Hydroperoxo complexes catalysts

The rate constants (ket) of electron transfer from Fc to [(TPP)M] agree well with those evaluated in light of the Marcus equations [91] for outer-sphere electron transfer (Eq. 4) [215]. Such agreement clearly demonstrates that electron transfer from Fc to [(TPP)M]+ in Scheme 15 proceeds via an outer-sphere pathway. In contrast to this, the ket value of the acid-catalyzed electron transfer from (TPP)Co to O2 is 10 -fold larger than that expected from an outer-sphere electron transfer [215]. Such huge enhancement of the observed rate relative to that calculated for outer-sphere electron transfer indicates the strong inner-sphere nature of acid-catalyzed electron transfer from (TPP)Co to O2 this should result in formation of the hydroperoxo complex, [(TPP)Co02H]+ (Scheme 15, M = Co). Other metalloporphyrins (M = Fe and Mn) can also act as efficient catalysts of the reduction of... 

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). 

A different type of chemistry has been realized in the process commercialized by EniChem in 1986, which is based on employment of and the titanium-silicalite TS-1 as heterogeneous catalyst [106, 111]. The hydroxy lation mechanism involves activation of hydrogen peroxide via the formation of a titanium hydroperoxo complex, TiOOH, followed by electrophilic oxygen atom transfer to phenol. Methanol and acetone are the solvents of choice to achieve high selectivity. The nature of solvent, phenol concentration, reaction time, and size of catalyst particles affect the CAT/ HQ ratio. The TS-l-based process offers clear advantages in terms of conversion, selectivity, efficiency (see Table 14.1), catalyst separation/recycling, and, hence, environmental impact. 

The activation of On by Co, Mn, and Mo porphyrins has been studied theoretically by Witko and coworkers [568]. In the case of the Co porphyrin, the active forms of the catalyst are found to include an end-on complex with dioxygen and the hydroperoxo form, but not an oxo O=Co(porph) complex because water will not easily dissociate. In the case of the Mn porphyrin, the side-on, hydroperoxo, and 0X0 types of ligands are possible. In the case of the Mo porphyrin, all forms, the side-on, end-on, hydroperoxo, and oxo forms, are possible. 

Cu BSP], the preoxidized form [Cu BSP(BF4)], and and the pre-reduced form [Cu BSP] . The alcohol first coordinates to the catalyst precursor [Cu BSP], leading to the formation of a metal-phenoxyl radical complex. This species undergoes the substrate C -H proton abstraction by the radical, followed by a rapid intramolecular electron transfer with reduction of Cu to Cu. After this rate-determining step, the copper(I) species reacts with dioxygen to form an hydroperoxo copper(II) with the release of the carbonyl product. Finally, dihydrogen peroxide is replaced by a new alcohol molecule to give back the active species (Fig. 11). 

Solid catalysts may also be used for reactions implying oxidants differ by dioxygen. The most popular case is that of titanium-silicalite catalysts for oxidation with hydrogen peroxide. The active species in the presence of water has been characterised to be a side-on peroxo complex characterised by a Raman-detected 0-0 stretching at 618 cm Upon drying, this species converts into a hydroperoxo species characterised using IR by an 0-0 stretching at 837 cm and a broad OH band at 3400 cm ... 

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