Intramolecular hydrogen bonding in some ortho-substituted N-methyl,N-benzyl and N-aryl-4-nitroanilines: a proton magnetic resonance study

Abstract

¹H n.m.r. spectra of a wide variety of N-methyl, N-benzyl and N-aryl 2-substituted 4-nitroanilines, where the 2-substituent is an electron-withdrawing group, namely acetyl, cyano, formyl or methoxy-carbonyl, reveal that long-range coupling (⁵J 0.65-0.70 Hz) occurs between the NH proton and the 5-proton of the nitroaryl ring in (D)chloroform solution; coupling is absent when the 2-substituent is trifluoromethyl. However, for some compounds (the nature of the 2-substituent is critical), NH,H₅ coupling is absent in (D₆)dimethyl sulfoxide solution. An examination of these phenomena leads to the conclusion (a) that intramolecular hydrogen bonding occurs between the NH group and the 2-substituent and (b) that, for these N,2-substituted 4-nitroanilines, the intramolecular hydrogen bond strength decreases in the following order:

NH...COOCH₃>NH...NO₂≈NH...COCH₃>NH...CHO>NH...CN

A parallel study involving some N-methyl 4-substituted 2-nitroanilines where the 4-substituent is an electron-donating group, namely t-butyl, methyl and methoxy, revealed long-range NH,H₅ coupling in both (D)chloroform and (D₆)dimethyl sulfoxide solution; when the substituent is dimethylamino, NH,H₅ coupling could not be detected in either solvent.