Azo compound

Azo compounds are widely used as radical initiators in organic synthesis [1], AIBN (2,2 -azobisisobutyronitrile) is the most commonly used initiator because of its high decomposition ability and stability. Azo compounds are decomposed by heat to the corresponding alkyl radicals and nitrogen [1]. As previously described, it is known that they undergo decomposition via the cis form by absorbing light [3]. 

Recently, azo-type radical initiators that work below room temperature have been discovered, and are applied in the stereoselective construction of a carbon-carbon bond [6], Furthermore, it was reported that azo compounds possessing the hydrophilic functional groups act as effective initiators in aqueous media [7]. Molecular design of practical water-soluble radical initiators has been performed recently [8]. In this section, the discussion is focussed on the more recent developments of representative and useful radical initiators. 

Thermolysis of peroxides has been used in the study of radical reactions for a long time. On heating, peroxides produce alkoxy radicals and acyloxy radicals by the cleavage of the peroxide bond. The nature of the radicals produced is generally electrophilic, although it is dependent on the structure of the radical species. A brief description of the widely used peroxides is given below. 

Aliphatic diacyl peroxides are generally less stable than their aromatic counterparts. Acetyl peroxide decomposes at 25°C, so that careful handling is required to avoid dangerous explosion. These compounds are sensitive to shock, light, heat and metals. 

Half-lives of the peroxides discussed so far [14] are given in Table 1. 

Azo-compounds are conveniently prepared by oxidation of hydrazo-compounds, and 1-chloroisatin is a new oxidant to effect this transformation. Seven- 

Francotte, R. Merenyi, B. Vandenbulcke-Coyette, and H.-G. Viehe, Helv. Chim. Acta, 1981, 64, 1208. 

The first preparation of both the cis- and trans-isomers of azocyclopropane have been described.  

A preparation of azo-compounds by the A -deoxygenation of azoxy-compounds utilizes Fe3(CO)i2 or Mo(CO)6 on alumina.  

From PE spectroscopy results for azomethane (Haselbach and Heilbron-ner, 1970) the energy of the n MO is known to lie between those of the n + and n orbitals. Thus, a HOMO-LUMO crossing results and the trans-cis 

In agreement with the theoretical results, the photoisomerization of simple azoalkanes is found to be rather effective. For azomethane in benzene at 25 C quantum yields of = 0.42 and = 0.45 have been observed (Thompson et al., 1979). Cis-azo compounds are moderately stable. Only tertiary ct -azoalkanes are thermally unstable and decompose to nitrogen and radicals. (See Section 7.2.2.) 

In azobenzene the cis-trans isomerization in the ( r.. i ) state apparently proceeds along a twisting path whereas in the (n, r ) state it proceeds along the inversion path. This has been suggested by the fact that for azobenzenes such 

3 Homolytic Bond Scission. - 2.3.1 Azo compounds. Azo compounds such as 2,2 -azobisisobutyronitrile (AIBN) are widely used as polymerization in-itiators. Upon UV excitation AIBN decomposes into a nitrogen molecule and a geminate pair of two 2-cyano-2-propyl radicals  

Laser flash experiments of trans-azocumene using TR EPR have been reported by Savitsky et and a reversed TM CIDEP was observed with AIBN. These time-resolved investigations were extended by triplet sensitization of symmetrical azoalkanes with acetone and benzophenone. The photosensitized dissociation of AIBN yields a pair of 2-cyano-2-propyl radicals [2 R-] in a triplet or singlet state according to the reaction scheme 

The radical-pair polarization (RPM) of the 2-cyano-2-propyl radical generated by laser flash photolysis of AIBN has been studied in solvents of various viscosities. The ratio of F-pair and geminate-pair polarization determined increases with higher viscosity. These results are discussed in terms of a viscosity-dependent cage effect. 

The triplet-sensitized photo-decomposition of azocumene into nitrogen and cumyl radicals was investigated by TR EPR and optical spectroscopy. The cumyl radicals observed carry an initial spin polarization and are formed with a yield which depends on both the solvent viscosity and the strength of the external magnetic field. This phenomenon is interpreted in terms of a depopulation-type triplet mechanism, i.e. competition between decay into radicals and fast triplet sub-level intersystem crossing back to the azocumene ground state. 

Similar to the cases with platinum metals, the rate of hydrogenolysis of hydrazobenzene appears to be smaller than that of azobenzene over Raney Ni as well,234 although Wisniak and Klein observed the formation of only azoxy- and azobenzenes, and no hydrazobenzene, during the course of hydrogenation of nitrobenzene over Raney Ni at 170°C and 1.38 MPa H2.235 

Hydrogenation of diazo coupling products of 3-hydroxypyridines has been shown to be a useful method for the preparation of 2-amino-5-hydroxypyridines. The synthesis was carried out with p-nitrobenzeneazo derivatives that were hydrogenated over Pd-C in acetic acid, as in the following example 241 

Space limitations make it impossible to thoroughly survey this extensive area of hetero Diels-Alder chemistry. The following sections will highlight some of the more recent advances and previous reviews which should be consulted for additional information. -  

Various azodicarboxylate esters have been used extensively as dienophiles. - Several representative examples of this type of cycloaddition are listed in Table 6-III. As mentioned above, these reactions appear to be concerted proeesses that retain the 1,3-diene configuration in the adducts. The more bulky esters tend to react more slowly with dienes. Thus, dimethylazodicarboxylate reacts 5-6 times more rapidly with cyclopentadiene than does the ethyl ester, whereas the fert-butyl compound reacts sluggishly.  

Kresze et al. have described an interesting pyrrole synthesis based on an azo Diels-Alder reaction [Eq. (16)].  

In this particular example, reduction of the initial adduct with zinc gave a trisubstituted pyrrole in 33% overall yield. The mechanism of this transformation is not immediately obvious. 

Diels-Alder adducts of various azodicarboxylates have been used extensively in synthesis of cyclic azoalkanes and pyridazine derivatives. - Since previous reviews have adequately dealt with this material, it will not be reiterated here. 

T0069 ARS Technologies, Inc., Ferox, Reduction of Chlorinated Organics in the Vadose Zone 

T0641 Radian International, L.L.C., AquadetoxySoil Vapor Extraction (SVE) T0833 Ultraviolet Oxidation (UV/Oxidation)—General T0889 White Rot Fungus—General 

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Azo-compounds can be obtained by reduction of nitro-compounds, or by oxidation of hydrazo-compounds. They are usually prepared, however, by reacting a phenol or amine with a diazonium salt. The coupling usually takes place in the position para to the hydroxyl or amino group, but if this position is occupied it goes to the ortho position, e.g. 

The azo-compounds are usually very stable, and can be directly chlorinated, nitrated and sulphonated. On vigorous reduction the molecule splits at the azo group to give two molecules of primary amines, e.g. bcnzene-azophenol gives PhNH2 and p-HOC H NHa. 

The diazoamino-compounds are usually yellow in colour, and do not dissolve in acid they can usually be isolated and crystallized without decomposition. When treated with HNO2 two molecules of diazonium salt are formed. Form an azo compound when warmed with an amine and its hydrochloride, e.g. 

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

When an aqueous solution of a diazonium salt is added to an alkaline solution of a phenol, coupling occurs with formation of an azo-compound (p. 188). If ho vc cr the ntiueous solution of the diazonium salt, t. . ., />-bromohenzene diazonium chloride, is mixed with an excess of an aromatic hydrocarbon, and aqueous sodium hydroxide then added to the vigorously stirred mixture, the diazotate which is formed, e.g., BrC,H N OH, dissolves in the hydrocarbon and there undergoes decomposition with the formation of nitrogen and two free radicals. The aryl free radical then reacts with the hydrocarbon to give a... 

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... 

It must be noted, however, that nitroso, azoxy and azo compounds when subjected to the same treatment yield res])ectively hydroxylamines, hydrazo and hydrazine compounds, all of which reduce ammoniacal silver nitrate solution in the cold. 

Azo compounds (ArN=NAr) are prepared bj the interaction of a diazonium salt with a phenol in the presence of sodium hydroxide, for example ... 

Compounds containing two primary amino groups attached to a benzene ring can be prepared by the reduction of dinitro compounds and of nitroanilines, usually with tin or stannous chloride and hydrochloric acid or with iron and very dilute hydrochloric acid. / ara-diamines may also be obtained by the reduction of aromatic amino-azo compounds (e.g., p-aminodimethylanihne from methyl orange, see Section IV,78). p-Phenylenediamine may also be prepared from p-nitroacetanilide reduction with iron and acid yields p-amino-acetaniUde,.which may be hydrolysed to the diamine. 

Azo compounds may be identified by examination of the amine(s) formed on reduction in acid solution (see Sections IV,76 and IV,78) ... 

Supplement 1942 195-449 Hydroxy-amines Aminoethyl alcohol, 274. Carbonyl-amines Aminoacetaldehyde, 307. Aminoacetone, 314. Hydroxy-carbonyl amines Glucosamine, 328. Aminocarboxylic acids Glycine, 333. Hydroxylamines, 534. Hydrazines, 546. Azo Compounds. 562. Oryano-metallic Compounds, 580. 

Supplement 1952 2504-2665 Furfuracrylic acid, 300. Sulphonic acids, 667. Amines, 683. Hydroxylamines, 637. Hydrazines, 639. Azo compounds, 643. Diazo compounds, 661. Carbon-metal compounds, 663. 

Supplement 1953 3242-3457 Hydroxy-carboxylic acids, 190 In i doxylic acid, 226. Carbonyl-carboxylic acids, 284. i Sulphonic acids, 386 Quinoline sul-phonic acid, 390. Amines, 419 2-Aminopyridine, 428. Amino-carboxylic acids, 541 Tryp- tophane, 545. Hydrazines, 563. Azo. compounds, 572. Diazo compounds, 590. ... 

Supplement 1955 3634-3793 Sulphonic acids Indigo-disulphonic acid (indigocarmine), 304. Amines, 308. Keto-ammes Pyramidone, 452. Allan-toin, 474. Murexide, 499. Amino-carboxylic acids Histidine, 513. j Hydrazines, 531. Azo compounds, 535. 1... 

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

Again, it is noteworthy that 4-substituted 5-hydrdxythiazoles (24) react like 5-hydroxy-THISs with alkynes to give pyrroles and sometimes with alkenes to give exo-cycloadducts (Scheme 22). In the latter case other processes compete with the cycloaddition, becoming dominant when 24 is treated with azo-compounds, enamines, or heterocumulenes (31). 

Azo compounds are often highly colored and many of them are used as dyes... 

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