Mechanism, radical aryldiazonium salts

There are reasons to explain the catalytic activity of copper(ll) in terms of cation-radical mechanism. This mechanism is confirmed by the unusual direction of Meerwein reaction in some cases, for example, when the replacement of halogen by an aryl radical occurs in the reaction of halo-styrenes with aryldiazonium salts (Obushak et al. 1991). A cation-radical in the system [olefin-Cu(II)] has been detected by UV spectroscopy (Obushak et al. 1991). In the cases of cis isomers of benzylidenacetone (Allard and Levisalles 1972) and maleic esters (Isaev et al. 1972), the unreacted... 

Analogous consideration of aryldiazonium salts and sulfur dioxide reaction with a-nitro olefins in the presence of cupric chloride gave rise to a conclusion that the process also includes the electron-transfer step and develops according to anion-radical mechanism (Bilaya et al. 2004). The reaction eventually leads to the formation of p-arylsulfonyl a-nitroethanes. 

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. 

Fig. 5.56. Nucleophilic substitution reactions on a masked aryldiazonium salt. The introduction of fluorine takes place by the SN1 mechanism in Figure 5.50, the introduction of iodine occurs by the radical mechanism of Figure 5.56.

With other nucleophiles (Figures 5.42-5.45) aryldiazonium salts react according to other mechanisms to form substitution products. These substitutions are possible because certain nucleophiles reduce aryldiazonium salts to form radicals Ar—N=N-. These radicals lose molecular nitrogen. A highly reactive aryl radical remains, which then reacts directly or indirectly with the nucleophile. 

Another useful approach to styrenes via sp -sp -coupling reactions, which is beyond the scope of this section, is Meerwein-type arylations in which aryldiazonium salts undergo usually copper(I)- or palladium-catalyzed couplings with electron deficient alkenes, indicating a radical-based mechanism.The method is so useful for the synthesis of stilbenes from unsubstituted styrenes 92 or p>silylstyrenes. 93... 

A paper on new approaches to the generation of arylphosphinidenes has been published. The stable bis-azide precursor (133) upon photolysis, or vapour phase thermolysis, gave (134), obviously via the phosphinidene (135) the same product was obtained by photolysis of the phosphaketene (136). The reduction of aryldiazonium salts to arenes with triethyl phosphite or triphenylphosphine is shown to proceed by a radical-chain mechanism. The previously described photo-Arbuzov rearrangement of benzyl phosphites has been used to prepare several acyclic phosphonate nucleotide analogues, e.g. (137). ... 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

The use of iodoacetic acid as an aryl radical trapping agent has confirmed the intermediacy of aryl radicals in some hydrodediazoniation reactions, whether these are initiated or not.4 Spontaneous hydrodediazoniation of aryldiazonium fluoroborates occurs in warm dimethylformamide (DMF). Detailed study5 of the conversion of the 4-nitro derivative into nitrobenzene indicates a homolytic mechanism in which H-atom abstraction occurs from both sites in DMF with a formyl methyl preference of 3.5 1.0. High yields of mixed perfluorinated biaryls may be obtained by the catalytic dediazoniation of pentafluorobenzenediazonium ions in acetonitrile containing aromatic substrates and small amounts of iodide salts. The catalytic role of iodide and the isomeric product distributions indicate that arylation proceeds through the pentafluorophenyl radical in an efficient homolytic chain process.6... 

Substituted Benzenes.— The stability of aryldiazonium fluoroborates under a variety of conditions has been investigated. These compounds are rendered more thermally stable by sodium meta(octa)molybdate, while complexes with 18-crown-6 polyether show enhanced stability over the uncomplexed salts in both thermal and photochemical de-diazoniations. In the latter procedure, products from an ionic mechanism are obtained at 313 nm, but at >330 nm protodedi-azoniation of fluoroborates occurs by a radical mechanism. The effect of solvents on the mechanism of nitrogen loss from diazonium salts has been reviewed. ... 

It is thought that the decomposition of aryldiazonium halides by copper salts, which gives aromatic halides (Sandmeyer reaction), proceeds via a free radical mechanism and that Cu—Ar intermediates arc probably not important. It is however proposed that copper-diazo intomediates such as Ar—N=N—Cu are formed [121]. These may be compared with the aryl-diazomolybdenum and tungsten complexes jr-C5H5M(CO)2N2Ar [122]. 

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