Copper catalysts Sandmeyer reaction

There are three areas of activity in the field of arenediazonium salts in interaction with metals and transition elements which have some similarities to metals. First is the use of copper in the reactions of Sandmeyer (1884), Pschorr (1896), Gomberg-Bachmann (1924), and Meerwein (1939). Other transition metal catalysts (Ti and Pd) have been used for such reactions since the 1970s (see Secs. 10.8 and 10.9). Up to now only one intermediate has been directly identified, the aryldiazenido palladium complex (ArN2Pd(PPh3)3]+BF4 (Yamashita et al., 1980 see Sec. 10.9, Scheme 10-64). 

In cases where it is not necessary to introduce the nitro group directly at a position of the azole ring poorly susceptible to electrophilic attack it is possible to use the Sandmeyer reaction or its modifications (Scheme 53). The use of copper salts as catalysts is not essential in a number of cases [392, 393],... 

A diazonium salt reacts with copper(I) chloride or copper(I) bromide to form an aryl chloride or aryl bromide, respectively. This is called the Sandmeyer reaction. It provides an alternative to direct chlorination and bromination of an aromatic ring using CI2 or Br2 and a Lewis acid catalyst. 

The Sandmeyer reaction probably involves radical-like intermediate stages1191,1192 or free radicals 240 see Pfeil et a/.1192,240 for details of the primary step. The role of the copper ion as redox catalyst is decisive. Increasing the concentration of halogen ions decreases the rate of the Sandmeyer reaction since, for example, the catalytically active copper(i) chloride and Cl- form a complex ion [CuCl3]2-.1192 However, increasing the halogen ion concentration hinders the side reactions (d) and (e) still more strongly,1192 so that the yield of aryl halide is increased in spite of the slower reaction. 

Copper is commonly used as a catalyst for a variety of organic reactions applying organometallic intermediates (114,115, 148-150). These processes include the Ullmann reaction, the Sandmeyer reaction, the Meerwein reaction, the Click reaction, a variety of atom transfer processes including pol5maerizations, etc. Many of these processes involve radicals and redox processes initiated by the copper species. All these processes are usually carried out in aprotic solvents and are therefore beyond the scope of this review. However, the mechanism of some of them was studied in aqueous solutions with the hope to perform them in this medium and in order of determining their detailed mechanisms. 

CuCl) A white solid, insoluble in water, prepared by heating copper(II) chloride in concentrated hydrochloric acid with excess copper turnings. When the solution is colorless, it is poured into air-free water (or water containing sulfur(IV) oxide) and a white precipitate of copper(I) chloride is obtained. On exposure to air this precipitate turns green due to the formation of basic copper(II) chloride. Copper(I) chloride absorbs carbon monoxide gas. It is used as a catalyst in the rubber industry. The chloride is essentially covalent in structure. In the vapor phase both dimeric and trimeric forms exist. It is used in organic chemistry in the Sandmeyer reactions. 

C6H5N2 + Cl- C HjCl + N2 This reaction was discovered by the German chemist Traugott Sandmeyer (1854-1922) in 1884. A variation of the reaction, in which the catalyst is freshly precipitated copper powder, was reported in 1890 by the German chemist Ludwig Gatterman (1860-1920). This is known as the Gatterman reaction (or Gat-terman-Sandmeyer reaction). 

CeHsNa+Cl- - CeHsCl + N2 The copper acts as a catalyst. The reaction is easier to perform than the Sandmeyer reaction and takes place at lower temperature, but generally gives lower yields. It was discovered in 1890 by the German chemist Ludwig Gattermaim (1860-1920). 

Another important class of reactions involves the introduction of a cyano group by substitution in an Ar-Z precursor. In fact, novel pathways leading to aromatic nitriles-for example, photosubstitution reactions-are desirable in view of the many applications of aryl cyanides as agrochemicals and pharmaceuticals. Today, the classical copper(l)-mediated Rosenmund-von Braun and Sandmeyer reactions, from aryl halides and aryldiazonium salts respectively, have been supplanted by reactions which employ palladium- or copper-catalysis [57]. The rather common use of excess cyanide anion may lead to a deactivation of the catalyst, and affect to a remarkable extent each of the key steps of the catalytic cycle [58aj. Although the use of complex iron cyanide has been shown to offer an effective solution to this limitation [58b,c], photocyanation provides an equally useful alternative [10],... 

As mentioned in Section 22-10, the Sandmeyer reactions, which entail the substitution of the nitrogen in arenediazonium salts by Cl, Br, or CN, require copper(I) ion as a catalyst and proceed by complex radical mechanisms. Why do such substitutions not follow an Sn2 pathway Why is the S l mechanism, which operates in the corresponding displacements with OH and I, not effective . 

Sandmeyer reaction A method of producing chloro- and bromo-substituted derivatives of aromatic compounds by using the DIAZONIUM SALT with a copper halide. The reaction starts with an amine, which is diazotized at low temperature by using hydrochloric acid and sodium nitrite (to produce nitrous acid, HNO2). For example QH5NH2 + NaNOi + HCl CgHjNi + Cl- + OH- + Na + H2O The copper halide (e.g. CuCl) acts as a catalyst to give the substituted benzene derivative ... 

Organometaiiic chemistry started some forty years ago and developed rapidly, particularly that part involving transition metals. This can be illustrated by the facts that, in the period 1981-1992, not less than 1319 stable organometaiiic compounds containing rhodium, a relatively rare member of the platinum group, were described (Sharp, 1995), or that stable complexes containing diazenido ligands with at least 19 transition metals are known (Sutton, 1993). On the other hand, no aryldiazenido complex of copper has been described, in spite of the fact that such coordination compounds may be formed in the Sandmeyer, Pschorr, and Meerwein reactions. The latter have been known, in part, for more than a century We are aware, of course, that the search for the structure of catalysts is methodically very different from that for stable compounds. This is likely to be the reason that in the majority of review papers either structures of stable compounds or catalysts are discussed but correlations between these areas of interest are not. ... 

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