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 HNEtPri2]CF3COO] (protonated Hünig'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]AlCl4] 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.