The Surprisingly Facile Thermal Dehalogenation of Chlorinated Aromatics by a Hydroaromatic Donor Solvent. Tautomerization of Chlorinated Phenols and Anilines

Abstract

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.

Anthrone and Related Hydroxyarenes: Tautomerization and Hydrogen Bonding

“…phenol ( 2 ) or 1-naphthol ( 4 ), is the exclusive tautomer . However, despite its (relatively) low concentration, the presence of the enone may be crucial for the opening of a certain reaction path, as for example has been demonstrated for the facile desubstitution of 2- and 4-chlorinated phenols at elevated temperatures . About 100 years ago, Meyer published the seminal papers on the tautomerization (desmotroperization) of anthrone ( 5 ) into 9-anthrol ( 6 ) (Scheme ). , It has been found that the nature of the solvent plays an important role on both the rate of tautomerization and on the thermodynamic equilibrium ratio. , In benzene, anthrone tautomerizes slowly to a small amount of 9-anthrol at equilibrium conditions while in pyridine the tautomeric equilibrium is established readily at ambient temperature, and the enol is the preferred tautomer. , …”

Syntheses of 2-substituted 1-amino-4-bromoanthraquinones (bromaminic acid analogues) – precursors for dyes and drugs

“…However, these conditions led to the loss of the bromine substituent at position 4, yielding compound 11 as the main product ( Scheme 2 ). The debromination might be promoted by the high reaction temperature (220 °C) using acetic acid as the potential hydrogen donor for the reaction [ 64 – 65 ].…”

Intramolecular Hydride Transfer Reactions in (Formazanate)Boron Dihydride Complexes

“…A potential factor involved in the feasibility of this reaction is related to the (redox-active) formazanate ligand being able to function as an electron reservoir. , Hydride migration to the N–Ar ortho -position is accompanied by formation of a neutral cyclohexadiene imine moiety ( J in Scheme ), with the negative charge positioned on a ligand N atom that is bound to the B center. The cyclohexadiene imine J can subsequently tautomerize (rearomatize), resulting in the formation of structure K . K is related to the monoanionic formazanate ligand (L – ) in the starting material via 2e/H + transfer and is thus formally equivalent to a 2e-reduced formazan (LH 2– ).…”