Ionic hydrogen-bonding interaction controlled electrophilicity and nucleophilicity: Mechanistic insights into the synergistic catalytic effect of lipase and natural deep eutectic solvents in amidation reaction

Abstract

To improve their catalytic activity, herein, three types of metal chloride hydrate were introduced into pure choline chloride-glycerol (CGly) for fabricating new three constituents-natural deep eutectic solvents (3c-NADESs) with enhanced Lewis acidity. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) confirmed that the Lewis acidity of CGly was significantly improved due to the addition of metal chloride hydrates. The catalytic performance of these new-designed 3c-NADESs was characterized via the amidation reaction of lauric acid and lysine. Compared with pure CGly, a significant enhancement of the yield of Gemini lauroyl lysine (GLL) was obtained. Interestingly, the control experiments suggested that there was an apparent synergistic effect between Lipase immobilized on acrylic resin from Candida antarctica (CALB) and NADESs. Quantum calculation studies suggested fatty acid and amino acid were activated by chloride ions via multiple hydrogen-bonding interactions, resulting in the enhancement of electron-attracting capacity of fatty acid and electron-loss ability of amino acid. The frontier molecular orbits analysis provided us with a cogent evidence that chloride ions activated both lysine and lauric acid and made the reaction easier to be performed. The intrinsic reaction coordinates (IRC) study indicated chloride ions act as a “freighter”, which transported the hydrogen atom of amino group to the hydroxyl group. This in-depth study may shed light on developing sustainable and efficient strategies of amidation and other nucleophilic or electrophilic reactions.