Hydrogen-transfer hydrodehalogenation of aromatic halides with a silica-supported palladium catalyst in alkaline 2-propanol: comparison between brominated and chlorinated anisoles

Abstract

Hydrogen-transfer hydrodehalogenation of brominated anisoles, chlorinated anisoles, and bromochloroanisoles was carried out at 30–40 °C in a solution of NaOH in 2-propanol/methanol (99:1 V/V) in the presence of a Pd/SiO2 catalyst. The dehalogenation reactions proceeded stepwise to afford anisole, and 2-propanol served as a hydrogen source. In single-substrate reactions, 2,4,6-tribromoanisole (TBA) was dehalogenated more slowly than 2,4,6-trichloroanisole, whereas in a competitive reaction, 2,4,6-TBA was preferentially dehalogenated. The analysis of lower-halogenated intermediates revealed that the ortho halogen was less reactive than the para halogen, and the isomer selectivity of the dehalogenation reaction strongly depended on the halogen atom. The conversion of bromochloroanisoles (3-bromo-4-chloroanisole, 3-bromo-5-chloroanisole, and 2-bromo-4-chloroanisole) to anisole occurred exclusively via the corresponding chloroanisoles. The difference in dehalogenation behavior between the brominated and chlorinated anisoles is discussed on the basis of carbon–halogen bond dissociation energies, steric effects, and substrate adsorption ability.