Superoxide olefin formation

The reaction of superoxide ion with carbon tetrachloride is important for olefin epoxidations. This reaction includes the formation of the trichloromethyl peroxide radical Oj" + CCI4 —> Cl + CI3COO. The trichloromethyl peroxide radicals formed oxidize electron-rich olefins. The latter gives the corresponding epoxides. This peroxide radical is a stronger oxidizing agent than the superoxide ion itself (Yamamoto et al. 1986). 

DMF)Ru(0EP)02 (8, 135) can be formed from molecular 02, while Ti(0EP)02 (136) and Mo(TPP)(02)2 (137) can be made from peroxide addition [TPP = tetraphenylporphyrin, OEP = octaethylporphyrin]. The Ru system is ineffective for oxidation of terminal olefins at least under the mild conditions (1 atm 02, 35°C) studied thus far even the ubiquitous substrate triphenylphosphine is not oxidized catalytically because of formation of a relatively inert Ru(OEP)(PPh3)2 complex (138). The catalytic potential for 02 activation by Ru(II) porphyrins compared with Fe(II) porphyrins seems considerable, at least in principle, in view of a more readily accessible oxidation state of IV (139) this could circumvent the unfavorable one-electron reduction of 02 to superoxide (140). Such systems seem promising generally in terms of the multi-electron redox processes that 02 displays (141). 

Among the suggestions for the [2 4- 2] cycloaddition mechanism of 02 to olefins, Foote etal. [39,40] proposed a nonconcerted pathway involving a rate-limiting electron-transfer process with the formation of an olefin radical cation and superoxide ion complex (Scheme 1). 

In the effort to find confirmation on Foote s original mechanistic proposal [84] and discriminate among these two different pathways, a great deal of experimental proofs were achieved. First of all, the DCA and/or 9-cyanoanthracene (CNA)-sensitized reactions on aryl-olefins were studied under inert atmosphere by flash spectroscopic techniques obtaining clear evident for the formation of both olefin radical cations and cyanoaromatic radical anions [95]. In the presence of oxygen, the cyanoaromatic radical anions were rapidly removed, supporting the very rapid formation of superoxide ion and so its direct involvement in these photoinduced oxygenations. 

Radical cations are probably involved in the photooxidation of olefins [1-7, 24, 50-54, 56, 61, 66] (Eqs. 2 and 5). Many oxidizable arenes and dienes are, similarly, oxidatively cleaved or rearranged by routes consistent with initial formation of a radical cation. As we saw above, the observed oxidative cleavage product probably derives from the combination of a surface-bound radical cation with superoxide or adsorbed oxygen. With alkanes or simply substituted alkenes, however, the capture of a photogenerated hole is often thermodynamically forbidden. Thus, instead of radical cations, radicals formed by hydrogen atom abstraction by an activated oxygen species dominate the observed chemistry. With alkanes, oxygenation at sites... 

As mentioned earlier in the discussion of exciplex formation, electron transfer between an excited state species and a ground state molecule (Equation 2.8 and Equation 2.9) is frequently observed in the photochemistry of systems containing an electron donor-acceptor combination. As a result, a pair of radical ions is formed that react with oxygen but with different rates. The reaction of ground state oxygen with radical anions occurs rapidly and yields superoxide anion (Equation 2.16). The superoxide then adds to the radical cation forming D02 (Equation 2.17). When D is an olefin, D02 is a dioxetan that is liable to cleave to yield ketones as products. 

Electrocatalytic oxidations (mainly epox-idation) of alkenes by manganese porphyrins [77, 78] and a Schiff-base [79] and iron and cobalt porphyrins [78] have been achieved. Hydrogen peroxide or the superoxide ion (O ) was generated electrochem-ically by reduction of dioxygen in solvents containing an acid or acid anhydride, the metal compounds as catalysts, and olefins as substrates, in the presence or absence of an axial base. The reaction was believed to take place through the formation of a high valent metal 0x0 porphyrin, produced... 

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