Photostimulated electron transfer

The cleavage of C—S bonds in C—SO2R anion radicals plays an important role in SrnI tyP processes ". Kornblum and coworkers described a photostimulated electron transfer chain substitution at a saturated carbon where the leaving group is PhSOj ... 

Using alkene radical cations generated under photostimulated electron-transfer conditions, Arnold and coworkers showed that the addition of an-... 

Kochi s group obtained similar results for the reaction between ArH and C(N02)4 (Sankararaman et al. 1987, Masnovi et al. 1985,1989). After photostimulated electron transfer, the related triad was formed... 

Fig. 15 Upper plot Photostimulated electron transfer activation induced by irradiation of DHA 19a in acetonitrile (c = 1(T3 mol dirT3) at working potential -1050 mV vs. Fc/Fc+.

Fig. 16 Schematic representation of electronic changes due to photostimulated electron transfer.

J. O M. Bockris and M. Schuaib, Trans. Adv. Electrochem. Sci. Tech. 13 4 (1978). Photostimulated electron transfer to and from photosystem 1 and photosystem 2 from ions in aqueous solutions. (First evidence for a photoelectrochemical mechanism in photosynthesis.)... 

The mechanism of the reduction process (formation of benzene) in the photostimulated reaction of benzeneselenate ion with iodobenzene was found to be photostimulated electron transfer from the nucleophile to the substitution product (equations 187-189)705. 

A few further examples of nucleophilic displacement of halogen from halogenothiophenes by a radical chain SrN, mechanism have been reported. The photostimulated reaction of 2-halo or 3-halothiophene with excess of tetrabutylammonium benzenethiolate in MeCN takes place by an SrN mechanism <87JOC5382>. The 2- or 3-(phenylthio)thiophene is formed in modest yields. The initiation and propagation steps are shown in Scheme 119. The first step may be the photostimulated electron transfer from thiolate anion to the halothiophene (ThX). The reaction does not take place in the dark. 

Photonucleophilic aromatic substitution reactions of phenyl selenide and telluride with haloarenes have also been proven to involve the S jlAr mechanism, with the formation of anion radical intermediates. Another photonucleophihc substitution, cyanomethylation, proves the presence of radical cations in the reaction mechanism. Liu and Weiss have reported that hydroxy and cyano substitution competes with photo substitution of fluorinated anisoles in aqueous solutions, where cation and anion radical intermediates have been shown to be the key factors for the nucleophilic substitution type. Rossi et al. have proposed the S j lAr mechanism for photonucleophihc substitution of carbanions and naphthox-ides to halo anisoles and l-iodonaphthalene. > An anion radical intermediate photonucleophilic substitution mechanism has been shown for the reactions of triphenyl(methyl)stannyl anion with halo arenes in liquid ammonia. Trimethylstannyl anion has been found to be more reactive than triphenylstannyl anion in the photostimulated electron- transfer initiation step. 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

An example of transfer of electron from nucleophile to substrate is seen in the formation of the radical anions (observable by ESR) of 5-halo-2A/,3W-benzotf>]thiophene-2,3-diones on treatment with nucleophiles.20 It has been proposed in some cases, that this single-electron transfer step takes place through a charge transfer complex between the nucleophile and the aromatic substrate.21-22 Some reactions occur spontaneously, i.e. without any catalysts or reagents other than the substrate and the nucleophile, but the initiation process is usually, although not invariably, photostimulated (near-ultraviolet radiation, 300-... 

The number of solvents that have been used in SrnI reactions is somewhat limited in scope, but this causes no practical difficulties. Characteristics that are required of a solvent for use in SrnI reactions are that it should dissolve both the organic substrate and the ionic alkali metal salt (M+Nu ), not have hydrogen atoms that can be readily abstracted by aryl radicals (c/. equation 13), not have protons which can be ionized by the bases (e.g. Nth- or Bu O" ions), or the basic nucleophiles (Nu ) and radical ions (RX -or RNu- ) involved in the reaction, and not undergo electron transfer reactions with the various intermediates in the reaction. In addition to these characteristics, the solvent should not absorb significantly in the wavelength range normally used in photostimulated processes (300-400 nm), should not react with solvated electrons and/or alkali metals in reactions stimulated by these species, and should not undergo reduction at the potentials employed in electrochemically promoted reactions, but should be sufficiently polar to facilitate electron transfer processes. 

Fig. 2. Schematic representation of the mechanism of photostimulated luminescence in BaBrF Eu. (A) Formation of a colour centre (F) under X-ray irradiation by trapping of an electron in a bromine vacancy with trapping of the hole formed in the valence band by a hole trapping centre (HT) in the vicinity (B) Release of the trapped electron by laser irradiation and transfer of the electron-hole recombination energy to Eu2+. Relaxations after electron transfers have not been represented.

As previously observed for the diphenyl diselenide (Scheme 2) [24], Pandey has found that the phenyl alkyl selenides also can be transformed into the corresponding radical cations by photostimulated single electron transfer to 1,4-dicyanonaphthalene (DCN). These intermediates, as proposed above, suffer fragmentation to afford diphenyl diselenide and a carbocation, which can be trapped by a nucleophile. On the basis of these observations Pandey [121] described a sequential one-pot selenenylation-deselenenylation of alkenes in methanol using only catalytic amounts of diphenyl diselenide. An example is reported in Scheme 41 which illustrates the single steps of this process... 

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