Oxygen reaction with copper complexes

Oxygen radical anion forms excited-singlet oxygen in different pathways, e.g. by a reaction with copper-cysteine-oxygen complex to yield the excimer (02)2- The computerized kinetic equations derived from this scheme allowed predictions in respect of the chemiluminescence intensity as a function of the oxygen and cysteine concentrations and as a function of time these were satisfactorily confirmed by the ex-... 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Recently analogous results were obtained in the study of copper chelate complexes containing hydrocarbons(30). It is well known that the latter are weak Lewis acids and direct reaction with copper chelate complexes is not established.However, the addition of small amounts of bases causes the decomposition of copper chelate complexes if they contain sulphur in their chromophore. In complexes containing oxygen and/or nitrogen atoms in the chromophore, similar reaction did not occur. The formation of copper(II) halide and a resin-like organic residue was established as a final reaction product. 

Haseloff, R.F., Ebert, B., Wischnewsky, G., 1991. Reactions of oxygen free radicals with copper complexes in pyridine differentiation between superoxide and hydroxyl radicals. Anal. Chim. Acta 243, 221—225. 

The reaction of binuclear copper complexes with oxygen as models for tyrosinase activity was also markedly accelerated by applying pressure (106408 ). Tyrosinase is a dinuclear copper protein which catalyses the hydroxylation of phenols. This reaction was first successfully modeled by Karlin and co-workers (109), who found that an intramolecular hydroxylation occurred when the binuclear Cu(I) complex of XYL-H was treated with oxygen (Scheme 5). 

Carbonyl ylides can be viewed as an adduct between a carbonyl group and a carbene and, in fact, some ylides have been prepared this way (see above). The application of carbonyl ylides to the synthesis of complex natural products has been greatly advanced by the finding that stabilized carbenoids can be generated by the decomposition of ot-diazocarbonyl compounds with copper and rhodium complexes. The metallocarbenoids formed by this method are highly electrophilic on carbon and readily add nucleophiles such as the oxygen of many carbonyl derivatives to form carbonyl ylides. This type of reaction is in fact quite old with the first report being the addition of diazomalonate and benzaldehyde (33,34). 

---