Alkenes by hydrogen peroxide

Ab initio calculations of the transition-state energies in the epoxidation of alkenes by hydrogen peroxide catalysed by titanosilicates have been carried out. They indicate a markedly lower energy barrier for attack of the alkene by the oxygen atom of the titanium(IV) hydroperoxide intermediate that is closer to the metal centre. 

In 1994, Mansuy and coworkers found that a simple ammonium salt, like ammonium acetate alone, is a very efficient cocatalyst for the metalloporphyrin-catalyzed epoxidation of simple alkenes by hydrogen peroxide ". Bases like sodium carbonate, sodium acetate or tetrabutylammonium hydroxide turned out to promote the porphyrin-catalyzed epoxidation without any other additive. Adducts of hydrogen peroxide (with Na2C03, urea, MesNO, PhsPO), which turned out to be particularly useful for reactions in which the concentration of H2O2 in solution needs to be controlled at a fixed level, have been employed by Johnstone and coworkers. 

The two-phase epoxidation of alkenes by hydrogen peroxide in water-dichloromethane system, catalysed by manganese(III)-porphyrin, is strongly accelerated by addition of catalytic amounts of a carboxylic acid and lipophilic imidazole or pyridine axial ligand365,366. Manganese(III)-porphyrin bound to colloidal anion-exchange particles is more active in the selective epoxidation of styrene by aqueous hypochlorite than the same catalyst in aqueous solution367. 

A number of workers have shown that a small amount of nitrogenous base with manganese metalloporphyrins greatly increases the stereospecificity of alkene epoxidation 49-51 Figure 2.18 illustrates the dual behaviour of imidazole during the epoxidation of alkenes by hydrogen peroxide.52... 

Figure 2.18 Dualfunction of imidazole in the epoxidation of alkenes by hydrogen peroxide catalysed by manganese (III) porphyrins.

Al-Ajlouni, A.M., and I.H. Espenson, Kinetics and Mechanism of the Epoxidation of Alkyl-Substituted Alkenes by Hydrogen Peroxide, Catalyzed by Methyhhenium Trioxide,/. Org. Chem. 61 3969-3976 (1996). 

Poriel C, Ferrand Y, Le Maux P, Rault-Berthelot J, Simonneaux G (2003) S5mtheses of manganese and iron tetraspirobifluorene porphyrins as new catalysts for oxidation of alkenes by hydrogen peroxide and iodosylbenzene. Tetrahedron Lett 44 1759-1761... 

A breakthrough in iron-catalyzed asymmetric epoxidation of aromatic alkenes using hydrogen peroxide has been reported by our group in 2008. Good to excellent isolated yields of aromatic epoxides are obtained with ee-values up to 97% for stilbene derivatives using diphenylethylenediamines 9 as ligands (Scheme 5) [45, 46]. 

On the other hand, Tilley et al. have reported a synthesis of a well-defined tris(tert-butoxy)siloxy-iron(lll) complex [13] as well as respective molecular siloxide complexes of cobalt [14] and copper [15], which appear to become precursors for their grafting onto silica and application as catalysts for oxidation of alkanes, alkenes and arenes by hydrogen peroxide. 

Epoxidation. 1,1,3,3-Tetrachloroacetone can mediate the epoxidation of olefins by hydrogen peroxide in the same way as hcxafluoroacctonc (9, 244-245). It has the advantage that it is probably less toxic, and it is inexpensive and commercially available. The actual oxidant presumably is the hydroperoxide 1, which is converted during the epoxidation into the unstable hydrate (2), from which the tctrachloroacetone can be recovered in >70% yield. Monosubstitutcd alkenes are epoxidized in low yields by this method more highly substituted alkenes are epoxidized in 65 85% yield (VPC).1... 

Molybdenum and tungsten compounds have long been known to catalyze the transformations of alkenes into epoxides and diols by hydrogen peroxide.171"173 This reaction was found to be suitable for the epoxidation of water-soluble alkenes such as allylic alcohols (equation 30)174,175 or unsaturated carboxylic acids (equation 31).171 Tungsten catalysts were found to be more active and selective in aqueous solution than molybdenum complexes. 

Intramolecular hydrosilylations of functionalized alkenes followed by hydrogen peroxide oxidation provide powerful methods for organic syntheses86-88. The reactions of allylic O-dimethylsilyl ethers 59 promoted by platinum catalysts, e.g. Karstedt s catalyst and Pt(PPh3)2(CH2=CH2), or rhodium catalysts, e.g. Rh(acac)(COD) and [RhCl(CH2=CH2)2]2> proceed via 5-endo cyclization to give oxasilacyclopentanes 60 with a couple of exceptions in which siloxatanes 61 are formed (Scheme ll)87,89. 

Studies on the epoxidation of unsaturated acids by hydrogen peroxide in the presence of phosphotungstic acid352 and of alkenes by alkyl hydroperoxides in the presence of molybdenum complexes353,354 and vanadium oxide355 have appeared. 

A titanium complex (1) with a salen ligand is an efficient catalyst for the enan-tioselective epoxidation of alkenes with hydrogen peroxide as the terminal oxidant. The participation of a titanium-peroxo species, activated by hydrogen bonding, in the reaction, has been postulated.73... 

E. J. Allain, L. P. Hager, L. Deng, and E. N. Jacobsen, Highly enantioselective epoxidation of disubstituted alkenes with hydrogen peroxide catalyzed by chloro-peroxidase, J. Am. Chem. Soc. 1993, 115, 4415-4416. 

Fig. 7.10. Partial benzonitrile hydrolysis of (A) accelerated by hydrogen peroxide under basic conditions. The intermediate E, a perimidic acid, is as suitable for the epoxidation of standard alkenes as MCPBA or HHPP (cf. Section 3.19), but unlike the latter two is also capable of epoxidizing keto alkenes (which would be oxidized by MCPBA or MMPP in the ketonic substructure in a Baeyer-Vil-liger reaction cf. Section 14.35).

In the laboratory, epoxidations of alkenes are usually accomplished by hydrogen peroxide or its derivatives. Hydrogen peroxide is applied in an alkaline medium so that it reacts as an anion (equation 54) [138, 142, 143, 146, 147, 148, 1099]. 

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