Synthesis of Long Chain 2-Alkyl-1-(2-hydroxyethyl)-2-imidazolines under Microwave in Solvent-Free Conditions

Rafael Martínez-Palou; Gerardo de Paz; Jesús Marín-Cruz; Luis Gerardo Zepeda

doi:10.1055/s-2003-41410

LETTER

Synthesis of Long Chain 2-Alkyl-1-(2-hydroxyethyl)-2-imidazolines under Microwave in Solvent-Free Conditions

Rafael Martínez-Palou*^a, Gerardo de Paz^a, Jesús Marín-Cruz^a, Luis Gerardo Zepeda^b
^a Competencia de Química Aplicada. Programa de Ingeniería Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas No. 152, San Bartolo Atepehuacan, 07730 México
Fax: +525(3003)6239; e-Mail: rpalou@imp.mx;
^b Departamento de Química Orgánica, Escuela Nacional de Ciencias Biológicas, Instituto Politécnico Nacional, Prol. de Carpio y Plan de Ayala, 11340 México

Abstract

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Abstract

An efficient method for the synthesis of long chain 2-alkyl-1-(2-hydroxyethyl)-2-imidazolines, and their amide precursors, by condensing aminoethylethanolamine and several fatty acids under non-solvent microwave irradiation using CaO as support, is described. Products were obtained in good yields and high purity in a few minutes, in both multimode microwave and monomode microwave oven. Evidences of non-thermal microwave effects are given.

Key words

microwave - solid-phase synthesis - cyclizations - imidazolines - amides

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References

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Microwave irradiations was carried out with a DMW (Panasonic NM-5460A, 800 W, 2450 MHz) and a MMW (MIC-I, 600 W, from SEV, México). For the reactions using a DMW the final temperatures were measured directly, immediately at the end of the reaction, with a contact thermometer embedded in the reaction mixture. For a comparison multimode vs monomode microwave oven see ref. [4b] and ref. [6a]

Typical Experimental Procedure: In an open Pyrex vessel (100 mL, for DMW) or sealed tube (100 mL, for MMW) were carefully mixed 0.52 g (5.0 mmol) of aminoethyl-ethanolamine (3), 5.0 mmol of the corresponding fatty acid (4a-d) and 2.5 g of CaO. The resulting mixture was irradiated using the power and the reaction time showed in Table [1] . The reaction mixture was allowed to reach room temperature, EtOAc (20 mL) was added and the mixture was heated until boiling and filtered off while hot, and the filtrated was concentrated under vacuum to dryness, yielding the corresponding product as a white solid.

[2-(2-Hydroxyethylamino)ethyl]heptadecylamide (2a): Recrystallization from EtOAc. Mp 102-103 °C (lit [3a] 103-105 °C). IR (KBr): 3300-3270, 2920, 2850, 1641 cm^-1. ¹H NMR (200 MHz, CDCl₃): δ = 0.86 (t, J = 6.0 Hz, 3 H), 1.23 (sa, 28 H), 1.45-1.63 (m, 2 H), 2.19 (t, J = 7.9 Hz, 2 H), 2.72-2.92 (m, 4 H), 3.39 (q, J = 6.0 Hz, 2 H), 3.70 (t, J = 5.0 Hz, 2 H), 4.10 (sa, 1 H). ¹³C NMR (50 MHz, CDCl₃): δ = 14.3, 22.8, 26.0, 29.4, 29.6, 29.7, 32.0, 36.8, 38.9, 48.6, 51.1, 60.4, 173.5.
2-Heptadecyl-1-(2-hydroxyethyl)-2-imidazoline (1a): Recrystallization from EtOAc. Mp 62-63 °C (lit [3a] 63-65 °C). IR (KBr): 3310, 2917, 2849, 1602 cm^-1 ¹H NMR (200 MHz, CDCl₃): δ = 0.86 (t, J = 6.0 Hz, 3 H), 1.23 (sa, 28 H), 1.45-1.65 (m, 2 H), 2.22 (t, J = 5.8 Hz, 2 H), 3.22 (t, J = 5.4 Hz, 2 H), 3.29-3.39 (m, 2 H), 3.61-3.71 (m, 4 H), 3.76 (sa, 1 H). ¹³C NMR (50 MHz, CDCl₃): δ = 14.3, 22.8, 26.6, 28.0, 29.5, 29.68, 29.72, 29.8, 49.4, 50.2, 51.6, 59.8, 168.0.

The thermal heating reaction protocol was as in the case of microwave methodology except that the glass tube containing the reagents and a magnetic stirring bar, was placed into a preheated oil bath at 150 ºC. The temperature of each was measured for 5 s using a thermometer embedded in the reaction mixture. The work-up procedure was as with the microwave methodology.