Dissociative electron transfer mechanisms

All these observations point to the occurrence of a 8 2 rather than an outer sphere, dissociative electron-transfer mechanism in cases where steric constraints at the carbon or metal reacting centres are not too severe. It is, however, worth examining two other mechanistic possibilities. One of these is an electrocatalytic process of the Sg -type that would involve the following reaction sequence. If, in the reaction of the electron donor (nucleophile), the bonded interactions in the transition state are vanishingly small, the alkyl radical is formed together with the oxidized form of the electron donor, D . Cage coupling (144) may then occur, if their mutual affinity is... 

L. Eberson, Problems and Prospects of the Concerted Dissociative Electron-Transfer Mechanism, Acta Chem. Scand. 1999, 53, 751-764. 

If, whatever the interest of conceiving electron-pair transfer reactions such as Sn2 substitution as an inner sphere electron-transfer process, single electron transfer is intended to qualify reactions in which the rate-determining step is an outer sphere, non-dissociative or dissociative electron-transfer step preceding the bond-formation step, then the answer is no. There are a number of cases where true SN2 mechanisms (in which the bond-breaking and bond-formation steps are concerted) do occur, even with nucleophiles that are members of reversible one-electron reversible redox couples. In terms of activation energy, the SN2 mechanism merges with the outer sphere, dissociative electron-transfer mechanism when the bonded interactions in the transition state vanish. Steric constraints at the electro-... 

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

As depicted in Scheme 1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. How the dynamics of concerted electron transfer/bond breaking reactions (heretofore called dissociative electron transfers) may be modeled, and particularly what the contribution is of bond breaking to the activation barrier, is the first question we will discuss (Section 2). In this area, the most numerous studies have concerned thermal heterogeneous (electrochemical) and homogeneous reactions. 

There is thus an apparent continuity between the kinetics of an electron transfer leading to a stable product and a dissociative electron transfer. The reason for this continuity is the use of a Morse curve to model the stretching of a bond in a stable product in the first case and the use of a Morse curve also to model a weak charge-dipole interaction in the second case. We will come back later to the distinction between stepwise and concerted mechanisms in the framework of this continuity of kinetic behavior. 

As depicted in Scheme 3.1, reductive and oxidative cleavages may follow either a concerted or a stepwise mechanism. RX is a commonly used designation for an alkyl halide. Many experimental studies of dissociative electron transfers have indeed taken as examples the reductive cleavage of alkyl halides. However, many other compounds have been investigated in the framework of reaction Scheme 3.1 in the organic and inorganic field, for reductions as well as for oxidations. 

The forward and reverse rate constants are thus equal at zero standard free energy. However, this will be difficult to check in practice, for both reactions are very slow, since a bond-breaking/bond-forming process endowed with a quite large internal reorganization is involved. The result is that dissociative electron transfer reactions are usually carried out with electron donors that have a standard potential largely negative to the dissociative standard potential. The reoxidation of the R, X- system is thus possible only with electron acceptors, D +, that are different from the D,+ produced in the reduction process (they are more powerful oxidants). There is no reason then that the oxidation mechanism be the reverse of the... 

Similarly, several examples of reactions of perliuoroalkyl halides have been demonstrated to follow an SrnI -type mechanism (Section 3, p. 75). Since, here too, dissociative electron transfer is likely under the conditions of the reactions [Section 2, pp. 54-56 (Andrieux et ai, 1990b)], the substitution process most probably also follows mechanism (134) rather than (103). The same is also likely to be true with alkyl mercurials (Russell, 1989). 

This nuance of the original Sr I mechanism may thus occur in quite a number of cases. Nomenclature purists may consider it necessary to find other symbols to name this mechanism and, presumably, to question the adequacy of the 1 in this case. Beyond symbols, if the Sr I mechanism is viewed as an outer sphere electron-transfer-induced nucleophilic substitution , a possible designation of the mechanism under discussion might be dissociative electron-transfer-induced nucleophilic substitution . The original designation of these reactions as nucleophilic reactions proceeding via anion radical intermediates (Komblum, 1975) would still apply to both nuances of the mechanism since, in the present case, RNu is an essential intermediate in the reaction, even if RX is not. 

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