Electron transfer dissociative

The chapter ends with a long-debated issue, that is, whether an electron donor molecule reacts as a single electron donor or as a nucleophile in an SN2 reaction. The experimental and theoretical aspects of this question are discussed in Section 3.7. 

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FIGURE 34.8 Free-energy surfaces for the dissociative electron transfer reaction (a) for the solvent polarization (b) along the coordinate r of the molecnlar chemical bond. corresponds to stable molecule in oxidized form. U" is the decay potential for the rednced foim. AFj and AF are the partial free energies of the transition determining mntnal arrangement of the two sets of the free-energy surfaces. 

German ED, Kuznetsov AM. 1994. Quantummechanicaltheory of dissociative electron transfer in polar solvents. J Phys Chem 98 6120-6127. 

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... 

Intramolecular dissociative electron transfer 146 Homolytic cleavage 152... 

Heterolytic cleavage, homolytic cleavage, dissociative electron transfer 155... 

Influence on the dynamics of dissociative electron transfer 158 Dissociative electron transfer to carbon tetrachloride 160 Other examples 162... 

Dichotomy and connections between SN2 reactions and dissociative electron transfers 177... 

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. 

Although the most numerous investigations of dissociative electron transfer have concerned thermal reactions, photoinduced dissociative electron transfer has also attracted a great deal of recent theoretical and experimental attention. As discussed in Section 6, one of the key questions in the field is whether photoinduced dissociative electron transfers are necessarily endowed with a unity quantum yield as one would predict on purely intuitive grounds. Quantum yield expressions for the concerted and stepwise cases are established and experimental examples are discussed. 

Since the publication of the review on Single Electron Transfer and Nucleophilic Substitution in this same series,1 reviews or research accounts have appeared concerning several particular points among those addressed here, namely, dynamics of dissociative electron transfer,2-6 single electron transfer and Sn2 reactions,2,7 9 and SRN1 reactions.10,11... 

The thermodynamics of dissociative electron transfer reactions were first characterized by Hush12 from thermochemical data in the case of the electrochemical reactions... 

Recently2 it has been asserted that the very existence of dissociative electron transfer reactions is ruled out by application of the principle of microscopic reversibility. The line of argument was as follows. In the reaction of the cleaving substrate RX, say, with an electron donor D (the same argument could be developed for an oxidative cleavage triggered by an electron acceptor),... 

The first attempt to describe the dynamics of dissociative electron transfer started with the derivation from existing thermochemical data of the standard potential for the dissociative electron transfer reaction, rx r.+x-,12 14 with application of the Butler-Volmer law for electrochemical reactions12 and of the Marcus quadratic equation for a series of homogeneous reactions.1314 Application of the Marcus-Hush model to dissociative electron transfers had little basis in electron transfer theory (the same is true for applications to proton transfer or SN2 reactions). Thus, there was no real justification for the application of the Marcus equation and the contribution of bond breaking to the intrinsic barrier was not established. 

Fig. 1 Morse curve modeling of the contribution of bond-breaking to the dynamics of dissociative electron transfer...

The validity of equation (12) has been checked for several families of alkyl halides for which D and E /x- are known (Ref. 32, see particularly figure 6 therein). It was thus found that for v = 0.1 V s the constant is equal to 0.3 eV at 20°C (expressing D in eV and the potentials in V). Equation (12) was then applied to the approximate determination of unknown BDEs in several series of compounds undergoing dissociative electron transfer, namely, TV-halosultams,32 sulfonium cations,33 vicinal dihalides,34 1,3-dihaloadamantes, 1,4-dihalo-bicyclo[2.2.2]octanes, and l,3-dihalobicyclo[l.l.l]pentanes.35 In the latter case, the mutual influence of the two halogens could be rationalized thanks to the conversion of the peak potential data to bond dissociation energies. 

The dissociative electron transfer model has been improved since then by a more accurate estimation of the activation entropy, taking into account that R and X are formed within a solvent cage from which they successively diffuse out.49 The free energy and entropy of activation are thus obtained from equations (14)... 

Expressions for the transition between adiabatic and non-adiabatic dissociative electron transfers have also recently been derived.54... 

Note also that additional information about the degree of adiabaticity can be derived from the investigation of photoinduced dissociative electron transfers, as discussed in Section 6. 

Note, however, that the concerted/stepwise dichotomy discussed here concerns cases in which the intermediate is unstable toward cleavage that occurs by means of intramolecular dissociative electron transfer. As discussed in the foregoing sections, for primary radicals that cleave homolytically, criteria based on the life-time of the intermediate may be pertinent. 

Describing the dissociative electron transfer step, RX e R + X involves determining the saddle point on the intersection of the two following free energy surfaces.22,31... 

Figures 8c and 8d represent the projection of the reaction pathways on the same plane, namely the front plane in which the dissociative electron transfer step is represented. This two-dimensional representation is easier to decipher than the 3D representation for determining the preferred pathway. They may however be misleading if it is not borne in mind that, in the 2D representation, the crossings between the three curves should not be considered as actual crossings of reaction pathways.

Carbonyl compounds are also suited to the investigation of the role of solvent reorganization in the dynamics of intramolecular dissociative electron transfer as observed in a series of phenacyl derivatives bearing various leaving groups.199... 

HETEROLYTIC CLEAVAGE, HOMOLYTIC CLEAVAGE, DISSOCIATIVE ELECTRON TRANSFER... 

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. 

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