Some new approaches to alkene epoxidation with manganese catalyst

Abstract

Epoxides are important starting materials in organic synthesis, as they can be easily transformed into other useful functionalities. Although there are many existing routes to prepare epoxides, many of these processes involve toxic chemicals and volatile organic solvents that cause pollution to the environment. The objective of this study is to develop some new approaches for alkene epoxidation that can minimize the use of toxic chemicals and solvents in the synthetic process. Manganese catalysts are chosen in this study because they are relatively non-toxic and many manganese compounds are known to be good catalysts for epoxidation. In the first part of the thesis, hydrogen peroxide (H2O2) was generated in aqueous bicarbonate solutions electrochemically for the in-situ epoxidation of alkenes. Dioxygen (O2) was directly reduced at the cathode to generate hydrogen peroxide, while epoxide was formed in-situ with a catalytic amount of manganese(II) sulfate (MnSO4). Water-soluble alkenes were epoxidized with excellent yields and high efficiencies. The system can be adapted for the epoxidation of lipophilic alkenes in a tert-butanol/water/Mn2+/HCO3- mixture. In the second part of the thesis, ionic liquids (ILs) were employed as reaction media to generate hydrogen peroxide electrochemically for the epoxidation of alkenes. Ionic liquids are non-volatile and highly conductive solvents in which hydrogen peroxide could be generated by electrochemical reduction of dioxygen in the presence of suitable amount of water. The active oxidant peroxymonocarbonate (HCO4-) was generated in the hydrogen peroxide/ionic liquid mixture by purging with carbon dioxide. Lipophilic alkenes were effectively epoxidized with the assistance of a catalytic amount of manganese(II) sulfate. The ionic liquids can be reused without any diminution in the efficiency of electrosynthesis and catalysis. In the third part of the thesis, a catalytic system based on simple manganese salt was developed for the efficient epoxidation of terminal aliphatic alkenes with peracetic acid. Terminal aliphatic alkenes are a challenging class of substrates in epoxidation because of their poor nucleophilicity. We found that terminal aliphatic alkenes are efficiently and selectively epoxidized at ambient temperature by peracetic acid/Mn2+/NH4HCO3- in acetonitrile/water mixture with low catalyst loading. The reaction is stereospecific with retention of the cis/trans configuration of the alkenes in the epoxide products.