Oxygen atom transfer nucleophiles

Acid-catalyzed ester hydrolysis can occur by more than one mechanism, depending on the structure of the ester. The usual pathway, however, is just the reverse of a Fischer esterification reaction (Section 21.3). The ester is first activated toward nucleophilic attack by protonation of the carboxyl oxygen atom, and nucleophilic addition of water then occurs. Transfer of a proton and elimination of alcohol yields the carboxylic acid (Figure 21.8). Because this hydrolysis reaction is the reverse of a Fischer esterification reaction, Figure 21.8 is the reverse of Figure 21.4. 

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. 

Early studies of these complexes focused primarily on the investigation of their oxygen-atom-transfer reactivity. Relatively little success was achieved, however. The peroxo fragments in these complexes exhibit nucleophilic character, and, therefore, they are generally ineffective oxidants for reactions with synthetically interesting electron-rich substrates such as alkenes and sulfides. Most of the known reactivity involves electrophilic substrates that, in many cases, insert into the Pd-0 bond of peroxopalladium(II) species (Fig. 4) [105,118]. 

Ans. Ethylene to acetaldehyde, cyclohexane to adipic acid, p-xylene to tere-phthalic acid, and propylene to propylene oxide. For the first, redox chemistry of palladium and nucleophilic attack on coordinated ligand. For the second and third, radical chain, and for the fourth, oxygen atom transfer. 

Oxygen atom transfer (OAT) reactions catalyzed by oxorhenium(v) compounds have been comprehensively reviewed. Some recent developments include a focus on nucleophilic assistance in this process, and a study in which the ligand-substitution reaction that precedes OAT is examined in detail. In the former report, oxygen atom transfer from pyridine A -oxide to ttiphenylphosphine catalyzed by [CH3ReO(mtp)PPh3] (ReX), where mtpH2 is 2-(mercaptomethyl)thiophenol, has been monitored. Reaction was carried out in benzene and following the absorbance of pyridine A -oxides, (PyO), by UV spectrophotometry. The rate law was established to be ... 

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