Oxygen atom transfer electronic factors

Perhaps the most interesting of the inner-sphere pathways are those which result in the net transfer of an oxygen atom. The factors governing the viability of this pathway are still speculative. For cobalt-02 adducts, thermochemical considerations suggest that oxygen atom transfer should be accompanied by electron transfer in the reverse direction. Similarly, this pathway should be enhanced for MO2 complexes in which the metal... 

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. 

Perhaps the most significant and at the same time the simplest reaction of the radicals under consideration is dimerization, and the following discussion is confined to this. Other reactions, such as electron or oxygen atom transfer, may be equally significant to their chemistry, but involve mechanistic factors extraneous to the present paper. 

In summary, we may add that bacterial utilization of quinoline and its derivatives as a rule depends on the availability of traces of molybdate in the culture medium [363], In contrast, growth of the bacterial strains on the first intermediate of each catabolic pathway, namely, the lH-2-oxo or 1 II-4-oxo derivatives of the quinoline compound was not affected by the availability of molybdate. This observation indicated a possible role of the trace element molybdenum in the initial hydroxylation at C2. In enzymes, Mo occurs as part of the redox-active co-factor, and all the Mo-enzymes involved in N-heteroatomic compound metabolism, contain a pterin Mo co-factor. The catalyzed reaction involves the transfer of an oxygen atom to or from a substrate molecule in a two-electron redox reaction. The oxygen is supplied by the aqueous solvent. Certainly, the Mo-enzymes play an important role in the initial steps of N-containing heterocycles degradation. 

C. Electronic Factors Influencing Oxygen Atom versus Hydroxyl Transfer 72... 

Jinnouchi and Okazaki performed AIMD studies of the first-electron transfer reaction with 1 hydronium ion, 9 water molecules, and 12 Pt atoms at 350 K as shown in Fig. 13.108 The adsorbed water molecules and the hydronium ion hydrated the adsorbed oxygen atoms, and proton transfer through the constructed hydrogen bonds frequently occurred. When the conformation of these species satisfied certain conditions, the oxygen dissociation with the proton transfer reaction was induced and three OH were generated on the platinum surface (Fig. 14). The authors concluded that the oxygen dissociation tendency is one of the dominant factors for the reactivity of the cathode catalyst. This work demonstrates the power of AIMD that does not require specific assumption in order to describe charge transfer. 

The reduction process of polycycles by lithium metal converts the neutral atoms to anions. The electron transfer is best achieved in ethereal solvents. This enables the stabilization of the lithium cation by coordination to the oxygen atoms of the solvent. The hydrocarbon anion and the cation are linked together by electrostatic forces in which the solvent molecules are also involved, therefore the ion-solvation equilibrium should be considered8. The limiting cases in this equilibrium are free ions and contact ion-pairs (CIP), and in between there are several forms of solvent separated ion-pairs (SSIP)9. In reality, anionic species of aromatic hydrocarbons in ethereal solvents exist between CIP and SSIP. Four major factors influence the ion-solvation equilibrium of lithium-reduced 7T-conjugated hydrocarbons, as observed by H and 7Li NMR spectroscopies8,10. 

That basicity is the dominant factor in determining nucleophilic reactivity in these cases cannot be concluded from these results. Because the nucleophiles are structurally similar, very probably E° would vary in the same way as p Ka does for all of them. If the oxygen atom is replaced by sulfur, the resultant ion is H -IO5 times more reactive, even though the sulfanion is much less basic. This result strongly suggests that electron transfer plays the dominant role. 

---