Hydroperoxides vicinal

Because osmium tetraoxide is regenerated in this step, alkenes can be converted to vicinal diols using only catalytic amounts of osmium tetraoxide, which is both toxic and expensive. The entire process is perfonned in a single operation by simply allowing a solution of the alkene and rerr-butyl hydroperoxide in terr-butyl alcohol containing a small fflnount of osmium tetraoxide and base to stand for several hours. 

The possibility that 34 and 35 were formed via 33 was eliminated and hence it must be concluded that, in contradistinction to the reaction with 2,3-dibromocyclo-pentyl hydroperoxide 38), the Ag02CCF3-induced dioxabicyclization of 3,4-dibromo-cyclopentyl hydroperoxide involves preferential displacement of the ciy-3-bromine. It seems highly probable that this process is assisted by the vicinal bromine, i.e. that the frans-bromonium ion 36 is an intermediate. Failure to observe the analogous mechanism with 2,3-dibromocyclopentyl hydroperoxide presumably reflects the disfavoured nature of the mode of ring closure needed in the corresponding species 37. 

These short-lived species, in turn, promote the propagation of chain peroxidation of membrane lipids, thus disrupting the cellular integrity, or they may oxidize also various xenobiotics that are in the vicinity. Indeed, the hydroperoxide dependent oxidation process has been implicated as one of the mechanisms of conversion of procarcinogens to carcinogens (6,9) ... 

The stereoselectivity of the allylic hydroperoxidation also depends on se eral factors. With chiral allylic alcohols or allylic amines 200, a hydrogen is developed between ]02 and the vicinal hydroxyl or amino group. The fac differentiation results from an approach of 102 in the transition state that mi mizes 1,3-allylic strain. 201 and 202 can be obtained with a diastereoisome excess higher than 90% in CCI4. As previously indicated for the formation dioxetanes and 1,4-endoperoxides, the selectivity decreases considerably in presence of hydroxylic solvents [123]. When hydrogen bonding is no more pos ble in 203, the stereofacial differentiation is steered by steric and electronic rep sion effects at the level of the possible diastereoisomeric transition states 204 is formed selectively (Scheme 54). 

In the epoxidation step, the peroxidic oxygen vicinal to the metal center is transferred to the double bond forming the oxirane ring (Figure 14, bottom line, for Mo(VI) catalysts). The active species is regenerated by reaction of the alkoxymetal species with the organic hydroperoxide. 

The oxidative cleavage of epoxides with hydrogen peroxide gives vicinal hydroxy hydroperoxides [178]. With dimethyl sulfoxide in the presence of trifluoromethanesulfonic acid and diisopropylethylamine, epoxides are converted into a-hydroxy ketones [1014], and with periodic acid, dicarbonyl compounds are formed [761] (equations 343 and 344). 

Similar electrophilic activation of coordinated peroxides or alkylperoxides can be observed for the metal ions in intermediate oxidation states. To give just one example, Sharpless epoxidation takes advantage of an electrophilic activation of alkyl hydroperoxides at titanium(IV). Notably, efficient epoxidation requires substrate binding in the vicinity of coordinated alkylperoxide, thus limiting the substrate scope of this reaction to allylic alcohols (alkoxy group acts as an anchor).1,45... 

Vicinal diols are normally prepared by hydroxylation of alkenes with osmium tetraoxide and ferf-butyl hydroperoxide. 

The value of AT (>10 Lmor ) and the independence of Wj from [ROOH] indicate that near the surface of the heterogeneous metal-polymer catalyst the local concentration of hydroperoxide is high. It has been shown that in the vicinity of catalytic centers there are always vacant carboxyl groups able to bind ROOH molecules by hydrogen bonds. Probably, the transfer of ROOH molecules to the active center is carried out initially via their migration across the two-dimensional catalyst surface, and only after their decay do the radicals formed escape into the bulk liquid where the chain radical oxidation of hexene takes place. 

In contrast to the results obtained with simple olefins, olefins containing alcohol functionality were epoxidized much more rapidly in the presence of vanadium complexes than with molybdenum [409,410]. The efficiency of the vanadium catalyzed epoxidation of allyl alcohol has been rationalized on the basis of an intermediate complex having a geometry which places the electron-deficient oxygen of the hydroperoxide in the vicinity of the double bond, equation (265). 

Figure 2 Intramolecular propagation and formation of vicinal hydroperoxides in PP and their decomposition.

Raman microscopy has been used to probe photo-oxidation of imstabilized PP (90). It was concluded that the catalyst residues, for the type of PP studied, tend to stabilize the polymer in the immediate vicinity, but also form reactive species that diffuse away from the catalyst to initiate oxidation. Localized oxidation was also confirmed by examining the distribution of polystyrene grafl ed onto the hydroperoxide sites in photo-oxidized PP (91). 

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