Electron transfer solvent dependence

The photoreactions of mono- and di-cyanonaphthalene derivatives with al-lyltrimethylsilane have been reported by Mizuno and Albini, independently [136,428,433,436], The product ratios of photocycloadducts via exciplexes and the allylated products via electron transfer were dependent on the solvent polarity and additives. The intramolecular photocycloaddition of the reductive allylated products efficiently gives tricyclic compounds in high yields [428,433,436] (Scheme 23). 

It is commonly assumed that solvent reorganization will dominate electron transfer kinetically. Depending on the thermodynamics of the electron exchange, it is possible to quantitatively predict a relationship between the free energy of activation for electron transfer and the free energy associated with solvent reorganization based on Marcus theory. 

The metal reduction of the polycyclic system is usually carried out in an ether solvent and by an alkali metal at low temperature (—78 °C). When potassium metal is applied it is best to prepare a metal mirror. Sodium and lithium react, either directly in the form of a metal wire, or after treatment in an induction furnace. Cesium is prepared by thermolysis of cesium azide. It has recently been found that the application of an ultrasonic bath facilitates the reaction and avoids side reactions. The reaction can be carried out in a modified NMR tube or in an ESR cavity. Diamagnetic ions are prepared in extended NMR tubes to which the metal is extruded and sealed under vacuum. Reaction rates are difficult to compare as the electron-transfer process depends on various experimental conditions such as concentration, temperature, the presence of impurities, the solvent and the nature of the metal surface. It may take from minutes to days to form the first radical-anion the second step then follows and can sometimes be rather slow 10 13). 

FIGURE 12.13 Gerischer-type diagram for interfacial electron transfer. The rate constants for interfacial electron transfer are dependent on the overlap of the sensitizer and the semiconductor density of states. Note that the density of states of the semiconductor is not a singular parameter and can shift with a change in environment, that is, pH, ionic strength, solvent, and so on. 

Clegg AD, Rees NV, Klymenko OV, Coles BA, Compton RG (2004) Marcus theory of outer-sphere heterogeneous electron transfer reactions dependence of the standard electrochemical rate constant on the hydrodynamic radius from high precision measurements of the oxidation of anthracene and its derivatives in nonaqueous solvents using the high-speed channel electrode. J Am Chem Soc 126(19) 6185-6192... 

Imahori H, Hagiwara K, Aoki M, Akiyama T, Taniguchi S, Okada T, Shirakawa M and Sakata Y 1996 Linkage and solvent dependence of photoinduced electron transfer in porphyrin-Cgg dyads J. Am. Chem. Soc. 118 11 771-82... 

Electron transfer reaction rates can depend strongly on tire polarity or dielectric properties of tire solvent. This is because (a) a polar solvent serves to stabilize botli tire initial and final states, tluis altering tire driving force of tire ET reaction, and (b) in a reaction coordinate system where the distance between reactants and products (DA and... 

The equation does not take into account such pertubation factors as steric effects, solvent effects, and ion-pair formation. These factors, however, may be neglected when experiments are carried out in the same solvent at the same temperature and concentration for an homogeneous set of substrates. So, for a given ambident nucleophile the rate ratio kj/kj will depend on A and B, which vary with (a) the attacked electrophilic center, (b) the solvent, and (c) the counterpart cationic species of the anion. The important point in this kind of study is to change only one parameter at a time. This simple rule has not always been followed, and little systematic work has been done in this field (12) stiH widely open after the discovery of the role played by single electron transfer mechanism in ambident reactivity (1689). 

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

Figure 2.1 (Plate 2.1) shows a classification of the processes that we consider they aU involve interaction of the reactants both with the solvent and with the metal electrode. In simple outer sphere electron transfer, the reactant is separated from the electrode by at least one layer of solvent hence, the interaction with the metal is comparatively weak. This is the realm of the classical theories of Marcus [1956], Hush [1958], Levich [1970], and German and Dogonadze [1974]. Outer sphere transfer can also involve the breaking of a bond (Fig. 2. lb), although the reactant is not in direct contact with the metal. In inner sphere processes (Fig. 2. Ic, d) the reactant is in contact with the electrode depending on the electronic structure of the system, the electronic interaction can be weak or strong. Naturally, catalysis involves a strong...

In a simple electron transfer reaction, the reactant is situated in front of the electrode, and the electron is transferred when there is a favorable solvent fluctuation. In contrast, during ion transfer, the reactant itself moves from the bulk of the solution to the double layer, and then becomes adsorbed on, or incorporated into, the electrode. Despite these differences, ion transfer can be described by essentially the same formalism [Schmickler, 1995], but the interactions both with the solvent and with the metal depend on the position of the ion. In addition, the electronic level on the reactant depends on the local electric potential in the double layer, which also varies with the distance. These complications make it difficult to perform quantitative calculations. 

In bulk solution dynamics of fast chemical reactions, such as electron transfer, have been shown to depend on the dynamical properties of the solvent [2,3]. Specifically, the rate at which the solvent can relax is directly correlated with the fast electron transfer dynamics. As such, there has been considerable attention paid to the dynamics of polar solvation in a wide range of systems [2,4-6]. The focus of this chapter is the dynamics of polar solvation at liquid interfaces. 

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

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