Oxygen atom, reviews

Structure. The straiued configuration of ethylene oxide has been a subject for bonding and molecular orbital studies. Valence bond and early molecular orbital studies have been reviewed (28). Intermediate neglect of differential overlap (INDO) and localized molecular orbital (LMO) calculations have also been performed (29—31). The LMO bond density maps show that the bond density is strongly polarized toward the oxygen atom (30). Maximum bond density hes outside of the CCO triangle, as suggested by the bent bonds of valence—bond theory (32). The H-nmr spectmm of ethylene oxide is consistent with these calculations (33). 

The most important oxirane syntheses are by addition of an oxygen atom to a carbon-carbon double bond, i.e. by the epoxidation of alkenes, and these are considered in Section 5.05.4.2.2. The closing, by nucleophilic attack of oxygen on carbon, of an OCCX moiety is dealt with in Section 5.05.4.2.1 (this approach often uses alkenes as starting materials). Finally, oxirane synthesis from heterocycles is considered in Section 5.05.4.3 one of these methods, thermal rearrangement of 1,4-peroxides (Section 5.05.4.3.2), has assumed some importance in recent years. The synthesis of oxiranes is reviewed in (B-73MI50500) and (64HC(19-1U). 

Electropositive fluorinating agents are categorized in distinct classes as (1) fluoroxy reagents, in which the fluorine is bound to an oxygen atom (for reviews, see references 1,2,3, 4, 5, and 6) (2) fluoraza reagents, in which the fluorine atom IS bound to the nitrogen atom of either an amide or ammonium ion structure (for a partial review, see reference 6), (3) xenon difluoride, in which the fluorine atoms are bound to xenon (for reviews, see references 5, 7, 8, and 114) and (4) perchloryl Iluoride, in which the fluorine atom is bound to the chlorine atom of the perchloryl function (for a review, see reference 9). 

The sulfur-rich oxides S 0 and S 02 belong to the group of so-called lower oxides of sulfur named after the low oxidation state of the sulfur atom(s) compared to the best known oxide SO2 in which the sulfur is in the oxidation state +4. Sulfur monoxide SO is also a member of this class but is not subject of this review. The blue-green material of composition S2O3 described in the older literature has long been shown to be a mixture of salts with the cations S4 and Ss and polysulfate anions rather than a sulfur oxide [1,2]. Reliable reviews on the complex chemistry of the lower sulfur oxides have been published before [1, 3-6]. The present review deals with those sulfur oxides which contain at least one sulfur-sulfur bond and not more than two oxygen atoms. These species are important intermediates in a number of redox reactions of elemental sulfur and other sulfur compounds. 

Carotene cleavage enzymes — Two pathways have been described for P-carotene conversion to vitamin A (central and eccentric cleavage pathways) and reviewed recently. The major pathway is the central cleavage catalyzed by a cytosolic enzyme, p-carotene 15,15-oxygenase (BCO EC 1.13.1.21 or EC 1.14.99.36), which cleaves p-carotene at its central double bond (15,15 ) to form retinal. Two enzymatic mechanisms have been proposed (1) a dioxygenase reaction (EC 1.13.11.21) that requires O2 and yields a dioxetane as an intermediate and (2) a monooxygenase reaction (EC 1.14.99.36) that requires two oxygen atoms from two different sources (O2 and H2O) and yields an epoxide as an intermediate. ... 

Thionyl chloride (SOCl2) is the most versatile of all sulfur transfer reagents in heterocyclic synthesis. The sulfur-oxygen bond renders the sulfur atom more electrophilic the possibility of ready removal by elimination of the oxygen atom allows easy aromatization of many of the initially formed sulfur heterocycles. Two general reviews of the chemistry of thionyl chloride are available.53 Both cover the literature up to about 1970. 

The reduction of furoxans and benzofuroxans has been reviewed in detail in CHEC(1984) and CHEC-II(1996). The main reducing agents to remove oxygen atoms from nitrogen without cleavage of the heterocyclic ring are phosphines and phosphites. These reactions, namely deoxygenation of furoxans and benzofuroxans, are discussed in Section 5.05.4.2.2. 

The epoxidation of electron-deficient alkenes, particularly a,P-unsaturated carbonyl compounds, continues to generate much activity in the literature, and this has been the subject of a recent concise review <00CC1215>. Additional current contributions in this area include a novel epoxidation of enones via direct oxygen atom transfer from hypervalent oxido-).3-iodanes (38), a process which proceeds in fair to good yields and with complete retention of... 

A new generation of oxorhenium compounds has now been prepared. They catalyze oxidation reactions of a different type, and appear to function by a different mechanism. They are oxorhenium(V) compounds that form usually metastable dioxorhenium(VII) intermediates. The mechanisms feature Rev(0)-to-ReVII(0)2 interconversions and catalyze oxygen atom transfer reactions. The mechanisms show a certain diversity as to the steps that enter in a kinetic sense. Yet the schemes presented in this review show a great deal of similarity in their overall mode of action. 

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