Cuprous salts mechanism

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 quinoline 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 quinoline and that arylcopper compounds are intermediates that can be isolated in some cases. Metallic silver has been used in place of copper, with higher yields. ... 

The mechanism by which a nucleophile displaces the diazonium group depends on the nucleophile. While some displacements involve phenyl cations, others involve radicals. Nucleophiles, e.g. CN , Cl and Br , replace the diazonium group if the appropriate cuprous salt is added to the solution containing the arene diazonium salt. The reaction of an arene diazonium salt with cuprous salt is known as a Sandmeyer reaction. 

An example of the anionotropic rearrangement is the reaction of propargyl halides with cuprous salts (Eq. 33). The mechanism and the role of the cuprous salt have not been-completely elucidated. 

Reactions involving diazo groups are also affected by heavy metals such as cuprous ion. For example, in the well known Sandemeyer reaction, cuprous ion can catalyze the decomposition of aryl diazonium salts. Although the exact mechanism is unknown and may involve free radical processes, the anion of the cuprous salt replaces the nitrogen of the diazonium salt. 

The mechanism of the copper-mediated cross-coupling of iodoarenes and perfluoroalkyl iodides is supposed to be similar to that of related reactions involving the interaction of halogenoarenes with cuprous salts of organic nucleophilic anions (for example CuCN) [55] (Scheme 2.122). First a solvated perfluoroalkyl copper(I) complex is formed. This then coordinates to the iodoarene followed by exchange of ligands [56]. Several electron-transfer steps are probably involved in this process. 

The most important finding for the synthetic applications of the Sandmeyer reaction is the clear experimental evidence of Galli that both oxidation states of copper ion are necessary for high yields. This claim is understandable on the basis of the reaction mechanism cupric ions are a ligand transfer reagent (see reviews ). The fact that the presence of Cu" ions was not realized much earlier is understandable, because cuprous salts are rarely completely free of cupric impurities. In aqueous systems they form cupric ions by equilibration as well as by air oxidation. The following comparative experiments of Gain in the chloro-de-diazoniation of benzenediazonium sulfate in water at room temperature are instructive. Yields of chlorobenzene are with 0.25 M CuCl 45% with 0.25 M CuCl -F 0.25 M Cu(N03)2 63% with 0.25 M Cu(N03)2 <0.1%. 

KCl and KBr cannot be used in place of CuCl and CuBr in Sandmeyer reactions. The cuprous salts are required, which indicates that the copper(I) ion has a role in the reaction. Although the precise mechanism is not known, it is thought that the copper(I) ion donates an electron to the diazonium salt, forming an aryl radical and nitrogen gas. 

For copper corrosion in hot H2SO4, Ross and Berry found that benzotriazole (BTA) inhibits the rate of copper dissolution flow rate and aeration change the inhibitor concentration profile, and a film composed of the cuprous salt of BTA is formed. Other effective inhibitors for copper in H2SO4 include 2,4-dinitrophenyl-hydrazine, benzimidazole, indazole, and quinoline. Various azoles appear to be effective corrosion inhibitors for brass, as well as copper. Finally, Subra-manyan et al. studied the corrosion inhibition mechanisms of the alkaloids quinine (C21H22N2O2) and strychnine (C21H22N2O2). Both compounds inhibit the corrosion of copper, in 1% H2SO4 at 86°F (30°C), relatively well, and both chemisorb and... 

The diazonium salt solution decomposes on standing and hence must be mixed with the Cuprous chloride solution without delay. Mechanical stirring is an advantage. 

Kochi (1956a, 1956b) and Dickerman et al. (1958, 1959) studied the kinetics of the Meerwein reaction of arenediazonium salts with acrylonitrile, styrene, and other alkenes, based on initial studies on the Sandmeyer reaction. The reactions were found to be first-order in diazonium ion and in cuprous ion. The relative rates of the addition to four alkenes (acrylonitrile, styrene, methyl acrylate, and methyl methacrylate) vary by a factor of only 1.55 (Dickerman et al., 1959). This result indicates that the aryl radical has a low selectivity. The kinetic data are consistent with the mechanism of Schemes 10-52 to 10-56, 10-58 and 10-59. This mechanism was strongly corroborated by Galli s work on the Sandmeyer reaction more than twenty years later (1981-89). 

The reaction with ammonia or amines, which undoubtedly proceeds by the SnAt mechanism, is catalyzed by copper and nickel salts, though these are normally used only with rather unreactive halides. This reaction, with phase-transfer catalysis, has been used to synthesize triarylamines. Copper ion catalysts (especially cuprous oxide or iodide) also permit the Gabriel synthesis (10-61) to be... 

The reaction between aryl halides and cuprous cyanide is called the Rosenmund-von Braun reactionP Reactivity is in the order I > Br > Cl > F, indicating that the SnAt mechanism does not apply.Other cyanides (e.g., KCN and NaCN), do not react with aryl halides, even activated ones. However, alkali cyanides do convert aryl halides to nitrilesin dipolar aprotic solvents in the presence of Pd(II) salts or copper or nickel complexes. A nickel complex also catalyzes the reaction between aryl triflates and KCN to give aryl nitriles. Aromatic ethers ArOR have been photochemically converted to ArCN. 

Nitro compounds can be formed in good yields by treatment of diazonium salts with sodium nitrite in the presence of cuprous ion. The reaction occurs only in neutral or alkaline solution. This is not usually called the Sandmeyer reaction, although, like 14-25, it was discovered by Sandmeyer. The BFJ ion is often used as the negative ion to avoid competition from the chloride ion. The mechanism is probably like that... 

Examples of the three mechanistic types are, respectively (a) hydrolysis of diazonium salts to phenols89 (b) reaction with azide ion to form aryl azides90 and (c) reaction with cuprous halides to form aryl chlorides or bromides.91 In the paragraphs that follow, these and other synthetically useful reactions of diazonium intermediates are considered. The reactions are organized on the basis of the group that is introduced, rather than on the mechanism involved. It will be seen that the reactions that are discussed fall into one of the three general mechanistic types. 

By-products from capture of nucleophilic anions may be observed.53 Phenols can be formed under milder conditions by an alternative redox mechanism.98 The reaction is initiated by cuprous oxide, which effects reduction and decomposition to an aryl radical, and is run in the presence of Cu(II) salts. The radical is captured by Cu(II) and converted to the phenol by reductive elimination. This procedure is very rapid and gives good yields of phenols over a range of structural types. 

The mechanism of the Kharasch-Sosnovsky reaction remains unclear. The generally accepted version, as proposed by Kochi and co-workers (94-96) and later improved by Beckwith and Zavitsos (97), is illustrated in Scheme 8. Cuprous ion reduces the perbenzoate to Cu(II)OBz (Bz = benzoyl) and free /-BuO radical. The radical abstracts an allylic hydrogen atom generating an allyl radical that combines with the cupric salt to form an allylcopper(III) species. Reductive elimination with... 

In addition to water, a variety of organic liquids, including amines, carboxylic acids, and hydrocarbons, have been used as solvents in the study of the homogeneous reactions of hydrogen with metal salts. In general, there is more uncertainty about the nature of the species present in such systems than in aqueous solution and, correspondingly, it is usually more difficult to elucidate the reaction mechanisms in detail. The most extensive solvent effect studies have been made on cupric, cuprous, and silver salts. A number of the more important results are considered below. 

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