Nitroenamines. Part 6. Carbon-13 nuclear magnetic resonance spectral studies on 1-amino-2-nitro- and 1,1-diamino-2-nitro-ethylenes

Abstract

¹³ C N.m.r. spectra of 1-amino-2-nitroethylenes (1a–c), 1,1-diamino-2-nitroethylenes (2a–c), and 2-acyl-1-aminoethylenes (3a and b) have been analysed. The chemical shifts of C-2 and 2-H are shown to correlate well with each other and with the chemical reactivity of the enamines. Nonequivalence was observed for the methyl groups in 1-dimethylamino-2-nitroethylene (1a) both in ¹³C and ¹H n.m.r. spectra and is explained on the basis of restricted rotation around the N–C-1 bond due to extensive delocalization. A similar phenomenon occurs in the pyrrolidinoenamine (1b) and to a less pronounced extent in the morpholinoenamine (1c). Nonequivalence of N–CH₃ groups is not observed in 1,1-bis(dimethylamino)-2-nitroethylene (2a), as well as in the enamino acyl compounds (3a and b), reflecting decreased double bond character around N–C-1.