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.
Reduction of nitrobenzene by excess organic electron donor, 12, affords diphenylhydrazine in a reaction where azobenzene oxide and azobenzene are likely intermediates. No cleavage of the N-N -bond is seen under photoactivation conditions, whereas traces are seen under thermal activation. Hydrazone derivatives were prepared to explore the cleavage of N-N -bonds; the results show that a low-lying LUMO assists the transition state for accepting an electron, and the stabilisation that the potential fragments from N-N bond cleavage afford to the fragments is important in determining whether cleavage is observed.
The [2,3]-Wittig rearrangement of propargyloxy acetates bearing a propargylic stereocenter opposite to the oxyacetic acid moiety gives α-hydroxy-γ-amino acids containing an allene unit under high stereocontrol. The resulting product displays a novel type of γ-amino acid with a potentially intriguing biological profile.
Computational studies have been performed on potassium alkoxide‐allenes, as well as potassium and lithium amido‐allenes to probe the mechanism of their cyclizations to dihydrofurans and to 2,5‐dihydropyrroles. A long‐standing proposal envisaged electron transfer from dimsyl anions (formed by deprotonation of the solvent DMSO) but this pathway shows an exceptionally high kinetic barrier, while direct 5‐endo‐trig cyclization of the alkoxides and amides is much more easily achievable. The energy profiles for 4‐exo‐trig cyclizations onto the allenes are also explored, and the preferred formation of the observed five‐membered products is rationalized.
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