxford University Press, 1997).

Publication Dates

• Publication in this collection
  27 Oct 2002
• Date of issue
  June 2002

History

• Received
  16 Nov 2001