Hydrogen peroxide asymmetric

Direct oxidation of hydrogen to hydrogen peroxide Asymmetric ceramic support Pd, Pd-Ag dense layers hydrophobic gas permeable polymer Tubular single channel O2 saturated liquid on the hydrophobized Pd dense layer H2 gas on the support side Choudhary et al. (2001)... 

Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

As mentioned in Sect. 2.2, phosphine oxides are air-stable compounds, making their use in the field of asymmetric catalysis convenient. Moreover, they present electronic properties very different from the corresponding free phosphines and thus may be employed in different types of enantioselective reactions, m-Chloroperbenzoic acid (m-CPBA) has been showed to be a powerful reagent for the stereospecific oxidation of enantiomerically pure P-chirogenic phos-phine-boranes [98], affording R,R)-97 from Ad-BisP 6 (Scheme 18) [99]. The synthesis of R,R)-98 and (S,S)-99, which possess a f-Bu substituent, differs from the precedent in that deboranation precedes oxidation with hydrogen peroxide to yield the corresponding enantiomerically pure diphosphine oxides (Scheme 18) [99]. 

Urea hydrogen peroxide adduct (UHP) was employed in metal-catalysed asymmetric epoxidation [98] and Baeyer-Villiger oxidation [99,100]. Since the presence of urea does not change the course of the reaction, this will not be described here. Conversion of epoxides to halohydrins with elemental... 

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]. 

With concentrated mineral acids azobenzene gives red salts, as may be shown by pouring hydrochloric acid on it. Addition of hydrogen leads to the re-formation of the hydrazo-compound. Oxygen is added on and the azoxy-compound formed by the action of hydrogen peroxide or nitric acid. The synthesis of asymmetrical aromatic azo-compounds from nitroso-compounds and primary amines was discussed above. 

The publication (70) in 1976 of the preparation of optically active epoxyketones via asymmetric catalysis marked the start of an increasingly popular field of study. When chalcones were treated with 30% hydrogen peroxide under (basic) phase-transfer conditions and the benzylammonium salt of quinine was used as the phase-transfer catalyst, the epoxyketones were produced with e.e. s up to 55%. Up to that time no optically active chalcone epoxides were known, while the importance of epoxides (arene oxides) in metabolic processes had just been discovered (71). The nonasymmetric reaction itself, known as the Weitz-Scheffer reaction under homogeneous conditions, has been reviewed by Berti (70). 

In the early 1980 s Julia and Colonna published a series of papers which, to some extent, filled the gap left by the natural biocatalysts. The Spanish and Italian collaborators showed that a, -unsaturated ketones of type 1 underwent asymmetric oxidation to give the epoxide 2 using a three-phase system, namely aqueous hydrogen peroxide containing sodium hydroxide, an organic solvent such as tetrachloromethane and insoluble poly-(l)-alanine, (Scheme 1) [12]. The reaction takes place via a Michael-type addition of peroxide anion (the Weitz-Scheffer reaction). 

In an early application, an enzyme electrode system was reported for the determination of creatinine and creatine, using a combination of creatinine amidohy-drolase, creatine amidinohydrolase and sarcosine oxidase, co-immobilized on an asymmetric cellulose acetate membrane. Thus, the hydrogen peroxide produced was detected to give a quantitative measure of creatine and creatinine in biological fluids [70]. 

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