Diazo radical

The phenomena enumerated in Section 2.4 do not, of course, fully describe all the differences between chemical and electrode processes of ion radical formation. From time to time, effects are found that cannot be clearly interpreted and categorized. For instance, one paper should be mentioned. It bears the symbolic title ir- and a-Diazo Radical Cations Electronic and Molecular Structure of a Chemical Chameleon (Bally et al. 1999). In this work, diphenyldiazomethane and its 15N2, 13C, and Di0 isotopomers, as well as the CH2-CH2 bridged derivative, 5-diazo-10,ll-dihydro-5H-dibenzo[a,d]cycloheptene, were ionized via one-electron electrolytic or chemical oxidation. Both reactions were performed in the same solvent (dichloromethane). Tetra-n-butylammonium tetrafluoroborate served as the supporting salt in the electrolysis. The chemical oxidation was carried out with tris(4-bromophenyl)-or tris(2,4-dibromophenyl)ammoniumyl hexachloroantimonates. Two distinct cation radicals that corresponded to it- and a-types were observed in both types of one-electron oxidation. These electromers are depicted in Scheme 2-28 for the case of diphenyldiazomethane. 

An intermediate reduction of aryldiazonium salts Ar-N =N to the diazo radicals Ar-N=N also occurs when aryldiazonium salts react with KI to yield aryl iodides (Figure 5.55). Therefore, aryl radicals Ar are obtained under these conditions, too. Their fate, however, differs from that of the aryl radicals, which are faced with nucleophiles in the presence of Cu(II) (cf. Figure 5.53) or H3P02 (cf. Figure 5.54) the iodination mechanism of Figure 5.55 is a radical chain reaction consisting of four propagation steps. 

Bases, Neutral Salts.— As a base it forms salts, in which form the diazo compound is obtained by diazotization, and which though also unstable has been isolated in small quantities and the composition and properties determined. Of the three salts, the sulphate, chloride and nitrate, the first is the most stable and the last is the least stable. They are colorless crystalline neutral compounds soluble in water, difficultly soluble in alcohol and insoluble in ether. After being prepared by the ordinary diazo reaction, with sodium nitrite in cold acid water solution, they may be precipitated in crystalline form by the addition of alcohol and ether. If the diazotization is effected in alcohol solution by means of amyl nitrite or ethyl nitrite the crystals of the diazonium salt separate at once. These salts of diazo benzene all show true salt characteristics, e.g., they lower the freezing point of solutions. The diazo radical, (CeHs—N2—) is thus basic toward strong acids, and the hydroxide, the non-isolated hypothetical diazo benzene, CeHs—N2—OH, is the free base. It may be considered as the simplest aromatic diazo compound and the mother substance of all other members of the class. 

But, in addition to the reaction-product of the diazo-radical, there is always formed One molecule of an amine. 

A number of kinetic studies were carried out to elucidate the interaction between a-tocopherol and ascorbic acid in liposomes. By using either water-soluble or oil-soluble diazo radical initiators, the effect of radicals can be compared when they are produced either in the phospholipid bilayer or in the aqueous phase. Oxidation of a soybean phosphatidylcholine liposome in the presence of radicals produced in the water phase with a water-soluble radical initiator [2,2 -azobis(2-amidinopropane) dihydrochloride, (AAPH)], showed an induction period with both a-tocopherol (vitamin E) and ascorbic acid (vitamin C) (Figure 10.9). With a mixture of vitamin E and vitamin C, the length of the induction was close to the sum of the individual induction periods. This result indicates an additive effect in suppressing oxidation by both vitamin E and vitamin C. Vitamin C apparently traps radicals in the water phase. When oxidation was induced by radicals produced in the lipid phase with an oil-soluble radical initiator [2,2 -azobis(2,4-dimethylvaleronitrile), (AMVN)] incorporated into the membrane, ascorbic acid alone had no effect, while a-tocopherol had a greater effect because it is lipophilic. However, the mixture... 

A plausible mechanism, equation (6-5), involves initially the gain of an electron by the diazonium cation to form a diazo radical. This subsequently decomposes to give N2 and an aryl radical, equation (6-6), which reacts with ferrocene, equation (6-7), leading to the final product, equation (6-8). Later... 

Hexafluoroacetone azine accepts nucleophiles (ROH, RSH, R NH) in positions 1 and 2 to yield hydrazones [27] Phosphites give open-chain products via a skeletal rearrangement [22] Radical addition reactions are also reported [22] Treatment of tnfluoropyruvates with tosylhydrazine and phosphorus oxychlo-ride-pyndme yields tnfluoromethyl-substituted diazo compounds [24] (equation 3)... 

Other electrophilic substitutions proceed with difficulty, or not at all. Nitrosation and diazo coupling require the presence of the strongly activating dimethylamino group (see Section VIII). Bromine adds, in the presence of sunlight, to give tetrabromotetrahydrobenzofuroxan (48) the initial attack is probably free-radical in nature. The product can be dehydrobrominated to form 4,7-, or a mixture of 4,5- and 4,6-dibromobenzofuroxan, depending upon the conditions. More conventional electrophilic bromination conditions have been tried in an attempt to obtain a monosubstituted product, but without success. 

The production of aryl radicals from peroxides normally provides a cleaner method of arylation than the methods based on the decomposition of azo and diazo compounds, and, in the case of benzenoid compounds, better yields of arylated products are obtained. The... 

In contrast to the acid, sodium nitrite should not in general be added in excess. Firstly, as far as the ratio of amine to nitrite is concerned, diazotization is practically a quantitative reaction. In consequence, it provides the most important method for determining aromatic amines by titration. Secondly, an excess of nitrous acid exerts a very unfavorable influence on the stability of diazo solutions, as was shown by Gies and Pfeil (1952). Mechanistically the reactions between aromatic diazonium and nitrite ions were investigated more recently by Opgenorth and Rtichardt (1974). They showed that the primary and major reaction is the formation of aryl radicals from the intermediate arenediazonitrite (Ar —N2 —NO2). Details will be discussed in the context of homolytic dediazoniations (Secs. 8.6 and 10.6). 

Coming back to the chain reaction sequence (Scheme 8-50) the inclusion of the final step shown here demonstrates clearly that the initial formation of the aryl radical from the diazo ether (Scheme 8-49) may be only an initiation step. The arguments of Broxton concerning whether the homolytic dediazoniation starts with the diazo ether or with the diazonium ion therefore become irrelevant. 

Diazenyl radical, see Aryldiazenyl radical Diazo acetates (covalent) 30, 115, 138 f.,... 

There is much evidence that the mechanism" of the 1-pyrazoline reactions generally involves diradicals, though the mode of formation and detailed structure (e.g singlet vs. triplet) of these radicals may vary with the substrate and reaction conditions. The reactions of the 3 f-pyrazoles have been postulated to proceed through a diazo compound that loses N2 to give a vinylic carbene." ... 

Diazonium salts can also be converted to halides by processes involving aryl free radicals. In basic solutions, aryl diazonium ions are converted to radicals via the diazo oxide.102... 

In the classical procedure, base is added to a two-phase mixture of the aqueous diazonium salt and an excess of the aromatic that is to be substituted. Improved yields can be obtained by using polyethers or phase transfer catalysts with solid aryl diazonium tetrafluoroborate salts in an excess of the aromatic reactant.177 Another source of aryl radicals is A-nitrosoacetanilides, which rearrange to diazonium acetates and give rise to aryl radicals via diazo oxides.178... 

Aromatic diazo compounds can be reduced in water via a radical process (Scheme 11.5).108 The reduction mechanism of arenediazo-nium salts by hydroquinone was studied in detail.109 Arenediazonium tetrafluoroborate salts undergo facile electron-transfer reactions with hydroquinone in aqueous phosphate-buffered solution containing the hydrogen donor solvent acetonitrile. Reaction rates are first order in a... 

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