Base-catalyzed 1-3 Prototropic Isomerization of Cycloheptatrienes

Abstract

The base-catalyzed proton migration (from the 7- to 2-position) and proton exchange (at the 7-position) of cycloheptatrienes carrying an electron-withdrawing group were studied in a medium of aqueous sodium hydroxide or alcoholic potassium hydroxide. The order of the effect of the various substituents upon the relative rates is approximately: CN>CONH₂>COO⁻, Ph. The base-catalyzed prototropic isomerization proceeds through: i) the abstraction of a proton of the 7-position by the base, ii) the migration of the resulting carbanion to the allylic position (the rate-determining step), and iii) the recovery of a proton from the solvent. The kinetic study of the isomerization of 7-carboxycycloheptatriene (Ia) discloses the rate of the isomerization (k[the cycloheptatriene][base]) and the parameters (E_a=22.6 kcal/mol, ΔH^\eweq=22.0 kcal/mol, ΔS^\eweq=−15.4 e.u. at 60°). The structure of the cycloheptatrienyl anions formed as intermediates is discussed.