Heteroatom oxidations oxygen

Heteroatoms nitrogen, oxygen, sulfur, and metals) are found in every crude oil, and the concentrations have to be reduced to convert the oil to transportation fuel. This is caused by the fact that if nitrogen and sulfur are present in the final fuel during combustion, nitrogen oxides (NO ) and sulfur oxides (SO ) form, respectively. In addition, metals affect many upgrading processes adversely, poisoning... 

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... 

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 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... 

Chapter 6 (Addition), Chapter 7 (Carbonylation), Chapter 8 (Acylation), and Chapter 10 (Heterosubstitution) deal with derivatization reactions to form carbon-heteroatom bonds. The important broad field of hydrocarbon oxidations is covered in Chapter 9 (Oxidation-oxygenation). Both the chemistry brought about by... 

Where the selectivity of BVMOs regarding the Baeyer-Villiger process versus a heteroatom oxidation is concerned, usually the oxygenation at the carbonyl center is favored. This was demonstrated in a study of heterocyclic substrates of type 6 to give good yields of the corresponding lactone product 7 (Scheme 21.3) [61, 62]. 

Most of the examples reviewed concentrate on instances in which the C X heteroatom is oxygen, and this reflects the dearth of work that has been reported on other heteroatoms. Thus, although numerous examples of the aromatization of nitrogen heterocycles exist, there is very little pertaining to other systems. This is an area where more exploratory work is needed, especisdly on oxazolines and related heterocycles.Reactions with C=cS compounds are even more rare, presumably because of the ease with which such systems are oxidized under dehydrogenation conditions. Opportunities exist to develop the dehydrogenation of such systems, however, as demonstrated with thioamides which have served as suitable intermediates for the dehydrogenation of otherwise difficult amides. ... 

Many flavoenzymes catalyze the oxidation of carbon—heteroatom bonds, where the heteroatom is usually nitrogen or oxygen. In these reactions, amines or alcohols are oxidized by the flavin. When nitrogen is the heteroatom, the resulting imine is hydrolyzed nonenzymatically. When the heteroatom is oxygen, a stable carbonyl is formed. 

The direct desaturation of carbon adjacent to heteroatoms, notably oxygen and nitrogen, has also been observed. These reactions include the desaturation of a tetrahydrofuran ring in the biosynthesis of aflatoxin and sterigmatocystin by a specific P450 enzyme, the P450-catalyzed desaturation of flavanones to flavones, and the conversion of ethylcarbamate to vinyl carbamate . In some instances, the desaturation may involve the carbon and the heteroatom instead of two carbon atoms. The CYP2B1-catalyzed oxidation of testosterone to androstenedione, which involves oxidation of the 17-hydroxy to a 17-keto function, is possibly such a reaction, because only... 

Cytochrome P450 enzymes are the most widespread, active, and most versatile in their xenobiotic Phase I transformation activity. These enzymes are composed of heme-containing enzymes in the ferric ion state. In transformations the ferric ion is reduced to the ferrous ion that can bind Oj and CO. These enzymes basically add oxygen or remove hydrogen in a step-wise process to generate Phase I biotransformation products. Most cytochrome P450 transformations require an additional enzyme (co-enzyme) to assist in the transfer of electrons. Cytochrome P-450 enzymes carry out many kinds of oxidations - hydroxylations, epoxidations, heteroatom oxidations, N-hydroxylations, dealkylations, ester hydrolysis, and dehydrogenation. 

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