Catalytic Aminolysis (Amide Formation) from Esters and Carboxylic Acids: Mechanism, Enhanced Ionic Liquid Effect, and its Origin

This paper describes the use of imidazolium-based ionic liquids {1-n-butyl-3-methylimidazolium tetrafluoroborate [BMI-BF4], 1-n-butyl-3-methylimidazolium hexafluorophosphate [BMI-PF6], and 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMI-NTf2]} as efficient supports for Lewis and Bronsted acids ,which are promoters of the aminolysis of some esters, fatty acids, and fatty acid esters (among others) to form amide derivatives. Some esters and carboxylic acids were tested to demonstrate the generality of the methodology, and the corresponding amides were obtained in high yields. Recycling reactions (at least eight reuses) without a notable loss in activity could be performed by using CdO and SnCl2 as catalysts in BMI-NTf2 as the ionic medium. Bronsted acids, such as H2SO4 and HCl, were also tested with impressive results; however, it was not possible to perform recycling reactions because of catalyst leaching. The same was true when using BF3 center dot OEt2 as the catalyst. Mechanistic insights and the ionic-liquid effect were investigated by using 13C{1H} NMR spectroscopy, which showed that there is a strong interaction of the imidazolium cation with the C=O and C=C bonds of methyl oleate, most likely through C-H...pi interactions, pi-stacking interactions, and ion-pair formation in the presence of a metal catalyst. Electrospray ionizationquadrupole time-of-flight experiments allowed a better understanding of the reaction mechanism. The results could explain the enhanced ionic-liquid effect on the stabilization of the formed intermediates, which facilitated the amide bond formation.