Peroxometal complex

The earliest attempts to obtain optically active sulfoxides by the oxidation of sulfides using oxidants such as chiral peracids did not fare well. The enantiomeric purities obtained were very low. Biological oxidants offered great improvement in a few cases, but not in others. Lately, some very encouraging progress has been made using chiral oxaziridines and peroxometal complexes as oxidants. Newer developments in the use of both chemical oxidants and biological oxidants are described below. 

Heterolytic oxygen transfer processes can be divided into two categories based on the nature of the active oxidant an oxometal or a peroxometal species (Fig. 4.10). Catalysis by early transition metals (Mo, W, Re, V, Ti, Zr) generally involves high-valent peroxometal complexes whereas later transition metals (Ru, Os), particularly first row elements (Cr, Mn, Fe) mediate oxygen transfer via oxometal species. Some elements, e.g. vanadium, can operate via either mechanism, depending on the substrate. Although the pathways outlined in Fig. 4.10... 

Similarly, for tertiary amines a distinction can be made between oxometal and peroxometal pathways. Cytochrome P450 monooxygenases catalyze the oxidative N-demethylation of amines in which the active oxidant is a high-valent oxoiron species. This reaction can be mimicked with some oxometal complexes (Ruv=0), while oxidation via peroxometal complexes results in oxidation of the N atom (Fig. 4.93 a and b) [261]. A combination of MTO/hydrogen peroxide can... 

V. Conte, F. Di Furia, Catalytic oxidations with hydrogen peroxide as oxidant. Peroxometal complexes derived from hydrogen peroxide. Some applications in organic synthesis. Catalysis by Metal Complexes 9 (1992) 223-252. 

For selective oxygen-transfer processes, as in, for example, epoxidation, Ru-0x0 species in lower oxidation states have been commonly applied. In general, catalytic systems for oxygen-transfer processes can be divided into two major categories, involving peroxometal and oxometal species as the active oxidant, respectively [1]. The peroxometal mechanism is generally observed with early transition elements whereby high-valent peroxometal complexes of, for example, Mo, and TF, are the active oxidants (Fig. 2, pathway a). Cataly-... 

A unified interpretation of the electronic spectra of 1 1 and 1 2 superoxo and peroxometal complexes has been dveloped by Lever and Gray [177]. The energy levels of the HO radical have been Used to discuss... 

The spectral characteristics of peroxometal complexes are summarized in Table XII. 

Bridging peroxometal complexes are mostly non-planar and have two... 

Doubly bridged peroxometal complexes also exhibit two LMCT bands in the same region (Table XII). Side-on peroxo peroxo complexes show a weak LMCT absorption, which sometimes has a shoulder, or a second, weak band, whose position is quite variable. 

The available 0 NMR data for peroxometal complexes and some nonmetallie peroxides are listed in Table XVI. 

NMR chemical shifts (ppm) and line-widths (Hz) of some "side-on" peroxometal complexes (after Camporeale et al, [259])... 

A number of peroxometal complexes, especially those of Mo and W have been shown by Mimoun et al. to be selective epoxidizing reagents [1-7]. These complexes are usually synthesized from the metal oxide and in the presence of an added ligand (HMPT, DMF,py) and are employed... 

CIS- and trans-isomers, 51 and 52, respectively) rather than cleavage of the double bond [226]. This has some mechanistic implications inasmuch as the side-on peroxometal complexes of this type (cf. Chapter 1) are not likely intermediates in tryptophan-2,3-dioxygenase model systems. 

A mechanistic study has revealed a weak electrophilic character for the competent oxidant, turning to a biphilic nature with respect to sulfoxide oxidation. This behavior is likely to be stimulated by the polynuclear framework, which may foster a dual activation of both the oxidant and the sulfoxide by coordination to proximal Lewis acid sites on the polyoxotungstate surface. The postulated asymmetric binding of the peroxo-ligand, not evolving to a bidentate j -coordination mode, could explain the atypical selectivity behavior, with respect to classical d -peroxometal complexes. 

Conte, V., Di Furia, F. and Modena, G. (1992). Transition Metal Catalyzed Oxidation. The Role of Peroxometal Complexes, in Ando, W. (ed.). Organic Peroxides. John Wiley Sons, Chichester, pp. 559-598. 

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