Peroxides heterolysis

Whereas TiCU interacts with the peroxide bridge yielding ethers, SnCl4 promotes a selective displacement of the alkoxide to form peroxides. Heterolysis of an 0-0 bond (Hock reaction) furnishes oxycarbenium ion intermediates via 1,2-shifts (path a), whereas acid-catalyzed C-O ionization affords carbenium ions (path b, Scheme 13). 

The central theme that the apoprotein facilitates the scission of the O—O bond is based on the established mechanisms of peroxide heterolysis 165). By invoking concerted proton transfer (s) in the transition state, such schemes illustrate that oxygen-oxygen heterolysis need not be attended by an electrostatically unfavorable charge separation. In addition, they offer some rationale for the observed high entropy of activation in the primary H202-catalase reaction (—25 cal mole" deg ) 166). This should be the case in a rigid lattice of interactions implied in Eq. (20) and formulas (VII) and (VIII). 

The dimethyl sulfide trapping experiments in the alkane functionalization reactions described above implicate the participation of a formally iron(V)-oxo species derived from peroxide heterolysis at the iron center. Similar species have also been proposed in other nonheme iron-catalyzed alkane oxidation systems. " Such species are characterized in heme-containing systems and best characterized as [(porphyrin)Fe=0]+ with oxidizing equivalents stored on the iron (+4 oxidation state) and the porphyrin (radical). However direct spectroscopic evidence for a corresponding nonheme iron complex has only been recently obtained, Interestingly this species is also best described as an iron(IV)-oxo species with a ligand cation radical. 

The well established chemistry of acyclic secondary-alkyl peroxides 12> suggested that bases should catalyse the isomerization of related bicyclic peroxides to cyclic hydroxyketones 62 via abstraction of bridgehead hydrogen and heterolysis of the peroxide bond (Eq. 48). 

Turning to the complexes of copper(II), copper(IV) is not stable and heterolysis of the 0—0 bond of the peroxide to form the copper(IV) oxo complex does not occur. In addition, the Lewis acidity of the copper(II) ion is not high enough to enhance the electrophilicity of the coordinated alkyl- or acylperoxide to promote direct oxo-incorporating reactions. With these points in mind, the inert activity of alkyl- and acylperoxo copper(II) complexes, experimentally observed, is understandable, and it is quite unlikely that the mechanism of copper monooxygenase parallels that of cytochrome P-450. 

The medium also has a strong effect on the peroxides. Acids induce heterolysis of the O—O bond and migration of the aryl or alkyl group to the electron-deficient oxygen [30]. 

The active species is an electrophilic peroxo-metal complex containing an intact hydroperoxide ligand (Figure 14). The high-valent metal acts as a strong Lewis centre facilitating the heterolysis of peroxidic oxygen. 

Metals that are capable of 2e redox changes, typically main group elements and 4d and 5d transition metals, can give heterolysis of a peroxide to form a diamagnetic oxidant that may avoid the radical pathways seen in the case of equation (14-15). O atom transfer to the substrate is possible in this way. Sharpless epoxidation provides an excellent example. In this case rBuOOH is the primary oxidant, Ti(i-OPr)4 is the catalyst precursor and a tartrate ester is the ligand that induces a high ee in the epoxy alcohol formed from an allylic alcohol. This reaction has been successfiiUy developed on an industrial scale. 

What does the peroxide do Why does its presence change the mechanism The peroxide undergoes homolysis of the weak 0-0 bond extremely easily, and because of this it initiates a radical chain reaction. We said that H-Cl in the gas phase undergoes homolysis in preference to heterolysis other types of bond are even more susceptible to homolysis. You can see this for yourself by looking at this table of bond dissociation energies (AGfor X-Y —> X + Y ). 

Davies, A.G. (1961) 0-0 heterolysis intermolecular nucleophilic substitution at oxygen, in Organic Peroxides, Butterworth, London, pp. 128-142. 

Figure 31. Selected stopped-flow data for the disappearance of (HPX)Fe acyl peroxide (X, ) 416 nm and the concomitant appearance of (HP X)Fe =0 (Xmax) 678 nm. Global analysis of the full spectral window (400-700 nm) for the disappearance and appearance traces using a first-order kinetic model gives kohs = (1-9 0.1) x 10 s for 0—0 bond heterolysis.

The second question is about how the the high-valent oxo intermediate forms in both enzymes. For catalase and peroxidase, the evidence indicates that hydrogen peroxide binds to the ferric center and then undergoes heterolysis at the... 

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