Ionic Liquid Solvation versus Catalysis: Computational Insight from a Multisubstituted Imidazole Synthesis in [Et2NH2][HSO4]

Author(s)

Abdullayev, Yusif ; Abbasov, Vagif ; Ducati, Lucas C. ; Talybov, Avtandil ; Autschbach, Jochen

• Document type: Journal article
• Source: CHEMISTRYOPEN; v. 5, n. 5, p. 460-469, OCT 2016.
• Web of Science Citations: 2

Abstract

The mechanisms of a tetrasubstituted imidazole {[}2-(2,4,5-tri-phenyl-1H-imidazol-1-yl)ethan-1-ol]synthesis from benzil, benzaldehyde, ammonium acetate, and ethanolamine in {[}Et2NH2] {[}HSO4] ionic liquid (IL) are studied computationally. The effects of the presence of the cationic and anionic components of the IL on transition states and intermediate structures, acting as a solvent versus as a catalyst, are determined. In IL-free medium, carbonyl hydroxylation when using a nucleophile (ammonia) proceeds with a Gibbs free energy (Delta G{*}) barrier of 49.4 kcal mol(-1). Cationic and anionic hydrogen-bond solute-solvent interactions with the IL decrease the barrier to 35.8 kcal mol(-1). {[}Et2NH2]{[}HSO4] incorporation in the reaction changes the nature of the transition states and decreases the energy barriers dramatically, creating a catalytic effect. For example, carbonyl hydroxylation proceeds via two transition states, first proton donation to the carbonyl (Delta G {*} = 9.2 kcal mol(-1)) from {[}Et2NH2](+), and then deprotonation of ammonia (Delta G{*} = 14.3) via Et2NH. Likewise, incorporation of the anion component {[}HSO4](-) of the IL gives comparable activation energies along the same reaction route and the lowest transition state for the product formation step. We propose a dual catalytic IL effect for the mechanism of imidazole formation. The computations demonstrate a clear distinction between IL solvent effects on the reaction and IL catalysis. (AU)

• FAPESP's process: 14/21930-9 - Study of the solvent effect on the spin spin coupling constant 1JPt-Tl in heavy metal complexes by ab initio molecular dynamics
• Grantee: Lucas Colucci Ducati
• Support Opportunities: Scholarships abroad - Research