Electrophilic nitration of aromatics in ionic liquid solvents

Potential utility of a series of 1-ethyl-3-methylimidazolium salts [emim][X]] with X = OTf-, CF3COO-, and NO3- as well as [HNEtPr2i][CF3COO] (protonated Hunig's base) ionic liquids were explored as solvent for electrophilic nitration of aromatics using a variety of nitrating systems, namely NH4NO3/TFAA, isoamyl nitrate/BF3. Et2O, isoamyl nitrate/TfOH, Cu[NO3]/TFAA, and AgNO3/Tf2O. Among these, NH4NO3/TFAA (with [emim][CF3COO], [emim][NO3] and isoamyl nitrate/BF3. Et2O, isoamyl nitrate/TfOH (with [emim][OTf]) provided the best overall systems both in terms of nitration efficiency and recycling/reuse of the ionic liquids. For [NO2][BF4] nitration, the commonly used ionic liquids [emim][A1C1(4)] and [emim][Al2Cl7] are unsuitable, as counterion exchange and arene nitration compete. [Emim][BF4] is ring nitrated with [NO2][BF4] producing [NO2-emim][BF4] salt, which is of limited utility due to its increased viscosity. Nitration in ionic liquids is surveyed using a host of aromatic substrates with varied reactivities. The preparative scope of the ionic liquids was also extended. Counterion dependency of the NMR spectra of the [emim][X] liquids:can be used to gauge counterion exchange; (metathesis) during nitration. Ionic liquid nitration is a useful alternative to classical;nitration routes due to easier Product isolation and recovery of the ionic liquid solvent, and because it avoids problems associated with-neutralization of large quantities of strong acid.