The nitration of aromatic compounds is a very important reaction in organic synthesis. It usually requires a harsh mixture of concentrated nitric acid and sulfuric acid. This approach produces large amounts of acidic waste during workup. However, there are catalytic systems which allow more efficient and “green” nitration reactions with only one equivalent of nitric acid.
Ruimao Hua and colleagues, Tsinghua University, Beijing, China, have developed a catalyst-free system for the nitration of arenes under mild conditions that uses solvent effects to promote the reaction. The team used one equivalent of nitric acid in HFIP (1,1,1,3,3,3-hexafluoroisopropanol) as a solvent to nitrate a variety of electron-rich and electron-poor arenes at room temperature.
The desired mononitrated products were obtained in good to high yields within one hour. The solvent can easily be recovered by distillation after the reaction. According to UV-Vis spectroscopy and quantum chemical calculations, HFIP stabilizes an intermediate π-complex of the aromatic compound and a nitronium ion and, thus, promotes the reaction.
- HNO3/HFIP: A Nitrating System for Arenes with Direct Observation of π-Complex Intermediates,
Le Lu, Huixin Liu, Ruimao Hua,
Org. Lett. 2018.