Research On Green Process Of Oxidation And Oxidative Halogenation

Oxidative halogenation have received great attention due to their important role in many fields such as military,medicine,pesticides and functional materials.The HBr/H₂O₂ system is a highly environmentally friendly and efficient oxidative halogenation system,which has been applied in the oxidation of alcohols,ethers or aldehyde and the oxidative bromination of active methylene groups,aromatic rings,aromatic side-chains or alkenes.Besides the biological activities halogenated pyrazolols,especially the 4-bromo derivatives,have been regarded as vital precursors for the synthesis of functionalized pyrazoles.Thus,4-bromo-pyrazolols have aroused great interests of researchers.Until now,however,an efficient method for direct access to 4-bromo-pyrazolol from pyrazolidinone are still rarely reported.In this paper,a series of bromopyrazol-3-ol compounds were prepared by one step approach for aromatization and oxidative bromination of pyrazolidin-3-one with HBr/H₂O₂system.In this paper,a series of pyrazoline-3-one compounds were firstly synthesized by the classical condensation reaction of hydrazine compounds and?,?unsaturated esters.In this reaction,we optimized the traditional process conditions.After the addition of the sample,we initially performed the reaction at 45^oC for 0.5 hours,then slowly heated to reflux.The optimum yield was obtained by reflux reaction for 2 hours.Under the optimized reaction conditions,pyrazoline-3-one compounds have been obtained in high yield from 65%to 89%.The synthesis mechanism of the reaction has been also discussed in this paper.All pyrazoline-3-one compounds have been characterized by NMR,IR and ESI-MS.Secondly,bromopyrazole-3-ol compounds have been synthesized by one-step aromatization and oxidative bromomination of pyrazoline-3-one with HBr/H₂O₂ system.The novel method is metal-free,atom economic and highly effective.Under the optimized reaction conditions,the aromatization and oxidative bromination of pyrazolidin-3-ones were undertaken in CHCl₃ at 60^oC for 30 min in the presence of 1 equivalent of HBr and 3equivalents of H₂O₂ to give great yield of bromopyrazole-3-ol from 86%to 95%.The reaction mechanism has been also researched in this paper.The structures of bromopyrazole-3-ol compounds have been confirmed by NMR,IR,MS and elemental analysis.This new method is environment-friendly,highly effective and economic which makes it a great potential for application in industrial production.