Nitration of Quinoline 1-Oxide : Mechanism of Regioselectivity

Abstract

The acidity dependence of orientation in the nitration of quinoline 1-oxide was investigated by using the trifluoromethanesulfonic acid (TFSA)-trifluoroacetic acid (TFA) system and the antimony pentafluoride (SbF₅)-TFSA system. These systems provide a wider range of acidity than that of aqueous sulfuric acid. Comparison of the behavior of quinoline 1-oxide and 1-methoxyquinolinium triflate in acidic and neutral media demonstrated that O-protonated quinoline 1-oxide is nitrated at the 5- and 8-positions, and the free (unprotonated) molecule is nitrated at the 4-position. This result is consistent with theoretical expectation. It was also discovered that nitration at the 5-position increasingly predominates over that at the 8-position as the acidity is increased.