Unsaturated hetero chains, IV. Synthesis, structure, and protonation of 1‐oxa‐3‐azahexatrienes

Differently substituted 1‐oxa‐3‐azahexatrienes 3 were synthesized either by acylation of N‐silyl‐1‐azabutadienes 1 or by a four‐step‐one‐pot procedure starting from the nitriles 5, involving a Wittig‐Horner‐Emmons‐type olefination as the key step. Both methods allow the isolation of the highly reactive compounds 3. The configuration and conformation of 31 in the crystalline state by X‐ray diffraction were determined, indicating an (E) configuration of the CC bond and a gauche conformation of the CNCO subunit. The reactivity of compounds 3 towards tetrafluoroboric acid was studied. Protonation takes place exclusively at the nitrogen atom leading to the 1‐oxa‐3‐azoniahexatriene salts 10. Quantum‐chemical ab initio model calculations for all possible structures of the parent substances C4H5NO 9 and C4H6NO+ 14–16 were performed in order to obtain quantitative data regarding the structural and dynamic behavior of these highly flexible molecules in the gas phase.