UNSATURATED HETERO CHAINS .4. SYNTHESIS, STRUCTURE, AND PROTONATION OF 1-OXA-3-AZAHEXATRIENES

Differently substituted 1-oxa-3-azahexatrienes 3 were synthesized eith er 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-Emm ons-type olefination as the key step. Both methods allow the isolation of the highly reactive compounds 3. The configuration and conformatio n of 31 in the crystalline state by X-ray diffraction were determined, indicating an (E) configuration of the C=C bond and a gauche conforma tion of the C=N-C=O subunit. The reactivity of compounds 3 towards tet rafluoroboric acid was studied. Protonation takes place exclusively at the nitrogen atom leading to the 1-oxa-3-azoniahexatriene salts 10. Q uantum-chemical ab initio model calculations for all possible structur es of the parent substances C4H5NO 9 and C4H6NO+ 14-16 were performed in order to obtain quantitative data regarding the structural and dyna mic behavior of these highly flexible molecules in the gas phase.