Of azo compounds

Aminoazobenzene is a member of the large class of azo-compounds, all of which contain the characteristic grouping, C-N N-C. Azo-compounds are of considerable technical importance, as they are all coloured, and the majority possess considerable stability. They may be prepared by the following methods ... 

The most noteworthy reaction of azo-compounds is their behaviour on reduction. Prolonged reduction first saturates the azo group, giving the hydrazo derivative (C NH-NH C), and then breaks the NH NH linkage, with the formation of two primary amine molecules. If method (1) has been employed to prepare the azo-compound, these two primary amines will therefore be respectively (a) the original amine from which the diazonium salt was prepared, and (6) the amino derivative of the amine or phenol with which the diazonium salt was coupled. For example, amino-azobenzene on complete reduction gives one equivalent of aniline, and one of p-phenylene diamine, NHaCeH NH benzene-azo-2-naphthoI similarly gives one equivalent of aniline and one of... 

Palladium-Catalyzed Decomposition of Azo Compounds, Azides, and Peroxides... 

The colors of azo compounds vary with the nature of the aryl group with its substituents and with pH Substituents also affect the water solubility of azo dyes and how well they... 

Although distibenes, the antimony analogues of azo compounds, have never been isolated as free, monomeric molecules (130), a tungsten complex, tritungsten pentadecacarbonyl[p.2-Tj -diphenyldistibene] [82579-41-7] C2yH2Q025Sb2W2, has been prepared by the reductive dehalogenation of phenyldichlorostibine (131) ... 

Another quite general source of free radicals is the decomposition of azo compounds. The products are molecular nitrogen and the radicals derived from the substituent groups ... 

IV-Nitrosqanilides are an alternative source of aryl radicals. There is a close mechanistie relationship to the decomposition of azo compounds. The JV-nitrosoanilides rearrange to intermediates that have a nitrogen-nitrogen double bond. The intermediate then decomposes to generate aryl radieals. ... 

We have two cases to consider here. If the reactants are nonpolar and the products are nonpolar, it is a reasonable hypothesis that the transition state also is nonpolar. In this case, the Hughes-Ingold hypothesis (Section 8.1) leads us to expect that the solvent will have little effect on the rate. The decomposition of azo compounds provides an example. The reaction is... 

The reactivity of the 5-position of 2-aminothiophene in diazo coupling, which is present also in the acylated derivatives, complicates the formation of a diazonium salt from 2-aminothiophene. Thus Steinkopf and Miiller obtained only an azo dyestuff, although they proved, through the isolation of small amounts of 2-thienyl diazonium chloride, the diazotizability of 2-aminothiophene which had earlier been denied. However, recent Russian work claims the preparation of 2-thienyldiazonium chloride by treating the double salt in 10% hydrochloric acid with sodium nitrite. Amazingly high yields (over 90%) of azo compounds were then achieved by coupling the diazonium salt solution with y9-naphtol, w-toluidine or with the 2-aminothiophene double salt. These authors have also studied the... 

The IV-phosphoryl group in azoxyfurazan 233 was removed under basic conditions (MeOH/KOH, -30°C) to give a mixture of azo compounds 234 and 235, albeit in low yield (Scheme 158) (93IZV609, 93RCB577). 

A general method for the preparation of azo compounds is reduction of azines to the hydrazine, followed by oxidation (12,33,128,132). Platinum... 

To incorporate a labile azo group as the essential active site to MAI, a series of azo compounds such as 2,2 azobisisobutyronitrile (AIBN), 4,4 -azobis(4-cyanopen-tanoyl chloride) (ACPC), 2,2 azobis (2-cyanopropanol) (ACPO), 2,2 azobis [2-methyl-N-(2-hydroxyethyl)prop-ionamide] (AHPA), etc., were used as starting materials for polycondensation with various diols, diamines, diacids, or diisocyanates. 

Azo-compounds and peroxides undergo photodecomposition to radicals when irradiated with light of suitable wavelength. The mechanism appears similar to that of thermal decomposition to the extent that it involves cleavage of the same bonds. The photodecomposition of azo-compounds is discussed in Section 3.3.1.1.2 and peroxides in Sections 3.3.2.1.2 (diacyl peroxides) and 3.3.2.3.2 (peroxyesters). Specific photoinitiators are discussed in Section 3.3.4. It is also worth noting that certain monomers may undergo photochemistry and direct photoinitiation on irradiation of monomer is possible. 

The kinetics and mechanism of the thermal and photochemical decomposition of dialkyldiazenes (15) have been comprehensively reviewed by Engel. The use of these compounds as initiators of radical polymerization has been covered by Moad and Solomon2 and Sheppard.50 The general chemistry of azo-compounds has also been reviewed by Koga et cr/./11 Koenig,3 and Smith.3J... 

Table 3.2 Selected Kinetic Data for Decomposition of Azo-Compounds ...

An important ramification of the photolability of azo-compounds is that, when using dialkyldiazenes as thermal initiators, care must be taken to ensure that the polymerization mixture is not exposed to excessive light during its preparation. 

While the rate of azo-compound decomposition shows only a small dependence on solvent viscosity, the amount of cage reaction (and hence f) varies dramatically with the viscosity of the reaction medium and hence with factors that determine the viscosity (conversion, temperature, solvent, etc.) 1... 

General concepts have been discussed in Section 3.1.8. General reviews on photoinitiation include those by Pappas,"6" 264 Bassi,26 Mishra300 and Osier and Yang267 and Gruber.268 The applications of azo-compounds and peroxides as photoinitiators are considered in the sections on those initiators (see 3.3.1.1.2, 3.3.2.1.2, 3.3.2.3.2). References to reviews on specific pholoinilialors are given in the appropriate section below. 

These conditions severely limit the range of initiators and monomers that can be used and require that attention to reaction conditions is of paramount importance. The relatively low incidence of side reactions associated with the use of azo-compounds (Section 3.3.1) has led to these initiators being favored for this application. Functional azo compounds used in telechelic syntheses include 9,19c> 198 10l99,2ml and ll20l,2<12. The acylazidc end groups formed with initiator 11 may be thermally transformed to isocyanate ends.201 2t, ... 

The long and controversial dispute of Bamberger and Hantzsch (see Sec. 1.1) led the majority of azo chemists to neglect the configuration of azo compounds by formulating stretched azo groups ( —N=N —) instead of bent structures... 

Mechanistically these isomerizations have been investigated very little, in contrast to those of azo compounds such as azobenzene and its derivatives. This subject is not... 

In 1988 Masoud and Ishak demonstrated that ( -arenediazo methyl ethers do not react with 2-naphthol in dry organic solvents such as dioxan, ethanol, 2-propanol, but only in the presence of water. The reactions are catalyzed by hydrochloric acid (even in the absence of water). Under such conditions almost quantitative yields of azo compounds were obtained. A careful and extensive kinetic investigation of the HCl-catalyzed dediazoniation of substituted benzenediazo methyl ethers, varying the HC1 concentration and the diazo ether/2-naphthol ratio (the latter either absent or in large excess), and comparing the observed rate constants with Hammett s acidity functions for dioxane and ethanol (see Rochester, 1970) indicated the mechanism shown in Schemes 12-8 to 12-10 (DE = diazo methyl ether, D+ = diazonium ion). 

A completely different method of synthesis of azo compounds from diazonium salts involving radical intermediates was found by Citterio et al. (1980, 1982 c), Cit-terio and Minisci (1982), and Fontana et al. (1988). It is a new general synthesis of arylazoalkanes based on the addition of an alkyl radical to an arenediazonium ion followed by reduction of the intermediate azo radical cation adduct by a metal salt (Scheme 12-80). The preferred source for the alkyl radical R in this reaction is an alkyl iodide, which gives rise to alkyl radicals cleanly in the presence of an arenediazonium salt and a Ti3+ or Fe2+ salt as in Scheme 12-81. The overall stoichiometric equation is therefore as given in Scheme 12-82. The yields vary between 36% and 79% (with respect to alkyl iodide). 

The thermolysis and photolysis of azo-compounds (R.N N.R) are well documented sources of radicals (for a review, see Strausz et al., 1972). As such they have been extensively used as radical sources in CIDNP studies (Fischer and Bargon, 1969 Gloss and Trifunac, 1969, 1970b, c Iwamura and Iwamura, 1970 Iwamura et al., 1970b Kasukhin et al., 1970). The application of CIDNP to unravel some of the mechanistic complexities of the decomposition of azo-compounds has now begun. 

In the stepwise decomposition of azo-compounds such as 4, products can arise from reactions within the primary diazenyl-alkyl radical pair or from the secondary radical pair produced by loss of nitrogen from the... 

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