Hydroperoxides as reagents

In the field of asymmetric oxidation reactions the epoxidation of a,P-unsatu-rated carbonyl compounds was investigated. In the case of 1,4-naphthoquinone derivatives and terf-butyl hydroperoxide as reagents enantioselectivities up to 78% ee were observed with quininium and quinidinium salts as PT catalysts [40]. 

Anomeric hydroperoxides are readily prepared by treatment of 2-deoxy sugars with H202 in the presence of acid (Fig. 59). They are used as reagents for enantioselective epoxidation of a,(Tunsaturated olefins (e.g. chalcone) in the presence of sodium hydroxide, the epoxidations showed exceptionally high asymmetric induction.76... 

Phenol is the major source of Bakelite and phenol resins, which are utihzed in many commodities worldwide phenol is also used as reagent for syntheses of dyes, medicines and so on. The industrial demand for phenol has increased every year and its production now exceeds 7.2 megaton year 94% of the worldwide production of phenol is processed in the cumene process. The cumene process involves the reaction of benzene with propene on acid catalysts like MCM-22, followed by auto-oxidation of the obtained cumene to form explosive cumene hydroperoxide and, finally, decomposition of the cumene hydroperoxide to phenol and acetone in sulfuric acid (Scheme 10.3) [73],... 

Enantioselective hydroxylation of 2-benzyl (3-ketoesters was catalysed by [RuCl(OEt3)(PNNP)]/aq. H O /CH Cy thus ethyl 2-benzyl-3-oxo-butanoate gave ethyl 2-hydroxy-2-benzyl-yoxo-butanoate. Better results were obtained with cumyl hydroperoxide as co-oxidant [14]. The reagent Ru(CO)(TPP) or Ru(CO) (TMP)/(Cl3pyNO)/aq. HBr/C Hy40°C oxidised 5 3-steroids to the corresponding Sp-hydroxy derivatives with retention of configuration [15]. 

Common alcohol oxidation methods employ stoichiometric amounts of toxic and reactive oxidants like Cr03, hypervalent iodine reagents (Dess-Martin) and peracids that pose severe safety and environmental hazards in large-scale industrial reactions. Therefore, a variety of catalytic methods for the oxidation of alcohols to aldehydes, ketones or carboxylic acids have been developed employing hydrogen peroxide or alkyl hydroperoxides as stoichiometric oxygen sources in the presence of catalytic amounts of a metal catalyst. The commonly used catalysts for alcohol oxidation are different MoAV(VI), Mn(II), Cr(VI), Re(Vn), Fe(II) and Ru complexes . A selection of published known alcohol oxidations with different catalysts will be presented here. 

The introduction of substituents into position 7 of a 2,4-disubstituted pteridine can be effected very cleanly by the use of acyl radicals typically and has been known for many years. Treatment of aldehydes with /-butyl hydroperoxide and iron(ll) generates acyl radicals which add selectively to the 7-position. A recent exploitation of this chemistry has provided a large number of new examples including both aryl and alkyl acyl radicals as reagents <2004PTR129> pA , data have been compiled (Section 10.18.4) and many nucleophilic substitution reactions of the 7-acylated pteridines and functional group modifications have been described (Section 10.18.7.2). 

This procedure, which is based entirely on commercially available reagents, is very easy to reproduce. Table 6.12 shows different aryl- and alkyl-substituted enones that can be epoxidised with high asymmetric induction with the in situ formed (/ )-BINOL-zinc-catalyst in diethyl ether, with cumene hydroperoxide as the terminal oxidant. 

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... 

Chiral epoxides are important intermediates in organic synthesis. A benchmark classic in the area of asymmetric catalytic oxidation is the Sharpless epoxidation of allylic alcohols in which a complex of titanium and tartrate salt is the active catalyst [273]. Its success is due to its ease of execution and the ready availability of reagents. A wide variety of primary allylic alcohols are epoxidized in >90% optical yield and 70-90% chemical yield using tert-butyl hydroperoxide as the oxygen donor and titanium-isopropoxide-diethyltartrate (DET) as the catalyst (Fig. 4.97). In order for this reaction to be catalytic, the exclusion of water is absolutely essential. This is achieved by adding 3 A or 4 A molecular sieves. The catalytic cycle is identical to that for titanium epoxidations discussed above (see Fig. 4.20) and the actual catalytic species is believed to be a 2 2 titanium(IV) tartrate dimer (see Fig. 4.98). The key step is the preferential transfer of oxygen from a coordinated alkylperoxo moiety to one enantioface of a coordinated allylic alcohol. For further information the reader is referred to the many reviews that have been written on this reaction [274, 275]. 

Chromium(VI) oxide can be used as a catalytic oxidant for alcohols with r-butyl hydroperoxide as the cooxidant. This reagent appears to be selective for allylic and benzylic over saturated alcohols, though ( )/(Z)-isomerization has been observed during the preparation of a,3-unsaturated aldehydes. This reagent is also a good oxidant for allylic and ben lic C—bonds these may be competing pathways in more sophisticated substrates. ... 

Oganoselenium reagents have been observed to exhibit selectivity for the oxidation of allylic alcohols, for example a catalytic amount of dimesilyl diselenide with r-butyl hydroperoxide as cooxidant will oxidize benzyUc and allylic alctdiols in the presence of saturated alcohols, as in the case of the diol (7 equation 4). 

Several procedures for this chemoselective oxidation utilize molybdenum-based catalysts, with either hydrogen peroxide or r-butyl hydroperoxide as the stoichiometric oxidant. These include ammonium molybdate in the presence of a ph e transfer reagent and hydrogen peroxide, which with pH control (potassium carbonate) will selectively oxidize a secondary alcohol in the presence of a primary alcohol without oxidizing alkenes. In addition hindered alcohols are oxidized in preference to less hindered ones (Scheme 18). 

To overcome the problems of toxicity and work-up associated with many inorganic oxidants, it would be advantageous to develop a catalytic supported oxidant. Towards this aim, chromium(III)-impregnated Nafion 511 (NAFK) has been used as a catalytic oxidant in the presence of f-butyl hydroperoxide. This reagent gives good yields of ketones (80-100%), but unfortunately oxidation of primary alcohols leads to the formation of complex mixtures. 

Recent literature refers to the stereoselective and asymmetric epoxidation of allylic alcohols with organoaluminium peroxides. PhaSiOOH epoxidizes olefins with a stereoselectivity similar to that with peracid. Reports have been made of a-substituted hydroperoxides (acids, esters, ketones, amides, and nitriles) as effective epoxidizing reagents and the application of hexachloroacetone, tetrachloracetone, and hexafluoroacetone hydroperoxide, as well as the HaOa-Vilsmeier reagent system. ... 

Table m. Carbon-Hydrogen Activation of C1-C3 Hydrocarbons with a Manganese-Substituted Keggin Ion Catalyst Using t-Butyl Hydroperoxide as the Monooxygen Transfer Reagent in Benzene a... 

A modified Sharpless reagent has been developed by Kagan [503, 814], Modena [502, 814] and their coworkers. This new catalyst is formed by mixing water, Ti(0/-Pr)4, and diethyltartrate in a ratio of 1/1/2. The modified catalyst promotes enantioselecfrve oxidation of arylalkylsulfides by fert-BuOOH, and chiral sulfoxides are produced with excellent enantiomeric excesses (> 90%). Lower selectivities are observed from dialkylsulfides. From (R,R) or (5 S)-diethyl tartrate, either sulfoxide enantiomer can be obtained. The use of cumene hydroperoxide as the oxidant may improve the enantioselectivity. Uemura and coworkers obtained similar results by replacing the tartrates in these complexes with binaph-thols [815],... 

Asymmetric epoxidation of allylic alcohols. This reaction has been accomplished by use of the ligand 1 with VO(acac)2 as catalyst and -butyl hydroperoxide as the epoxidation reagent. Optical yields as high as 50% have been obtained substantially lower inductions were obtained with cumene hydroperoxide. Molybdenum complexes with 1 give low asymmetric inductions. ... 

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