Metal oxenes

Figure 2.16 Formation of a metal oxene species in the presence of hydrogen peroxide.

Metal oxene compounds are formed from the interaction of a metal centre with a peroxygen and this reaction is classed as a two-electron transfer oxidation (Figure 2.16). 

All of these characteristics must be present in the hydroxylating metal-oxene system. An interesting idea is that the oxene... 

We will see that similar conclusions with respect to the mechanism (possibility of insertion as well as hydrogen atom abstraction) may be made for reactions of oxygen atoms bound to metal ( oxenes ) with alkanes in enzymatic and model chemical reactions. The excited nitrogen atom, N( D), reacts analogously [8f,g]. 

Metal oxenes. Here, the reaction consists of a two-electron formal oxidation of a metal centre by a peroxygen through mono-oxygen transfer ... 

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

The active metallic species is thought to be the oxene, Por—M=0 the mechanism is discussed in terms of the competing reactions of this species and the superior performance of the Fe(III) over the Mn(III) system is attributed to the faster oxygen transfer from the oxene to the sulfide or sulfoxide. 

Metal(V) species derived from the complexes in Table I are rare. In fact, only one such species, L1Cr(V) (presumably a dioxo or hydro-oxo species), has been observed and characterized by ESR and UV-visible spectroscopies (45,69), Figs. 5 and 6. This Cr(V) species, which has a lifetime of several seconds at room temperature, was generated from a hydroperoxo precursor by an intramolecular transformation that closely resembles the proposed, but so far unobserved step in the chemistry of cytochrome P450, whereby the hydroperoxoiron(III) is transformed to the FeIV(P + ) form also known as oxene (P += porphyrin radical cation). All the steps in Scheme 1 for the L1Cr(H20)2+/02 reaction have been observed directly (45,69). 

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. 

As discussed above, iodosylbenzene (Phl=0) oxidizes various transition-metal ions (Mn+) such as manganese, iron, ruthenium, and chromium ions to the corresponding oxo-metal species (0=Mtransfer agents. Likewise, fM-(p-toluenesulfonyl)imino]-phenyliodinane(PhI=NTs) also oxidizes these metal ions to give the corresponding tosylimino-metal species (TsN=M(n+2)+) that undergo nitrene-transfer reaction such as aziridination (Scheme 6B.28) [73],... 

Late transition metal ions that can accommodate a two-electron rise in their oxidation state, like Cr(III), Mn(III), and Fe(III), and likely Ru(I), operate by a redox mechanism of epoxidation. They receive an oxygen atom from a TO to form an oxene species (MO) which then transfers the oxygen to an olefin by the intermediacy of a metallacycle, or a radical or cation species. Interestingly, these systems are not inhibited by water or alcohol as are the Lewis acid metals. 

The high degree of electrophilicity of the oxene intermediate would facilitate hydrogen-atom abstraction from substrates such as the methyl group of N,N-dimethylaniline to generate a "crypto-hydroxyl metal center able to undergo the well-known "oxygen rebound mechanism.22... 

A truly enormous number of different enzymes has been characterized as mixed function oxidases (MFOs) by Mason (162) and as monooxygenases by Hayaishi (107). They can catalyze a wide variety of oxidation reactions including hydroxylation of aromatic and aliphatic compounds, epoxidation of olefins, oxidative decarboxylation, lactoniza-tion etc. (104). According to Hamilton (105), MFO-catalyzed reactions proceed via an oxene mechanism and there is now ample evidence that the oxinoid species is a reduced form of the enzyme (or an enzyme with a reduced cofactor, e.g. metal ions) which reacts with oxygen and the substrate either in one step or more than one (273). 

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