Nitroenamines hydrogen bonding

Complex 1 1 is considered the only complex present, but the hydrogen bond may be either two (76) or three center (77). Nitroenamines are more prone to complex with 4-fluorophenol than the nitroanilines and they form the strongest hydrogen bond presently known for nitro-bases. In particular, l-piperidino-2-nitroethylene (78) and 1-dimethylamino-2-nitroethylene (79) (both in E form) present a hydrogen bond basicity comparable to that of tributylamine. 

Nitroenamines are affected by the isomerism204 shown in equilibrium 22. Compounds with primary (R1 = R2 = H) or secondary amino (R1 = H, R2 = alkyl) groups exist as mixtures of the intramolecularly hydrogen bonded Z and E forms, in an equilibrium (equation 22) depending on the solvent properties. The E form of a nitroenamine with a tertiary amino group is the more populated form. 

Primary and secondary nitroenamines (9b-9e) have been studied by IR spectroscopy, and band doubling indicated them to be mixtures of E and hydrogen-bonded Z forms36. 

Nitroenamines (36) have also been the subject of much research. They have been studied both theoretically67 and experimentally68 using such diverse techniques as IR, Raman, H-NMR and 13C-NMR spectroscopy, and two-bond deuterium isotopic effects. The results obtained show that, with R1 = Me and R2 or R3 = H, tautomer Z (stabilized by hydrogen bonding) is the major form. However, with R1 = H and R2 (R3) = H, the two forms are in equilibrium, whereas when R2 = R3 = H, the E isomer is the sole form present. 

Nitroenamine radical 53 exists in trans (A) or cis (B) forms form A crystallizes from alcohol, and on dissolution in chloroform it passes into form B, stabilized by the intramolecular hydrogen bond. Treatment of compound 53 with sodium ethylate leads to the formation of the sodium salt of aciform 54, and the action of an equimolar amount of bromine or of excess bromine leads, correspondingly, to mono- and dibromo derivatives 55 and 56. 

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