Amine oxidation. Part 14. Acid-catalysed deoxygenation of some NN-dimethylaniline N-oxides and reactions of the resultant iminium-benzenium dications

Abstract

The reactions of NN-dimethylaniline N-oxide and three substituted derivatives (4-CH₃, 4-CH₃O, and 4-NO₂) in strong acids have been followed by ¹H and ¹³C n.m.r. spectroscopy and product studies. These N-oxides can be deoxygenated in strong acids to give NN-dimethyliminium-benzenium dications. With the 4-methoxy-substituted N-oxide the reaction, which is helped by the electron-releasing methoxy-substituent, proceeds at room temperature in trifluoroacetic acid (TFA). By contrast the NN-dimethylaniline N-oxide with an electron-withdrawing 4-nitro-substituent is stable in TFA and requires the stronger acid fluorosulphonic acid for reaction to occur. The mechanisms of these reactions are discussed.