Collect. Czech. Chem. Commun. 1990, 55, 2692-2700

Synthesis, kinetics and mechanism of cyclization of 1-(2-aryloxycarbonylphenyl)-3-phenyltriazenes

Oldřich Pytela and Zdeněk Bahník

Department of Organic Chemistry, Institute of Chemical Technology, 532 10 Pardubice


Twelve substituted 1-(2-aryloxycarbonylphenyl)-3-phenyltriazenes have been synthetized and kinetics of their reactions have been measured in 52.1% (by mass) aqueous methanol at pH 3 to 11. Plots of kobs vs pH show three regions: noncatalyzed cyclization (pH 4 to 7), acid-catalyzed splitting of the triazene chain, and base-catalyzed cyclization. The non-catalyzed cyclization exhibits a kinetic isotope effect, the reaction constant ρ = 2.69 (σ-p), and β1g = 1.02, which indicates a mechanism of E1cB type with intramolecular proton transfer and a transient formation of a ketene intermediate. The base-catalyzed cyclization, on the other hand, exhibits the reaction constant ρ = 1.05 (σ-p), β1g = 0.4, and distinct steric effects, which indicates a cyclization by BAc2 mechanism with rate-limiting formation of the tetrahedral intermediate.