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

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 [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 (ΔG≠) 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 (ΔG≠=9.2 kcal mol-1) from [Et2NH2]+, and then deprotonation of ammonia (ΔG≠=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.

4-(1-Allyl-4,5-diphenyl-1H-imidazol-2-yl)-N,N-dimethyl-aniline.

The title compound, C26H25N3, crystallizes with four independent mol-ecules, 1-4, in the asymmetric unit of the triclinic unit cell. The allyl substituents on the imidazole rings adopt similar conformations in all four mol-ecules. The imadazole and the 4-and 5-substituted phenyl rings of two pairs of molecules in the asymmetric unit stack parallel to (110). In contrast, the dimethyl-aniline systems in these pairs of mol-ecules are almost normal to one another, with dihedral angles of 85.84 (10) and 85.65 (10)° between the benzene rings of the two dimethyl-aniline fragments of mol-ecules 1 and 2, and 3 and 4, respectively. The crystal structure features an extensive series of C-H⋯π inter-actions that link the mol-ecules into undulating rows along the c axis. The crystal studied was a pseudo-merohedral twin with twin law [-100, 0-10, 111] and the BASF parameter refined to 0.513 (3).

2-(4-Meth-oxy-phen-yl)-1-pentyl-4,5-di-phenyl-1H-imidazole.

The title compound, C27H28N2O, is a lophine (2,4,5-triphenyl-1H-imidazole) derivative with an n-pentyl chain on the amine N atom and a 4-meth-oxy substituent on the benzene ring. The two phenyl and meth-oxy-benzene rings are inclined to the imidazole ring at angles of 25.32 (7), 76.79 (5) and 35.42 (7)°, respectively, while the meth-oxy substituent lies close to the plane of its benzene ring, with a maximum deviation of 0.126 (3) Šfor the meth-oxy C atom. In the crystal, inversion dimers linked by pairs of C-H⋯O hydrogen bonds generate R2(2)(22) loops. These dimers are stacked along the a-axis direction.