Copper compounds mechanisms

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). 

Both the copper and nickel snlfamates (2-hydrates) are obtained by similar procedures and in comparable yield. The only losses are mechanical. The salts are respectively blue and green in color. Both the cobalt and the nickel salts dissolve in 96% alcohol but the copper compound does not. 

All of the neutral (organo-)copper compounds just listed can be arylated by aryl iodides and/or -bromides Ar-Cu and R-C=C-Cu can also be alkynylated by alkynyl iodides and/or -bromides. Table 16.2 summarizes the whole spectrum of corresponding products. After having grasped the mechanism of these reactions by way of Figure 16.3, we will discuss their synthetic potential by means of the examples given in Figures 16.4-16.9. 

The Ullmann reaction (Figure 13.4) represents another synthesis of substituted biphenyls. In this process an aryl iodide or—as in the present case—an aryl iodide/aryl chloride mixture is heated with Cu powder. It is presumed that under standard conditions the aryl iodide reacts in situ with Cu to form the aryl copper compound. Usually, the latter couples with the remaining aryl iodide and a symmetric biphenyl is formed. In a few instances it is also possible to generate asymmetric biaryls via a crossed Ullmann reaction. In these cases one employs a mixture of an aryl iodide and another aryl halide (not an iodide ) the other aryl halide must exhibit a higher propensity than the aryl iodide to couple to the arylcopper intermediate. It is presumed that the mechanism of the Ullmann reaction parallels the mechanism of the Cadiot-Chodkiewicz coupling, which we will discuss in Section 13.4. 

Neophyl(tri- -butylphosphine)copper decomposes at temperatures between 30° and 120°C mainly, if not totally, by a free-radical mechanism. The usual products resulting from rearrangement of the neophyl radical were isolated [Eq. (37)], and no evidence was obtained to indicate the formation of an intermediate phenyl-substituted tert-hu. y -copper compound, C6H5CH2(CH3)aCCu. 

Two reviews have detailed the reactions of aryl halides with copper and copper compounds these are (1) the Ullmann biaryl synthesis, by Fanta 113) and (2) copper-promoted reactions, by Bacon and Hill (9). The mechanism of the Ullmann biaryl synthesis was discussed the possible routes are shown in Eq. (110). No evidence had been obtained... 

The decomposition of alkyl and aryl copper compounds has been the subject of much debate. Initially free radical mechanisms were advanced, such theories being supported by the reduced yields of hydrocarbons in the presence of benzoquinone or other free-radical scavengers (24, 150). However, under all conditions very poor yields of dimeric alkanes are obtained. These would be the likely products of a free-radical decomposition. 

Despite its synthetic importance, the mechanism of the copper-quinoline method has been studied very little, but it has been shown that the actual catalyst is cuprous ion. In fact, the reaction proceeds much faster if the acid is heated in quinohne with cuprous oxide instead of copper, provided that atmospheric oxygen is rigorously excluded. A mechanism has been suggested in which it is the cuprous salt of the acid that actually undergoes the decarboxylation. It has been shown that cuprous salts of aromatic acids are easily decarboxylated by heating in quinohne and that aryl-copper compounds are intermediates that can be isolated in some cases. Metallic silver has been used in place of copper, with higher yields. ... 

The products of reaction are consistent with this mechanism. The presence of the copper salts has certain advantages so far as interpretation is concerned, since some of the species formed can be trapped out in the insoluble copper compound formed. 

Crivello (5) also found that these photocatalysts could be activated thermally when used in combination with copper compounds. For those conditions, Crivello has proposed a cationic polymerization mechanism which does not... 

In this chapter, the effect of a series of transition metal stearates on the thermal oxidation of polypropylene in homogeneous solution is examined, and the results obtained are compared with that in bulk reported previously (16). In addition, the effects of the anion of copper compounds, the concentration of copper, the solvent, and the additives on the copper compound-catalyzed thermal oxidation of polypropylene are studied, and the mechanism of the copper catalysis in solution is discussed. 

The mechanism of the Ullmann reaction has not been fully clarified, however, it should proceed via aryl-copper intermediates [13]. This proposal was supported by an improvement of the method by Koten and Noltes, who reacted an aryl-copper compound with... 

S.H. Lee, Y.-S. Her, and E. Matijevic Preparation and Growth Mechanism of Uniform Colloidal Copper Compounds by the Controlled Double-Jet Precipitation. J. Colloid Interface Sci., 186, 193-202 (1997). 

Pd-catalyzed homocoupling of terminal alkynes proceeds in the presence of a catalytic amount of Cu(l) salts and amines. During this process a terminal alkyne reacts with copper(I) salt in the presence of an amine to give an alkynyl copper compound that reacts with the Pd catalyst. Therefore, the homocoupling of terminal alkynes falls into the category of homocoupling of organometals and follows the same reaction mechanism. 

Based largely on the Li NMR results of Yamakawa et al., in combination with diffraction and electrochemical analysis, the following reaction mechanisms were proposed for the binary copper compounds. In the Cup2 system, a simple single-step conversion reactimi of the form... 

Rapid development of the Cu-mediated reactions has led to several excellent catalytic systems based on different copper compounds as catalyst precursors. Further studies of Cu-catalyzed reactions are expected to focus on the mechanism of catalytic reactions to allow rational catalyst design. 

---