H2O2-urea oxidant

Torisawa [17] developed an alternative oxidative amidation of aldehydes using palladium chloride (PdCl2)-xantphos complex as a catalyst. The use of hydrogen peroxide (H2O2)-urea complex as oxidant prevents the formation of imine from the carbinolamine intermediate and minimizes the level of benzoic acid side... 

Dinuclear or polynuclear manganese complexes of salen-type ligands were also tested in the epoxidation of olefins with H2O2 as oxidant. Enantioselective epoxidation of several olefins with urea-H202 and a dinuclear Mn-salen type of complex has been reported by Kureshy and co-workers (130). Conversions of more than 99 % ee were obtained with chromenes and... 

In 2010, Tanaka s group investigated whether a chiral cyclic a-amino acid included in an oligopeptide chain could catalyze the epoxidation of different enones with high enantiomeric excess. They demonstrated that the a-helical secondary structure of the peptide catalyst is directly related to the chosen a,a-disubstituted amino acid [134]. Thus, they found that 5 mol% of a-helical nonamer 92 with urea-H2O2 as oxidant can catalyze the reaction with ee > 95% (Scheme 12.18). 

Other methods of removing [NOz] involve oxidation to [NOs] (using [OCl] or H2O2 as oxidant), and removing the [NOs] as detailed above, or using urea or sulfamic acid to reduce [NOz] to N2 (see end-of-chapter problem 15.41). 

The low solubility of oxygen in most ionic liquids limits its application in oxidation catalysis in these liquids. However, oxidation by H2O2 or organoperoxide is not subject to this limitation when the ionic liquids are properly chosen. An example of catalytic oxidation is the methyltrioxorhenium (MTO)-catalyzed epox-idation of alkenes with the urea-H202 adduct in [EMIMJBF4 (228). High conversions and yields were obtained. 

The effect of structural variation and the use of different caboxylate salts as cocatalysts was investigated by Pietikainen . The epoxidation reactions were performed with the chiral Mn(III)-salen complexes 173 depicted in Scheme 93 using H2O2 or urea hydrogen peroxide as oxidants and unfunctionalized alkenes as substrates. With several soluble carboxylate salts as additives, like ammonium acetate, ammonium formate, sodium acetate and sodium benzoate, good yields (62-73%) and moderate enantioselectivities (ee 61-69%) were obtained in the asymmetric epoxidation of 1,2-dihydronaphthalene. The results were better than with Ai-heterocycles like Ai-methylimidazole, ferf-butylpyridine. 

V-methylmorpholine 7V-oxide or 4-phenylpyridine TV-oxide as cocatalysts. The yields and enantioselectivities obtained with H2O2 or urea hydrogen peroxide were comparable, with slightly better yields for the epoxidation with H2O2 (73% versus 68% for the epoxide of 1,2-dihydronaphthalene in the presence of NH4OAC). 

EP, epoxy allylbenzene or styrene oxide PAD, phenylacetaldehyde BD, benzaldehyde Diols, 3-phenyl-1,2-propanediol or styrene diol, including some high-boiling products HP, H2O2 (45 wt% aqueous) U + HP, urea and H2O2 mixture (1 1 mol ratio) UHP, uiea-H202 adduct. 

For catalytic purposes, MTO has also been supported on zeolites, niobia and polymers a useful means of preparing quinones in high yields (see Supported Organotransition Metal Compounds).Other useful variations use the urea-H2O2 adduct as an oxidant in water-free reactions or ionic liquids as solvents. 

Conjugated dienes are oxidized to epoxides (or diols, if water is present) with the MTO/H2O2 system [11]. Urea/H202 avoids the subsequent epoxide ring opening. Electron-rich and conjugated dienes are more easily oxidized than electron-poor dienes and dienes with isolated double bonds. According to kinetic measurements complex 3 plays no important role as catalyst in this case. Compound 2 is an active species [11a]. 

Fig. 2.16 Catalytic enantioselective Baeyer-Villiger oxidation of prochiral ketones, using H2O2 (in the form of a urea-adduct) as oxygen equivalent in stoichiometric amounts (a) 3-phenylcyclobutanone, (b) tricyclic cyclo-butanone.

Chromogenic detection of horseradish peroxidase POase is widely used in enzyme immunoassays (EIA) and many suitable chromogens (which are oxidized by the enzyme in the presence of the peroxide or urea peroxide substrates) have been developed. Peroxide is the usual substrate, particularly on solid phases since its reduction results in the formation of inert water near the solid phase (Fig. 7.9). It should be realized that POase has a very pronounced optimum concentration of H2O2 substrate (Tijssen et al., 1982). Activity is low at low substrate concentrations, but inhibition is considerable at high substrate concentrations. The universally used POase, C isozyme, has an optimum in solution of 0.003% peroxide but higher concentrations are usually required on a solid phase. 

Picolinic acid also accelerates the H2O2 oxidations but less efficiently than pyrazine-2-carboxylic acid. It has been demonstrated recendy that the vanadium complex with picolinic acid, VO(PA)2 , encapsulated into the NaY zeolite retains solution-like activity in the liquid-phase oxidation of hydrocarbons [16a], It is noteworthy that pyrazine-2-carboxylic acid accelerates the hydrocarbon oxidation catalyzed by CH3Re03 [25 b]. Employing a (+)-camphor derived pyrazine-2-carboxylic acid as a potential co-catalyst in the CHsReOj-catalyzed oxidation of methyl phenyl sulfide with urea-H202 adduct, the corresponding sulfoxide was obtained with an e.e. of 15% [16b]. 

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