Peroxo complexes oxygen transfer mechanism

The olefin oxygenations carried out with dioxygen seem to be metal-centered processes, which thus require the coordination of both substrates to the metal. Consequently, complexes containing the framework M (peroxo)(olefin) represent key intermediates able to promote the desired C-0 bond formation, which is supposed to give 3-metalla -l,2-dioxolane compounds (Scheme 6) from a 1,3-dipolar cycloinsertion. This situation is quite different from that observed in similar reactions involving middle transition metals for which the direct interaction of the olefin and the oxygen coordinated to the metal, which is the concerted oxygen transfer mechanism proposed by Sharpless, seems to be a more reasonable pathway [64] without the need for prior olefin coordination. In principle, there are two ways to produce the M (peroxo)(olefin) species, shown in Scheme 6, both based on the easy switch between the M and M oxidation states for... 

Despite of the common reaction mechanism, peroxo complexes exhibit very different reactivities - as shown by the calculated activation energies -depending on the particular structure (nature of the metal center, peroxo or hydroperoxo functionalities, type and number of ligands). We proposed a model [72, 80] that is able to qualitatively rationalize differences in the epoxidation activities of a series of structurally similar TM peroxo compounds CH3Re(02)20-L with various Lewis base ligands L. In this model the calculated activation barriers of direct oxygen transfer from a peroxo group... 

To analyze the epoxidation activity of these peroxo complexes, we characterized for each complex TSs of oxygen transfer to the model olefin ethene. We compared the two mechanisms mostly discussed in the literature, namely insertion [2, 8, 9, 97] and a direct attack of the peroxo group on the olefin [11] (see Section 1). Direct oxygen transfer can be envisaged to occur... 

All these data are in favor of a homolytic mode of oxygen transfer from Vv alkyl peroxides to hydrocarbons, and the mechanism suggested in Scheme 4, based on that of oxidation by Vv-peroxo complexes (Scheme 2), was tentatively attributed to a biradical V17 — OR—O species which can add to arenes and abstract hydrogen atoms from alkanes. It is probable that the absence of a releasable coordination site adjacent to the triangular alkyl peroxide group in (22) precludes the possibility of the alkene coordination to the metal and therefore prevents its heterolytic epoxi-dation. 

The mechanism of oxygen transfer from the peroxide (29) to the substrate still remains a matter of controversy. Current opinions favor the formation of a high-valent FeO (or Fe =0 or Fe —0-) (30) active species, which acts as a homolytic hydrogen abstractor from the substrate. An alternative mechanism considers the Fe -peroxide complex as the actual hydroxylating reagent, by analogy with the reactivity of vanadium(V)-peroxo and -alkylperoxo complexes and that of chromium(Vl)-peroxo complexes (see below). ... 

The reaction mechanism for the oxidation of olefins by metal ions / complexes in homogeneous medium is studied widely and the formation of peroxo and oxo complexes was suggested to be responsible for the transfer of oxygen to the substrate. Vaska et al., have reported formation of peroxo complexes when dioxygen is bound covalently to the metal centre [16]. The formation of 0X0 complex and the transfer of oxygen via this route has been suggested by Taqui Khan et al., in the oxidation of olefins catalysed by Ru(III) complex in homogeneous medium [17]. On the basis... 

In spite of intensive efforts, the knowledge accumulated to date about polar oxygen-transfer reactions on peroxo, hydroperoxo, and alkylperoxo complexes is not yet sufficient to decide whether to agree with the direct attack of the olefin at the positively polarized oxygen center [9] ( butterfly mechanism without an organometallic intermediate, step A in Scheme 1) discussed by Chong and Sharpless, or with the mechanism postulated by Mimoun based on an olefin coordination. 

A similar process mediated by a catecholato-bipyridyl (phenanthrolyl) Cu(II) complex (Scheme 8) was found to be oxygen-dependent in keeping with intra-complex dioxygen transfer 47). Once again, other mechanisms are possible. Oxygen could be reduced to hydroperoxide and react with the appropriate o-quinone outside the co-ordination sphere 48). The binuclear p-peroxo complex (8) could also be implicated and, if so, it would behave similarly to the catalytically active species encountered in other oxygen-dependent cuprous chloride/pyridine oxidations 49). 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

These copper-mediated reactions very often involve dinuclear intermediates, but detailed mechanistic studies on stoichiometric systems are relatively few. The key features are the formation of p-peroxo or p-superoxo complexes by electron transfer from cop-per(i) to dioxygen. The co-ordinated oxygen may then act as an electrophile to the aromatic ring. A possible mechanism for the ortho-hydroxylation of phenol by dioxygen in the presence of copper catalysts is shown in Fig. 9-29. 

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