Niobium chloride in 1−n−butyl−3−methylimidazolium chloride ionic liquid as a catalyst for biginelli reaction

Abstract

In this study, NbCl5 in a liquid ionic medium was evaluated as a catalyst for Biginelli reaction. Firstly, the ionic species found in mixtures of NbCl5 and 1−butyl−3−methylimidazolium chloride (BMImCl) at various molar ratios (x) were determined. Samples with x ranging from 0.2 up to 0.4 were constituted of BMIm+, Cl− and [NbCl6]− ions in different proportions. When x was 0.5, BMIm+ and [NbCl6]− were the only species. When the molar ratio was 0.6 a mixture of BMImNbCl6 and Nb2Cl10 was observed. The identification of the ionic species provides useful information about the chemical nature of the niobium catalyst in the Biginelli reaction and other chemical or electrochemical transformations. The Biginelli reaction carried out using ethyl acetoacetate, benzaldehyde and urea in the presence of NbCl5 and xNbCl5_BMImCl mixtures with x varying from 2 to 5 yielded almost 90% of 3,4-dihydropyrimidinone, which means the acid C (2) hydrogen in the BMIm+ with [NbCl6]- as the counterion acted as Brønsted acid capable of bonding to the CO of benzaldehyde and ethyl acetoacetate, following the iminium mechanism.