The Copper Cycle

The copper cycle begins with the formation of a 71 complex between Cu(I) and a terminal alkyne. 

The 71 complex then reacts with the amine to give a copper(I)-alkyne adduct. 

The copper(I)-alkyne adduct undergoes a transmetallation reaction in which the alkyne group becomes bonded in a Pd(II) complex. The alkyne group is initially trans to the species to which it will be coupled. 

The next series of steps then continues as part of the palladium cycle described above. 

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The electrons are now passed from PQH2 via the cytochrome bf complex (also called cytochrome b6f complex) to plastocyanin (PC). PC is a copper-containing protein that accepts electrons by the copper cycling between Cu2+ and Cu+ states ... 

The blue copper sites seem generally to be involved in electron transfer, with the copper cycling between the +1 and +2 states. The mechanistic details of the reversible redox cycle can, however, display unexpected complexity.34... 

These species play an important role in the copper cycle as they can behave either as oxidants or as reducing agents. The Cu(I) oxidation proceeds in two steps slow (Eq. 44) and fast (Eq. 45). 

Dissolved Cu(II) compormds show exceptional susceptibility to scavenge hydroperoxide (HO2 ) and superoxide (O2 ) radicals (230). Peroxide and superoxide species are thus not only formed in the copper cycle but they have also an influence on the Cu(I)/ Cu(II) interconversion, which is of importance in natural waters. 

It is now realized that copper as metal next to iron and chromium participates in photoredox cycles and its role cannot be ignored. The most important part of the cycle is photoreduction of Cu(II) to Cu(I) induced by solar light and oxidation of ligands to their environmentally benign forms. Then Cu(I) is easily re-oxidized to Cu(II), which can coordinate the next ligand molecule, and thereby the Cu photocatalytic cycles contribute to continuous environmental cleaning. Besides oxida-tion/reduction, other critical processes relevant to the copper cycles are adsorption/desorption and precipitation/dissolution... 

A group of cytochromes (labeled a, b, and c, depending on their spectra) serve as oxidation-reduction agents, converting the energy of the oxidation process into the synthesis of adenosine triphosphate (ATP), which makes the energy more available to other reactions. Copper is also involved in these reactions. The copper cycles between Cu(II) and Cu(I) and the iron cycles between Fe(nl) and Fe(II) during the reactions. Details of the reactions are available in other sources. ... 

In the second step, the palladium cycle and the copper cycle intersect, and a transmetallation reaction occurs. 

The Sonogashira reaction is believed to take place through two independent cycles the palladium cycle and the copper cycle, as depicted in Scheme 3.13 [58,59]. The Sonogashira reaction commences with the oxidative addition of the vinyl or aryl halide (R-X) to the Pd" species, followed by... 

The copper contained in the electrolyte, anodes and cathodes, and the normal circulating scrap load is equal to about 10% of the annual copper production of a typical electrorefinery. Some refineries (27) have changed from the traditional 25—30 day anode cycle to a 9—14 day cycle using smaller anodes to reduce the copper inventory in spite of the higher resulting scrap load. 

The CASS Test. In the copper-accelerated acetic acid salt spray (CASS) test (42), the positioning of the test surface is restricted to 15 2°, and the salt fog corrosivity is increased by increasing temperature and acidity, pH about 3.2, along with the addition of cupric chloride dihydrate. The CASS test is used extensively by the U.S. automobile industry for decorative nickel—chromium deposits, but is not common for other deposits or industries. Exposure cycle requirements are usually 22 hours, rarely more than 44 hours. Another corrosion test, now decreasing in use, for decorative nickel—chromium finishes is the Corrodkote test (43). This test utilizes a specific corrosive paste combined with a warm humidity cabinet test. Test cycles are usually 20 hours. 

Some metals are soluble as atomic species in molten silicates, the most quantitative studies having been made with Ca0-Si02-Al203(37, 26, 27 mole per cent respectively). The results at 1800 K gave solubilities of 0.055, 0.16, 0.001 and 0.101 for the pure metals Cu, Ag, Au and Pb. When these metal solubilities were compared for metal alloys which produced 1 mm Hg pressure of each of these elements at this temperature, it was found drat the solubility decreases as the atomic radius increases, i.e. when die difference in vapour pressure of die pure metals is removed by alloy formation. If the solution was subjected to a temperature cycle of about 20 K around the control temperamre, the copper solution precipitated copper particles which grew with time. Thus the liquid metal drops, once precipitated, remained stable thereafter. 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

The original Sonogashira reaction uses copper(l) iodide as a co-catalyst, which converts the alkyne in situ into a copper acetylide. In a subsequent transmeta-lation reaction, the copper is replaced by the palladium complex. The reaction mechanism, with respect to the catalytic cycle, largely corresponds to the Heck reaction.Besides the usual aryl and vinyl halides, i.e. bromides and iodides, trifluoromethanesulfonates (triflates) may be employed. The Sonogashira reaction is well-suited for the synthesis of unsymmetrical bis-2xy ethynes, e.g. 23, which can be prepared as outlined in the following scheme, in a one-pot reaction by applying the so-called sila-Sonogashira reaction ... 

A. niger normally produces many useful secondary metabolites citric and oxalic acids are stated as the dominant products. Limitation of phosphate and certain metals such as copper, iron and manganese results in a predominant yield of citric acid. The additional iron may act as a cofactor for an enzyme that uses citric acid as a substrate in the TCA cycle as a result, intermediates of the TCA cycle are formed. 

When cleaning boilers containing iron-copper deposits (with, say, hydrochloric acid), unless special precautions are taken, the cupric oxide dissolves and the cupric ion is reduced to copper, which then replates onto the steel surface, thus beginning the corrosion cycle again. 

Although various alkaline citrates and inorganic oxidizing cleaners are sometimes used, the standard procedure, where HC1 is employed, is to add thiourea. This method circumvents the copper corrosion cycle and permits the simultaneous removal of iron and copper deposits. 

In this final section, the global cycles of two metals, mercury and copper, are reviewed. These metals were chosen because their geochemical cycles have been studied extensively, and their chemical reactions exemplify the full gamut of reactions described earlier. In addition, the chemical forms of the two metals are sufficiently different from one another that they behave differently with respect to dominant... 

Fig. 15-18 The global copper cycle. Units are 10 g Cu (burdens) and 10 g Cu/yr (fluxes). (Reprinted with permission from J. O. Nriagu (1979). "Copper in the Environment, Part I Ecological Cycling", Wiley-Interscience, NY.)... 

It is noteworthy that metallic copper or cuprous bromide used under nitrogen atmosphere shows only a very short induction time. This last result points out the inhibitor role of the oxygen of the air atmosphere and most likely the important role taken either by reduced species or by radical intermediates in the catalytic cycle. 

Blue copper proteins. A typical blue copper redox protein contains a single copper atom in a distorted tetrahedral environment. Copper performs the redox function of the protein by cycling between Cu and Cu. Usually the metal binds to two N atoms and two S atoms through a methionine, a cysteine, and two histidines. An example is plastocyanin, shown in Figure 20-29Z>. As their name implies, these molecules have a beautiful deep blue color that is attributed to photon-induced charge transfer from the sulfur atom of cysteine to the copper cation center. 

For the copper/aluminum catalyst analyses later described, the sample mount was transfered from the XPS system following treatment and analysis to an inert atmosphere dry box without air exposure. From there individual pellets were transferred to the SAM for subsequent analysis without air exposure. The reverse process was employed for the next reaction cycle. 

Similar experiments with copper dispersed on AI2O3 did not show any unusual behavior of the Al(ls) or Cu(2p) photolines. In this case, the copper could be easily cycled between CuO under oxidative conditions, to Cu metal during reducing conditions. We observed only a slight shift (<0.4 eV) of the aluminum (Is) line upon initial heating, which was attributed to the loss of water in the alumina matrix. 

Normally, copper-catalysed Huisgen cycloadditions work with terminal alkynes only. The formation of a Cu-acetylide complex is considered to be the starting point of the catalyst cycle. However, the NHC-Cu complex 18 was able to catalyse the [3-1-2] cycloaddition of azides 17 and 3-hexyne 23 (Scheme 5.6). 

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