Reactions and Dissociative Electron Transfers

DICHOTOMY AND CONNECTIONS BETWEEN SN2 REACTIONS AND DISSOCIATIVE ELECTRON TRANSFERS 

ELECTRON TRANSFER, BOND BREAKING, AND BOND FORMATION 

Additional and systematic experimental and theoretical investigations have followed, leading eventually to a reasonably sound overall picture. 

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

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. 

Most of the free-radical mechanisms discussed thus far have involved some combination of homolytic bond dissociation, atom abstraction, and addition steps. In this section, we will discuss reactions that include discrete electron-transfer steps. Addition to or removal of one electron fi om a diamagnetic organic molecule generates a radical. Organic reactions that involve electron-transfer steps are often mediated by transition-metal ions. Many transition-metal ions have two or more relatively stable oxidation states differing by one electron. Transition-metal ions therefore firequently participate in electron-transfer processes. 

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. 

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. 

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 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. 

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.

However, a more accurate comparison between the experimental reaction kinetics and the predictions of the dissociative electron transfer theory revealed that the agreement is good when steric hindrance is maximal (tertiary carbon acceptors) and that the reaction is increasingly faster than predicted as steric hindrance decreases.31 These results were interpreted as indicating an increase... 

It is a pleasure to acknowledge the essential contribution of Dr. C. P. Andrieux to most of the work reported above as well as that of Dr. D. Lexa in the field of porphyrins, Professor Moiroux and Dr. A. Anne to cation radical reactivity, Dr. M. Robert to photoinduced dissociative electron transfer and to the stepwise/concerted competition and Drs. P. Hapiot and Medebielle to recent work on thermal SRn1 reactions. Many students from our group have also contributed effectively to the work, namely, C. Costentin, G. Delgado, V. Grass, A. Le Gorande, C. Tardy and D. L. Wang. Fruitful and pleasant... 

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. 

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