Heteroatoms oxidative

The oxidation of heteroatoms and, in particular, the conversion of sulfides to asymmetric sulfoxides has continued to be a highly active field in biocatalysis. In particular, the diverse biotransformations at sulfur have received the majority of attention in the area of enzyme-mediated heteroatom oxidation. This is particularly due to the versatile applicability of sulfoxides as chiral auxiliaries in a variety of transformations coupled with facile protocols for the ultimate removal [187]. 

Heteroatom Oxidation, Dehydrogenation Electrooxidative kinetic resolution of rac alcohols mediated with a catalytic amount of an optically active A-oxyl was performed in an undivided cell at constant current conditions. A high enantiomeric purity for the recovered alcohol was found, which could be increased by electrolysis at lower temperatures. The optically active A-oxyl was recovered and used repeatedly without change in efficiency and selectivity [368]. Cyclovoltammetry with the A-oxyl (GR, 7S, 10/f)-4-oxo-2,2,7-trimethyl-10-isopropyl-l-azaspiro[5.5]undecane-A-oxyl as catalyst showed for rac-1-phenylethanol a highly enhanced catalytic... 

Bifunctional systems In the case of bifunctional systems (or molecules) only two alternatives are possible the bifunctional relationships are either "consonant" or "dissonant" (apart from molecules or systems with functional groups of type A to which we have referred to as "assonant"). In the first case, the synthetic problem will have been solved, in principle, in applying the "heuristic principle" HP-2 that is to say, the molecule will be disconnected according to a retro-Claisen, a retro-aldol or a retro-Mannich condensation, or a retro-Michael addition, proceeding if necessary by a prior adjustment of the heteroatom oxidation level (FGI). 

The mechanism of cytochrome P450 catalysis is probably constant across the system. It is determined by the ability of a high valent formal (FeO) species to carry out one-electron oxidations through the abstraction of hydrogen atoms or electrons. The resultant substrate radical can then recombine with the newly created hydroxyl radical (oxygen rebound) to form the oxidized metabolite. Where a heteroatom is the (rich) source of the electron more than one product is possible. There can be direct recombination to yield the heteroatom oxide or radical relocalization within the... 

Fig. 7.1 Heteroatom oxidation of drugs by cytochrome P450 leading to heteroatom oxides or dealkylation products. 

Contemporary dioxirane chemistry Epoxidations, heteroatom oxidations and CH insertions ... 

With this brief preamble on the more important current theoretical results for the general structural and electronic characteristics of dioxiranes, we shall now examine the computed transition structures of the oxygen transfer in epoxidations, heteroatom oxidations and CFI insertions. Since each reaction type exhibits its individual mechanistic features, these oxyfunctionalizations shall be presented separately. 

Since dioxiranes are electrophilic oxidants, heteroatom functionalities with lone pair electrons are among the most reactive substrates towards oxidation. Among such nucleophilic heteroatom-type substrates, those that contain a nitrogen, sulfur or phosphorus atom, or a C=X functionality (where X is N or S), have been most extensively employed, mainly in view of the usefulness of the resulting oxidation products. Some less studied heteroatoms include oxygen, selenium, halogen and the metal centers in organometallic compounds. These transformations are summarized in Scheme 10. We shall present the substrate classes separately, since the heteroatom oxidation is quite substrate-dependent. 

The oxidation of organic substances by cyclic peroxides has been intensively studied over the last decades , from both the synthetic and mechanistic points of view. The earliest mechanistic studies have been carried out with cyclic peroxides such as phthaloyl peroxide , and more recently with a-methylene S-peroxy lactones and 1,2-dioxetanes . During the last 20 years, the dioxiranes (remarkable three-membered-ring cyclic peroxides) have acquired invaluable importance as powerful and mild oxidants, especially the epoxidation of electron-rich as well as electron-poor alkenes, heteroatom oxidation and CH insertions into alkanes (cf. the chapter by Adam and Zhao in this volume). The broad scope and general applicability of dioxiranes has rendered them as indispensable oxidizing agents in synthetic chemistry this is amply manifested by their intensive use, most prominently in the oxyfunctionalization of olefinic substrates. 

HPCA), transition metal peroxides, 1116 Heteroarenes, dioxirane epoxidation, 1143-4 Heteroatom oxidations... 

Niobia-supported MTO has been prepared either by the deposition of sublimed MTO onto the support, or by the impregnation of the support by a solution of MTO, and has been well characterised [54]. A large variety of oxidation reactions were efficiently performed with niobia-supported MTO, such as olefin metathesis catalysis [53,54], reactions of ethyl diazoacetate, heteroatom oxidation (amine and phosphine oxidations) and olefin epoxidation with hydrogen peroxide [55] (Scheme 13). 

Volume I. Liquid-Phase, Base-Catalyzed and Heteroatom Oxidations, Radical Initiation and Interactions, Inhibition... 

The mono-oxygenases which catalyse a series of oxidations such as hydroxylation, epoxidation, heteroatom oxidation and Baeyer-Villiger oxidation (Figure 2.24), depend on NADH or NADPH and cofactors usually Fe or Cu. A particularly important reaction is the direct incorporation of molecular oxygen into non-activated carbon centres, such as in synthesis of important steroidal drags by microbial 11 dr-hydroxylation of... 

Figure 2.24 Reactions catalysed by mono-oxygenases, hydroxylation of carbon centres, aromatic hydroxylation, epoxidation of alkenes, heteroatom oxidation and Baeyer-Villiger oxidation of a ketone.

Reaction of tricoordinated phosphorus compounds with heteroatomic oxidizing agents... 

The chemoselectivity, regioselectivity, diastereoselectivity, and enantioselectivity of heteroatom oxidations, epoxidations and CH insertions by dioxiranes have been reviewed.26 The selective ring-opening reactions of epoxides at high pressures have been reviewed (in Japanese).27... 

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