KOtBu : a privileged reagent for electron transfer reactions?

Barham, Joshua P. and Coulthard, Graeme and Emery, Katie J. and Doni, Eswararao and Cumine, Florimond and Nocera, Giuseppe and John, Matthew P. and Berlouis, Leonard E. A. and McGuire, Thomas and Tuttle, Tell and Murphy, John A. (2016) KOtBu : a privileged reagent for electron transfer reactions? Journal of the American Chemical Society, 138 (23). pp. 7402-7410. ISSN 0002-7863 (https://doi.org/10.1021/jacs.6b03282)

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Many recent studies have used KOtBu in organic reactions that involve single electron transfer; in the literature, the electron transfer is proposed to occur either directly from the metal alkoxide or indirectly, following reaction of the alkoxide with a solvent or additive. These reaction classes include coupling reactions of halobenzenes and arenes, reductive cleavages of dithianes and SRN1 reactions. Direct electron transfer would imply that alkali metal alkoxides are willing partners in these electron transfer reactions, but the literature reports provide little or no experimental evidence for this. This paper examines each of these classes of reaction in turn, and contests the roles proposed for KOtBu; instead, it provides new mechanistic information that in each case supports the in situ formation of organic electron donors. We go on to show that direct electron transfer from KOtBu can however occur in appropriate cases, where the electron acceptor has a reduction potential near the oxidation potential of KOtBu, and the example that we use is CBr4. In this case, computational results support electrochemical data in backing a direct electron transfer reaction.