Peroxo-oxygens

The electropositive metal center polarizes the peroxo group so that is much more electrophilic than free peroxide anion. Evidence has been accumulated, and it will be summarized later, that such polarization happens to the extent that nucleophilic reagents can attack a peroxo oxygen when it is coordinated to a d° metal. In other words, this situation... 

The transition state in the above scheme differs from the cyclic titanium peroxo complex proposed earlier (217). In the earlier mechanism, any of the two peroxo oxygens in the Ti-O-O-H (bound end-on) could have been inserted into the C=C bond, and accordingly two isomers would be possible. They have never... 

An important finding is that all peroxo compounds with d° configuration of the TM center exhibit essentially the same epoxidation mechanism [51, 61, 67-72] which is also valid for organic peroxo compounds such as dioxiranes and peracids [73-79], The calculations revealed that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen center (via a TS of spiro structure) is preferred because of significantly lower activation barriers compared to the multi-step insertion mechanism [51, 61-67]. A recent computational study of epoxidation by Mo peroxo complexes showed that the metallacycle intermediate of the insertion mechanism leads to an aldehyde instead of an epoxide product [62],... 

In summary, one can identify three factors that mainly affect the epoxidation activity of a TM peroxo complex (i) the strength of the M-0 and 0-0 interactions, (ii) the electrophilidfy of the peroxo oxygen centers and the olefin, and (iii) the interaction between the tt(C-C) HOMO of the olefin and the peroxo a (0-0) orbital in the LUMO group of the metal complex to which we will also refer as the relevant unoccupied MO, RUMO (Figure 5). 

Density functional calculations reveal that epoxidation of olefins by peroxo complexes with TM d° electronic configuration preferentially proceeds as direct attack of the nucleophilic olefin on an electrophilic peroxo oxygen center via a TS of spiro structure (Sharpless mechanism). For the insertion mechanism much higher activation barriers have been calculated. Moreover, decomposition of the five-membered metallacycle intermediate occurring in the insertion mechanism leads rather to an aldehyde than to an epoxide [63]. 

At odds with other similar peroxo metal complexes, V0(02)pic(H20)2, 36, performs non-selective epoxidation reactions. On this occasion Mimoun proposed a mechanism where a homolytic rupture of one metal-peroxo oxygen bond produces the active oxidant (Scheme 14). When aromatic substrates are allowed to react with 36, hydroxylation reaction takes place by way of the same active species as indicated in Scheme 15. 

A modification of the mechanism that involves the hydroperoxo titanium complex and one solvent molecule has been proposed that involves the formation of a stable cyclic titanium peroxo complex (Clerici et al., 1993). In this case, the two peroxo oxygens are not equivalent, and thus two intermediates would be possible ... 

SCHEME 10.5 Hydrogen bond between one peroxo oxygen and a hydrogen of the amino group. 

The current understanding for molybdenum and rhenium is that the involved species are bisperoxo- and hydroperoxo complexes, and that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen is significantly preferred over the two-step insertion mechanism proposed by Mimoun. 

Both of these species are believed to function through an intermediate known as the Venturello complex (Figure 2.26) 84 one of the peroxo oxygens is... 

Selective epoxidation of olefins by vanadium(V) alkyl peroxo complexes has also been reported (76). These complexes are very effective stereo-selective reagents for the transformation of olefins into epoxides. The mechanism consists of binding of the olefin to the metal to displace one of the peroxo-oxygen atoms, nucleophilic attack of the bound oxygen atom on the coordinated electron-deficient olefin, dissociation of the epoxide, and reaction of the remaining vanadium intermediate with... 

The consumption of protons in the conversion of halide to hypohalous acid, [Equation (4.10a)], and the fact that protons are needed as a co-catalyst in the oxygenation of sulfides to sulfoxides [Equation (4.11)] is suggestive of an essential role of protonation in the activation of the peroxo form of the haloperoxidases [Equation (4.12)]. For the peroxo form, the best agreement between calculations and structural data is obtained for a strongly distorted trigonal bipyramid, with the Ne of the imidazole moiety in the axial and one of the peroxo oxygens in the pseudoaxial position (a in Scheme 4.4).[ ] Protonation can occur on the pseudoaxial or the equatorial peroxo oxygen, or on the second... 

Activation of peroxide through formation of a peroxo and hydroperoxo intermediate (centre), and catalysis of the oxygenation of bromide (bottom) and sulfide (top). In both reaction paths, a hydroperoxo complex is the active catalyst and oxygen transfer occurs through direct attack of the substrate to the nonprotonated peroxo oxygen (Scheme 4.4 , and Scheme 4.9). 

The current understanding of oxygen activation by P-450 is summarized as follows (Scheme II) (Ic, 5a, b) (i) incorporation of a substrate to the resting ferric state (1) of the active site of the enzyme to afford the ES complex (2) (//) one-electron reduction of the heme from NAD(P)H via an associated reductase enzyme (Hi) reaction of the reduced heme (3) with O2 to form an oxy complex (4a, 4b) (iv) one-electron reduction of the oxy complex to yield a peroxo complex (5) (v) protonation (or possibly acylation) of the peroxo oxygen (vi) the formation of active species, the so-called oxenoid [FeO + (7)], by heterolytic 0-0 bond cleavage of (6) (v/7) oxygen transfer to the substrate. Thus, the overall stoichiometry can be expressed as in Eq. (2), where R is orR C(0) ... 

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