Synthesis of 4-[(4-dimethylaminobenzylidene)amino]phenyl tetradecanoate

The discovery of Schiff base 4-methoxybenzylidene-4’-butylaniline and the application of its room temperature nematic phase in displays sparked some renewed interest in liquid crystals [1]. In our earlier studies, the results revealed that ester and azomethine linking units are useful structural components for generating mesomorphism in two and three aromatic rings thermotropic liquid crystals [2,3,4,5]. Therefore, continuing investigations have been carried out which focused on the synthesis of a new Schiff base ester, 4-[(4-dimethylaminobenzylidene)amino]phenyl tetradecanoate.

The title compound was synthesized according to a previously reported method [6,7,8,9,10]. A solution of 4-dimethylaminobenzaldehyde (1.19 g, 8 mmol) and 4-aminophenol (0.87 g, 8 mmol) in absolute ethanol (40 mL) was heated under reflux for 3 hours. Compound 1 thus obtained was recrystallized from absolute ethanol. Then, Schiff base 1 (0.48 g, 2 mmol) in dimethylformamide (DMF) (1 mL), was added to a solution of tetradecanoic acid (0.46 g, 2 mmol) and 4-dimethylaminopyridine (DMAP) (0.12 g, 1 mmol) in dichloromethane (20 mL). The resulting mixture was stirred in an ice bath. To this solution, N,N’-dicyclohexylcarbodiimide (DCC) (0.41 g, 2 mmol) dissolved in dichloromethane (10 mL) was added dropwise while stirring in the ice bath for an hour. The resulting mixture was subsequently stirred at room temperature for another 3 hours. Then, the reaction mixture was filtered and the excess solvent was removed from the filtrate by evaporation. Recrystallization from absolute ethanol gave the Schiff base 2 as a yellow solid (0.19 g, 21%).

Melting point: 105.7 ^oC.

EI-MS m/z (rel. int. %): 451 (6) [M+1]⁺, 450 (20) [M]⁺, 240 (100).

IR (KBr, cm^-1): 2953, 2918, 2850 (C-H aliphatic); 1752 (C=O ester); 1608 (C=N); 1590, 1498 (C=C aromatic).

¹H NMR (300 MHz, CDCl₃): δ/ppm 0.90 (3H, t, J = 6.5 Hz, CH₃), 1.28-1.42 {m, 20H, CH₃(CH₂)₁₀-}, 1.77 (quint, 2H, J = 7.3 Hz, -CH₂CH₂COO-), 2.58 (t, 2H, J = 7.4 Hz, -CH₂COO-), 3.08 {s, 6H, ‑N(CH₃)₂}, 6.74 (d, 2H, J = 8.9 Hz, Ar-H), 7.07 (d, 2H, J = 8.8 Hz, Ar-H), 7.20 (d, 2H, J = 8.8 Hz, Ar-H), 7.76 (d, 2H, J = 8.9, Ar-H), 8.33 (s, 1H, CH=N).

¹³C NMR (75 MHz, CDCl₃): δ/ppm 14.63 (CH₃), 23.17, 29.57, 29.74, 29.83, 29.93, 30.06, 30.11, 30.14, 32.38 for methylene carbons (CH₃(CH₂)₁₀-), 25.41 (-CH₂CH₂COO-), 34.85 (-CH₂COO-), 40.63 {N(CH₃)₂}, 111.92, 122.18, 122.48, 124.53, 130.91, 148.48, 150.90, 152.89 for aromatic carbons, 160.93 (CH=N), 173.09 (COO).

Elemental analysis: Calculated for C₂₉H₄₂N₂O₂: C, 77.29%, H, 9.39%, N, 6.22%; Found: C, 77.37%, H, 9.47%, N, 6.16%.