Structures and Reactivities of Iminium Ions Derived from Substituted Cinnamaldehydes and Various Chiral Imidazolidin-4-ones

• Published in: Asian journal of organic chemistry 2014-04, Vol.3 (4), p.550-555
• Main Authors: An, Feng, Paul, Shyeni, Ammer, Johannes, Ofial, Armin R., Mayer, Peter, Lakhdar, Sami, Mayr, Herbert
• Format: Article
• Language: English
• Subjects: cinnamaldehydes; imidazolidinones; kinetics; linear free energy relationships; Organic chemistry; organocatalysis
• ISSN: 2193-5807; 2193-5815
• DOI: 10.1002/ajoc.201402009

A series of α,β‐unsaturated iminium ions derived from substituted cinnamaldehydes and C2‐ and C5‐substituted chiral imidazolidin‐4‐ones were isolated and characterized in solution and in the solid state. The kinetics of the reactions of the iminium ions with silyl ketene acetals were determined in dichloromethane at 20 °C. The resulting second‐order rate constants were used to estimate the electrophilicity E of the iminium ions according to the linear free energy relationship log k2(20 °C)=sN(N+E). The kinetics for the reactions of two of the iminium ions with tributylphosphine were studied by laser flash spectroscopy and their second‐order rate constants were found to agree within a factor of 2.2 with those calculated by using the linear free energy relationship above. Worth its salt: Iminium salts derived from substituted cinnamaldehydes and imidazolidinones have been synthesized and characterized by X‐ray crystallography. Kinetic investigations of their reactions with cyclic ketene acetals were performed to quantify their electrophilic reactivity.