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.