'Push-pull' effects in nitroethenamines

(E)-N-methyl-1-(methylthio)-2-nitroethenamine, C4H8N2O2S, is a near-planar molecule with significant pi-electron delocalization from the ethylene bond into the enamine C-N bond. In the two nitrothioacrylamide derivatives, N-benzoyl-3,3-bis(methylamino)-2-nitrothioacrylamide, C12H14N4O3S, and N-cinnamoyl-3,3-bis(dimethylamino)-2-nitrothioacrylamide, C16H20N4O3S, the formal ethylene bond has single-bond properties with a length of about 1.49 Angstrom, while the magnitudes of the torsion angles about this bond approach 90 degrees. The enamine N-C bonds have significant double-bond character and there is significant electron delocalization in the nitrothioacrylamide moiety of each molecule. These compounds can be described as 'push-pull' ethylenes rather than as enamines and are best represented by a zwitterionic formulation in which the charges are accumulated near the opposite ends of the ethylene bond, A strong intramolecular hydrogen bond involving the amide and nitro groups in each compound maintains a rigid conformation, which may explain why these compounds will not undergo cyclization reactions to the corresponding thioxopyrimidines. S-methyl N-benzoyl-3-(N-methyl-N-phenylamino)-2-nitro-3-(phenylamino)thioacrylimidate C24H22N4O3S, exhibits similar 'push-pull' characteristics, but the absence of an intramolecular hydrogen bond gives the molecule flexibility and enables it to adopt a conformation in which it undergoes a cyclization reaction to the corresponding nitropyrimidine.