Isomerisations akin to the Dimroth rearrangement. Part II. The equilibria of 4-mercapto-1,2,3,5,7-penta-azaindenes with 4-amino-1-thia-2,3,5,7-tetra-azaindenes

Abstract

When 1-benzyl-4-mercapto-1,2,3,5,7-penta-azaindene or 4-benzylamino-1-thia-2,3,5,7-tetra-azaindene is boiled in an alcohol or maintained in the molten state, the equilibrium mixture consists largely of the thia-isomer. Analogues in which the benzyl group is replaced by a phenyl or p-substituted phenyl group behave similarly. For each pair of isomers, the first-order rate constant for equilibration in butanol–methoxyethanol, and the equilibrium constant are recorded. The plot, log forward component of the rate constant versus Hammett's sigma value for the p-substituent, is rectilinear. Equilibrium constants also vary with the substituents and with temperature: the thiaisomer is more strongly favoured by electron withdrawal and by higher temperatures. In dimethyl sulphoxide solution equilibrium lies about midway; in aqueous sodium hydroxide, the penta-azaindene (anion) is favoured. Each compound was prepared by a direct route as well as by isomerisation.