Nucleophilic oxygen, description

The description of the carbon-oxygen double bond is analogous, but in addition to the cr-bonds there are unshared pairs of electrons on oxygen so that two excited states are possible, n-n and n-n. For n-n excitation the resultant half-vacant orbital on oxygen should possess electrophilic reactivity, and the electron rich -system should have nucleophilic characteristics. 62>... 

The electron-donor N -oxide oxygen atom of a nitrone makes it suitable for com-plexation and protonation. Such properties of nitrones have been widely used to influence their reactivity, using Lewis acids and protonation in nucleophilic addition reactions (see Section 2.6.6). In this chapter, the chemistry of nitrones with various metal ions [Zn (II), Cu(II), Mn (II), Ni (II), Fe (II), Fe (III), Ru (II), Os (II), Rh (I), UO2 2 ] (375, 382, 442-445), and diarylboron chelates is described (234—237, 446). Accurate descriptions of the structures of all complexes have been established by X-ray analysis. 

This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H—Si(lll) has a band gap of about 1.1 eV while the HOMO LUMO gap in (Me3Si)3SiH is within 8-11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter-... 

Dimesityldioxirane, a crystalline derivative, has been isolated by Sander and colleagues and subjected to X-ray analysis. The microwave and X-ray data both suggest that dioxiranes have an atypically long 0—0 bond in excess of 1.5 A. Those factors that determine the stability of dioxiranes are not yet completely understood but what is known today will be addressed in this review. A series of achiral, and more recently chiral oxygen atom transfer reagents, have been adapted to very selective applications in the preparation of complex epoxides and related products of oxidation. A detailed history and survey of the rather remarkable evolution of dioxirane chemistry and their numerous synthetic applications is presented in Chapter 14 of this volume by Adam and Cong-Gui Zhao. Our objective in this part of the review is to first provide a detailed theoretical description of the electronic nature of dioxiranes and then to describe the nuances of the mechanism of oxygen atom transfer to a variety of nucleophilic substrates. 

Although pyrylium cations in combination with an anion of a strong acid are stable, the presence of a formally charged oxygen atom renders them susceptible to reactions with nucleophiles and the valence bond description indicates that C-2(6) and C-4 are the potential targets for attack (Scheme 4.1). 

Extensive studies of AChE have resulted in the purification and amino acid sequencing of the enzyme from several sources as well as the description of its quaternary structure from x-ray crystallographic and molecular modeling studies (38). To understand the mode of action of AChEls, it is necessary to examine the mechanism by which AChE catalyzes hydrolysis of acetylcholine. This enzymatically controlled hydrolysis parallels the two chemical mechanisms for hydrolysis of esters. The first mechanism is acid-catalyzed hydrolysis, in which the initial step involves protonation of the carbonyl oxygen. The transition state is formed by the attack of a molecule of water at the electrophilic carbonyl carbon atom. Collapse of the transition state affords the carboxylic acid and the alcohol (Fig. 12.11). The second mechanism, base-catalyzed hydrolysis, involves the nucleophilic attack ... 

The C-0 and C-N bond in the product of the oxidations with oxygen and nitrogen donors forms by either nucleophilic attack on the coordinated olefin or by insertion of the olefin into a palladium-oxygen or palladium-nitrogen bond. More detailed descriptions of the nucleophilic attack on coordinated ligands were provided in Chapter 11 and a more detailed description of migratory insertions was provided in Chapter 9. These reactions are discussed in the context of the effect of additives on the stereochemistry of the catalytic processes in several earlier sections on the Wacker process. Henry conducted the same stereochemical study for reactions of alcohols with the resolved allylic alcohol in Scheme 16.24 as was conducted for reactions of water. The results of these experiments were similar to those on the reactions of water. ... 

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