Oxygen atom transfer, direct

The above scheme includes the formation of the oxoiron(IV) porphyrin cation radical via 0-atom transfer from PhIO within an intermediate complex. The double bond approaches from the side and parallel to the porphyrin ring. This geometry is clearly favorable for the cis-olefins. The epoxidation step consists in oxygen atom transfer directly from the oxoiron(IV) to the olefin within an intermediate that has been detected with mCPBA as oxidant [46]. According to... 

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... 

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... 

We have attempted to find conditions which will allow us to watch the species build up, to isolate it kinetically, and to observe its reactions directly. That does not imply that the intermediate is unreactive. The remarkable feature of the intermediate in your rhenium system is the apparently facile oxygen atom transfer. 

We point out that the mechanism sketched in path A of Scheme 11 is in agreement with the kinetic and spectroscopic data collected from several research groups. On the other hand, a series of contradictions was encountered in fitting the experimental data into the mechanism proposed in path B. Furthermore, several other papers have appeared in the last decade, based on both experimental results and theoretical calculations, supporting an epoxidation mechanism involving a direct oxygen atom transfer to olefins. For selected examples, see References 34, 145-155. 

The direct production of SOz in a reaction such as (73b) was suggested by the independence of the measured S02 yields on the oxygen concentration. NOz is not produced in significant yields (<5%) in the N03-CH3SH reaction (Dlugokencky and Howard, 1988), ruling out a simple oxygen atom transfer or decomposition of the adduct to N02 and other products. 

Subsequently, RR was used to successfully detect structural changes between the oxidized and reduced forms of both DMSOR and BSOR that are consistent with the proposed oxygen atom transfer mechanism of the catalytic reaction (95, 97). These experiments make use of the readily measurable isotopic shifts in vibration frequency between ieO=Mo and lsO=Mo to follow the fate of the oxygen atom removed from DMSO (or BSO) by the Mo. In this way, the clean transfer of 180 from DMSlsO to Mo(IV) to yield the oxidized form of the active site as Mo(VI)=180 was directly observed as well as the substrate-bound intermediate, (DMS180)Mo(IV). Further discussion of the technique of RR applied to metal dithiolenes and dithiolene-containing enzymes is included in Chapter 4 in this volume (98). 

As for the reactions catalyzed by members of this third family of molybdenum enzymes, there are several variations from the principal theme of oxygen atom transfer. Formate dehydrogenase from E. coli catalyzes the oxidation of formate to CO2, a reaction that isotope studies have shown does not pass through a bicarbonate intermediate (Khangulov et al., 1998). Instead, it appears likely that C02is formed by direct hydride transfer from substrate to the molybdenum center. Polysulfide reductase is another molybdenum enzyme that catalyzes a non-canonical reaction the... 

---