Copper catecholate systems

Brown and coworkers observed reactions that may parallel the biological ring oxidation process in early studies on copper catecholate systems (2). 

Some mononuclear Cu(II) complexes have been also reported to act as catalysts in the catechol oxidation 146,151,165). The efficiency of these systems is very low and in general it is always possible that the actual species responsible for catechol oxidation is a dimeric species resulting from aggregation of two mononuclear units, as it has been recently shown for an amino-carbohydrate-copper(II) system 156). 

Model studies and much of the developmental research on synthetic catechol oxidation systems has been concentrated on iron and copper catecholate complexes. 

There has been enormous activity in the field of copper(I)-dioxygen chemistry in the last 25 years, with our information coming from both biochemical-biophysical studies and to a very important extent from coordination chemistry. This has resulted in the structural and spectroscopic characterization of a large number of copper dioxygen complexes, some of which are represented in Figure 14.2. The complex F, first characterized in a synthetic system was subsequently established to be present in oxy-haemocyanin, and is found in derivatives of tyrosinase and catechol oxidase, implying its involvement in aromatic hydroxylations in both enzymes and chemical systems. 

Fukuzumi and co-workers described spectroscopic evidence for a ix-rf- ] -peroxo-(Cu )2 species stabilized with a fcidentate nitrogen ligand, but no (catalytic) oxidation behavior towards catechol was noted (a related trinu-clear copper species converted 2,4-di-ferf-butylphenol stoichiometrically towards the biphenol derivative) [224], Stack et al. have described a similar ] -peroxo-(Cu )2 species (28, vide supra) that could be considered a structural and functional model for tyrosinase-activity, as it efficiently reacted with catechol, benzyl alcohol and benzylamine to yield quinone (95%), benzaldehyde (80%) and benzonitrile (70%) [172,173]. This dinuclear per-0X0 species is generated by association of two monomeric copper centers, in contrast to the systems based on dinucleating Ugand scaffolds described above. 

Terminally metallizable dyes (30) are obtained by the interaction of a diazonium salt and a coupling component containing a chelating system, for example salicylic acid, catechol, salicyl-aldoxime or 8-hydroxyquinoline, and their coordination chemistry is typical of these compounds. Such dyes were rarely used as preformed metal complexes but were usually applied to cotton and then converted to their copper complexes on the fibre to improve their fastness to wet treatments. A typical example is the blue dyestuff (31). 

Other reactions show an even greater resemblance to those which occur in biological systems. A typical example is seen in the smooth oxidation of catechol by dioxygen in the presence of mixed pyridine/methanol solutions containing copper(i) chloride (Fig. 9-28). The cleavage products in this reaction are derived from an intermediate 1,2-quinone. 

In conclusion, the mechanism of catechol oxidation by the model compounds is very intricate, which obviously explains often contradictory literature reports on the catalytic behavior of copper(II) complexes. However, despite being sometimes controversial, studies on model compounds offer stimulating results, which improve our knowledge of the structure-activity relationships in natural systems. There is little doubt that the combination of distinct but complementary disci-... 

In special circumstances the metal and a ligand can compete for the spin in a paramagnetic complex. As this alternative involves oxidation-state changes, it is referred to as valence tautomerization or redox isomerization [78]. Such behavior is observed for o-semiquinone complexes of cobalt and manganese [78] recently, a copper(I)-semiquinone-copper(II)-catecholate equilibrium system (7) of biochemical interest has been analyzed by temperature-dependent ESR [79]. 

Further ternary complexes, e.g. [Cu(L-aspartateXL-ornithine)], have been studied spectrophotometrically. A range of spectral techniques have been used to look at the copper(ii) complex of DL-3,4-dihydroxyphenylalanine (dl-DOPA) and copper(ii)-amino-acid-catechol, e.g. (142), systems. 2f-Ray absorption edge spectrometry... 

Dioxygenase enzymes are known that contain heme iron, nonheme iron, copper, or manganese.The substrates whose oxygenations are catalyzed by these enzymes are very diverse, as are the metal-binding sites so probably several, possibly unrelated, mechanisms operate in these different systems. For many of these enzymes, there is not yet much detailed mechanistic information. However, some of the intradiol catechol dioxygenases isolated from bacterial sources have been studied in great detail, and both structural and mechanistic information is available. These are the systems that will be described here. 

Among the enzyme systems performing redox reactions oxidations copper containing oxidases perform the irreversible oxidation of phenols. Two main types of phenol oxidases are known, the catechol oxidases and the laccases of which the latter is supposed to be... 

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