Copper redox catalyst

Electron transfer from the substrates to 02 proceeds by a redox cycle that consists of copper(II) and copper(I). The high catalytic activity of the copper complex can be explained as follows (1) The redox potential of Cu(I)/Cu(II) fits the redox reaction. (2) The high affinity of Cu(I) to 02 results in rapid reoxidation of the catalyst. (3) Monomers can coordinate to, and dissociate from, the copper complex, and inner-sphere electron transfer proceeds in the intermediate complex. (4) The complex remains stable in the reaction system. It may be possible to investigate other catalysts whose redox potentials can be controlled by the selection of ligands and metal species to conform with these requisites several other suitable catalysts for oxidative polymerization of phenols, such as manganese and iron complexes, are candidates on the basis of their redox potentials. 

We have already hypothesised a reaction mechanism based on the redox chemistry of the Cu-ZSM5 catalyst [9]. According to that scheme, we proposed that N2O isothermal oscillations are related to the two possible roles assumed by N2O molecule in its interaction with catalyst copper sites. Based on the well known evidence that in Cu-ZSM5 the copper sites can exist in two different oxidation states (Cu and Cu") depending on the experimental conditions, this assumption expects that N2O could act both as an oxidant to transform Cu to Cu" and as a reductant to promote the reverse process and attributes reaction rate oscillations to the changes of the oxidation state of these sites, according to the following reactions ... 

Copper occims in foin oxidation states, zero, monovalent, divalent, and trivalent (f). Its divalent species is the most common and most stable. Copper, particularly copper(I) and its complexes, is a catalyst in many industrial and biological processes. The majority of reactions in which copper is used as a catalyst, usually redox processes, involve monovalent copper as an intermediate amd often as the initiator of the catalytic process. The role of copper in biological processes as well as the green chemistry desire to perform catalytic processes in water stresses the importance of Cu(I) in aqueous media. 

The action of redox metal promoters with MEKP appears to be highly specific. Cobalt salts appear to be a unique component of commercial redox systems, although vanadium appears to provide similar activity with MEKP. Cobalt activity can be supplemented by potassium and 2inc naphthenates in systems requiring low cured resin color lithium and lead naphthenates also act in a similar role. Quaternary ammonium salts (14) and tertiary amines accelerate the reaction rate of redox catalyst systems. The tertiary amines form beneficial complexes with the cobalt promoters, faciUtating the transition to the lower oxidation state. Copper naphthenate exerts a unique influence over cure rate in redox systems and is used widely to delay cure and reduce exotherm development during the cross-linking reaction. 

The main by-products of the Ullmaim condensation are l-aniinoanthraquinone-2-sulfonic acid and l-amino-4-hydroxyanthraquinone-2-sulfonic acid. The choice of copper catalyst affects the selectivity of these by-products. Generally, metal copper powder or copper(I) salt catalyst has a greater reactivity than copper(Il) salts. However, they are likely to yield the reduced product (l-aniinoanthraquinone-2-sulfonic acid). The reaction mechanism has not been estabUshed. It is very difficult to clarify which oxidation state of copper functions as catalyst, since this reaction involves fast redox equiUbria where anthraquinone derivatives and copper compounds are concerned. Some evidence indicates that the catalyst is probably a copper(I) compound (28,29). 

A third mechanism involves redox processes,87 and is particularly likely to operate in reactions in which copper salts are used as catalysts.88... 

Catalyst activity (in terms of KAJRp) is also intrinsically dependent on the redox potential of the metal complex. The latter, in turn, depends on the relative stability of the higher (MtM+1/L) and lower (Mt"/L) oxidation states. For the case of relatively stablel 1 copper complexes, the redox potential can be calculated using the following equation [98,144,145,146] ... 

Earlier results regarding the kinetics and mechanism of the copper(II) catalysis are controversial. Reaction orders for [02], [Cu11], [H2A] and [H+] were reported in the following respective ranges 0.5 to 1, 0.5 to 1,0 to 1, and —2 to +1 (8). It is also disputed whether the redox cycling of the catalyst includes oxidation states +1 and +2 or +2 and +3. The discrepancies are too marked to be explained only by the differences in the experimental conditions applied. 

In non-aqueous solution, the copper catalyzed autoxidation of catechol was interpreted in terms of a Cu(I)/Cu(II) redox cycle (34). It was assumed that the formation of a dinuclear copper(II)-catecholate intermediate is followed by an intramolecular two-electron step. The product Cu(I) is quickly reoxidized by dioxygen to Cu(II). A somewhat different model postulated the reversible formation of a substrate-catalyst-dioxy-gen ternary complex for the Mn(II) and Co(II) catalyzed autoxidations in protic media (35). 

Wainwright, Tomsett, Trimm, and coworkers/Mellor, Copperthwaite, and coworkers—Raney copper catalysts for WGS and methanol synthesis. In 1995, Wainwright and Trimm295 reviewed Raney178 copper catalysts for both water-gas shift and methanol synthesis applications and discussed the possibility of either a redox mechanism or a formate mechanism for Raney copper catalysts. Formates, they indicated, rapidly decompose to C02 and H2 over metallic copper surface. They... 

The initial screen of potential catalysts by these workers revealed that several Lewis acids are capable of effecting nitrenoid transfer to alkenes. In particular, SmLOf-Bu, a species that is unlikely to participate in redox processes, was found to work well for 7ra s-p-methylstyrene aziridination. Although the generality of this catalyst fell far short of the copper system, it raises the intriguing possibility that the Cu(II) species formed in the aziridination acts at least in part as a Lewis acid. The considerable Lewis acidity of cationic Cu(II) complexes has since been extensively exploited (cf. Section V). 

This equilibrium has a buffer-like effect stabilizing the presence of cationic copper species in the structure even in a highly reductive atmosphere. The above scheme of copper oxide-ceria interactions indicates clearly that the catalyst is mutually promoted, i.e., both copper and ceria cooperate in the redox mechanism. 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

R G. Harrison, I. K. Ball, W. Azelee, W. Daniell, and D. Goldfarb, Nature and surface redox properties of copper(ll)-promoted cerium(lV) oxide CO oxidation catalysts, Chem. Mater. 12, 3715-3725 (2000). 

M. M. Gunter, T. Ressler, R. E. lentoft, and B. Bems, Redox behavior of copper oxide catalysts in the steam reforming of methanol studied by in situ X-ray diffraction and absorption spectroscopy, J. Catal. 203, 133-149 (2001). 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

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