Piperidine-appended imidazolium ionic liquids as task-specific catalysts: computational study, synthesis, and multinuclear NMR

Abstract

Imidazolium ionic liquids (IMILs) with a piperidine moiety appended via variable length methylene spacers (with n = 1–4) were studied computationally to assess their potential to act as internal base for N-heterocyclic carbene (NHC) generation. Proton transfer energies computed by B3LYP/6-311+G(2d,p) were least endothermic for the basic-IL with n = 3, whose optimized structure showed the shortest C2-H----N(piperidine) distance. Inclusion of counter anion (Cl or NTf2) caused dramatic conformational changes to enable close contact between the acidic C2-H and the anions. To examine the prospect for internal C2-H----N coordination, multinuclear NMR data (1H, 15N, and 13C) were computed by gauge independent atomic orbitals–density functional theory (GIAO-DFT) in the gas phase and in several solvents by the PCM method for comparison with the experimental NMR data for the basic ILs (with n = 2–4) synthesized in the laboratory. These studies indicate that interactions with solvent and counter ion are dominant forces that could disrupt internal C2-H----N coordination/proton transfer, making carbene generation from these basic-ILs unlikely without an added external base. Therefore, the piperidine-appended IMILs appear suitable for application as dual solvent/base in organic/organometallic transformations that require the use of mild base, without the necessity to alkylate at C-2 to prevent N-heterocyclic carbene formation.