Sn2 Substitution versus single electron transfer

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 

This conception of an SN2 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 SN2 —still remains, whatever intimate description of the SN2 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 al., 1981), an actively investigated question in other areas of chemistry, particularly that of coordination complex chemistry (Taube, 1970 Espenson, 1986). 

Three main sources of information are available for solving the ET versus Sn2 problem, namely, comparative kinetic studies, stereochemistry and cyclizable radical-probe experiments. 

Among comparative kinetic studies, the kinetic advantage method has been used systematically in several cases. It has been developed for the first time for investigating the ET versus SN2 problem in the reaction of iron(i) and cobalt(n) porphyrins with primary butyl halides (Lexa et al., 1981), yielding the corresponding a-butyl-iron(m) and cobalt(m) complexes according to the overall reaction (142). 

Quite similar results have been found recently in the reaction of the cobalt(i) form of vitamin B12 (B12s) with alkyl halides with n-butyl iodide, bromide and chloride, ethyl bromide and benzyl chloride the representative data point of vitamin B12s falls several orders of magnitude above the outer sphere dissociative electron-transfer line (Walder, 1989). 

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