Metal-oxo species

The formation of ROO and RO radicals, and M" =0 species is expected when phthalocyanines and porphyrins are used as catalysts (21, 22). The formation of the epoxide, Fignre 49.1, may be associated with the attack of metal oxo species (Fe = O) to the double bond (23). For a-pinene this attack is possibly favored by its rigid structure that causes an orbital overlapping, making the allylic hydrogen abstraction difficult (24). 

H. B. Gray Work on the excited states of soluble metal oxo species that in a sense are the homogeneous-solution analogues of Ti02-type materials is a promising direction to take, in my view. 

Terminal metal oxo species are stable and ubiquitous. (d° and d configurations)... 

Terminal metal oxo species were unknown until 2004. 

Terminal metal oxo species (mesityl)3lr-oxo are important and intensely investigated. 

The results prompted Jefford and coworkers to re-examine the iron(II) degradation of artemisinin in aqueous acetonitrile with iron(II) chloride (Scheme 10), a system they suggested was closer to the physiological conditions than iron(II) bromide in THF. They reported that iron(II) chloride catalysed isomerization of artemisinin to afford the same products identified by Posner (13 and 21), except that deoxyartemisinin 3 was not observed. When the reaction was carried out in the presence of cyclohexene, none of the expected epoxide was produced, which suggested (in sharp contrast to Posner s results) that a high-valent metal oxo species was not involved. 

These PVP polymers provide a "proximal effect" without addition of free pyridine in the reaction mixture. Different studies have shown that only one pyridine per manganese catalyst is sufficient to enhance the rate of the catalytic oxygen atom transfer from the high-valent metal-oxo species to the organic substrate. The advantage of PVP polymer over a cationic Amberlite resin (see Scheme II for structures) have been recently illustrated in the modeling of ligninase (11). 

Any catalytic sequence needs to fulfill certain thermochemical boimdary conditions as far as the elementary steps are concerned. For illustration, consider the oxygenation of an organic substrate S by a transition-metal oxo species [M]0 according to reaction 1, where [M] stands for a bare or Hgated, neutral or charged metal fragment ... 

It goes without saying that mononuclear metal-oxo species can only be considered as first-order models for the processes occurring in partial oxidations... 

Some of the early transition metals are known to form mononuclear peroxo complexes. These complexes are not formed by reacting a metal complex in a low oxidation state with dioxygen, as this usually results in the formation of metal oxo species, but rather by reaction of a metal complex in a high oxidation state, eg. TiIV, Nbv or MoVI, with the peroxide anion. This frequently leads to more than one peroxide ligand per metal centre. 

Iodosylbenzene has been extensively utilized over the last few years for its ability to cleanly transfer oxene oxygen atoms to metals and possibly generate high-valent reactive metal-oxo species. PhIO has been successfully used instead of 02+NADPH in conjunction with cytochrome P-450 to hydroxylate alkanes,81 and has found a variety of interesting applications as a two-electron oxidant in the presence of first-row transition metal porphyrins. 

The metathetical reaction of C02 with metal (Ti(IV), U(V)) arylimido-complexes led to an arylisocyanate and a metal-oxo species [4g, 115a]. In these processes, which take place under very mild conditions (ambient temperature and pressure), the metal center acts as the sink for unrequired oxygen. A common feature which characterizes these transformations from a mechanistic point of view is the intermediate formation of a metal carbimato-species through the [2+2] cycloaddition of C02 with the metal-imido complex (Scheme 6.23). By cycloreversion, the four-membered aza-metalla-cycle then converts into isocyanate and a stable metal-oxo complex. These processes, at least formally, are reminiscent of the reaction of imino-phosphoranes with C02 to give isocyanates and carbodiimides [115b,c]. 

Figure 13 Formation of high-valent metal-oxo species by heterolytic cleavage of the O—O bond of reduced 02 (or of H202) advantageous role of H+ and acylating agents. (M = Fe or Mn.)...

The condensation of metal-oxo species with hydrazines TEq. (18)1 (dtc = S2CNMe2 ). 

With electron-withdrawing groups substituted at the phenyl groups of the MePc macrocycles like tetra-nitro [Pc(N02)4] and perchloro-phthallocyanine (PcCli4), the metal-oxo species of the MePc complex becomes considerably more active, also as... 

The characteristic feature that becomes apparent is nature s use of high-valent metal-oxo species that form during dioxygen activation. Such species are aggressive oxidants. Hence, their creation is tightly coupled to the presence of substrate in a confined space, which is absolutely critical for the reactions to proceed in a controlled manner. 

Fig. 11 Phase transitions in mesoporous solids a-d, lamellar-hexagonal e-f, hexagonal-cubic. The circular objects around the surfactant assemblies are the metal-oxo species (from Neeraj and...

Fig. 4 Schematic representation of lamellar—.hexagonal phase transformation (a through d) and the hexagonal—reubic transformation (e and f). The shaded circles around the surfactant aggregates represent the inorganic species (generally metal alkoxides or other metal-oxo species).

The cis-trans alkene isomerization observed in the transformation of conjugated alkenes gave rise to proposals of several different reaction mechanisms. Four different pathways for the epoxidation of alkenes by the manganese metal-oxo species have been proposed (Scheme 9) in order to explain the experimental results. The two concerted pathways (b,c) involve formation of a three-membered or four-membered transition state that involves the alkene and either the transferring oxygen or the Mn=0, respectively, of the formal Mn(V) species. The other pathways are stepwise mechanisms one proceeds via bond formation between the alkene and the... 

Ogliaro, F., De Visser, S.P., Groves, J.T., and Shaik, S. (2001) Chameleon states. Fligh-valent metal-oxo species of cytochrome P450 and its ruthenium analogue, Angew. Chem., Intern. Ed. 40, 2874-2878. 

Oxidative cyclizations of dienes and polyenes mediated by transition-metal-oxo species to yield tetrahydrofurans and tetrahydropyrans have been reviewed by Piccially <2007S2585>. A multigram synthesis of diastereomerically pure tetrahydrofuran diols that applies the oxidation of 1,5-dienes has been published <2007S2751>. 

Theoretical calculations imply that gaseous oxygen is adsorbed onto the catalyst as an q -peroxo species. However, it is thought that an superoxo species may be the dominant form, if the oxide species is stabilized by interactions with neighboring metal-oxo species [1]. 

Groves has shown how a variety of model complexes, notably iron porphyrins, can catalyze a series of oxidation reactions of the monooxygenase type with 0x0 donors such as PhIO as primary oxidants. The intermediacy of metal-oxo species is indicated. 

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