Outer-sphere ET process

Another example is the reaction of indoles with nitrosoarenes in the presence of acids. The redox potentials of the reactants cannot justify an outer-sphere ET process, thus the formation of the phenylaminoxyl detected for the reaction carried out in the ESR cavity, could be more likely justified by an inner-sphere ET mechanism95. In fact the reaction of quinoline N-oxide with primary alkyl Grignards for which an outer-sphere mechanism was earlier proposed, takes place through classical nucleophilic addition96. 

Before we can enter a discussion of the redox processes involved in the two mechanisms defined above, we need a simple theoretical background which provides relevant insights into the phenomenon of ET. The Marcus theory of outer-sphere ET provides such a framework for the delineation of mechanistic domains, thanks to its origin in a simple model and its classical nature (Marcus, 1964 Marcus and Sutin, 1985 for applications in organic chemistry, see Eberson, 1982b, 1987). 

Organometallic donors and acceptors are particularly suitable to elucidate outer-sphere and inner-sphere ET processes as follows First, they exist as neutral molecules as well as cationic or anionic species, and thus form both molecular EDA complexes and ion-pair salts (see Section 2.3). Molecular complexes with organometallic donors and acceptors exhibit a wide range of donor-acceptor interaction from very... 

The radical clock experiments as well as the stereochemical outcome of the reaction along with the reactivity profiles observed pointed to an ET process as the operating mechanism. Linear-free energy relationships were also consistent with this mechanistic pathway (see succeeding text). ET may proceed in two ways, usually referred to as inner-sphere and outer-sphere ET, which can be contemplated as the two extremes of a continuous mechanism [204]. Both processes are dissociative in nature for alkyl halides and presumably do not involve a discrete radical anion, RX" [205]. The situation may, however, be different for aryl halides. Radical anions do exist, and aryl halides probably undergo a stepwise reaction with an electron donor to give rise to RX [206]. 

Electrochemical reductions have been referred to as compulsory ET processes, and for inert smooth Pt and some C, electrodes are assumed to be outer-sphere ET. For a dropping Hg electrode, which is the case considered here, the situation has been discussed Ref. 194, p 79. 

The first estimations of for photoinduced processes were reported by Dvorak et al. for the photoreaction in Eq. (40) [157,158]. In this work, the authors proposed that the impedance under illumination could be estimated from the ratio between the AC photopotential under chopped illumination and the AC photocurrent responses. Subsequently, the faradaic impedance was calculated following a treatment similar to that described in Eqs. (22) to (26), i.e., subtracting the impedance under illumination and in the dark. From this analysis, a pseudo-first-order photoinduced ET rate constant of the order of 10 to 10 ms was estimated, corresponding to a rather unrealistic ket > 10 M cms . Considering the nonactivated limit for adiabatic outer sphere heterogeneous ET at liquid-liquid interfaces given by Eq. (17) [5], the maximum bimolecular rate constant is approximately 1000 smaller than the values reported by these authors. 

Chemical reactivity of unfunctionalized organosilicon compounds, the tetraalkylsilanes, are generally very low. There has been virtually no method for the selective transformation of unfunctionalized tetraalkylsilanes into other compounds under mild conditions. The electrochemical reactivity of tetraalkylsilanes is also very low. Kochi et al. have reported the oxidation potentials of tetraalkyl group-14-metal compounds determined by cyclic voltammetry [2]. The oxidation potential (Ep) increases in the order of Pb < Sn < Ge < Si as shown in Table 1. The order of the oxidation potential is the same as that of the ionization potentials and the steric effect of the alkyl group is very small. Therefore, the electron transfer is suggested as proceeding by an outer-sphere process. However, it seems to be difficult to oxidize tetraalkylsilanes electro-chemically in a practical sense because the oxidation potentials are outside the electrochemical windows of the usual supporting electrolyte/solvent systems (>2.5 V). 

Section 3, are commonly classified as inner-sphere (contact) ion pairs (Kochi, 1988). Accordingly, in organic and organometallic processes a strong distinction must be made in their behaviour from that of other less common outer-sphere ion pairs that are pertinent to the Marcus treatment of electron-transfer dynamics (Eberson, 1987 Lee et al., 1991). 

This conception of an 8, 2 reaction as an electron-shift process is obviously equivalent to its conception as an inner sphere electron transfer, i.e. a single electron transfer concerted with the breaking of the R—X bond and the formation of the R—Nu bond. Faced with an experimental system, however, the first question—ET or 8 2 —still remains, whatever intimate description of the 8, 2 reaction one may consider most appropriate. If this is thought of in terms of inner sphere electron transfer, the question thus raised is part of the more general problem of distinguishing outer sphere from inner sphere electron-transfer processes (Lexa et ai, 1981), an actively investigated question in other areas of chemistry, particularly that of coordination complex chemistry (Taube, 1970 Espenson, 1986). 

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