Dehalogenation and desulfurization

Cytochromes P450 catalyze oxidative and reductive reactions. Oxidative biotrasfor-mations are more frequent and include aromatic and sidechain hydroxylation, N-, 0-, S-dealkylation, N-oxidation, sulfoxidation, N-hydroxylation, deamination, dehalogenation and desulfuration. The majority of these reactions require the formation of radical species this is usually the rate-determining step for the reactivity process [24],... 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

While the cytochrome P-450 monooxygenase reaction described in Eq. (1) often involves hydroxylation of carbon, many other reactions are catalyzed by these enzyme systems. These reactions include oxidation of nitrogen and sulfur, epoxidation, dehalogenation, oxidative deamination and desulfuration, oxidative N-, O-, and S-dealkylation, and peroxidative reactions (56). Under anaerobic conditions, the enzyme system will also catalyze reduction of azo, nitro, N-oxide, and epoxide functional groups, and these reductive reactions have been recently reviewed (56, 57). Furthermore, the NADPH-cytochrome P-450 reductase is capable of catalyzing reduction of quinones, quinonimines, nitro-aromatics, azoaromatics, bipyridyliums, and tetrazoliums (58). 

Catalytic dehalogenation (mostly by H2/Pd) and desulfurization (by Raney-Ni) are important tools for structural analysis. This way chlorine can be removed from positions 2, 5, and 7 [64CPB204 66JCS(C)2031]. Zinc was used for dechlorination at C-7 (59YZ903). The 5,7-dichloro-TPs (61CPB801) and 6,7-dichloro-TPs (59CPB903) in the presence of H2/Pd first lose the more reactive chlorine from C-7 the remaining 6-chloro compound can be dechlorinated only by Raney-Ni. 

Raney nickel is very probably the most commonly used nickel catalyst. It is also the most versatile of catalysts. In one or other of its several modifications it has been used for hydrogenations over a wide range of pressures varying from high to subatmospheric, for desulfuration, for dehalogenation, and for very many other reactions. 

Figure 8 Various forms of replacement reactions oxidative dehalogenation, deamination, and desulfuration.

The most common reaction catalysed by P450 is hydroxy lation. However, it is also involved in a wide spectrum o reactions including epoxidation, 0-, N-, and. 5-dealkylation deamination, desulfuration, dehalogenation and peroxidation. 

Aryl and alkyl hydroxylations, epoxide formation, oxidative dealkylation of heteroatoms, reduction, dehalogenation, desulfuration, deamination, aryl N-oxygenation, oxidation of sulfur Oxidation of nucleophilic nitrogen and sulfur, oxidative desulfurization Oxidation of aromatic hydrocarbons, phenols, amines, and sulfides oxidative dealkylation, reduction of N-oxides Alcohol oxidation reduction of ketones Oxidative deamination... 

Microsomal oxidations may be subdivided into aromatic hydroxylation aliphatic hydroxylation alicyclic hydroxylation heterocyclic hydroxylation N-, S-, and O-dealkylation N-oxidation N-hydroxylation S-oxidation desulfuration deamination and dehalogenation. 

A characteristic of the liver P4so enzymes is their almost total lack of substrate specificity, distinguishing them from the adrenal gland enzymes which are much more specific (B-74MI11003). In addition to hydroxylation of hydrocarbons, which involves the conversion of C—H bonds to C—OH bonds and C=C bonds to epoxide rings, a multitude of other types of reaction are catalyzed. These include iV-oxidation, 5-oxidation, N-, S- and O-dealkylation, peroxidation, deamination, desulfuration and dehalogenation, as well as... 

The classic ionic methods for the removal of an unwanted hydroxyl group are summarized in detail in the book by Larock [6]. One method involves—for primary and unhindered secondary alcohols—the synthesis of the corresponding mesylates or tosylates. These compounds are prepared readily and then transformed into the corresponding deoxy compounds by reduction [7], Alternatively, introduction of a thiolate or halogen by a nucleophilic reaction can also be used. These compounds can then be readily desulfurized or dehalogenated (Scheme 1). Tertiary alcohols present no problem either, because a... 

Organocopper reagents, 207 Organovanadium reagents, 219 By desilylation of vinylsilanes Tetrabutylammonium fluoride, 286 By desulfurization of vinyl sulfides and related sulfur compounds Sodium dithionite, 281 Elimination reactions By dehalogenation Dichlorobis(cyclopentadienyl)-titanium, 102... 

Epoxidation and hydroxylation A-Dealkylation O-Dealkylation -Dealkylation -Oxidation A-Oxidation P-Oxidation Desulfuration Dehalogenation Nitro reduction Azo reduction Cytochrome P450 (CYP) Aflatoxin, aldrin, benzo[a]pyrene, bromobenzene, naphthalene Ethylmorphine, atrazine, dimethylnitrocarbamate, dimethylaniline p-Nitroanisole, chlorfenvinphos, codeine Methylmercaptan Thiobenzamide, phorate, endosulfan, methiocarb, chlorpromazine 2-Acetylaminofluorene Diethylphenylphosphine Parathion, fonofos, carbon disulfide CCLt, CllCb Nitrobenzene O-Aminoazotoluene Flavin-Containing Monooxygenase (FMO)... 

Raney nickel was recommended as a convenient reagent for desulfurization of phenothiazine, and it has been used in many structural investigations. More recently, the method has been applied to various phenothiazines, - in its classical form, that is Raney nickel and hydrogen, as well as in some modified variants. Thus, working with a special Raney nickel, without hydrogen, 1,3,7,9-tetrachlorophenothiazine was desulfurized without dehalogenation the usual procedure leads, for example, to diphenylamine, when applied to 3,7-dichlorophenothiazine. ... 

Cytochrome P-450 is a name for a family of heme proteins that perform hydroxylation reactions, as well as epoxidation, peroxygenation, desulfuration, dealkylation, deamination, and dehalogenation reactions. Most vertebrate genomes contain more than 40 different structural genes for cytochrome P-450. The proteins resemble mitochondrial cytochrome oxidase in being able to bind both 02 and carbon monoxide. Cytochrome P-450 proteins are usually found in the endoplasmic reticulum of eukaryotic cells. 

The main hydrotreating reaction is that of desulfurization, but many others take place simultaneously to various degrees, such as denitrogenation, various hydrogenations and hydrocracking (in some cases deoxidation and dehalogenation) (see Table 1). 

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