Electron-transfer reactions/reagents

Scheme 36). Interestingly, the higher order cuprate 206 underwent conjugate addition with only moderate selectivity. This is likely due to the intervention of an electron transfer pathway. Competing electron transfer reactions involving a-alkoxymetal reagents of this type have also been reported by Cohen [81]. 

This iron-ate complex 19 is also able to catalyze the reduction of 4-nitroanisole to 4-methoxyaniline or Ullmann-type biaryl couplings of bis(2-bromophenyl) methylamines 31 at room temperature. In contrast, the corresponding bis(2-chlor-ophenyl)methylamines proved to be unreactive under these conditions. A shift to the dianion-type electron transfer(ET)-reagent [Me4Fe]Li2 afforded the biaryl as well with the dichloro substrates at room temperature, while the dibromo substrates proved to be reactive even at —78°C under these reaction conditions. This effect is attributed to the more negative oxidation potential of dianion-type [Me4Fe]Li2. 

Although electron transfer reactions, kinetically eh behaves as a classical nucleophilic reagent. 

Bridging of macrolides.1 A number of natural products are known to consist of oxapolycyclic systems. A potential route to such systems involves bridging of the more readily available macrolides (or the corresponding dithionolides, available by thionation with Lawesson s reagent) by electron-transfer reactions initiated with sodium naphthalenide (Scheme I). 

Thioglycosides can also be activated by a one-electron transfer reaction from sulfur to the activating reagent tris-(4-bromophenyl)ammoniumyl hexachloroanti-monate (TBPA+) [102,103]. The use of this promoter was inspired by an earlier report where activation was achieved under electrochemical conditions to give an intermediate S-glycosyl radical cation intermediate [104], and the reactivity and mechanism have also been explored [105,106]. 

Titanium catalysts have long been used in electron transfer reactions involving epoxides, mostly as stoichiometric reagents. Gansauer et al. have developed a catalytic version of these reactions using titanocenes along with zinc metal to generate the active catalyst (Scheme 60). In situ reduction of Ti(IV) with zinc metal provides Ti(III) species 231, which coordinates... 

In order to probe these effects, a number of studies on the kinetics of electron transfer between small molecule redox reagents and proteins, as well as protein-protein electron transfer reactions, have been carried out (38-41). The studies on reactions of small molecules with electron transfer proteins have pointed to some specificity in the electron transfer process as a function of the nature of the ligands around the small molecule redox reagents, especially the hydrophobicity of these... 

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

The most important application of organolithium reagents is their nucleophilic addition to carbonyl compounds. One of the simplest cases would be the reaction with the molecule CO itself, whose products are stable at room temperature. Recently, it was shown that a variety of RLi species are able to react with CO or f-BuNC in a newly developed liquid xenon (LXe) cell . LXe was used as reaction medium because it suppresses electron-transfer reactions, which are known to complicate the reaction . In this way the carbonyllithium and acyllithium compounds, as well as the corresponding isolobal isonitrile products, could be characterised by IR spectroscopy for the first time. 

Indirect electrochemical processes are hybrids in a certain sense they combine an electrochemical and therefore heterogeneous electron transfer reaction with a homogeneous redox process. The redox reagent undergoes a homogeneous reaction with the substrate and is subsequently regenerated in its active form at the electrode (see Fig. 1). 

Similar considerations apply to the role of spin equilibria in electron transfer reactions. For many years spin state restrictions were invoked to account for the slow electron exchange between diamagnetic, low-spin cobalt(III) and paramagnetic, high-spin cobalt(II) complexes. This explanation is now clearly incorrect. The rates of spin state interconversions are too rapid to be competitive with bimolecular encounters, except at the limit of diffusion-controlled reactions with molar concentrations of reagents. In other words, a spin equilibrium with a... 

In view of the lack of a clear understanding of the physical picture of the process, the thermal diffusion model has no predictive power. On its basis, for example, the manner in which the kinetic curves for low-temperature electron transfer reactions should change with changing concentration or kind of spatial distribution of reagents cannot be predicted. By contrast, the model of electron tunneling permits such predictions, and these predictions have been shown above (see, for example, Chap. 6, Sect. 3) to agree with the experiments. 

It is now generally admitted that this reaction involves both one-electron and two-electron transfer reactions. Carbonyl compounds are directly produced from the two-electron oxidation of alcohols by both Crvl- and Crv-oxo species, respectively transformed into CrIV and Crm species. Chromium(IV) species generate radicals by one-electron oxidation of alcohols and are responsible for the formation of cleavage by-products, e.g. benzyl alcohol and benzaldehyde from the oxidation of 1,2-diphenyl ethanol.294,295 The key step for carbonyl compound formation is the decomposition of the chromate ester resulting from the reaction of the alcohol with the Crvl-oxo reagent (equation 97).296... 

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... 

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

The rate expressions 6.25-6.29 are all of the general form shown in Equation 6.30, where X is a reagent, here phenol. (Note that, in the electrochemical literature, the electron transfer reaction is sometimes written as A + e B, rather than O + e 4> R, and the reaction orders as a and cb rather than o and V we are using this aspect of the original notation.) Thus, the first task in the analysis of the kinetics of a reaction is to determine the values of... 

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