Oxene mechanism

Filatov, M., Shaik, S., 1998b, Theoretical Investigation of Two-State-Reactivity Pathways of H-H Activation by FeO+ Addition-Elimination, Rebound , and Oxene-Insertion Mechanisms , J. Phys. Chem. A, 102, 3835. 

Biological systems have learned to generate and tame highly reactive chemical species such as NO, FT, 0 2, HO, and oxene, in other words to produce and trap them in nano-environments where they can react productively rather than deleteriously. But biological systems have also learned a contrasting strategy, which is to enhance the chemical reactivity of a mild molecule such as water. As illustrated in this book, a diversity of enzyme mechanisms have been discovered by nature to render water more nude-... 

The active metallic species is thought to be the oxene, Por—M=0 the mechanism is discussed in terms of the competing reactions of this species and the superior performance of the Fe(III) over the Mn(III) system is attributed to the faster oxygen transfer from the oxene to the sulfide or sulfoxide. 

Formic acid treatment of a mixture of epoxide 478 with 475 provided the cephalosporin C lactone 480 together with 478. The detailed mechanism involving the addition of iron(IV)-oxene to the double bond shown in equation 285 has been proposed567. 

Figure 6.2 The mechanism of cytochrome-c-peroxidase complex formation, (a) Native enzyme, (b) Activated complex with the acid-base catalytic function of distal histidine (His) and stabilization of negative charge by arginine (Arg) residue of the active site, (c) Hypothetic intermediate oxene complex, (d) Complex I after intramolecular electron regrouping of oxene complex with Fe4+ and free radical X fragment formation.

By analogy with carbenes, this mechanism is known by the name oxene mechanism . The diagram shows that the oxygen molecule reacts with the reduced flavin is activated and... 

Ferrvl rebound mechanisms suggests that the formation of the ferryl-oxene structure is similar to those for Compound I (Por-FeIV-0) in the peroxidase reaction (Fig. 3.8). This mechanism (Groves and McClusky, 1976 Groves and Subramanian,... 

However, a significant fraction of the prodncts from this substrate (75% for HOOH and 80% for t-BnOOH, Table 10) is the resnlt of a dioxygenation to give PhCHO and MeCHO. When m-ClC6H4C(0)00H is the oxidant, only 10% of the PhCH=CHMe that reacts is dioxygenated. These resnlts are consistent with the proposition that the end-on configuration of the Fe [m-ClC6H4C(0)00H] + adduct has the most oxene character and favors O-atom transfer to PhCH=CHMe and that the Fe (HOOH) + and Fe (t-BnOOH) + addncts react via a biradical mechanism (12, Scheme 3). 

The trioxolane was also shown to effect epoxidation of olefins86). This was differentiated from the peracid pathway by generating the intermediate at — 100 °C. Warming this system to — 50 °C in the presence of olefins resulted in the formation of epoxides. The corresponding experiment with peroxyacetic acid at — SO °C showed no epoxidation of the olefins. The trioxolane was also shown to effect hydroxylation of tert-butylphenol to tert-butylcatechol, in agreement with an oxene transfer mechanism. 

Late transition metal ions that can accommodate a two-electron rise in their oxidation state, like Cr(III), Mn(III), and Fe(III), and likely Ru(I), operate by a redox mechanism of epoxidation. They receive an oxygen atom from a TO to form an oxene species (MO) which then transfers the oxygen to an olefin by the intermediacy of a metallacycle, or a radical or cation species. Interestingly, these systems are not inhibited by water or alcohol as are the Lewis acid metals. 

The high degree of electrophilicity of the oxene intermediate would facilitate hydrogen-atom abstraction from substrates such as the methyl group of N,N-dimethylaniline to generate a "crypto-hydroxyl metal center able to undergo the well-known "oxygen rebound mechanism.22... 

Oxene is a rather electrophilic species it is neutral but has only six electrons in its outer shell. Its detailed reaction mechanisms are beyond the scope of this chapter, but some indications will be given when discussing the various reactions of oxidation catalyzed by cytochromes P450. 

Scheme V. Proposed mechanism for CCP compound I formation, (a) The native enzymes (b) the activated complex with the distal histidine operating as an acid-base catalyst and the active site arginine stabilizing a developing negative charge on RO-OFe (c) the hypothetical oxene intermediate (d) Compound I after the intramolecular electron rearrangement of (c) to give Fe(IV) and a hee radical, X.

As mentioned in the introduction, Mb also forms an Fe =0 heme on reaction with peroxides (7). Of particular interest is the mechanism of proton delivery to the oxene ligand on reduction because the distal heme pocket of Mb lacks proton donors (Figure 2b) and is isolated from the bulk solvent (43), unlike CCP where the hydrated substrate channel connects the distal heme cavity to the solvent (44). Thus, it is expected that solvent-assisted PT to the heme of Mb at physiological pH is severely restricted. A spectacular example of solvent-assisted PT to a buried redox center has been highlighted in the X-ray structure of the bacterial reaction centers from Ehodobacter sphaeroides (45). A narrow hydrated channel extends from the cytoplasmic side of the reaction center to quinone Qb, which is buried -23 A in the L-subunit. 

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