KX/H2O2 - An efficient and non-polluting halogenating reagent in organic synthesis

Abstract

(A) Kirihara et al. reported chemoselective mono-bromination of active methylene and methine compounds using potassium bromide, hydrochloric acid and hydrogen peroxide.10 (Chemical Equation Presented) (B) α-Halo β-keto sulfones were synthesized by reaction of β-keto sulfones with potassium halide in the presence of hydrogen peroxide and acetic acid in aqueous medium.11 Further, the α-halo β-keto sulfones on treatment with SO2Cl2/Br2 afforded α,α-symmetrical and asymmetrical dihalo β-keto sulfones, 12 which on further treatment with aqueous alkali underwent base-induced cleavage to afford α,α-symmetrical and asymmetrical dihalomethyl sulfones, respectively. This method is very simple, highly economical and eco-friendly for the synthesis of a wide variety of α-halo and α,α-dihalo β-keto sulfones. (Chemical Equation Presented) (C) Parida and co-workers reported an oxidative bromination of phenols using metal-supported heteropoly acid such as zirconia,13 titanium phosphate (TiP),14 zirconium phosphate (ZrP),15 titania pillared zirconium phosphate (ZrP)16 and titanium phosphate (TiP)17. In these reactions KBr acts as brominating agent and hydrogen peroxide as an oxidant in the presence of acetic acid or mineral acids at room temperature. The reaction proceeds through the formation of a bromonium ion, which attacks the phenol ring to form different brominated products. (Chemical Equation Presented) (D) Vanadate-dependent haloperoxidase complexes as selective bromination agents of phenols have been reported. Starting from salicylaldehyde in the presence of KBr/H2O2, the anionic complex catalyses the oxidative bromination in water to afford 5-bromosalicylaldehyde in 40% yield and 87% selectivity.18 (Chemical Equation Presented) (E) An efficient, simple, mild and regioselective liquid-phase bromination of phenols using potassium bromide and hydrogen peroxide over zeolites CrZSM-5(30) has been reported. The reaction proceeds via electrophilic substitution of phenols with an electrophilic bromonium ion generated in situ from KBr in the presence of hydrogen peroxide as an oxidant.19 (Chemical Equation Presented) (F) Vanadium-catalyzed oxidative bromination using dilute mineral acids and hydrogen peroxide has been reported. In this system, kinetic studies show that HBr can be replaced with a combination of an alkali bromide salt (NaBr/KBr) and a dilute mineral acid (e.g., HCl, H2SO4, HNO3, H3PO 4). The salt/acid/peroxide system may be used for in situ or ex situ oxidative bromination of various aromatic compounds. The application of the above concept towards the recycling of industrial acid waste is discussed. 20 (Chemical Equation Presented) (G) A simple, efficient, regioselective and environmentally safe liquid-phase regioselective bromination of aromatic compounds over HZSM-5 catalyst has been reported. The electrophilic bromine was generated from KBr using HZSM-5 zeolites as a catalyst and H 2O2 as an oxidant.21 (Chemical Equation Presented). © Georg Thieme Verlag Stuttgart.