Synthesis and structural analysis of open-chain zwitterionic Stenhouse salts derived from furfural, morpholine, and tetracarbonitrile derivatives

The condensation of furfural, morpholine, and activated methylene compounds represents a versatile synthetic route to functionalized heterocyclic and extended conjugated systems. In this study, we systematically investigated the reaction of furfural, morpholine, and various substituted 2-arylpropane-1,1,3,3-tetracarbonitriles in ethanol at ambient temperature. Although classical mechanistic considerations suggested the formation of bicyclic adducts via intramolecular cyclization, comprehensive spectroscopic analysis unequivocally revealed an alternative structural outcome.Detailed ¹H and ¹³C NMR spectroscopy, complemented by FTIR data, demonstrated the complete absence of characteristic aliphatic bicyclic signals. Instead, key diagnostic resonances consistent with an open-chain zwitterionic architecture were identified: a carboxylate/carbonyl signal at ~170–185 ppm (¹³C NMR), an iminium carbon resonance at ~160–165 ppm, and preserved tetracyano functionality.
The compounds exist in solution as dynamic E/Z-isomeric mixtures, as evidenced by signal multiplicity and line broadening phenomena.Four derivatives bearing para-Cl, para-OMe, para-NEt₂, and ortho-NO₂ substituents were successfully synthesized and fully characterized, confirming the broad generality of this transformation. The proposed zwitterionic Stenhouse-type structure features an extended conjugated polyene chain linking a carboxylate terminus, a tetracyano-substituted central unit, and a morpholinium iminium moiety.
This work substantially revises the mechanistic understanding of furfural–amine–tetracarbonitrile condensations and establishes a reliable spectroscopic protocol for distinguishing open-chain zwitterionic products from potential cyclic isomers. The obtained highly functionalized conjugated systems demonstrate significant promise as versatile precursors for advanced materials science and medicinal chemistry applications.