Aromatic compounds photoadditions

A number of other aromatic compounds have been observed to undergo photoaddition to anthracene to yield products similar in structure to the anthracene dimer ... 

Photoadditions that arise by initial excitation of the aromatic compound are not common. Benzvalenes are readily attacked by hydroxylic compounds, and so irradiation of benzene in aqueous solutions of acetic acid, for example, results in the formation of a bicydic product (and an isomer derived from it by subsequent photoisomerizationl as a result of addition to the initially formed valence isomer (3.38). A different kind of photoaddition occurs when benzenes react photochemically with amines cyclohexa-T, 4-dienes are the major products (3.39), accompanied by cyclohexa-1.3-dienes, and unlike many of the photochemical reactions of benzene this does not suffer loss of efficiency in scaling-up. 

A. Gilhert, in W. M. Horspocl (ed.). Synthetic Organic Photochemistry, Plenum 11984). Photoaddition, photocycloaddition and photocyclization processes of aromatic compounds are all covered in this account of synthetic aspects of aromatic photochemistry. 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Photoaddition and substitution of electron-deficient aromatic compounds such as o-dicyanobenzene (o-DCNB), p-DCNB, and TCNB by use of group 14 organometallic compounds are classified to the reaction of the radical anions of electron-deficient aromatic compounds with carbon radical species generated... 

Mizuno K, Maeda H, Sugimoto A, Chiyonobu K. Photocycloaddition and photoaddition reactions of aromatic compounds. In Ramamurthy V, Schanze KS, eds. Molecular and Supramolecular Photochemistry. Vol 8. New York Marcel Dekker, Inc., 2001 127-316. 

The efficiency and selectivity of direct photoaddition reactions to aromatic compounds are usually not so high. Havinga reviewed direct photosubstitution reactions on aromatic compounds [55]. 

Photoaddition of neutral radicals R to aromatic rings occurs efficiently, especially for aromatic compounds having electron-withdrawing substituents. The addition of R to the radical anions of aromatic compounds affords the anion intermediates. The anion intermediates are converted to reductive addition products by protonation and to substitution products by elimination of an... 

The photoaddition of radicals to benzene rings usually gives substitution products as major products. The photosubstitution and reductive photoaddition occur competitively in the case of polycyclic aromatic compounds. The reductive addition becomes predominant over the substitution with increasing the number of the component rings. 

The present widespread interest in photoaddition reactions of aromatic compounds originates from observations made over 25 years ago that irradiation of maleic anhydride solutions in benzene produced the 2 1 adduct (33). Numerous accounts concerning the mechanism and scope of the reaction have since appeared and a kinetic model for the process has been proposed. "" The rate of reaction was found to be dependent on the concentrations of benzene, sensitizer, and oxygen but was essentially independent of temperature change in this last parameter does, however, have a pronounced effect on the distribution of 2 1 photoadduct isomers produced from maleic anhydride and methylbenzenes. Subsequent to the early findings on the maleic anhydride system several maleimides were also shown to form 2 1 adducts analogous to (33) with benzene. It is therefore most interesting that the 2 1 adduct (34) of... 

The analogous primary amine had been earlier studied by Wubbels and gave only the photo-Smiles reaction, but this process was not observed for the present system and the reaction is suggested to proceed by a photo-electron transfer mechanism to yield the radical ion pair. Photoaddition of N-H bonds to aromatic compounds has been known for some years, and the intramolecular process which leads to cyclization products has been reported for the reaction of 9-(aj-anilinolalkyl)phenanthrenes (220). The formation of the spiro-compounds (221) by addition of the N-H bond across the phenan-threne 9,10-positions is suggested to arise from the intramolecular exciplex in... 

Alkynes undergo cycloaddition on irradiation with benzene or naphthalene derivatives or with other aromatic compounds. With a benzene derivative the product is usually a cyclo-octatetraene which results from thermal electrocyclic ring-opening of the bicyclo-octatriene formed initially by 1,2-addition of the alkyne to the benzene ring (equation 61) . The intermediate can be trapped using a dienophile such as tetracyanoethylene (equation 62) ". The first step of the photoaddition process involves excitation of the alkyne , and orbital symmetry considerations suggest that concerted 1,2-addition is allowed if the alkyne is excited but not if the benzene is excited ... 

Several reviews have appeared during the year. The photoaddition reactions of aromatic compounds have been discussed with special attention to the mechanistic and structural features governing the outcome of the interaction of photoexcited arenes with alkenes and other arenes. Morrison has published an account of the photoactivation of distal functional groups in polyfunctionai... 

Mizuno, K., Maeda, H., Sugimoto, A., Chiyonobu, K. Photocycioaddition and photoaddition reactions of aromatic compounds. Molecular and Supramolecular Photochemistry 2001, 8,127-241. 

Numerous systems have been examined for photochromic activity and many of these involve the isomerisation of aromatic ring systems a monograph has been published which surveys many of these rearrangements. A short review, written in Chinese, on the photo-dimerisation of aromatic compounds and the photoaddition of enones and alkynes to aromatic rings has been reported. ... 

The well known photoalkylation of A-heterocyclic aromatic compounds has been extended to jjw-triazolo[4,3- ]pyridazine (39) the substitution can be effected by photoaddition of an alcohol followed by heating the initially formed adduct. 

Details are now presented of studies on the benzene-furan system, in which a [2 + 2]cycloadduct (43) and a [4 + 4]adduct (44) are the major products adduct (44) can be converted thermally or photochemically into (43). This photoaddition is the first example involving two monocyclic aromatic compounds benzene and thiophen give products in only very low yield on irradiation. 

Reviews relevant to this section which have been published within the year describe general photo-substitution reactions,72 and the mechanism of photoaddition-substitution reactions of six-membered aza-aromatic compounds,73 but both of these are in Russian. The reactions of aromatic nitro-compounds via their triplet states have received a very comprehensive review treatment hydrogen abstraction, reduction, incorporation of solvent fragments, and addition and substitution processes have been described for a wide variety of systems.74 In view ... 

This chapter deals with the photoisomerization, photoaddition and cycloaddition, photosubstitution, intramolecular photocyclization, intra- and inter-molecular photodimerization, photorearrangement reactions of aromatic compounds and related photoreactions. 

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. 

Table 6.3. Relative Rates of 2 1 Photoaddition of Maleic Anhydride to Some Aromatic Compounds at 30°C, Unsensitized, and Sensitized by Benzophenone"...

Other aromatic compounds like naphthalene, its derivatives, anthracene, its derivatives and other compounds shows photoaddition reactions. The photoaddition reaction may be between same compound or different compounds. 

We discuss here photoadditions to aromatic compounds, note certain characteristic features of Norrish Type II processes, and discuss further rearrangements of unsaturated ketones which are of considerable synthetic importance. Elsewhere examples of [2-1-2] photoadditions, ... 

Arnold, D. R., Wong, A. J., and Cameron, T. S., Radical ions in photochemistry. 12. The photoaddition of olefins to cyano aromatic compounds in polar solvents, PureAppl. Chem., 52,2609,1980. 

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