ELECTRON-TRANSFER-INDUCED REDUCTIVE DEMETHOXYLATION OF ANISOLE - EVIDENCE FOR CLEAVAGE OF A RADICAL-ANION

The electron-transfer induced cleavage of anisole with alkali metals (mostly potassium) in the absence of proton donors was studied in solvents of low polarity under a variety of conditions. In THF exclusive demethylation to phenol was observed, regardless of the temperature, whereas in solvents of very low dielectric constant (aliphatic hydrocarbons, toluene, tributylamine, dioxane) demethoxylation to benzene was the main reaction pathway, particularly at the lowest temperatures investigated. Investigation of the mechanism of demethoxylation, with the aid of labeling experiments, provided strong evidence for fragmentation of the radical anion of anisole to methoxide anion and phenyl radical; in a subsequent step, the latter abstracts a hydrogen atom from components of the reaction medium or undergoes one-electron reduction to phenyl anion. The influence of solvent and temperature on the regioselectivity of cleavage is discussed.