Photoinduced radical reactions electron transfer

The nucleophilic photosubstitution reactions of benzylic halides with a nitro group at the ortho- or / ara-position of the aromatic ring such as / -nitrocumyl chloride (26) occur via the S l mechanism. The reactions have been reviewed in References 6-10. The initiation and propagation steps of this photoinduced radical-chain electron transfer process have already been depicted in this review in equations 6 to 9 in Section II. A. Equation 37... 

Sekiguchi S, Kobori Y, Akiyama K and Tero-Kubota S 1998 Marcus free energy dependence of the sign of exchange interactions in radical ion pairs generated by photoinduced electron transfer reactions J. Am. Chem. Soc. 120 1325-6... 

It has been reported that Cgo and its derivatives form optically transparent microscopic clusters in mixed solvents [25, 26]. Photoinduced electron-transfer and photoelectrochemical reactions using the C o clusters have been extensively reported because of the interesting properties of C o clusters [25,26]. The M F Es on the decay of the radical pair between a Cgo cluster anion and a pyrene cation have been observed in a micellar system [63]. However, the MFEs on the photoinduced electron-transfer reactions using the Cgo cluster in mixed solvents have not yet been studied. 

Other radical cyclization approaches to the synthesis of piperidines include a CAN-mediated stereoselective cyclization of epoxypropyl cinnamyl amines <06TL705> and a cyclization of (-trimethylsilylmethylamine radical cation, generated via a photoinduced electron transfer reaction to a tethered -functionality <06JOC8481>. 

It is a pleasure to acknowledge the essential contribution of Dr. C. P. Andrieux to most of the work reported above as well as that of Dr. D. Lexa in the field of porphyrins, Professor Moiroux and Dr. A. Anne to cation radical reactivity, Dr. M. Robert to photoinduced dissociative electron transfer and to the stepwise/concerted competition and Drs. P. Hapiot and Medebielle to recent work on thermal SRn1 reactions. Many students from our group have also contributed effectively to the work, namely, C. Costentin, G. Delgado, V. Grass, A. Le Gorande, C. Tardy and D. L. Wang. Fruitful and pleasant... 

The importance of tertiary amines in the photochemically induced electron transfer reactions has also been addressed5. Direct irradiation of aromatic or aliphatic amines often leads to the scission of C—N, N—H or C—H bonds that lead to the subsequent chemical reactions by radical pathways6. In this section, photochemical reactions of amines reported since 1978 will be considered with emphasis on photoinduced electron transfer. Photochemical reactions of inorganic and organometallic compounds will not be included unless photochemistry of amine moieties is the primary interest. 

The amide functionality plays an important role in the physical and chemical properties of proteins and peptides, especially in their ability to be involved in the photoinduced electron transfer process. Polyamides and proteins are known to take part in the biological electron transport mechanism for oxidation-reduction and photosynthesis processes. Therefore studies of the photochemistry of proteins or peptides are very important. Irradiation (at 254 nm) of the simplest dipeptide, glycylglycine, in aqueous solution affords carbon dioxide, ammonia and acetamide in relatively high yields and quantum yield (0.44)202 (equation 147). The reaction mechanism is thought to involve an electron transfer process. The isolation of intermediates such as IV-hydroxymethylacetamide and 7V-glycylglycyl-methyl acetamide confirmed the electron-transfer initiated free radical processes203 (equation 148). 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

The photochemical reduction of 1-methylquinolinium ions by (TMS)3SiH proceeds regioselectively to afford the corresponding 1,4-dihydroquinones in a water-acetonitrile solvent system (Reaction 4.47) [83]. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from the silane to the singlet excited states of 1-methylquinolinium ions to give the silane radical cation-quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. Silyl cations are quenched by water. 

Until now, the isotopic effect was discnssed only in relation to the reactants. In electron-transfer reactions, the solvent plays an eqnally important role. As mentioned, different solvate forms are possible for reactants, transition states, and products. Therefore, it seems important to find a reaction where the kinetic effect resulting from the introduction of an isotope would be present for solvents, but absent for reactants. For a published work concerning this problem, refer Yusupov and Hairutdinov (1987). In this work, the authors studied photoinduced electron transfer from magnesium ethioporphyrin to chloroform followed by a dark recombination of ion-radicals in frozen alcohol solutions. It was determined that the deuteration of chloroform does not affect the rate of transfer, whereas deuteration of the solvent reduces it. The authors correlate these results with the participation of solvent vibrational modes in the manner of energy diffraction during electron transfer. 

An useful alternative to the already known retropinacol reactions is presented by Liu and co-workers [7], This works demonstrates that pinacols bearing (dimethylamino)phenyl substiments can be subjected to fast oxidative fragmentation via photoinduced electron transfer with chloroform as the electron acceptor in yields up to 80%. The extremely fast dechlorination of the chloroform radical anion inhibits back-electron transfer and thus leads to effective fragmentation of the pinacol radical cation (Scheme 8). 

Photoinduced single-electron transfer followed by fragmentation of the radical cation is an efficient method for generating carbon-centered radicals under exceptionally mild conditions. The fate of the thus formed radicals depends primarily on their interaction with the acceptor radical anions. Typically observed reactions are either back-electron transfer or radical coupling, but from the synthetic point of view, another most intriguing possibility is the trapping of the radical with suitable substrates such as olefins (Scheme 16). 

Mattay et al. examined the regioselective and stereoselective cyclization of unsaturated silyl enol ethers by photoinduced electron transfer using DCA and DCN as sensitizers. Thereby the regiochemistry (6-endo versus 5-exo) of the cyclization could be controlled because in the absence of a nucleophile, like an alcohol, the cyclization of the siloxy radical cation is dominant, whereas the presence of a nucleophile favors the reaction pathway via the corresponding a-keto radical. The resulting stereoselective cis ring juncture is due to a favored reactive chair like conformer with the substituents pseudoaxial arranged (Scheme 27) [36,37]. 

In addition to the former example, Pandey et al. achieved efficient a-aryla-tion of ketones by the reaction of silyl enol ethers with arene radical cations generated by photoinduced electron transfer from 1,4-dicyanonaphthalene. Using this strategy various five-, six-, seven-, and eight-membered benzannulated compounds are accessible in yields in the range 60-70% [39],... 

Zimmerman and Hoffacker also observed a regioselective reaction subjecting various aryl-substituted 1,4-pentadienes to photoinduced electron transfer using DCN and DCA. The radical cations produced underwent a regioselective cyclization wherein one electron-deficient aryl group of one diarylvinyl moiety bonds to the (3-carbon of the second diarylvinyl group (Scheme 30) [41]. 

The photoinduced reaction of chloranil with various 1,1-diarylethenes is another example of an intramoleclar [2 -I- 2] cycloaddition as reported by Xu and co-workers [86]. Although not interesting from the preparative point of view, the diverse reaction outcomes caused by parallel reaction pathways with and without single-electron transfer and various secondary reactions of the primary products show that the photochemistry involving haloquinones is far from being explored. Another interesting example in this context is the reaction of dichlorobenzoqui-none with various diarylacetylenes in the solid phase via photoinduced electron transfer as reported by Kochi and co-workers [87]. Here, time-resolved spectroscopy revealed the radical ion pair of the two reactants to be the first reactive intermediate that then underwent coupling. 

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

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