Diazo anion radicals

It is appropriate to start the discussion on electron-transfer processes to and from diazo compounds with polarographic results on electron additions to a-diazo ketones. For such a reaction one expects the formation of a diazo anion radical, in which the negative charge is localized mainly on the O-atom and to give diazenyl radical character to the diazo group (9.51). 

In the presence of hydroxy lie compounds and other proton donors, these reactions are more complex see Bethell and Parker (1986) for a mechanism involving a complex of a hydrogen-bonded water molecule to the diazo anion radical 9.57. 

In most cases in which dimerization is not dominant, proton transfer to the diazo anion radical is the reaction responsible for the majority of final products. These pathways were studied in detail with diphenyldiazomethane in aprotic solvents without added protic compounds (Bethell and Parker, 1981, 1982 Van Galen et al., 1984). Benzophenone hydrazone and diphenylmethane were the major products in this case (9-31). ... 

These products demonstrate that C- and A -protonation of the diazo anion radical are feasible. Therefore, there is a certain similarity to the nucleophilic character of these atoms in diazoalkanes. We agree, however, with the statement in the review of Bethell and Parker (1988) Further investigation of the relationship between charge distribution in RR CNJ" and protonation is clearly necessary . 

The carbon dioxide anion-radical was used for one-electron reductions of nitrobenzene diazo-nium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik and Okhlobystin 1979). The double bonds in maleate and fumarate are reduced by CO2. The reduced products, on being protonated, give rise to succinate (Schutz and Meyerstein 2006). The carbon dioxide anion-radical reduces organic complexes of Co and Ru into appropriate complexes of the metals(II) (Morkovnik and Okhlobystin 1979). In particular, after the electron transfer from this anion radical to the pentammino-p-nitrobenzoato-cobalt(III) complex, the Co(III) complex with thep-nitrophenyl anion-radical fragment is initially formed. The intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand. 

Principally the same, but chemically simpler, sequence was used to prepare arylnitro anion-radicals from arylamines, in high yields. For instance, aqueous sodium nitrite solution was added to a mixture of ascorbic acid and sodium 3,5-dibromo-4-aminobenzenesulfonate in water. After addition of aqueous sodium hydroxide solution, the cation-radical of sodium 3,5-dibromo-4-nitro-benzenesulfonate was formed in the solution. The latter was completely characterized by its ESR spectrum. Double functions of the nitrite and ascorbic acid in the reaction should be underlined. Nitrite takes part in diazotization of the starting amine and trapping of the phenyl a-radical formed after one-electron reduction of the intermediary diazo compound. Ascorbic acid produces acidity to the reaction solution (needed for diazotization) and plays the role of a reductant when the medium becomes alkaline. The method described was proposed for ESR analytical determination of nitrite ions in water solutions (Lagercrantz 1998). 

For the anion radicals intermediates and final products were identified. They correspond at least to four reaction types, namely a) dimerization of the diazo anion... 

When radical anions and dianions leads to very slow SET, their nucleophilic properties have to be taken into account. Reactions of electrogenerated species toward CO2, acetic anhydride, acetic chloride, and some classic RX compounds as electrophiles can be explained on the basis of nucleophilic substitutions, but extreme cares have to be taken on the intrinsic nature of the mechanism (Sn vs. radical coupling). Thus reactions involving azo and diazo compounds in the presence of CO2 could be explained by nucleophilic addition (formation of A-functionalized azine and diazines [13]). Several reactions [15,215,229] implying CO2 and acetic anhydride [230] were described. 

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. 

We use the term diazo compounds not only to name specific structures according to lUPAC rules (e. g., diazomethane, see below), but also as a class name (as meant in the title of this book and in chapter headings), including neutral, cationic, anionic, and radical compounds with the group — N2 or — N2 —, but excluding azo compounds, i.e., compounds in which the — N2— group is bound on both sides to C-atoms. 

Bethell and coworkers have used isotope effects to examine the decomposition reaction of alkyl diazo compounds. In particular, these workers wished to determine whether a carbene radical anion, 3, was generated from an alkyl diazo compound (equation 21). 

The radical anion has been identified as the initial intermediate formed in the electrochemical reduction of the diazo compound and several authors have suggested, on the basis of product studies of reduction reactions of diazo compounds, that the carbene radical anion is formed from the radical anion by the loss of nitrogen. However, other work suggests that the radical anion reacts with a hydrogen before the loss of nitrogen and that the carbene radical anion is not formed in the reaction (equation 22). 

Three events are involved with chain-growth polymerization catalytic initiation, propagation, and termination [3], Monomers with double bonds (—C=C—R1R2—) or sometimes triple bonds, and Rj and R2 additive groups, initiate propagation. The sites can be anionic or cationic active, free-radical. Free-radical catalysts allow the chain to grow when the double (or triple) bonds break. Types of free-radical polymerization are solution free-radical polymerization, emulsion free-radical polymerization, bulk free-radical polymerization, and free-radical copolymerization. Free-radical polymerization consists of initiation, termination, and chain transfer. Polymerization is initiated by the attack of free radicals that are formed by thermal or photochemical decomposition by initiators. When an organic peroxide or azo compound free-radical initiator is used, such as i-butyl peroxide, benzoyl peroxide, azo(bis)isobutylonitrile, or diazo- compounds, the monomer s double bonds break and form reactive free-radical sites with free electrons. Free radicals are also created by UV exposure, irradiation, or redox initiation in aqueous solution, which break the double bonds [3]. 

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