Azobenzenes, cyclization benzo cinnolines

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... 

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... 

---