Concerted oxygen transfer

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... 

The V(V) oxidation is about 10 and 10 times slower than those by Mn(ril) sulphate and pyrophosphate respectively. The [V(V)] dependence indicates two concerted one-equivalent oxidations one possible mechanism involves oxygen-transfer, viz. 

The allyl 2-nitrophenyl sulphoxide is apparently not an intermediate in this reaction as the MS of the latter compound is dominated by a cleavage reaction giving rise to C3H5+, whereas the ions corresponding to a loss of the HSC>2 radical apparently are of low intensity86. Thus, it was concluded that the double oxygen transfer to sulphur most probably should be formulated as a concerted reaction86. 

One mechanistic matter that has caused quite a bit of general consternation about a decade ago concerns the experimental evidence for the involvement of diradical intermediates (proposed as sources for the observed radical products) in dioxirane epoxidations, which were thought to be formed through induced peroxide-bond homolysis by the alkene. Nonetheless, rigorous experimental and high-level theoretical work disposed such radical chemistry in the epoxidation of alkenic substrates. The latter computations unequivocally confirm the established concerted mechanism, in which both CO single bonds in the incipient epoxide are concurrently formed by way of an asynchronous, spiro-structured transition state for the oxygen transfer. 

Discussions of the mechanism of the oxygen transfer to the double bond have led to controversy. Depending on the substrate and additives, the formation of side products with trans stereochemistry points to a radical mechanism, whereas alkyl-substituted olefins stereoselectively give only cis products via a concerted mechanism. 

The observed ESR spectra strongly support the interpretation that no electron transfer occurs between a skatole anion and Fe(III)-porphyrin in the binary system in toluene at room temperature. On the other hand, under the same conditions electron transfer from skatole anion to oxygen occurs in the ternary complex (II). This is an activation mechanism of molecular oxygen in the Fe(II)-porphyrin-catalyzed oxygenation of skatole. We have termed this type of electron transfer the cooperative (or concerted) electron transfer (COET) see Reaction 5). When the electron accepting power (electron affinity) of the accep-... 

The transfer of oxygen to alkene may occur by several different mechanisms (Scheme 1.15). Oxygen radicals may be formed as intermediates when radical stabilizing groups are attached. This mechanism is supported by the fact that czs-alkene gives both cis- and frans-epoxide. The formation of the metallaoxetane as an intermediate is also proposed by Norrby et alP However, concerted oxygen delivery has also been proposed. 

The mechanism of concerted electrophilic epoxidation is still a matter of debate. Two exemplary transition-state structures, (a) and (b), for symmetric oxygen transfer from peroxy compounds (or oxaziridines) to the C-C double bond are the most generally accepted. In (a) the double bond is located in the plane of the breaking bonds of the transferred oxygen in (b) it is perpendicular to the plane of the breaking bonds of the transferred oxygen. 

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