Photocyclizations of azobenzenes

Oxidative photocyclization of benzylideneaniline appears to proceed efficiently only in the presence of strong acid. The phenanthridine 31, however, has been prepared by irradiation of the imine 3228 few other examples of the photocyclization of arylimines have been reported.29 Strong acid is also required for successful photocyclization of azobenzenes to benzo-[c]cinnolines. Here, protonation is claimed to lower the reactive n, n excited state below the level of the unreactive n, it state. 2-Phenylazopyridine,... 

Corma [208] and Dutta [209] and their coworkers reported the photocyclization of azobenzene in zeolite HY and microporous aluminophosphate, respectively. They also demonstrated that the acid groups in these media directly intervene in the photochemical reactions of azobenzene. 

The reaction is prevented in the case of low-lying rot excited states, which are responsible for the lack of photocyclization of azobenzenes and benzylidene anilines. However, when the electronic configuration switches to a low-lying mt excited state upon complexation with Lewis acids, photocyclization takes place under these conditions [30]. Similarly, the presence of perchloric acid allows that, upon irradiation, (E)-4-benzyloxy-l,l,3,4-tetraphenyl-2-azabuta-l,3-diene is transformed in an isoquinoline derivative (Scheme 9.21) [31, 32]. 

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... 

Tung, C.-H., and Guan, J.-Q. (1996). Modification of photochemical reactivity by Nafion. Photocyclization and photochemical cis-trans isomerization of azobenzene. /. Org. Chem. 61, 9417-9421. 

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... 

The photochemistry of aromatic compounds is classified into the same categories adopted in the previous reviews in the series. The photoisomerization of arylalkenes, photoaddition and cycloaddition to aromatic rings, photosubstitution, photorearrangement reactions have less appeared in the period (2010-2011) considered. On the other hand, the photo-chromism including photoisomerization of azobenzenes and intramolecular photocyclization and cycloreversion of 1,2-diarylethenes, and the photodimerization have been widely developed. Supramolecular Photochemistry as a series of Molecular and Supramolecular Photochemistry was edited by Ramamurthy and Inoue in the period. In addition, it should be noteworthy that so many photochemical reactions in solid and/or crystalline states have appeared and developed. 

The light-induced yellowing of musk ambrette 48 is simulated41 by photolysis of 48 in 0.1 N methanolic sodium hydroxide solution to give the azobenzene 50 (through the intermediacy of azoxybenzene) and by-products 51 and 52, by intramolecular photocyclization (equation 36). 

In a reaction analogous to that of stilbene, protonated azobenzene iso-merizes at room temperature under irradiation with ( )e, z = 0.27 and = 0.25. Subsequently, the dehydro-photocyclization starts from the photo-stationary E/Z mixture to give protonated benzo[c]cinnoline with a yield of (j) = 0.02. This is parallel to the stilbene-to-phenanthrene reaction, and it proceeds not in concentrated sulfuric acid where the thermal Z -> E reisomerization is fast, but best in ca. 66% acid. Protonated benzo[c]cinnoline and hydrazobenzene are formed, and the latter undergoes the benzidine rearrangement. The complexation with metal ions in azobenzene crown ethers under participation of the n-electrons of the azo group leads to an increase of the E Z quantum yield from 0.25 of the metal free compound to 0.4 to 0.6 in the Ba and Ca "" complexes. The Z E yield decreases from 0.18 to 0.13 and 0.05. If the crown is larger, the values increase to 0.35 and (pz-iE = 0.45. " ... 

Electrocyclization provides an efficient way to synthesize a large variety of heterocyclic compounds. Among the many photocyclization reactions of aromatic compounds, electrocyclization is among the most frequently studied. An example of this is that illustrated in Scheme 1. This involves cyclization between the arene moieties of the protonated azobenzene 1, which reacts from the jtJt state reaction and is controlled by the Woodward-Hoffmann rules. Six rr-electrons are involved in the conrotatory cyclization. The reaction was not observed when unprotonated azobenzene was irradiated because in this case, the ntt state is populated. Since the photochemical electrocyclization is reversible, a consecutive trapping reaction is needed to obtain the final products in good yields. In the present case, as in many other reactions, a rearomatization step via oxidation took place to afford the final product 2. In the same way, the imine 3 can be cyclized to the phenanthridine derivative 4. In this case, the oxidation of the... 

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