Electron-Transfer Photooxidation

Despite the fact that singlet oxygen oxidizes tetramethylethylene (16) via the ene pathway, with abstraction of one of the 12 a protons, under electron-transfer photooxidation conditions dioxetane 32 (R = Me) has been generated [29]. The electron-deficient sensitizer 9-mesityl-10-methylacridinium ion (Acr+-Mes) in its excited state Acr -Mes + was generated, in which the alkene radical cation and 

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With tetraphenylcyclopropane, electron transfer photooxidation apparently gives easier fragmentation (126), for rearrangement products can be isolated, eq. 46 (127) ... 

In 2004, a report appeared that 9,10-dimethylanthracene-9,10-endoperoxide (48) arose from an electron-transfer photooxidation of 9,10-dimethylanthracene (38) with the use of the sensitizer Acr+-Mes in 02-saturated CH3CN at 0 °C [32]. Subsequent coupling of the anthracene radical cation and the superoxide ion generated endo-peroxide 48. Endoperoxide 48 was detected during the initial stage of the photooxidation, but was not isolated over time, the reaction yielded 10-hydroxanthrone, anthraquinone, and H202. 

Griffin s group [105], demonstrated the formation of ozonides by electron-transfer photooxidation of small-ring cycloalkenes, a further significant achievement to the mechanistic pathways involved in these processes. 

Some alkynes are also subject to electron-transfer photooxidation. Thus diphcnylacetylene is converted to benzil and benzoic acid, the product of further oxidation (equation 11). ... 

A time resolved CIDEP study of radical pair systems derived by electron-transfer photooxidation of carbazole derivatives using maleic anhydride has revealed that the singlet state of the radical ion-pair has an energy which is greater than that of the triplet state. From this evidence the conclusion is drawn that the sign of the apparent exchange interaction is positive. 

Wagner J.R., van Lier J.E., Johnston L.J., Quinone sensitized electron transfer photooxidation of nucleic acids Chemistry of thymine and thymidine radical cation in aqueous solution, Photochem. Photobiol., 1990,52, 333-343. 

Barton and coworkers have shown that proteins can in fact modulate the DNA electron transfer [168]. Methyltransferases are enzymes that recognize distinct DNA sequences, e.g., 5 -G CGC-3, and effect methylation by extrading the target base cytosine ( C) completely out of the DNA duplex while the remainder of the double helix is left intact. The methyltransferase Hha 1-DNA complex is a well-characterized example, revealing that the structure of the DNA is significantly but locally distorted [169,170]. In a recent study, Raj ski et al. used DNA duplex 20 containing the M.Hha I binding site between two oxidizable 5 -GG-3 sites [168] (Fig. 20). The duplex contains a complementary strand, selectively 5 -modified with a Rh intercalator that can function as a photooxidant. Upon... 

FIG. 20 DNA duplex 20 used for studies regarding the protein-modulated DNA electron transfer. Methyltransferase M.Hhal is capable of binding to the shadowed recognition site between two oxidizable 5 -GG-3 sites (outlined letters). The complementary strand of the duplex contains the Rh intercalator, [Rh(phi)2bpy ], at its 5 end, which can function as a photooxidant. (Adapted from Ref. 168.)... 

Li, X. and Ramamurthy, Y. (1996). Electron transfer reactions within zeolites photooxidations of stilbenes. Tetrahedron Lett. 37, 5235-5238... 

Lakshminarasimhan, P., Thomas, K.J., Johnston, L.J. and Ramamurthy, V. (2000). Wavelength dependent oxygen mediated electron-transfer reactions within M+ Y zeolites photooxidation and reduction of 1,1-diarylethylenes. Langmuir 16, 9360-9367... 

In a number of cases ITIES can be used to separate the products of a photoinduced electron-transfer reaction. An early example is the work by Willner et al. [7] at the water/toluene interface, who studied the photooxidation of [Ru(bpy)3]2+ in the aqueous phase. The excited state was quenched by hexadecyl- 4,4 bipyridinium, which becomes hydrophobic on reduction and crosses to the toluene phase. There are other examples and mechanisms at the present time their main interest resides in their chemistry, and in the separation of products that can be achieved at the interface. 

The photo-oxidation of the aryl-substituted cycloheptatrienes 7-(/ -methoxy-phenyl)cycloheptatriene and 7-, 1- and 3-(/ -dimethylaminophenyl)cycloheptatrienes to the corresponding radical cations in de-aerated acetonitrile solution was accomplished by electron transfer to the electronically excited acceptors 9,10-dicyanoanthracene, iV-methylquinolinium perchlorate, iV-methylacridinium perchlorate and l,T-dimethyl-4,4-bipyridinium dichloride. In the case of l- p-methoxyphenyl)cycloheptatriene (62), deprotonation of the radical cation occurs successfully, compared with back electron transfer, to give a cycloheptatrienyl radical (63) which undergoes a self-reaction forming a bitropyl. If the photooxidation is done in air-saturated acetonitrile solution containing HBF4 and one of the acceptors, the tropylium cation is formed. Back electron transfer dominates in the / -dimethylaminocycloheptatrienes and the formation of the cycloheptatrienyl radical is prevented. 

The oxidative behaviour of the acridinium carbocations 61 was also explored by the group of Lacour in the photoinduced electron transfer reaction [160]. In the amount of 2 mol%, the achiral hindered acridinium salt 61 catalyzed the aerobic photooxidation of the primary benzylic amine to benzylimine in the yield of 74% (Scheme 63). 

A macromolecular complex that allows plants to harvest the sun s photic energy by absorbing photons and using their energy to catalyze photooxidation of plastocyanin, the copper protein situated in the lumen of thylakoid membranes, which undergoes subsequent electron transfer reactions. These reactions are illustrated in Fig. 1. 

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