Hydroperoxides as oxidants

The vanadium(IV) complex of salen in zeolite was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butyl hydroperoxide as oxidant.88 Well-characterized vanadyl bis-bipyridine complexes encapsulated in Y zeolite were used as oxidation catalysts.101 Ligation of manganese ions in zeolites with 1,4,7-triazacyclononanes gives rise to a binu-clear complex stabilized by the zeolites but allows oxidation with excellent selectivity (Scheme 7.4). 

The heme peroxidases oxidize a variety of structurally diverse substrates by using hydrogen peroxide or hydroperoxides as oxidants, whereby the oxidants are reduced to water or alcohol. In the following, the present state of such peroxidase-catalyzed reactions in organic synthesis is described, which unquestionably is important for preparative work. 

Epoxidation of olefins over Mo containing Y zeolites was studied by Lunsford et al. [86-90]. Molybdenum introduced in ultrastable Y zeolite through reaction with Mo(C0)g or M0CI5, shows a high initial activity for epoxidation of propylene with t-butyl hydroperoxide as oxidant and 1,2-dichloroethane as solvent [88]. The reaction is proposed to proceed via the formation of a Mo +-t-butyl hydroperoxide complex and subsequent oxygen transfer from the complex to propylene. The catalyst suffers however from fast deactivation caused by intrazeolitic polymerization of propylene oxide and resulting blocking of the active sites. 

SCHEME 101. Uncatalyzed asymmetric sulfoxidation with chiral hydroperoxides as oxidants... 

An overview of the obtained results of the titanium-catalyzed asymmetric sulfoxidation of various sulfides with different optically pure hydroperoxides as oxidant and asymmetric inductor is given in Table 29. 

The use of a chiral hydroperoxide as oxidant in the asymmetric Baeyer-ViUiger reaction was also described by Aoki and Seebach, who tested the asymmetric indnction of their TADOOH hydroperoxide in this kind of reaction . Besides epoxidation and snlfoxidation, for which they found high enantioselectivities with TADOOH (60), this oxidant is also able to induce high asymmetry in Baeyer-ViUiger oxidations of racemic cyclobntanone derivatives in the presence of DBU as a base and LiCl as additive (Scheme 174). The yields and ee values (in parentheses) of ketones and lactones are given in Scheme 174 as... 

In Studying asymmetric oxidation of methyl p-tolyl sulfide, employing Ti(OPr-/)4 as catalyst and optically active alkyl hydroperoxides as oxidants, Adam and coworkers collected experimental evidence on the occurrence of the coordination of the sulfoxide to the metal center. Therefore, also in this case the incursion of the nucleophilic oxygen transfer as a mechanism can be invoked. The authors also used thianthrene 5-oxide as a mechanistic probe to prove the nucleophilic character of the oxidant. 

The haem peroxidases are a superfamily of enzymes which oxidise a broad range of structurally diverse substrates by using hydroperoxides as oxidants. For example, chloroperoxidase catalyses the regioselective and stereoselective haloge-nation of glycals, the enantioselective epoxidation of distributed alkenes and the stereoselective sulfoxidation of prochiral thioethers by racemic arylethyl hydroperoxides [62]. The latter reaction ends in (i )-sulfoxides, (S)-hydroperoxides and the corresponding (R)-alcohol, all In optically active forms. 

Therefore, we conclude that surface modification of Ti-MCM-41 materials by trimethylsilylation yields to catalysts up to nine times more active for epoxidation of olefins using organic hydroperoxides as oxidants than analogous non-si lylated Ti-MCM-41 catalysts. 

Silylation of Ti-MCM-41 materials produces highly active and selective for epoxidation ol olefins using organic hydroperoxides as oxidants, it has been found that the controlling parameters of the final catalytic activity of silylated Ti-MCM-41 materials are the hydrophobicity and the concentration of free silanol groups on the external surface of the mesopores that built up the Ti-MCM-41 structure. 

An interesting behavior of the Padua reagent (Ti(0-i-Pr)4/(/ ,i )-DET = 1 4) was described by Scretti et al. [42,43], who used racemic furylhydroperoxides 1 instead of cumyl hydroperoxide as oxidant. The enantioselectivities in the oxidation of methyl aryl sulfides are very good. For example, methyl p-tolyl sulfoxide was obtained in 75% yield and >95% ee together with about 15% of sulfone by using hydroperoxide 1(R =OEt,R = /-PrandR3 = Me) Simultaneously there is a kinetic resolution of the racemic hydroperoxide takes place is used in excess (2 mol equiv. with respect to sulfide). Thus in the example mentioned above, the enantiopurity of the residual hydroperoxide was 81% ee. It has also been established that some kinetic resolution of... 

An example of the synthesis of CCs from olefins in a single reactor has been reported by Srivastava [187, 188], by using titanosilicalite as catalyst and hydroperoxide as oxidant in the form of H202 or TBHP. The reaction was carried out in two steps, in which the olefin was first epoxidated at 233 K using H202 or TBHP. The C02 was then added in presence of N,N-dimethylamionopyridine as cocatalyst, to afford a 33% yield of the CC at 293 K. 

Composition and preparation conditions of mesoporous materials containing titanium are screened to optimise the catalytic activity and selectivity in the epoxidation of cyclohexene using tert-butyl hydroperoxide as oxidant. Important parame-... 

Jia, C. Lu, W. Kitamura, T. Fujiwara, Y. Highly efficient Pd-catalyzed coupling of are-nes with olefins in the presence of tert-butyl hydroperoxide as oxidant. Org. Lett. 1999, 1, 2097-2100. 

A. Supported Mo Catalysts with Alkyl Hydroperoxides as Oxidants... 

Immobilized CPO on the mesoporous molecular sieve SBA-15 CPO-SBA-15 was able to oxidize indole to 2-oxoindole using H202 or tert-butyl hydroperoxide as oxidants and it showed better conversion and pH range performance compared to native CPO [8]... 

Wong s group32 investigated the chloroperoxidase-catalyzed oxidation of p-sub-stituted alkyl phenyl sulfides by hydrogen peroxide or racemic alkyl hydroperoxides as oxidant in aqueous buffer. Slow addition of H202 to the reaction mixture afforded nearly enantiopure sulfoxides (97-99% ee). Interestingly, when racemic alkyl hydroperoxides were used as oxidant, optically active alcohols and alkyl hydroperoxides were obtained (Scheme 4). 

The oxidation of aniline was carried out in the liquid phase over a series of transition metal - substituted molecular sieves. For low oxidant/aniline ratios, azoxybenzene (AZY) was the major product formed over Ti-containing catalysts, the reaction was limited by diffusion for medium pore zeolites like TS-l and mesoporous silicas were preferred as they permitted the use of both H2O2 and tert-butyl hydroperoxide as oxidants. Higher oxidant/aniline ratios (>2) led to the formation of nitrobenzene (NB), whose selectivity was proportional to the catalyst concentration. In contrast, vanadium containing molecular sieves were only active with TBHP and aniline was converted very selectively into nitrobenzene for all oxidant concentrations. 

The well-known Sharpless system for the enantioselective epoxidation of allyl alcohols has been investigated [23]. This system employs a tetra-alkoxy titanium precursor, a dialkyltartrate as an auxiliary, and an alkyl hydroperoxide as oxidant, to effect the enantioselective epoxidation. The key intermediate is thought to be a dimeric complex in which titanium is simultaneously coordinated to the chelating tartarate ligand, the substrate in the form of an oxygen bound / -allyl-oxide and an -tert-butylperoxide. 

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