Preparation of 5-methyl-3,5-dipropyl-2-pyrazoline catalyzed by chloroaluminate ionic liquids

Abstract

5-methyl-3,5-dipropyl-2-pyrazoline is an intermediate for the preparation of new high energy fuels. In this paper, chloroaluminate ionic liquids ([Bmim]Cl-xAlCl3, x = 1, 1.2, 1.4, 1.6, 1.8, 2.0) were applied as green catalysts for the synthesis of 5-methyl-3,5-dipropyl-2-pyrazoline through the intramolecular cyclization reaction of 2-pentyl ketazine. The effects of temperature, acidity and dosage of ionic liquids on the cyclization reaction were studied in detail and the highest catalytic performance was under the conditions as 110 °C, [Bmim]Cl-1.6AlCl3 and 10% of 2-pentyl ketazine. With the theoretical simulations, the possible catalytic mechanism was proposed as the hydrogen-like interaction between Al ([Bmim]+AlCl4−) and N1 atoms, which increases the charge of N2 atoms, thereby improving its nucleophilicity and further making it undergo intramolecular addition reaction with methyl. This study will provide guidance for the green preparation of 5-methyl-3,5-dipropyl-2-pyrazoline.