Electron transfer photosensitization

The photosensitization of diaryliodonium and triarylsulfonium salts can also occur by an electron transfer process. 

In this mechanism, depicted above for diaryliodonium salts, an electron is transferred from the excited photosensitizer to the onium salt with perhaps the involvement of an excited state complex (exdplex). The result is that a photoredox reaction occurs in which the photosensitizer is oxidized to a cation-radical while the oniiun salt is reduced 

The electron transfer mechanism best explains the phenomenon of onium salt photosensitization in most cases. This conclusion is supported hy the following observations. Analysis of the products formed by the photosensitized photolysis of diaryliodonium and triarylsulfonium salts are those which are predicted on the basis of the above mechanism. Moreover, these products are identical to those formed by the electrolytic reduction of these same onium salts which give rise to the same diaryliodide and triarylsulfide radical intermediates. 

Because they have lower oxidation potentials, diaryliodonium salts are more easily photosensitized than triarylsulfonium salts. Thus, while diaryliodonium salts are photosensitized by a variety of aryl ketones, condensed ring aromatic hydrocarbons and dyes, useful photosensitizers for triarylsulfonium salts, are limited to those photoinitiators with high triplet energies and low oxidation potentials. Condensed ring aromatic hydrocarbons such as anthracene, rubrene, coronene, perylene and certain heterocyclic compounds as phenothiazine derivatives meet these requirements. 

5 kcal/mol. Calculated from half wave potentials (kcal/mol.) ° DSC determination with 1 % photoinitiator and 0.1 % photosensitizer in 3,4-epoxycyclohexylmethyl-3, 4 -epoxycyclohexane carboxylate using excitation wavelength = 366 nm 

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The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Acid-redox, acid-base, multi-electron transfer, photosensitivity, etc. 

Organic radical cations generated by electron transfer photosensitization are species with greatly enhanced pK a s compared with their closed shell precursors (142). If a C-H bond is located adjacent to the site of predominant charge density, rapid deprotonation will ordinarily ensue, producing a radical whose fate determines the identity of the isolated products. ... 

The electron transfer photosensitized reactions of diolefins results in the formation of [4 + 2]cycloadducts. For example, irradiation of octafluoronaphtha-lene [129] or dicyanoanthracene [130-132] in polar solvents containing cyclohexa-diene leads to the formation of endo- and exo-dicyclohexadiene. 

Triphenylthiopyrylium salts have been employed as efficient electron transfer photosensitizers to promote the [4+2] cycloaddition between thiobenzophenone and substituted styrenes. A radical cation derived from the thiobenzophenone is involved in the formation of separable diastereoisomeric mixtures of 1,3,4-trisubstituted isothiochromans <2007OL3587>. Interest has been maintained in the photosensitizing properties of thiopyrylium salts for blood disinfection <2007BMCL4406>. Several new thioxanthylium salts have been reported <2007JOC2647, 2007JOC2690>. 

Miranda, M.A. and Garda, H. (1994) 2,46-Triphenylpyrylium tetrafluoroborate as an electron transfer photosensitizer. Chemical Reviews, 94, 1063—1089. 

A sub case in this category is the benzylation of enolates through the Srn 1 process. In this way, both a-nitro- and a-chloro-4-nitrocumene are alkylated by the enolates of 2-nitropropane, diethylmalonate, or diethyl 2-butylmalonate. A particular case of benzylic C-C fragmentation is the electron transfer photosensitized Cope rearrangement of 2,5-diphenyl-1,5-hexadienes [218,219]. 

ELECTRON TRANSFER PHOTOSENSITIZED BY ZINC PORPHYRINS IN REVERSED MICELLES... 

In conclusion, whereas Co(NH3)63+ is useless because it undergoes a fast photodecomposition reaction, the analogous Co(sep)3+ complex may be employed as an electron transfer photosensitizer because of its intrinsic stability in the excited state and in the reduced form. In the same way, one can think to use cage-type polypyridine ligands for Ru complexes, so as to prevent ligand dissociation reactions. 

When the reaction of a non-absorbing quencher is induced by electron transfer (not energy transfer) using an excited light-absorbing sensitizer, the process is called electron transfer photosensitization. Then interaction between sensitizer and quencher consists of photoinduced electron transfer ... 

CO-NMe insertion to give (86) and (87). The same workers have also shown that in methanol solution and in the presence of 1,1-diphenylethylene, (79) acts as a typical electron transfer photosensitizer for the anti-Markovnikoff addition and 2,2-diphenylethyl methyl ether is formed in 70% yield with 93% of (79) being recovered. 

To really achieve photoinduced dethreading, a different approach has been devised [98, 99], based on the use of an external electron transfer photosensitizer (P) and a sacrificial reductant (Red), as illustrated in Figure 22. The photosensitizer must be able to (i) absorb light efficiently and (ii) have a sufficiently long-lived and reductant excited state, so that its excitation (process 1) in the presence of the pseudorotaxane will lead (process 2) to the transfer of an electron to a bipyridinium unit of the cyclophane. The relatively fast back electron transfer from the reduced cyclophane component to the oxidized photosensitizer is prevented by the sacrificial reductant, which, if... 

In order to really achieve photoinduced dethreading, a different approach has been devised [25], based on the use of an external electron-transfer photosensitizer (P) and a sacrificial reductant (Red), as illustrated in Fig. 9. The photosensitizer must... 

Jeon, Y. T., Lee, C. P, Mariano, P. S., Radical Cyclization Reactions of a Silyl Amine a,P Unsaturated Ketone and Ester Systems Promoted by Single Electron Transfer Photosensitization, J. Am. Chem. Soc. 1991, 113, 8847 8863. 

Electron-transfer photosensitization (2,4,6-triphenylpyrylium tetrafluorobor-ate) is reported to induce a photo-Diels Alder reaction between A-arylimines (55) and styrene or a-methylstyrene. The reaction is considered to proceed by attack of the styrene radical cation onto the arylimine and affords both dia-stereoisomers (56) and (57) in reasonable yields, although amounts of the quinoline (58) and the amine (59) are formed in some cases. 

Electron transfer photosensitization of iodonium salts and sulfonium salts is similar primary differences are a result of the stronger oxidizing power of iodonium salts compared to sulfonium salts ( (red) = —0.7 V and —1.2V versus SCE for diphenyliodonium and triphenylsulfonium salts respectively, see below). Triplet sensitization is again similar, including the triplet energies, and has been discussed above. 

Scheme 4 describes the electron transfer photosensitization of iodonium salts by anthracene [61,70,91-94]. Singlet anthracene reacts with diphenyliodonium cation by diffusion controlled electron transfer in acetonitrile solution. In-cage decomposition of diphenyliodo radical competes with rapid back electron transfer to yield the singlet radical pair of anthracene cation... 

Table 4. Electron Transfer Photosensitized Oxygenation of Benzylic Derivatives ...

Chart 10.5 Typical electron-transfer photosensitizers that may be applied in conjunction with onium salts [54]. 

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