Bicyclic imidazolium-based ionic liquids: synthesis and characterization

Abstract

We previously reported the use of imidazole as starting compound for preparing a bicyclic imidazolium ionic liquid, [b-3C-im][NTf 2], with an overall 29% isolated yield in four synthetic steps. This new room temperature ionic liquid was shown to be far more chemically stable than commonly used [bmim][PF 6], [bdmim][PF 6], and [bdmim][NTf 2]. Because of this intriguing chemical stability, it prompted us to develop a more generalized and high yielding synthesis so that molecular diversity of bicyclic ionic liquids may be explored. In this work, we amended the previous synthetic route by employing 4-chlorobutyronitrile or 5-chlorovaleronitrile as starting materials and successfully developed a five-step synthesis of a series of novel bicyclic imidazolium-based ionic liquids in 40–53% overall isolated yields. We investigated intrinsic reactivity of all bicyclic ionic liquids prepared and found that, under strongly basic conditions, among all tested ionic liquids the 5,5-membered [R-3C-im][NTf 2] ionic liquids were most stable to solvent deuterium isotope exchange while the previously reported [bdmim][NTf 2] ionic liquid was 50% deuterium exchanged at its C-2 methyl in 30 min at ambient temperature. Under identical condition, the commonly used [bmim][NTf 2] ionic liquid was deuterium exchanged instantaneously at its C-2 hydrogen. In the absence of bases, only [bmim][PF 6] was deuterium exchanged (50% within 1 h) and all other ionic liquids gave no detectable exchanges even after 25 days at ambient temperature. Moreover, both [bmim][NTf 2] and [bdmim][NTf 2] ionic liquids were readily methylated at C-2 position with methyl iodide under basic condition at room temperature. Under the same condition, [R-3C-im][NTf 2] and [R-4C-im][NTf 2] ionic liquids were completely stable and chemically inert. We envisioned that [R-3C-im][NTf 2] should be well suited as solvents for organic synthesis.