2-Amino-1,4-naphthoquinone imines. New syntheses and studies of their protonation in strong acid
4-Amino-1,2-naphthoquinone, 4-dialkylamino-1,2-naphthoquinones, and 4-ethoxy-1,2-naphthoquinone react smoothly with a variety of aliphatic amines to give preparatively useful yields of 2-alkylamino-N(4)-alkyl-1,4-naphthoquinone monoimines (II). 4-Alkylamino and 4-dialkylamino-1,2-naphthoquinones, after O-alkylation with triethyloxonium tetrafluoroborate, may also be converted into (II) by treatment with amines. As determined by proton resonance studies, compounds (II) show an equilibrium between ortho- and para-quinone imine at room temperature. In strong acid, imines (II) are protonated at N-4, giving vinylogues of amidinium salts. In addition, the C-3 vinyl proton undergoes ready exchange. The related 2-amino-1,4-naphthoquinones and 4-amino-1,2-naphthoquinones also undergo C-3 vinyl proton exchange in strong acid. The rate of this exchange is considerably greater with these latter quinones than with the 2-amino-1,4-naphthoquinone imines.