Oxides dioxygen adducts

The dioxygen adduct so formed is highly reactive and will catalyse the oxidation of phosphine to phosphine oxide and isocyanide to isocyanate (77). But clearly a survey of these reactions is not within the scope of this review and is provided elsewhere (7, 77). 

Oxidation of 3,5-di-tert-butyl catechol to 3,5-di-tert-butyl-o-benzoquinone by 02 is catalyzed by the Ir111 catecholate complex (124).212 The authors suggest that formation of the dioxygen adduct, as in reaction Scheme 16, is an important step in the process. 

Once the oxidative-addition reaction of dioxygen to metal d -ions has occurred, the essentially electrophihc dioxygen becomes a nucleophilic peroxide ligand. Since the oxidation of substrates is associated with electron transfer from the substrate to the oxidant, i.e. in this case the dioxygen adduct, effective oxygenations require a further activation to transform the nucleophihc peroxide into an electrophihc species prior to the oxygen transfer. 

Pentadentate macrocyclic ligands L13-L15 accommodate Ni(II) ion with simultaneous dissociation of the two amide protons to form the square-pyramidal high-spin complex of [NiH 2L]°, 27. These complexes are air-sensitive and show a very low Ni(II)/Ni(III) oxidation potential of +0.24 V vs SCE (72, 73). They form 1 1 dioxygen adducts, 27-02, at room temperature in aqueous solution, which are formulated as Nilu-02 based on the EPR spectral data. The magnetic moment of the 1 1 02 adduct is 2.83 BM, which is interpreted in terms of weak interactions of Ni(III) with the superoxide where the spin coupling is weak. The oxygen uptake reaction is first-order with respect to both [02] and [NiH 2L]° in aqueous solutions and yields a second-order rate constant... 

These studies are part of a rare family of examples of the chemoselec-tive oxidation of catechols (150). The identification of the catalyst and the interception of the catalyst-dioxygen adduct are of particular relevance when the chemistry of catechol dioxygenase and tyrosinase enzymes is concerned. 

Dioxygen oxidizes VC13 or VCl(salen) to vanadium(IV) with the intermediate formation of dioxygen adducts,235 with equilibrium constants of the order of 102 they are involved in the catalysis of oxygenation of 3,5-di-r-butylpyrocatechol.236... 

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