SRN1 reactions single electron transfer

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... 

A large number of radical reactions proceed by redox mechanisms. These all require electron transfer (ET), often termed single electron transfer (SET), between two species and electrochemical methods are very useful to determine details of the reactions (see Chapter 6). We shall consider two examples here - reduction with samarium di-iodide (Sml2) and SRN1 (substitution, radical-nucleophilic, unimolecular) reactions. The SET steps can proceed by inner-sphere or outer-sphere mechanisms as defined in Marcus theory [19,20]. 

The SRN1 reaction thus appears as a reaction in which single electron transfer plays a pre-eminent role but is by no means a single elementary step. A different problem is that of the possible involvement of single electron transfer in reactions that are not catalysed by electron injection (or removal). A typical example of such processes is another substitution reaction, namely,... 

The question we address now is that of the possible role of single electron transfer in substitution reactions that, unlike SRN1 reactions, are not catalysed by electron injection. The problem is twofold. One side of it consists in answering the questions do bond breaking and bond formation belong to two different and successive processes, i.e. (135) followed by (136), or, more... 

An Sn2 reaction, depicted in equation 1, seems to come about by an electron pair on the nucleophile displacing a second electron pair—the R-X a bond. Four valence electrons appear to be involved. The problem with this representation is that the electronic rearrangement seems radically different from that in a SET process, such as the initiation step of the SRN1 process (7), equation 2. Clearly, just a single electron has been transferred from the nucleophile to RX. As a consequence of these quite different descriptions, the relationship between the two processes becomes obscure. What factors encourage one pathway over the other is not clear. 

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