The various synthetic methods discussed in this review reveal that Oxone is a versatile reagent used in organic synthesis. Oxone is a cheap commercially available oxidant that easily oxidizes numerous functional groups. It is an efficient single oxygen-atom donor since it contains a nonsymmetrical O-O bond which is heterolytically cleaved during the oxidation cycle. It is an inexpensive reagent ($0.02-0.04/g), which compares favorably with hydrogen peroxide and bleach. Its byproducts do not pose an immediate threat to aquatic life upon disposal, and unlike chromium trioxide and bleach, it does not emit pungent vapors or pose a serious inhalation risk. The aqueous components of an organic Oxone reaction are oxidizing and acidic and should thus be quenched with sodium bisulfite followed by neutralization with sodium bicarbonate, thereby resulting in formation of a mixture of nonhazardous sulfate salts in water. These features make Oxone attractive for large-scale applications. Uses of other oxidizing agents lack the desired ingredients to attract the interest of industry because of tedious purification processes from their deoxygenated counterparts. The dioxirane (generated from reaction between Oxone and a ketone) epoxidation offers many advantages over traditional methods of epoxidation. Oxone is about one-half as expensive as m-chloroperoxybenzoic acid (mCPBA) and converted to KHSO4. KHSO4 during the reaction, while being relatively acidic, can easily be neutralized with dilute NaOH solution to produce nontoxic Na2SO4. Furthermore, the reaction conditions require the use of relatively nontoxic organic solvents plus water. Another advantage of dioxirane epoxidation is that acetone is recycled in the reaction, which means all of the extra oxygen in Oxone is incorporated into the respective alkenes. Dioxirane is also capable of oxidizing very unreactive olefins, and thus, isolation of some relatively unstable epoxides produced from glycals is possible. This represents a major advantage over the Sharpless and mCPBA protocols, which only epoxidize electron-rich olefins and allylic or homoallylic alcohols. These latter reagents also require a directing group. One drawback that dioxirane does have is the fact that it can also oxidize very reactive heteroatoms, hydroxyl groups, and unactivated C-H bonds during the epoxidation procedure. Oxone does have some disadvantages: (a) it is insoluble in organic solvents, (b) buffering is needed due to its acidity, and (c) it sometimes bleaches the metal catalysts and donor ligands during oxidation reactions. To overcome the need for aqueous conditions, some authors have used ionic liquids as solvent, and additionally, several tetraalkylammonium salts of Oxone have been reported. It has been found that when the cation in Oxone (i.e., K+) is changed to, e.g., n-Bu4N+, the oxidant also shows higher solubility in organic solvents, especially in dichloromethane. Tactical utilization of Oxone in synthetic plans is that it may replace tedious organic transformations with simpler routes. One other drawback which needs to be mentioned is that a relatively large excess of Oxone may be required in some reactions to consume all of the starting material. However, militating against this is that Oxone can be reused when it is in stoichiometric excess. Owing to the discovery of a variety of novel applications, Oxone is becoming an increasingly important reagent in synthetic organic chemistry. We hope that this review may act as a catalyst in boosting applications of Oxone in organic synthesis.
展开▼
