A new type of banana shape bifunctional monomer of ester chalcones

A synthesis of a new type of chalcone analogue banana-shaped liquid crystals compounds is described. The approach involved the reaction of hydroxyl chalcones with isophthaloyl dichloride to form benzene1,3-dicarbonyl-bis-1-(4-alkoxyphenyl)-3-(4-hydroxylphenyl)prop-2-en-1-one (7a-c and 8a-c) having two chalcone units via esterification reaction. The hydroxyl chalcones, which differ in the length of alkyl group, CnH2n+1, where n= 10, 12 and 14 were synthesised via Claisen Schmidt condensation prior to the reaction with isophthaloyl dichloride. The reaction was a catalyst dependence. No sign of product was formed by employing NaH (60%). It was only occurred in the presence of NaH (95%) to afford banana shape bi-functional monomer of ester chalcone 7a-c and 8a-c. The synthesized compound was characterized by 1H and 13C Nuclear Magnetic Resonance (NMR), and Fourier Transform Infrared (FTIR).


INTRODUCTION
Liquid crystal technology has been widely applied in device technology. Both fluidity and crystalline properties of liquid crystal facilitate better control over alignment when applied to device. The advantages of liquid crystals for these applications such as faster switching times, a wider viewing angle, continuous gray level, improved transmittance of the clear state, approximately no threshold voltage and low power consumption [1].
Series of linear shape of mesogens based on chalcone has been studied for their liquid crystal behavior [2,3]. Chalcone derivatives were reported for excellent nonlinear optic property [4] and liquid crystal displays [5,6]. Chalcones has been used in promoting light transmittance [7] and crystallability [8].
In recent years, banana-shaped liquid crystals have been given much attention in various aspects such as fast polarization reorientation, ferroelectricity and antiferroelectricity, and small threshold voltage when applied to liquid crystal display mode [9]. Banana-shaped liquid crystals have also been identified as interesting compounds due to their unexpected electro-optical properties, polarity and chirality [10].

Materials and characterizations
4-hydroxybenzaldehyde, 4-hydroxyacetophenone, 1-bromoalkane, 4-hydroxychalcone and isophthaloyl dichloride were obtained from Merck Company and used as received. THF was dried over sodium and benzophenone. All other reagents and solvent were used as received. Infrared spectra were recorded on a Perkin Elmer 1605 Fourier Transform Infra-Red Spectrophotometer. 1 H and 13 C NMR spectra were be recorded using JEOL ECA 500 Spectrometer operating at 500 MHz for 1 H and 125 MHz for 13 C with chemical shifts reported relative to CDCl3 as standard reference.

Synthesis of 4-hydroxychalcones
Potassium hydroxide (KOH) (1.68 g, 30 mmol) in ethanol (90 mL, 95%) was stirred for 30 min. 4hydroxyacetophenone (4.08 g, 30 mmol) and benzaldehyde (3.66 g, 30 mmol) were added to the solution mixture respectively. The reaction mixture was stirred at room temperature for 18 h. The mixture was cooled in an ice bath and acidified with hydrochloric acid (HCl) (12 M

Strategy 2
4-hydroxychalcone (0.11 g, 0.50 mmol) and THF (30 mL) was added to sodium hydride (NaH) (0.01 g, 0.50 mmol, 95 %) and stirred for 1 h at room temperature. Benzene-1,3-dicarbonyl dichloride (0.05 g, 0.25 mmol) in THF (20 mL) was added to the mixture. The mixture was refluxed for 8 h. The mixture was cooled to room temperature, filtered, and washed with distilled water and methanol. Dichloromethane (DCM) and distilled water were used to extract the organic layer. The organic layer was dried, filtered and concentrated in vacuo. The crude was purified from hexane: DCM (3:1) to give 2 as pale yellowish oil (0.

General procedure
A mixture of 4-hydroxybenzaldehyde (12.5 mmol) and 3a-c (12.5 mmol) in 35 mL of methanol was added under stirring to a solution of KOH (2.52 g) in methanol (10 mL). The mixture was heated at reflux for 10 h. The reaction was cooled to room temperature and acidified with cold diluted HCl (2 N). The resulting precipitate was filtered, washed, and dried. The crude was recrystallized from hexane:ethanol (7:1) to give 4a-4c.

General procedure
Bromoalkane (60 mmol), 4-hydroxybenzaldehyde (60 mmol), K 2 CO 3 (60 mmol), and TBAI (6 mmol) in MEK (200 mL) were heated at reflux for 5 h. The mixture was filtered and cooled to room temperature. Water (30 mL) was added to the filtrate, and the layers separated. The aqueous layer was extracted with dichloromethane (2 x 30 mL). The combined layers were washed with water (2 x 20 mL), dried (MgSO4), filtered, and concentrated in vacuo. The crude product was purified by column chromatography (eluting with 1:20 ethyl acetate:petroleum ether).The same general procedure gave compounds 5a-c.
The FTIR and NMR data were consistent with the reported literature [12].

General procedure
A mixture of 4-hydroxyacetophenone (30 mmol) and 6a-c (30 mmol) in 90 mL of methanol was added under stirring to a solution of KOH (6.06 g, 108 mmol) in methanol (10 mL). The mixture was heated at reflux for 10 h. The reaction was cooled to room temperature and acidified with cold diluted HCl (2 N). The resulting precipitate was filtered, washed, and dried. The crude was recrystallized from hexane:ethanol (7:1) to give 6a-6c.
The chemical structure of 2 was found to be consistent with FT-IR and 1 H NMR spectroscopic methods and showed the peaks corresponded to the structures.
The IR spectrum of 2 is shown in Fig. 1. The IR spectrum showed the disappearance of -OH peak at 3140 cm -1 and the appearance of absorption band at 1727 cm -1 , which were attributed to the formation of ester bond. The structure of 2 were further confirmed by 1 H NMR spectrum (Fig. 2), which showed peaks attributed to trans vinylic proton at δ 6.96 and 7.89, with a coupling constant, Jab 15 Hz. New peak attributed to center aromatic H 1 appeared as singlet at δ 8.49. The peaks corresponded to H3&4 were appeared at δ 7.57-8.08. This proton resonates more downfield from other resonances due to the attachment with two chalcones moieties. Several methods have been employed in the synthesis of chalcones due to ease of chalcone structure itself to be substituted [13,14]. A series of chalcone derivatives (E)-1-[4-(alkyloxy)phenyl]-3-[4hydroxyphenyl ] prop-2-en-1-one (4a-4c) and (E)-3-(4alkyloxyphenyl)-1-(4-hydroxyphenyl)prop-2-en-1-one (6a-6c) were prepared via Claisen-Schmidt condensation by the route depicted in Scheme 2.
Compound 3a-c and 5a-c were reacted with 4hydroxybenzaldehyde and 4-hydroxyacethophenone, respectively, under refluxing methanol to afford hydroxylated chalcones 4a-c and 6a-c as yellow crystals. IR  The structures of compound 4a-c and 6a-c were further verified using 1 H NMR and 13 C NMR analysis. 1 H NMR spectra showed the presence of resonances attributed to CH3 and CH2 at δ 0.87 and 1.26-1.83. The trans vinylic proton α and β centered at δ 7.42 and 7.74 as two doublets peak with Jab 15.45 Hz. The 13 C NMR spectra showed the peak attributed to C=O at δ 189.1 and -COH at δ 162.9.
Compounds 7a-c and 8a-c with two chalcone units were finally prepared by the reaction of 4a-c and 6a-c with isophthaloyl dichloride, respectively. The reactions were performed in the presence of NaH (95%) under refluxing THF to afford 7a-c and 8a-c as viscous yellow oil. The preparation of 7a-c and 8a-c is depicted in Scheme 3.
The IR spectrum of 7a (Fig 3) showed the presence of vc=o for ester at 1746 cm -1 and vc-o-c attributed to alkyl aryl ether at 1245 cm -1 . The 1 H NMR spectrum of 7a (Fig. 4) showed peaks attributed to CH 3 and CH 2 at δ 0.85 and 1.24-1.63, while peak attributed to OCH2 at δ 3.97. The trans vinylic proton α and β were assigned at δ 7.60 and 7.78 with Jab 15.45 Hz. The attachment of two chalcone units onto the benzene ring was supported by the appearance of a resonance as a triplet and a singlet at δ 7.37 and 8.85 respectively. Scheme 3 Synthesis of banana-shaped chalcones ester 7a-c and 8a-c

CONCLUSION
We have synthesised new homologues series of banana-shaped compounds 7a-c and 8a-c comprising bifunctional chalcone units in the presence of NaH (95%). The low yield of 7a-c and 8a-c might due to the steric effect of hydroxylated chalcone 4a-c bearing long alkyl chains. Compounds 7a-c and 8a-c could be useful in the study of banana shaped liquid crystals properties.