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Potential utility of a series of 1-ethyl-3-methylimidazolium salts emimX with X = OTf-, CF3COO-, and NO3- as well as HNEtPri2CF3COO (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 emimCF3COO, emimNO3) and isoamyl nitrate/BF3.Et2O, isoamyl nitrate/TfOH (with emimOTf) provided the best overall systems both in terms of nitration efficiency and recycling/reuse of the ionic liquids. For NO2BF4 nitration, the commonly used ionic liquids emimAlCl4 and emimAl2Cl7 are unsuitable, as counterion exchange and arene nitration compete. EmimBF4 is ring nitrated with NO2BF4 producing NO2-emimBF4 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 emimX 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.
Laali et al. (Thu,) studied this question.
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