Oxenoid

A more detailed study of the biological oxidation of sulphoxides to sulphones has been reported165. In this study cytochrome P-450 was obtained in a purified form from rabbit cells and was found to promote the oxidation of a series of sulphoxides to sulphones by NADPH and oxygen (equation 56). Kinetic measurements showed that the process proceeds by a one-electron transfer to the activated enzymatic intermediate [an oxenoid represented by (FeO)3+] according to equation (57). 

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

The hydroperoxide could react easily with Fe , followed by the formation of the oxenoid species Fe =0. The formation of trimethylsilanol, which dehydrates to hexam-ethyldisiloxane, is the driving force for the reaction. ... 

These results, as well as rate studies " and kinetic isotope effects ", support a concerted, 5ptra-structured oxenoid-type transition state for the CH oxidations". The original oxygen-rebound mechanism has been discounted (see the computational work in Section I.D). Recently, however, the stepwise radical mechanism was revived in terms of the so-called molecule-induced homolysis , but such radical-type reactivity was severely criticized on the basis of experimental" and theoretical grounds. 

A concerted, spiro-structured, oxenoid-type transition state has been proposed for C-H oxidation by dioxiranes (Scheme 5). This mechanism is based mainly on the stereoselective retention of configuration at the oxidized C-H bond [20-22], but also kinetic studies [29], kinetic isotopic effects [24], and high-level computational work support the spiro-configured transition structure [30-32], The originally proposed oxygen-rebound mechanism [24, 33] was recently revived in the form of so-called molecule-induced homolysis [34, 35] however, such a radical-type process has been experimentally [36] and theoretically [30] rigorously discounted. 

Scheme 5. The concerted oxenoid versus the stepwise oxygen-rebound mechanism for the C-H oxidation by dioxi ranes.

Hamilton674 proposed the oxenoid mechanism shown in Scheme 5 for both the model and enzymatic systems. (The iron-dioxygen complex is assumed to react as an oxenoid species and transfer an oxygen atom to the substrates S.)... 

The oxenoid mechanism implicates the insertion of an oxygen atom (oxenoid) across the C-H bond of the hydrocarbon (Hamilton et al., 1973). This mechanism is evidently energetically preferable, since it is accompanied by the formation of three bonds, two of which, O-H and C-O, are extremely strong and compensate the rupture of the relatively weak C-H and 0-0 bonds. The transition state, however, involves the formation of a three-membered ring with oxygen, whose formation is accompanied by a strain with an energy of about 30 kcal/mole. More over, the insertion of O to C-H or H-H bonds is a symmetrically forbidden process. 

Dioxiranes, generated by the oxidation of ketones with KHSOs, insert an oxygen atom into alkane C—H bonds with retention of configuration by an oxenoid mechanism related to that found for peracids. Tertiary C—H bonds are hydroxylated and react faster than secondary CH2 groups, which are completely oxidized to the ketone. Conversions of up to 50% have been observed." CF3(Me)C02 is a more recently developed reagent of the same type. These easily prepared reagents have considerable promise for organic synthesis. 

Oxenoid K, Kim HJ, Jacob J, Sonnichsen ED, Sanders CR. NMR assignments for a helical 40kDa membrane protein. J. Am. Chem. 

Roosild TP, Greenwald J, Vega M, Castronovo S, Riek R, Choe S. NMR structure of Mistic, a membrane-integrating protein for membrane protein expression. Science 2005 307 1317-1321. Oxenoid K, Chou JJ. The structure of phospholamban pentamer reveals a channel-like architecture in membranes. Proc. Natl. Acad. Sci. U.S.A. 2005 102 10870-10875. 

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