Azoarenes

Possibly the most significant discovery in the metabolism of aromatic azo compounds had implications that heralded the age of modem chemotherapy. It was shown that the bactericidal effect of the azo dye Prontosil in vivo was in fact due to the action of its transformation product, sulfanilamide, which is an antagonist of 4-aminobenzoate that is required for the synthesis of the vitamin folic acid. Indeed, this reduction is the typical reaction involved in the first stage of the biodegradation of aromatic azo compounds. 

Aromatic azo compounds, many of which are sulfonated, are components of many commercially important dyes, colorants, and pigments, so that attention has been directed to their degradation and transformation. These compounds are often considered recalcitrant, although then-transformation has been accomplished by reduction to amines with scission of the Ar-N=N-Ar bond to produce arylamines. The amines may then be degraded, for example, 6-aminonaphthalene-2-sulfonate by dioxygenation and ring fission to 5-aminosalicylate (Hang et al. 1991). This is then 

Bliimel S, M Contzen, M Lutz, A Stolz, H-J Knackmnuss (1998) Isolation of a bacterial strain with the ability to ntilize the snlfonated azo compound 4-carboxy-4 -snlfoazobenzene as the sole source of carbon and energy. Appl Environ Microbiol 64 2315-2317. 

Bliimel S, H-J Knackmnss, A Stolz (2002) Molecnlar cloning and characterization of the gene coding for the aerobic azorednctase from Xenophilus azovorans KF46. Appl Environ Microbiol 68 3948-3955. 

Chen H, SL Hopper, CE Cerniglia (2005) Biochemical and molecular characterization of an azoreductase from Staphylococcus aureus, a tetrametric NADPH-dependent tlavoprotein. Microbiology (UK) 151 1433-1441. 

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Open-chain and cyclic compounds containing azo groups (-N2 —), such as azoalkanes, azoarenes, pyrazolines, triazolines, etc. may also eliminate N2, but these reactions are called azo-extrusions (IUPAC, 1989 a). The terms denitrogenation and nitrogen extrusion, both used by Adam et al. (1992, 1993) and by Adam and Sengelbach (1993) should not be used. They are superfluous and ambiguous. 

The results were interpreted on the basis of a mechanism that starts with the photolytic formation of a radical cage consisting of an aryldiazenyl and and arylthiyl (Ar - S ) radical, followed by diffusion of both radicals out of the cage. Three reactions of the aryldiazenyl radical are assumed to occur bimolecular formation of the azoarene and N2, or of biphenyl and N2 (Scheme 8-37), the monomolecular dediazoniation (Scheme 8-38), and recombination with the thiyl radical accompanied by dediazoniation (Scheme 8-39). In addition, two radicals can react to form a di-phenyldisulfide (Scheme 8-40). 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

The dicobalt octacarbonyl-catalyzed transformation of azoarenes into 2-arylindazolin-3-ones by carbonylation has been known for many years157 high pressures of carbon monoxide are required and under more forcing conditions the products are quinazoline-2,4-diones (Scheme 92). Reactions... 

In reactions closely related to the carbonylation processes described above, the dimeric azoarene palladium complexes (78) can be transformed efficiently in two steps into 3-imino-2-phenylindazolines (Scheme 95).162... 

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... 

In contrast to thermal uncatalyzed reactions of /V-acylsulfinylamines with aryl isocyanates which give rise to azoarenes,188 the cobalt or iron carbonyl-catalyzed process gives additionally 3,5-dioxo-l,2,4-triphenyl-1,2,4-triazolidine (Scheme 124).189 The only possible restriction on this simple urazole synthesis would be the expectation that the substituents on the reactants must be the same to prohibit exchange. 

Azo- and azoxyarenes have been repeatedly observed during reactions of nitrosoarenes with thiols5-7,11,29 33 35 36 38. The latter family presumably emerges from the interaction of the TV-hydroxy arylamine with unreacted nitrosoarene, a reaction proceeding even in neutral solutions2,6 58. The formation of azoarenes may be due to condensation of the end-product arylamine with still unreacted nitrosoarene59. 

Catalysed oxidation of primary and secondary amines generally has little synthetic value. Primary amines yield either a mixture of nitriles and amides (ca. 30%) or, in the case of arylamines, the azo derivatives (42-99%) [39], Symmetrical and non-symmetrical azoarenes are also produced in good yields ( 60%) from the reaction of acetanilides with nitroarenes under basic solidtliquid conditions, although higher yields are obtained using TDA-1 [40],... 

Cp has been discussed [170b]. Alkylation with haloalkanes (often iodoalkanes), triflates (alkyl, benzyl, cyclopropyl), or [RsO] (R = Me, Et) is often the best entry to vinylidenes of any particular system. Other common electrophiles, such as halogens (Cl, Br, I), acylium ([RCO] ), azoarenes ([ArN2] ), tropylium ([C7H7] ), triphenylcarbenium (trityl, [CPhs] ), arylthio (ArS) and arylseleno (ArSe) have also been used. 

Dihydroxy-azoarene sind durch oxidative Knpfernng aus Hydroxy-azoarenen zugang-lich. Die Farbstolfe konnen durch halbstundiges Kochen in konzentrierter Salzsaure frei-gesetzt werden, s. Bd. X/3, S. 458-461 (1965). 

Unter modifizierten Reaktionsbedingungen wird die Wallach-Umlagerung zur Herstellung halogenierter Azoarene, die sich bei der Reaktion mit Chlorsulfonsaure als Neben-produkte bilden, zur Hauptreaktion ... 

The proximity effect of the functional groups in 47 is instructive. Thus, A-aryl aldimines of the type, ArCH=NAr, are reported to give azoarenes [Ar N=NAr ] and aldehydes [ArCH=0] with DAIB (77IJC(B)376), while phenols are oxidized to quinones, quinol ethers, or quinone acetals, depending on the nature of the reaction medium (92MI2, 01OR327). 

A recent X-ray structural determination of [(azb)2Rh(acetate)] provides the first direct confirmation that chelating azoarenes bond through the nitrogen lone pair rather than the N—N ir-system. The two Rh—C bonds are cis and the two Rh—N bonds are trans to one another 157). 

A marked distinction from the behavior of aliphatic azocompounds is the great thermal stability of azoarenes. Aryl radicals can be produced by thermolysis only with the assistance of a stable companion radical such as the triphenylmethyl radical in the case of phenylazotriphenyl methyl. 

As in thermolysis, aliphatic azocompounds readily decompose in photolysis to alkyl radicals and nitrogen, while azoarenes undergo facile geometrical isomerization. 

NUroarene reduction. Either azoarenes or azoxyarenes can be obtained on the reaction of nitroarenes with Bi-KOH in methanol. 

Deoxygenation. Ultrasonically dispersed potassium reduces azoxyarenes to azoarenes at room temperature. 

Oxidations. Allylic and benzylic alcohols are converted to the aldehydes with great selectivity by a Cr-mediated oxidation with sodium percarbonate. The phase-transfer oxidation of anilines with KMn04 results in azoarenes. ... 

Eliminations. Treatment of o-alkyl A -(r-butylthio)azoarenes with r-BuOK accomplishes cyclization and delivers l//-indazoles. Unsaturated a-amino nitriles are obtained on alkylation of 2-(N-methylanilino)-2-benzenesulfenylacetonitrile, due to elimination of benzenethiol in situ. 

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