Aromatics photooxygenation

From the oxidation of enamines with aromatic nitro compounds a-keto-enamines were obtained in modest yields (70J). Photooxygenation led to cleavage of the enamine double bond (706,707). 

Kondo et al. (182,183) reported a conversion of a fully aromatized phenolbetaine to a phthalide skeleton through photooxygenation. Reduction of norcoralyne (54) with zinc in acetic acid afforded dihydronorcoralyne (374), which was oxidized with m-chloroperbenzoic acid to the fully aromatized phenolbetaine 375 (Scheme 67). Photooxygenation of 375 in the presence of Rose Bengal, followed by reduction with sodium borohydride, gave rise directly to the phthalideisoquinoline 376 in 70% yield. The same phthalide (376) was also obtained from 2 -acetylpapaveraldine (129) (Section III,B,1). 

Kojima, M., Nakajoh, M., Matsubara, C. and Hashimoto, S. (2002). Photooxygenation of aromatic alkenes in zeolite nanocavities. J. Chem, Soc. Perkin Trans. 2, 1894-1901... 

The main types of substrates investigated so far are polycyclic aromatic compounds, aryl substituted carbo- and heterocyclic pentadienes, cyclic 1,3-dienes, furans, and olefins. It has turned out that type II photooxygenation of these compounds in solution occurs via the oxygen-activation mechanism. 

There are several reviews on various aspects of photooxygenation reactions. Bowen2 has reviewed type II direct photooxygenation reactions of polycyclic aromatic compounds in solution. 

The extension of direct photooxygenation reactions to polycyclic aromatic hydrocarbons as well as to aryl-substituted carbocyclic and heterocyclic pentadienes is due to the (exclusively preparative) work of Dufraisse and Etienne.5-20-29 Investigations on the mechanisms of these reactions were made by Bowen,2 Livingston,3 and Cherkasov and Vember.30-31... 

Cyclopentadienes, 1,3-cyclohexadienes, 1,3-cycloheptadienes, as well as furan and aklyl-substituted furans, have been investigated as substrates of photosensitized oxygenation reactions, while aromatic compounds such as anthracenes and tetracenes as well as aryl-substituted carbo-and heterocyclic pentadienes were studied in direct and indirect (photosensitized) photooxygenation reactions. 

On this ground, DCA was found a suitable sensitizer to induce the photooxygenation of a great variety of organic compounds such as alkenes [84,94-98], alkynes [99,100], sulfides [84,98,101], dienes [84], sulfoxides [102], cycloalkanes [103,104], cycloalkenes [105,106], epoxides [107,108], aziridines [109], allenes [110], dioxenes [111], p-dioxins [111,112], aromatic substrates [113], tertiary amines [114], and of great interest from the mechanistic point of view, sterically hindered olefines [97,115-117]. 

The cyanoaromatic-sensitized electron-transfer photooxygenation (Foote-type) is a useful preparative reaction with a very limited class of unsaturated olefins, namely those substituted by aromatics and, at the same time, totally inert towards singlet oxygen. On the other hand, in the previous sections, it has been many times underlined that electron-transfer reactions may compete with singlet oxygen formation and, above all, that the reactions of 02 may be the only observable outcome in the presence of singlet-oxygen acceptors. 

Since 1981 Santamaria has been reporting that the DCA-sensitized photooxygenation of certain aromatic compounds proceeds by two distinct mechanisms, each one beginning by an electron-transfer process [113,209,210]. In the first one, superoxide ion is involved, and in the second one singlet oxygen, produced through an unusual process, is the active oxygen species. In the same communication the author reported that several dinitro aromatic derivatives may behave as electron acceptors in solvents such as acetone or tetrahydrofuran. 

Redox photosensitization or co-sensitization by aromatic hydrocarbons has been utilized for enhancement of the efficiency of photoinduced electron transfer reactions. For example, the efficiency of the 9,10-dicyanoanthracene-sensitized photooxygenation of 1,2-diphenyloxirane in acetonitrile is enhanced appreciably by adding biphenyl as a co-sensitizer, giving 3,5-diphenyl-1,2,4-trioxolane in good yield [31-32]. This photoreaction does not take place in the absence of biphenyl. Schaap proposed that in this photoreaction the primary electron transfer reaction occurs from biphenyl (BP) to DCA to produce biphenyl radical cation BP and DCA . The secondary electron transfer from the oxirane to BP produces BP and the radical cation of the oxirane which is converted into the trioxolane (Scheme 5). 

A new electron-transfer mediator for photooxygenation of alkenes (to give allylic hydroperoxides) and methylarenes (to give aromatic aldehydes) is 9-mesityl-10-methylacridinium perchlorate (1). ... 

Photooxidation of the triphenylmethyl cation (as its tetrafluoroborate salt) in the presence of an aromatic donor was similarly found to afford bis(triphenyl-methyl) peroxide [31]. The mechanism was proposed to proceed through initial electron transfer from the aromatic donor to the singlet-excited triphenylmethyl cation to give the triphenylmethyl radical. Reaction of the radical with triplet oxygen, and subsequent coupling with another triphenylmethyl radical gave the observed peroxide. It was noted that electron transfer from the tetrafluoroborate counterion to the excited state cation could not be completely excluded, because the cation was slowly photooxygenated in the absence of an electron donor. 

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