Transition metal peroxides alkenes

Transition metal peroxides. Synthesis and role in oxidation reactions 1087 2. Electron-poor alkenes... 

C-H bond unreactive to insertion, 1160 flavonoid, 548-9, 550 hydroperoxide oxidation, 692 a-hydroxylation, 519-21, 522-3 IR spectrophotometry, 662 oxidation of alkenes, 521-5 Baeyer-VilUger, 784-7 transition metal peroxides, 1108-14 a,/S-unsaturated, 199-202, 246-7, 285,... 

Lewis acids such as SnCl4 also catalyze the reaction, in which case the species that adds to the alkenes is H2C —O— SnC. Montmorillonite KIO clay containing zinc(IV) has been used to promote the reaction. The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form allylic alcohols. A typical example is. ... 

Chromylchloride, Cr02Cl2, the main subject of the publication which led to the original discussion about the mechanism [12], shows a very different reactivity compared to the other transition metal oxides discussed above. Even in the absence of peroxides, it yields epoxides rather than diols in a complex mixture of products, which also contains cis-chlorohydrine and vicinal dichlorides. Many different mechanisms have been proposed to explain the great variety of products observed, but none of the proposed intermediates could be identified. Stairs et al. have proposed a direct interaction of the alkene with one oxygen atom of chromylchloride [63-65], while Sharpless proposed a chromaoxetane [12] formed via a [2+2] pathway. 

Transition metals and their complexes can be immobilized in the mesopores or incorporated in the structure to make silica-supported metal catalysts. For instance, titanium catalysts for selective oxidation can be formed by modifying the mesoporous structure with either Ti grafted on the surface (Tif MCM-41) or Ti substituted into the framework (Ti->MCM-41). The grafted version makes the better catalyst for the epoxidation of alkenes using peroxides, and has good resistance to leaching of the metal. 

Further studies verified the intermediate formation of free radicals, as demonstrated by the electron-spin resonance spectra obtained during autooxidation of cellulose,75 and hydrogen peroxide was identified as a byproduct in the autooxidation of D-glucitol. Similar oxidations of cellulose in the presence of alkenic monomers afforded graft copolymers. The autooxidation of cellulose and of the cello-oligosaccharides was shown to be more extensive in the presence of transition-metal cations. 

Transition metal catalysts not only increase the reaction rate but may also affect the outcome of the oxidation, especially the stereochemistry of the products. Whereas hydrogen peroxide alone in acetonitrile oxidizes alkenes to epoxides [729], osmic acid catalyzes syn hydroxylation [736], and tungstic acid catalyzes anti hydroxylation [737]. The most frequently used catalysts are titanium trichloride [732], vanadium pentoxide [733,134], sodium vanadate [735], selenium dioxide [725], chromium trioxide [134], ammonium molybdate [736], tungsten trioxide [737], tungstic acid [737],... 

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