Abstract
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.
Highlights
Alkali metal tert-butoxides (KOtBu, NaOtBu) play key roles in numerous organic transformations, acting as powerful bases
KOtBu has a dual role: (i) acting in combination with a wide variety of organic additives to initiate the process by converting aryl halides 1 into aryl radicals 3 and (ii) deprotonating radical 4 to form the radical anion 5; this radical anion transfers an electron to another molecule of aryl halide 1, thereby propagating a chain reaction
In 2015 using KOtBu in DMF as solvent, Taillefer et al reported[12] SRN1 coupling reactions between potassium enolates 10 and aryl radicals 3, the latter being formed from aryl halides 1
Summary
Alkali metal tert-butoxides (KOtBu, NaOtBu) play key roles in numerous organic transformations, acting as powerful bases In recent years, they have seen widespread use in transition metalfree coupling reactions of haloarenes 1 with arenes to afford biphenyls 2 (here the arene is the solvent)[1−9] or with styrenes to afford stilbenes.2e,3a,6c The mechanism for biaryl formation is shown in Scheme 1A.10. DMF was uniquely useful in accomplishing these reactions Through computational studies, they proposed that, following deprotonation of DMF to form carbamoyl anion 8, electron transfer occurred to the aryl halide 1, thereby generating an aryl radical 3 and carbamoyl radical 9, no experimental evidence for electron transfer was presented. This paper examines, in turn, the evidence in each of these cases
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