The surprisingly facile thermal dehalogenation of chlorinated aromatics by a hydroaromatic donor solvent. Tautomerization of chlorinated phenols and anilines

Up to 600 K, chlorinated benzenes and naphthalenes are inert in a hydrogen atom donating solvent such as 9,10-dihydroanthracene. However, when a hydroxyl or amine group is attached to the 2 or 4 position relative to chlorine, a surprisingly facile and selective hydrodehalogenation occurs at temperatures between 530 and 630 K. These features are the result of the onset of tautomeric equilibria for the chlorophenols or -anilines, creating the corresponding enones or imines, respectively, as reactive intermediates. The mechanism of dehalogenation involves the rate-determining transfer of a hydrogen atom from the solvent to the reactive intermediate according to a reverse radical disproportionation (RRD), followed by elimination of chlorine. The reactivity of mono- and dichlorophenols, chloroanilines, 4-chloro-1-naphthol, and 4-chloro-1-naphthylamine dissolved in 9,10-dihydroanthracene has been investigated. By means of density functional theory (DFT) calculations, the energies for the tautomers and their hydrogen affinities have been established. The experimental selectivities could be adequately reproduced by the computed data.