Synthesis and properties of heterocyclic carbonium systems

Abstract

Bis(4,4,5,5-tetramethyl-2-dioxolano) trimethylidynecyanine was synthesized by condensation of 2-methyl-1,3-dioxolanium perchlorate with 2-formylmethylene-1, 3-dioxolane by heating (90–100‡C) in acetic anhydride. It is shown that the 1,3-dioxolane also condenses under similar conditions with pyrylium salts containing active methylene and methyl groups to give unsymmetrical dioxolanopyrylocyanines. The synthesized cyanines react with HClO₄ to give unstable diperchlorates, which readily lose a molecule of HClO₄ to give the starting cyanines. Concentrated HCl decomposes one of the dioxolane rings of the symmetrical cyanine, and refluxing with dilute HCl brings about its destruction to low-molecular-weight unidentified compounds. The unsymmetrical cyanine from the pyrylium salt is resistant to the action of acids but is hydrolyzed by alkalis with cleavage of the pyrylium ring to a diketone. The action of Br₂ on the dioxolanocyanine leads to the addition of bromine to the double bonds and replacement of the ClO₄ ⁻ ion by Br⁻.