Azobenzenes, formation

Azobenzene formation signals reaction of the triplet nitrene, substituted 3H-azepines come from the trapping of dehydroazepines. Clearly substituents on aryl azides affect the formation and reactivity of these intermediates. The precise nature of the substituent effects was revealed by application of time-resolved absorption experiments that will be described later. However, from the perspective of product yields and synthetic applications, two noteworthy trends should be mentioned here. 

In this account httle has been said about the spin states of the carbenes and nitrenes which react, and it hcis been tacitly assumed that rearrangement occurs from singlet states, whereas amine and azobenzene formation most probably occurs from triplets. Since we are dealing with thermal reactions, the law of spin conservation 88) would predict that the first-formed species are singlets. However, this is claimed not to be absolutely necessary, for the direct formation of triplets. 

Jones R, Tredgold R H, Hoorfar A, Allen R A and Hodge P 1985 Crystal-formation and growth in Langmuir-Blodgett multilayers of azobenzene derivatives—optical and structural studies Thin Solid Films 134 57-66... 

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... 

Chlorination of the azobenzene complex 463 with chlorine produces mono-chloroazobenzene with regeneration of PdCN. Then complex formation takes place again with the chlorinated azobenzene. By this sequence, finally tetra-chloroazobenzene (503) is obtained using a catalytic amount of PdCT. The reaction, carried out by passing chlorine gas into an aqueous dioxane solution of azobenzene and PdCf for 16 h, gives a mixture of polychlorinated azoben-zenes[455]. 

Oxidation. Aromatic amines can undergo a variety of oxidation reactions, depending on the oxidizing agent and the reaction conditions. For example, oxidation of aniline can lead to formation of phenyUiydroxylamine, nitrosobenzene, nitrobenzene, azobenzene, azoxybenzene or -benzoquinone. Oxidation was of great importance in the early stages of the development of aniline and the manufacture of synthetic dyes, such as aniline black and Perkin s mauve. 

We have noted above that benzocrowns may be nitrated quite readily. This approach was used in the formation of a photoresponsive bis-crown (see Sect. 3.8) wherein the nitrobenzo crowns reductively dimerize to the corresponding azobenzene. Kikukawa, Nagira and Matsuda have utilized 4-nitrobenzo-15-crown-5 in a somewhat different way during the synthesis of 4 -vinylbenzo-l 5-crown-5Nitration is effected using nitric acid in a mbcture of chloroform and acetic acid. 

Heat a small quantity with Fehling s solution and observe the formation of cuprous oxide. The hydrazobenzene is oxidised to azobenzene. 

When the substituent groups in the polyphosphazenes were azobenzene [719] or spiropyran [720] derivatives, photochromic polymers were obtained, showing reversible light-induced trans-cis isomerization or merocyanine formation, respectively. Only photocrosslinking processes by [2+2] photo-addition reactions to cyclobutane rings could be observed when the substituent groups on the phosphazene backbone were 4-hydroxycinnamates [721-723] or 4-hydroxychalcones [722-724]. 

Organic synthesis 38 [OS 38] Formation of an azobenzene derivative from N,N-dimethylaniline with 4-nitrobenzenediazonium tetrafluoroborate... 

Organic synthesis 39 [OS 39) Formation of azobenzene derivatives for pigment production... 

Sljukic B, Bank CE, Compton RG (2004) The search for stable and efficient sonoelectroca-talysts for oxygen reduction and hydrogen peroxide formation azobenzene and derivatives. Phys Chem Chem Phys 6 4034-4041... 

Subtle differences in the behavior of azoarenes toward cobalt carbonyl derivatives are observed in regard to metal-complex formation. Azobenzene is transformed by dicobalt octacarbonyl in processes of orthometallation and carbonyl insertion into 2-phenylindazolin-3-one (see Section IV,D,2). In contrast, cyclopentadienylcobalt dicarbonyl effects N—N bond cleavage, and carbonylation of the isolable complex 88a provides 1 -phenylbenzimid-azolin-2-one (Scheme 106).171... 

The oxidative properties of ditelluronium and diselenonium dications are similar to S-S dications. For instance, interaction of thiophenol or 1,2-diphenylhydrazine with either the diselenonium dication 113 or the ditelluronium dication 114 leads to almost quantitative formation of diphenyldisulfide or azobenzene and reduced bis-chalcogenides.113,114,124 Reduction of 113 and 114 with NaBH4 gives the same products as above but proceeds quantitatively via direct electron transfer (Scheme 47).128... 

For obtaining the information on fabrication of noncentrosymmetric LB films with highly efficient second-order optical nonlinearity, six azobenzene-linked amphiphiles were synthesized as a model compound, and their molecular hyperpolarizabilities (3, monolayer-formation at the air-water interface, and molecular orientation and second-order susceptibilities of the azobenzene-linked amphiphiles LB films were evaluated. The molecular structures of the azobenzene-linked amphiphiles are shown in Fig.2. 

The simplest azo compound, azobenzene, exists as a mixture (Scheme 4-18) of a stable trans (4.19) and an unstable cis (4.20) form [38,39]. Formation of the cis isomer is induced by exposure to light, the quantum yield of the process depending upon the wavelength of the light employed [40]. The proportion of cis isomer can be appreciable in an equilibrium mixture. Thus a concentration of 24% of this unstable form builds up within a few hours when an acetic acid solution of azobenzene is exposed to sunlight in shallow white trays. Reversion to the trans form occurs readily on heating and is catalysed by a variety of substances that can function as electron donors or acceptors [41]-... 

The mixed liposomal solutions were prepared by the ethanol-injection method(13) in order to obtain completely transparent solutions. It is interesting to note that miscibility of the photochromic amphiphiles with DPPC depend on the position of bulky azobenzene. If azobenzene is incorporated close to the end of long alkyl chain, a stable mixed bilayer state cannot be formed. On the other hand, when the azobenzene moiety is located near the head group or at the center of the hydrocarbon tail, the azobenzene amphiphiles are successfully incorporated into the bilayer membrane. No individual micelle formation nor phase separation in the bilayer was observed at 25 °C by absorption spectroscopy. However, the microstructure of the mixed liposomes depends on the type of azobenzene amphiphiles. 

Weener et al. prepared photo-responsive monolayers from azobenzene modified polypropylene imine) dendrimers which also hold promise in the area of optical data storage [74], A fifth generation polypropylene imine) dendrimer was functionalized with equal amounts of palmitoyl and azobenzene containing alkyl chains, resulting in the formation of an amphiphilic copolymer with a random shell structure (Figure 16.5). 

With concentrated mineral acids azobenzene gives red salts, as may be shown by pouring hydrochloric acid on it. Addition of hydrogen leads to the re-formation of the hydrazo-compound. Oxygen is added on and the azoxy-compound formed by the action of hydrogen peroxide or nitric acid. The synthesis of asymmetrical aromatic azo-compounds from nitroso-compounds and primary amines was discussed above. 

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