Electron-transfer reactions, solvent effects radical

Santamaria, J. and Jroundi, R., Electron transfer activation — a selective photooxidation method for the preparation of aromatic aldehydes and ketones. Tetrahedron Lett., 32, 4291, 1991. Bokobza, L. and Santamaria, J., Exciplex and radical ion intermediates in electron transfer reactions. Solvent effect on the photo-oxygenation of 1,4-dimethylnaphthalene sensitized by 9,10-dicyanoan-thracene,/. Chem. Soc., Perkin Trans. 2, 269, 1985. 

Bokobza, L. and Santamaria, J., Exciplex and radical ion intermediates in electron transfer reactions solvent effect on the photo-oxygenation of 1,4-dimethylnaphthalene sensitized by 9,10-dicyanoan-... 

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. 

The remarkable solvent isotope effect on the kinetics of oxidation of guanine by 2AP radicals has been detected in H2O and D2O solutions [14]. In H2O, the rate constants of G(-H) formation are larger than those in D2O by a factor of 1.5-2.0 (Table 1). This kinetic isotope effect indicates that the electron transfer reaction from guanine to 2AP radicals is coupled to deprotonation/ protonation reactions of the primary electron-transfer products (Scheme 1). 

A review has focused on differentiation between polar and SET mechanisms through kinetic analysis.82 hi two separate reviews, the effects of solute-solvent interactions on electron-transfer reactions have been described.83,84 A review of the behaviour of radical cations in liquid hydrocarbons has given particular emphasis to those with high mobility.85 A paper presents selected studies in the formation of radicals by oxidation with manganese- or cerium-based reagents and then- application to C—C bond formation by SET processes.86... 

A subsequent study ° from the Arnold group showed an intriguing stereoelectronic effect in oxidative benzylic carbon-hydrogen bond cleavage reactions of substrates 8 and 9 (Scheme 3.7). In this study, electron transfer reactions were conducted in the presence of a nonnucleophilic base. Radical cation formation also weakens benzylic carbon-hydrogen bonds, thereby enhancing their acidity. Deprotonation of benzylic hydrogens yields benzylic radicals that can be reduced by the radical anion of dicyanobenzene to form benzylic anions that will be protonated by solvent. This sequence of oxidation, deprotonation, reduction, and protonation provides a sequence by which epimerization can be effected at the benzylic center. In this study, tram isomer 10 showed no propensity to isomerize to cis isomer 11 (equation 1 in Scheme 3.7), but 11 readily converted to 10 (equation 2 in Scheme 3.7). The reactions were repeated in deuterated solvents to assure that these observations resulted from kinetic rather than thermodynamic factors. Trans isomer 9 showed no incorporation of deuterium (equation 3 in Scheme 3.7) whereas cis isomer 11 showed complete deuterium incorporation. The authors attributed this difference in reactivity to... 

It has been established that for solvents in which specific solvation is not dominant, a small solvent effect implies an atom-transfer reaction and a large solvent effect suggests an electron-transfer reaction between neutral species. The high solvent sensitivity of electron-transfer reactions between neutral molecules should provide a useful test of their occurrence [215, 570]. From Table 5-11, it can be concluded that atom-transfer, according to Eq. (5-66), is the rate-limiting step in the reaction of pyridinyl radical with... 

This review article deals with addition and cycloaddition reactions of organic compounds via photoinduced electron transfer. Various reactive species such as exdplex, triplex, radical ion pair and free radical ions are generated via photoinduced electron transfer reactions. These reactive species have their characteristic reactivities and discrimination among these species provides selective photoreactions. The solvent and salt effects and also the effects of electron transfer sensitizers on photoinduced electron transfer reactions can be applied to the selective generation of the reactive species. Examples and mechanistic features of photoaddition and photocycloaddition reactions that proceed via the following steps are given reactions of radical cations with nucleophiles reactions of radical anions with electrophiles reactions of radical cations and radical anions with neutral radicals radical-radical coupling reactions addition and cycloaddition reactions via triplexes three-component addition reactions. 

Photochemical electron transfer reactions of electron donor-acceptor pairs in polar solvents provide a convenient and effective method for the generation of radical cations which can be trapped by complex metal hydrides. One of the most effective systems is based on irradiation of a solution of substrate, sodium borohydride and 1,4- or 1,3-dicyanobenzene. A range of bi- and poly-cyclic aromatic hydrocarbons has been converted into the dihydro derivatives in this way. An especially important aspect of this route to dihydroaromatic compounds is that it may give access to products which are regioisomeric with the standard Birch reduction products. Thus, o-xylene is converted into the 1,4-dihydro product (229) rather than the normal 3,6-dihydro isomer (228). The m- and p-xylenes are similarly reduced to (230) and (231), respectively. ... 

As discussed earlier, deprotonation of a-carbon forms a major reaction pathway for the disappearance of the amine radical cation. Studies of photoinduced electron-transfer reactions of tertiary amines by Lewis [7, 11] and by Mariano [5, 10] have contributed significantly towards our understanding of the factors that control this process. Lewis and coworkers used product-distribution ratios of stilbene-amine adducts to elucidate the stereoelectronic effects involved in the deprotonation process [5, 10, 121, 122]. In non-polar solvents, the singlet excited state of tran -stilbene forms non-reactive but fluorescent exciplexes with simple trialkylamines. Increasing solvent polarity brings about a decrease in the fluorescence intensity and an increase in adduct formation. For non-symmetrically substituted tertiary amines two types of stilbene-amine adduct can be formed, as is shown in Scheme 9, depending on whether the aminoalkyl radical adding to the stilbene radical is formed by de-... 

Quite different behavior is found for the electron transfer reaction between tetrathiofulvalene (TTF) and its cation radical. For this system, there is no correlation between In et and the Pekar factor y (fig. 7.22). However, there is a strong correlation between Inkgt and IniL. These results are consistant with an adiabatic reaction with a dynamic solvent effect. The estimate of A G[s for this... 

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