Redox photosensitization

The second direction in which redox properties of sulfones and sulfoxides could manifest themselves in photochemistry is redox photosensitization " "". In such a photosensitization the photosensitizer is transformed by light into a short-lived oxidant or reductant able to react with the substrate to be activated. Tazuke and Kitamura" have discussed the parameters to play with when one... 

Tab. 4.5.1. Most important redox-photosensitizers, radicals and radical ions used in the studies on the oxidative repair of pyrimidine cyclobutane dimers.

Further studies have shown that, to obtain an efficient amination, it is necessary to take into account both the difference in oxidation potential between the photocatalysts and the substrates and the positive charge distribution in the cation radicals of the olefin. The synthetic utility of the method was proven by the successful preparation of an aminotetraline, itself an intermediate for the synthesis of a compound with biological activities such as 2-aminoindan (26, Scheme 3.16b) [39]. As with the last synthesis, the redox-photosensitized amination occurred with no need for acids or bases, as usually required when using general protocols. 

Yasuda, M., Kojima, R., Tsutsui, H., Utsunomiya, D., Ishii, K., Jinnouchi, K., Shiragami, T., and Yamashita, T. (2003) Redox-photosensitized animations of 1,2-benzo-l,3-cycloalkadienes, arylcyclo-propanes, and quadricyclane with ammonia. Journal of Organic Chemistry, 68, 7618-7624. 

Majima T, Pac C, Nakasone A, Sakurai H. J Am Chem Soc 1981 103 4499. Pac et al. were probably the first who introduced redox photosensitization which is synonymous to co-sensitization. ... 

An indirect PET methodology known as redox photosensitization has been developed by Pac [45] and Tazuke [64] for achieving higher yields of nucleophile addition product to alkene cation radicals. One recent example of this approach may be mentioned by illustrating anti-Markonikov alcohol addition (e.g. 61-62) to non conjugated olefin 61 using biphenyl as cosensitizer [65]. More examples on this topic can be found in Farid [5] and Mariano s [11] reviews. 

Aromatic hydrocarbons also affect the efficiency of photoinduced electron transfer reactions. In such cases, aromatic hydrocarbons often act as redox photosensitizers or co-sensitizers. The role of these sensitizers is shown in Scheme 4. The primary photoinduced electron transfer occurs from ArH to A (or ArH to A ) to give ArH" and A". The succeeding secondary electron transfer from D to ArHproduces D and ArH in which D" is a real reactive... 

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). 

For example, rhenium(I) complexes have been used as emitter materials in electroluminescent devices 17-20) and biological probes (11), as dye for dye-sensitized solar cells (21), as chromophores for photochemical electron or energy transfer studies (14,17,22), and as a redox photosensitizer (23). 

As noted above (Section II), rhenium(I) complexes have relatively long excitation lifetimes in solution at room temperature and can be used as redox photosensitizers that drive... 

Reactions using rhenium complexes as redox photosensitizers for hydrogen generation have also been reported. The coexistence of Co complexes (76,77) or Fe complexes (78) as catalyst induced efficient hydrogen generation. 

Photocatalytic CO2 reduction of a supramolecule with a Zn porphyrin unit, which is a redox photosensitizer that can absorb even wider ranges of visible light, connected to a rhenium complex (ZnTMP-ReCl) was considered (101,102). In this system, ultrafast (l.SxlO s ) electron transfer from the S2 excited state of the ZnTMP unit to the rhenimn imit was observed. Reduction of CO2 proceeded with generation of the OER species of the rhenium unit by the reduction of this intramolecular charge-transfer state by TEA. 

Explain the following concepts and keywords photochromism photoswitching photoinitiator photoaffinity labelling photo-Wolff rearrangement photoactivatable compound Barton reaction photochemical trigger photolabile linker photoamination redox photosensitization. 

Majima, T., Pac, C., Nakasone, A., Sakurai, H., Redox photosensitized Reactions. 7. Aromatic Hydrocarbon photosensitized Electron transfer Reactions of Furan, Methylated Furans,l,l Diphenylethylene, and Indene with p Dicyanobenzene J. Am. Chem. Soc. 1981, 103, 4499 4508. 

PET is the key feature of many other photo-oxidations involving aromatic substrates. It has been shown, for example, that pyrene and anthracene which are covalently attached to silica, gold or indium-doped tin oxide (ITO), undergo a photo-oxidation forming dihydroxy/dione derivatives. The reaction involves 02, formed by ET between excited pyrene, or anthracene, and O2, and it is suggested that the implications of such a photodegradation need to be considered when polycyclic aromatic hydrocarbons (PAHs) are used as spectroscopic probes in surface adlayers. The redox photosensitized amination of 1,2-benzo-1,3-cycloalkadienes, arylcyclopropanes, and quadricyclane with ammonia and primary amines, using 1,2,4-triphenylbenzene (1,2,4-TPB) or 2,2 -methylenedioxy-1,1 -binaphthalene in the presence of m- or p-dicyano-benzene (DCB), has been described (Scheme 51). The process involves the formation of the radical cation of 1,2,4-TPB, for example, by PET to the DCB, followed by hole transfer from the radical cation to the substrate, the latter... 

Also operating in PET reaction pathways is the rapid back electron transfer from A to D. This process which leads to reduced quantum efficiencies can be partly avoided by use of redox photosensitization... 

DCB". Secondary electron transfer then occurs from the allylsilane D to ArH or the Jt-complexation of ArH wifli D, [ArH D]. The radical cation species D or [ArH D] reacts to produce the allyl radical, which reacts wifli /i-DCB to give the substitution product shown in Scheme 2. Aromatic hydrocarbons such as phenanthrene, naphthalene, triphenylene, and />-terphenyl (Table 3) are effective in the redox photosensitization in the photosubstitution of E)CB by allyltrimethylsilane. ... 

Chen et al. reported efficient photosensitization of onium salts by various compounds containing a carbazole nucleus. Both diaryliodonium and triarylsulfonium salts are photosensitized by sueh eompounds. Thus, the polymer of N-vinylcarbazole was found by them to be an excellent electron-transfer photosensitizer for various onium salts. They also found that poly(9-vinylcarbazole) yields similar results. Poly(2-vinyl carbazole) turned out to be the most efficient photosensitizer among various polymers with carbazole tested. In addition, Chen et al., concluded that the redox photosensitization by the carbazole molecule or its N-alkylated derivatives occurs predominantly from the singlet excited states. On the other hand, the carbazole derivatives with carbonyl substituents sensitize onium salts via triplet excited states. This follows... 

Ishitani O, Pac C, Sakurai H (1984) Redox-photosensitized reactions. 11. Ru(bpy)3 L photosensitized reactions of 1-benzyl-1,4-dihydronicotinamide with aryl-substituled enmies, derivatives of methyl cinnamate, and substituted cinnamonitriles electron-transfCT mechanism and structure-reactivity relationships. J Org Chem 49 26-34... 

The unsubstituted styrene derivatives such as indene (6p), 2-methylindene (6q), 3,4-dihydronaphthalene (6r), and l,2-benzo-l,3-cyloheptadiene (6s) have weak absorption in near UV region, showing that direct irradiation could not induce the efficient photoamination. Therefore, redox-photosensitization was applied. Our efforts were paid to find arenes as sensitizers which are inert toward NHj and amines, because many arenes readily react with NHj under the conditions of photoamination. Moreover, the redox-photosensitizers are required to have relatively higher oxidation potentials than the substrates. The polyphenylbenzenes have relatively high oxidation potentials near 1.50 V and are inert toward NH3 under photoamination conditions. However, many polyphenylbenzenes are poorly soluble in aqueous MeCN solutions. As the sensitizers that were moderately soluble in an aqueous MeCN solution, therefore, we selected 1,2,4-triphenylbenzene (1,2,4-TPB), 1,3,5-triphenylbenzene (1,3,5-TPB), m-terphenyl (m-TP), and p-terphenyl -TP) (Scheme 6.9B) [48]. Also 2,2 -methylenedioxy-l,l -binaphthalene (BN) (E, = 1.20 V) was used as redox-sensitizer having lower oxidation potential. 

In order to find a more effective sensitizer among the polyphenylbenzenes, the control experiments were performed for photoamination of 6p with NHj. Irradiation of an ammonia-saturated MeCN-H O (19 1,50 ml) containing 6p (2 mmol), m-DCB (3.75 mmol), and the polyphenylbenzene (0.75 nunol) for 5 to 13 h gave 29a (Scheme 6.28). When 1,2,4-TPB and p-TP were used as the sensitizer, photoamination of la proceeded in good yields. However, p-TP deposited during the irradiation because of the poor solubility in MeCN-HjO solution. Therefore, we selected 1,2,4-TPB as the sensitizer for photoamination. The redox-photosensitized amination of 6p-s are shown in Scheme 6.28. 

The redox-photosensitized amination was applied to the amination of simple alkenes and 1,3-alkadienes (8) with NHj (Scheme 6.30) [52]. Photoamination of 2,3-dimethyl-2-bntene (8a) gave the Type 11 aminated products, 32a and 32a. It is noteworthy that photoamination of 8 occurred accompanied by the incorporation of 4-cyanophenyl gronps. In the cases of 2,4-dimethyl-2,4-hexadiene (8b) and 2,4-hexadiene (8c), the aminated compounds (32b-i) in which 1,4-addition of the amino and 4-cyanophenyl groups occurred were obtained in fairly good yields. The similar photoamination of l,2,3,4-tetramethyl-l,3-cyclopentadiene (8d) gave selectively 1-amino-4-(4-cyanophenyl)-l,2,3,4-tetramethyl-2-cyclopentene (32j) in 95%. Similar photoamination can be applied to 2,3-dihydrofuran (8e) and 2,3-dihydropyran (8f). 

In the case of redox photosensitization (Scheme 6.9B), the Stem-Volmer quenching of fluorescence of the sensitizer (S) at a nearly diffusion-controlled rate and negative free-energy changes from the excited singlet state of S to A is required for the confirmation of the electron transfer from the exited singlet of 5 to A. The hole transfer from the resulting cation radical of S to D depends on the difference between the sensitizer and the substrates in the oxidation potentials. 

In the case of the redox-photosensitized amination of the substrates whose E, are lower than those of S, the efficient hole transfer would occur. However, the hole transfer from S - to the substrates whose Ej " are much higher than that of S would not occur. Moreover, 1,2,4-TPB-photosensitized amination of 9a was inefficient. 

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