2-(4-Heptyloxyphenyl)benzothiazole

Benzothiazole-based liquid crystals are known to be good hole-transporting materials with a low ionization potential, making them of potential interest as hole-transporting materials in organic light emitting devices [1,2,3]. In view of the importance of these compounds, chemists are prompted to generate the derivatives by introducing different substituents into the existing skeleton of the molecule [4,5,6]. Although the smectogenic 2-(4-heptyloxyphenyl)benzothiazole was reported in a few patents [7,8,9,10], the use of dimethyl sulfoxide (DMSO) as a solvent for the formation of benzothiazole is less convenient as DMSO is difficult to be removed from the product (high b.p. of DMSO, 187^oC). In this paper, we report the convenient preparation of 2-(4-heptyloxyphenyl)benzothiazole using ethyl alcohol (more volatile solvent) to ensure easy separation of the product. Furthermore, the title compound was obtained in two steps: cyclization to benzothiazole followed by Williamson etherification which is a reverse method reported in the literature. Thus, we provide an alternative synthetic pathway of 2-(4-heptyloxyphenyl)benzothiazole.

2-Aminothiophenol (5.01 g, 40 mmol) and 4-hydroxybenzaldehyde (4.88 g, 40 mmol) in absolute ethanol (40 mL) was heated under reflux for 6 hours. The reaction mixture was subsequently cooled to room temperature, then distilled water (60 mL) was added slowly until the mixture turned cloudy. It was kept overnight at about 20 °C and the solid formed was filtered and washed with cold ethanol:water (1:1.5) and dichloromethane.

In analogy to a recently published procedure [9], benzothiazole 1 (4.55 g, 20 mmol) in acetone (40 ml), was added to a solution of potassium hydroxide (1.12 g, 20 mmol) in distilled water (5 ml). This was followed by addition of a small amount of potassium iodide into the mixture. The reaction mixture was heated under reflux for an hour with stirring. 1-Bromoheptane (4.48 g, 25 mmol) was then added to the flask and reflux was continued for 20 hours. The solid obtained was repeatedly recrystallized from absolute ethanol whereupon the pure compound was isolated as a white solid (3.58 g, 55%).

Melting point: 86.9 °C.

EI-MS m/z (rel. int. %): 325 (38) [M⁺], 227 (100), 198 (10), 108 (5), 57 (7).

IR (KBr, cm^-1): 2921, 2852 (C-H aliphatic); 1603 (C=N); 1259, 1036 (C-O ether).

¹H NMR (400 MHz, CDCl₃): δ/ppm 0.9 (t, J = 6.8 Hz, 3H, CH₃-), 1.2-1.5 (m, 8H, CH₃-(CH₂)₄-CH₂-CH₂-O-), 1.8 (qt, J = 7.4 Hz, 2H, -CH₂-CH₂-O-), 4.0 (t, J = 6.6 Hz, 2H, -CH₂-O-), 7.0 (d, J = 8.8 Hz, 2H, Ar-H), 7.4 (t, J = 8.1 Hz, 1H, Ar-H), 7.5 (t, J = 8.1 Hz, 1H, Ar-H), 7.9 (d, J = 8.0 Hz, 1H, Ar-H), 8.0 (m, 3H, Ar-H).

Elemental analysis: Calculated for C₂₀H₂₃NOS: C, 73.81%, H, 7.12%, N, 4.30%; Found: C, 73.72%, H, 7.15%, N, 4.39%.