Redox Sandmeyer

On the basis of these redox potentials it seems likely that direct electron release to the benzenediazonium ion takes place only with iodide. This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light, or other special procedures (Sec. 10.6). For bromo- and chloro-de-di-azoniations, catalysis by cuprous salts (Sandmeyer reaction, Sec. 10.5) is necessary. For fluorination the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen (see Sec. 10.4). With astatide (211At-), the heaviest of the halide ions, Meyer et al. (1979) found higher yields for astato-de-diazoniation than for iodo-de-diazoniation, a result consistent with the position of At in the Periodic System. It has to be emphasized, however, that in investigations based on measuring yields of final products (Ar-Hal), the possibility that part of the yield may be due to heterolytic dediazoniation is very difficult to quantify. 

Halo-de-diazoniations are a series of reactions in which the replacement of the dia-zonio group changes from a heterolytic de-diazoniation in the case of the fluorination (Balz-Schiemann reaction) to transition metal-catalyzed chlorination and bromination (Sandmeyer reaction) and finally to iodination and astatination where no catalyst is necessary due to the favorable redox potentials of I and At- (I- E° = 1.3 V). 

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. 

W. A. Waters (Oxford University) Investigations of the extent to which complexes such as (CuCl)+ and undissociated CuCL affect the chain length in the polymerization associated with the Sandmeyer reaction are in progress at Oxford. It is well known that ions that complex well with cupric, e.g., (CN) , can be introduced into aryl nuclei by the Sandmeyer procedure in preference to chloride even when diazonium chlorides have initially been taken. The system, however, is complicated by the fact that the complexing of cuprous and cupric salts alters the redox potential, and this affects the facility of both stages 1 and 3 of the reaction sequence. The effects of introducing polar substituents into the aryl nuclei (Table I) indicates the importance of such effects. 

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. 

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