Collect. Czech. Chem. Commun. 1998, 63, 2075-2084

Steric Effects in the Base-Catalyzed Cyclization of 1-[2-(Methoxycarbonyl)phenyl]-3-(2-substituted Phenyl)triazenes

Miroslav Ludwig and Ingrid Bauerová

Department of Organic Chemistry, University of Pardubice, 532 10 Pardubice, Czech Republic


Eleven model 1-[2-(methoxycarbonyl)phenyl]-3-(2-substituted phenyl)triazenes were synthesized and their cyclization kinetics examined in aqueous-methanolic buffer solutions (51 wt.% methanol) at various pH values. 3(2-Substituted phenyl)benzo[d][1,2,3]triazin-4(3H)-ones were identified as the cyclization products. The log kobs vs pH plot was linear with a slope of unity. Investigation of the steric and electronic effects of substituents in the ortho position revealed that substituents at the ring which is bonded to the N(3) nitrogen affect the cyclization rate through their steric effect only, while their electronic effects are statistically insignificant. This fact was explained in terms of the ring being tilted from the plane in which the remaining part of the conjugate base anion of the model substrate lies. The assumed and confirmed BAc2 mechanism involving specific base catalysis begins by deprotonation of the triazene giving rise to the conjugate base, continues with formation of a tetrahedral intermediate, and ends with elimination of the methanolate ion. Other mechanisms, such as the elimination-addition mechanism via a ketene intermediate or the mechanism involving general base catalysis, are unlikely.

Keywords: Triazenes; Base catalyzed cyclization; Steric effect; Substituent effect; Reaction kinetics.