Open AccessIonic Liquid Solvation versus Catalysis: Computational Insight from a Multisubstituted Imidazole Synthesis in [Et 2 NH 2 ][HSO 4 ]
Author(s) -
Abdullayev Yusif,
Abbasov Vagif,
Ducati Lucas C.,
Talybov Avtandil,
Autschbach Jochen
Publication year - 2016
Publication title -
chemistryopen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.644
H-Index - 29
ISSN - 2191-1363
DOI - 10.1002/open.201600066
Subject(s) - chemistry , transition state , catalysis , ionic liquid , deprotonation , cationic polymerization , nucleophile , medicinal chemistry , imidazole , protonation , solvation , gibbs free energy , nitromethane , solvent , photochemistry , inorganic chemistry , organic chemistry , ion , physics , quantum mechanics
The mechanisms of a tetrasubstituted imidazole [2‐(2,4,5‐triphenyl‐1 H‐imidazol‐1‐yl)ethan‐1‐ol] synthesis from benzil, benzaldehyde, ammonium acetate, and ethanolamine in [Et 2 NH 2 ][HSO 4 ] 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 (Δ 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 . [Et 2 NH 2 ][HSO 4 ] 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 (Δ G ≠ =9.2 kcal mol −1 ) from [Et 2 NH 2 ] + , and then deprotonation of ammonia (Δ G ≠ =14.3) via Et 2 NH. Likewise, incorporation of the anion component [HSO 4 ] − 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.