Meerwein and Sandmeyer Reactions

Both the Sandmeyer and Meerwein reactions have been interpreted by heterolytic and homolytic mechanisms. Both reactions resemble halide replacements by involving solutions of complexes of Cu(I) with the reacting species, a diazonium compound. Cohen and Lewin have reported that a mixture of p-tolyldiazonium tetrafluoroborate and copper benzoate in aprotic solvents rapidly evolves nitrogen and forms toluene, bi(/)-tolyl), />-tolyl benzoate, and p,p-dimethylazobenzene (54). As was earlier suggested by Kochi 177), the azobenzene derivative is believed to arise from a reaction between an arylcopper species and the diazonium compound. A similar mechanism was suggested for the analogous reac-... 

Until the end of the 1970 s, interest in such reactions concentrated on catalysis by copper salts (review Burke and Grieco, 1979), obviously influenced by the long, broad, and successful experience with copper +- and copper-ions in aromatic diazo chemistry (Sandmeyer, Pschorr and Meerwein reactions, see Zollinger, 1994, Chapts. 8 and 10). A landmark was the discovery of Salomon and Kochi (1973), who found that cyclopropanations with diazomethane in the presence of copper(i) trifluoromethanesulfonate (triflate OTf) resulted in reduction of Cu + to Cu +, and that the rate of dediazoniation is inversely proportional to the alkene concentration. These results strongly indicate that formation of an alkene-Cu+ complex (8-47 2) precedes the complex formation with the diazoalkane. 

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. ... 

In this context two observations reported by Rondestvedt (1960, p. 214) should be mentioned (i) Meerwein reactions proceed faster in the presence of small amounts of nitrite ion. Meerwein reactions in which N2 evolution ceased before completion of the reaction can be reinitiated by addition of some NaN02. (ii) Optimal acidity for Meerwein reactions is usually between pH 3 and 4, but lower (pH — 1) with very active diazonium compounds such as the 4-nitrobenzenediazonium ion or the diphenyl-4,4 -bisdiazonium ion. At higher acidities more chloro-de-diazoniation products are formed (Sandmeyer reaction) and in less acidic solutions (pH 6) more diazo tars are formed. 

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). 

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). 

By single electron transfer from an electron donor, e.g. a transition metal ion, a trivalent phosphorous derivative or a base, followed by dissociation of the intermediate diazenyl radical in an aryl radical and dinitrogen. The aryl radical reacts with the solvent or with added reagents in various ways, as shown by the relatively large number of classical named reactions (Sandmeyer, Pschorr, Gomberg-Bachmann, Meerwein reactions). 

Cuprous-catalyzed addition of a diazonium salt to activated double bonds of alkenes and related compounds is known as the Meerwein reaction it competes with the Sandmeyer reaction. 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

Thus adding acetone increases the preparative value of the Sandmeyer reaction. It is assumed240 that the Sandmeyer and the Meerwein reaction occur by way of aryl radicals. 

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. 

Meerwein-Gomberg reactions between pyridinediazonium salts and furans fail because the Sandmeyer reaction supervenes, and the isopentyl nitrite method also gives poor results.264 Fortunately, photolysis of 3-... 

We chose arenediazonium salts as the simplest and most versatile electron acceptors to partner TTF. The use of diazonium salts as electron acceptors is not new and many fundamental and long-established transformations have developed based on the electron affinity of these reagents. For example, the Meerwein, Pschorr, and Gomberg-Bachmann and Sandmeyer reactions as well as the hypophosphorous acid-mediated dediazoniation are well-established radical reactions. 

Interactions between transition metals and arenediazonium ions were already known in the early history of diazo chemistry. Since the discoveries of Sandmeyer (1884), Pschorr (1896), Meerwein et al. (1939), and others, various metal-catalyzed replacements of the diazonio group by other substituents became important synthetic methods in organic chemistry. We have discussed these reactions in several sections of our first book (Zollinger, 1994, Chapts. 8 and 10). 

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