Several 1-phosphono-2-aza-1,3-butadienes, 1 and 13-20, were evaluated in the reaction with different enolate-type nucleophiles to induce addition at the 1- or the 4-position of the azadiene. 1-Phosphono-2-azadienes 1 react with sodium malonate at the 1-position, leading to the formation of bisenamines 12 after elimination of the phosphonate moiety. On the contrary, sodium malonate adds at the 4-position of 1-aryl-1-phosphono-2-azadienes 14-19 when the azadienes bear a halogenated phenyl substituent, and the resulting addition products 21-26 are easily transformed into the corresponding phosphonylated gamma-lactams 35-40. The regioselectivity of the addition is explained by reversal of polarization of the azadiene due to the electron-withdrawing character of the halogenated phenyl substituents.