A new synthesis and the reactivity of stable selenaazadienes are discussed. Their dienic, electrophilic and nucleophilic character are used to prepare five and six membered selenoheterocycles. The easy access to selenazine, selenazinone, selenopyran, selenazoline, selenazole and selenophene heterocycles from N-selenoacylamidines is described. An interpretation of the reactivity of the selenaazadiene systems, experimentaly compared to their thia analogues, is based on their physico-chemical parameters and correlated to the theoretical calculations of their frontier molecular orbital energy levels.

Dipole moments of selenobenzamide, of eight N′-thio (oxo or seleno) acylamidines and of two N′-thioacyliminoethylidenetriphenylphosphoranes in carbon tetrachloride, benzene or dioxane solutions were measured or examined. Calculations of molecular moments for the sp (CS and NC on the same side) and ap conformations and for the canonical structures implied by conjugation were performed according to a recently proposed method. A comparison between the calculated and the experimental values shows that only strong conjugations with significant statistical percentages ps(±) of the 1,5 (or 1,6) zwitterionic forms (X−CN−CN+ or X−CNCCP+ with X  O, S or Se) can explain observed moments. The μ differences between compounds are related to ps(±) variations which can give information about the molecular structures and their relationship with reactivities.

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N′-selenoacylamidines were isolated by condensation of selenobenzamide with orthoamides. [4+2] cycloadditions with electrophilic dienophiles were performed leading to 1,3-selenazine derivatives. Selenoamide vinylogs obtained by thermolysis of 4H-1,3-selenazines, were the precursors of functionalised selenopyrans or selenophenes.