Alkenes transition metal peroxide oxidation

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,... 

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],... 

The first examples of metal (Os, Mo, W, or V)- and PT-catalyzed biphasic oxidations of alkenes with H2O2 have been described by Starks [59]. The PT agent probably transports H2O2 molecules into the organic phase and prevents the decomposition of the peroxide. The direction of the reaction depends on the nature of the transition metal. 

In oxidation reactions, however, osmium is significantly more selective than catalysts derived from other transition metals. Osmium-based catalysts for the hydroxylation and amination of alkenes are very widely used in organic synthesis.With alkaloid-derived ligands, the hydroxylation and amination reactions are highly enantioselective see Enantioselectivit. The use of bleach, hydrogen peroxide, ferric cyanide, and oxygen have been reported as secondary oxidants for some of these reactions. 

Tertiary alkylhydroperoxides are used most often as oxidizing agents with alkenes since primary or secondary alkylhydroperoxides are susceptible to rearrangement and decomposition. Alkylhydroperoxides are relatively soluble in organic solvents, are more stable, and are easier to handle than hydrogen peroxide.256 Both TBHP and cumyl hydroperoxide are commercially available and widely used. As with hydrogen peroxide, reaction of alkenes with hydroperoxides usually requires transition metal catalysts in order to form... 

Sulfides react faster with hydrogen peroxide and alkyl hydroperoxides than do alkenes. For this reason, transition metal catalysts are rarely necessary, but these reactions are acid catalyzed and first order in both sulfide and peroxide. The acid (HX) can be as weak as alcohol or water but the "effectiveness (of the oxidation) is determined by the pXa of the acid. Sulfides also react faster with peroxides than do ketones (see the Baeyer-Villiger reaction, sec. 3.6). Formation of the sulfone in these reactions is straightforward, but requires more vigorous reaction conditions. It is usually easy to isolate the sulfoxide from oxidation of a sulfide. Direct conversion of a sulfide to a sulfone requires excess peroxide and vigorous reaction conditions (heating, long reaction times, more concentrated peroxide). 

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