Azobenzenes, cyclization

Treatment of the substituted azobenzene (561) with triethyl phosphite gave the triazapen-talene (563). It is likely that the 2-substituted benzotriazole (562) was initially formed which then underwent deoxygenative cyclization to (563) (74CL951). 

Indoles can be formed by metal-catalyzed cyclizations of azobenzenes on disubstituted alkynes (Equation (106)). 

Azobenzene is oxidatively cyclized to benzo(c)cinnoline (CIX) when exposed to sunlight in strongly acidic solutions,172 or in an acetic acid and ferric chloride mixture.124 As irradiation of azobenzene in neutral solutions is known to result in cis-trans equilibration, it seems that the... 

The substituted azobenzene (160) with triethyl phosphite gives the triazapentalene (162) via the 2-substituted benzotriazole (161) which undergoes a second deoxygenative cyclization (74CL951). 

Azobenzene readily undergoes both sensitized and direct cis-trans photoisomerization. However, the sensitized photostationary cis/trans ratios with high-energy sensitizers do not predict those obtained upon direct irradiation, indicating that much of the direct photoisomerization occurs from states other than the lowest triplet.455 As with stilbene, cyclization of the cis isomer apparently is a singlet-state reaction.456... 

This cyclization has also been observed in a variety of substituted azobenzenes,344 and is sometimes accompanied by the expulsion of methyl, chloro, iodo, and carbethoxy substituents from the position of cyclization. In the photocyclization of 2,4,6-trimethylazobenzene (328), a methyl migration is observed leading to the formation of... 

Six membered heterocyclics can be synthesized in various ways. Nitrogen analogues of stilbene, like the anilides of benzaldehyde or acetophenone sometimes form azaphenanthrenes 19). Similarly the plasma reaction of azobenzene yields a small amount of diazaphenanthrene 19>. Better results are observed in the cyclization of hydroxy, amino or nitro compounds. Suitable starting materials are molecules with two phenyl groups linked by a carbon or hetero atom. 

Tetraphenylethylene cyclizes anodically to 9,10-diphenylphenanthrene analogously to its photooxidative cyclization. The attempted anodic cyclization of cis- or frans-stilbene to phenanthrene however failed due to electrophilic reaction of the intermediate radical cation with the solvent 37S Primary aromatic amines are oxidized to radical cations which, depending on the pH of the electrolyte couple to aminodiphenylamines (C-N coupling (84) in Eq. (172) ), yield benzidines (85) at low pH (C-C coupling) or dimerize to hydrazobenzene (86) (N-N coupling) which is subsequently oxidized to azobenzene (Eq. (172) ) 2 5,376,377)... 

For all Azo-PURs, the quantum yields of the forth, i.e., trans—>cis, are small compared to those of the back, i.e., cis—>trans, isomerization—a feature that shows that the azo-chromophore is often in the trans form during trans<->cis cycling. For PUR-1, trans isomerizes to cis about 4 times for every 1000 photons absorbed, and once in the cis, it isomerizes back to the trans for about 2 absorbed photons. In addition, the rate of cis—>trans thermal isomerization is quite high 0.45 s Q 1 shows that upon isomerization, the azo-chromophore rotates in a manner that maximizes molecular nonpolar orientation during isomerization in other words, it maximizes the second-order Legendre polynomial, i.e., the second moment, of the distribution of the isomeric reorientation. Q 1 also shows that the chromophore retains full memory of its orientation before isomerization and does not shake indiscriminately before it relaxes otherwise, it would be Q 0. The fact that the azo-chromophore moves, i.e., rotates, and retains full orientational memory after isomerization dictates that it reorients only by a well-defined, discrete angle upon isomerization. Next, I discuss photo-orientation processes in chromophores that isomerize by cyclization, a process that differs from the isomeric shape change of azobenzene derivatives. 

There are only a few examples of carbene reactions that afford pyrazole derivatives. First, we mention the reaction of azobenzene with isopropylide-necarbene, which proceeds via an azomethine imine and its cyclization in indazole system 505. In the case of the unsymmetrically substituted azobenzenes, both isomeric products were obtained (77JA2597 84JA6015). Isopyrazoles are accessible via a convenient route with nucleophilic carbene... 

An early German patent first described the cyclodehydrogenation of azobenzene, 3,3 -dimethylazobenzene, and what was presumably 3,3 -dimethyl-4,4 -diaminoazobenzene to benzo[c] cinnolines by heating in melts of aluminum, potassium, and sodium chlorides in the presence or in the absence of air or oxidizing agents. The cyclization must proceed through... 

In a series of papers Lewis and co-workers have reported a comprehensive study of the photochemical reactions of azobenzenes in sulfuric acid. In the case of azobenzene itself, the products obtained were benzo[c]cinnoline (48%) together with benzidine (35%), and other azobenzenes gave products derived from the corresponding hydrazo compounds. The mechanism proposed is outlined in Scheme 2. There is an initial rapid establishment of a cis-trans equilibrium of monoprotonated azobenzene, with cyclization involving an excited form (14) of the cis-isomer, the lowest transition now being n-n. In the final step, dehydrogenation of the photocyclization product, 5,6-dihydrobenzo[c]cinnoline, is brought about by a second molecule of azobenzene, which is itself reduced to hydrazobenzene and... 

Cyclization of azobenzene and its derivatives tends only to occur under acid conditions. The reaction of the conjugate acids of azobenzene with anhydrous aluminium chloride, stannic chloride, and ferric chloride in 1,2-dichIoroethane has now been reported in detail.144 Benzo[c]cinnoline is formed, but the production of benzidine suggests that the reaction involves photodisproportionation. Similar cyclization is observed with the conjugate acids of azobenzenedicar-boxylic acids.145 With these compounds the reaction also occurs in 98% sulphuric acid to yield, from the 2,2 -isomer, benzo[c]cinnoline-4,7-dicarboxylic acid (90) and indazolo[2,1 - ]indazole-6,12-dione (91) in equal amounts the 3,3 -and 4,4/-dicarboxylic acids of azobenzene likewise give the corresponding cinnolines.145 It is important to note here that benzo[c]cinnoline is light-labile in 148... 

Diazaphenanthrenes are obtained by irradiation of benzylidenaminopyridines in concentrated sulphuric acid 27 the failure of this cyclization to occur in organic solvents is attributed to a rapid photochemical cis -> trans isomerization. Cyclization of 3-phenylazopyridine to give 2,9,10-triaza- and 4,9,10-triaza-phenanthrene has also been observed in sulphuric acid,28 whereas irradiation of azobenzene-2,2,-dicarboxylic acid (32) under the same conditions yields equal quantities of the expected benzo[c]cinnoline (33) and the indazolo[2,l-a]indazole... 

The photochemical reactivity of this herbicide class is well known and has been studied in aqueous media U), vapor phase CIO), organic solvents C21) and on soil surfaces C12. 13). Photoreduced Ce.g., amines and azobenzene and azoxybenzene dimers), -dealky-lated and cyclized derivatives appear to be the predominant photochemical transformation products. In practical application these pesticides must be soil Incorporated due in large part to their photochemical instability. Interestingly, the dinitroanilines are potent photostabilizers capable of protecting photo-labile insecticides on surfaces (14). 

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