Electron Transfer Reactions: KO tBu (but not NaO tBu) Photoreduces Benzophenone under Activation by Visible Light.

Long-standing controversial reports of electron transfer from KO tBu to benzophenone have been investigated and resolved. The mismatch in the oxidation potential of KO tBu (+0.10 V vs SCE in DMF) and the first reduction potential of benzophenone (of many values cited in the literature, the least negative value is -1.31 V vs SCE in DMF), preclude direct electron transfer. Experimental and computational results now establish that a complex is formed between the two reagents, with the potassium ion providing the linkage, which markedly shifts the absorption spectrum to provide a tail in the visible light region. Photoactivation at room temperature by irradiation at defined wavelength (365 or 400 nm), or even by winter daylight, leads to the development of the blue color of the potassium salt of benzophenone ketyl, whereas no reaction is observed when the reaction mixture is maintained in darkness. So, no electron transfer occurs in the ground state. However, when photoexcited, electron transfer occurs within a complex formed from benzophenone and KO tBu. TDDFT studies match experimental findings and also define the electronic transition within the complex as n → π*, originating on the butoxide oxygen. Computation and experiment also align in showing that this reaction is selective for KO tBu; no such effect occurs with NaO tBu, providing the first case where such alkali metal ion selectivity is rationalized in detail. Chemical evidence is provided for the photoactivated electron transfer from KO tBu to benzophenone: tert-butoxyl radicals are formed and undergo fragmentation to form (acetone and) methyl radicals, some of which are trapped by benzophenone. Likewise, when KOC(Et)3 is used in place of KO tBu, then ethylation of benzophenone is seen. Further evidence of electron transfer was seen when the reaction was conducted in benzene, in the presence of p-iodotoluene; this triggered BHAS coupling to form 4-methylbiphenyl in 74% yield.

KOtBu as a Single Electron Donor? Revisiting the Halogenation of Alkanes with CBr4 and CCl4.

The search for reactions where KOtBu and other tert-alkoxides might behave as single electron donors led us to explore their reactions with tetrahalomethanes, CX4, in the presence of adamantane. We recently reported the halogenation of adamantane under these conditions. These reactions appeared to mirror the analogous known reaction of NaOH with CBr4 under phase-transfer conditions, where initiation features single electron transfer from a hydroxide ion to CBr4. We now report evidence from experimental and computational studies that KOtBu and other alkoxide reagents do not go through an analogous electron transfer. Rather, the alkoxides form hypohalites upon reacting with CBr4 or CCl4, and homolytic decomposition of appropriate hypohalites initiates the halogenation of adamantane.

Evidence of single electron transfer from the enolate anion of an N,N'-dialkyldiketopiperazine additive in BHAS coupling reactions.

A designed N,N'-dialkyldiketopiperazine (DKP) provides evidence for the role of DKP additives as initiators that act by electron transfer in base-induced homolytic aromatic substitution reactions, involving coupling of haloarenes to arenes.

Effect of solvent on radical cyclisation pathways: SRN1 vs. aryl-aryl bond forming mechanisms.

A recent paper identified a series of alternative cyclisation pathways of aryl radicals that resulted from electron transfer to various tethered haloarene-acetylarene substrates, in either benzene or DMSO as solvent. The electron transfer occurred from one of two enolates that were formed in the presence of KOtBu: either the enolate of the acetylarene, within the haloarene-acetylarene substrate, or the enolate 7 of the N,N'-dipropyl diketopiperazine (DKP) additive 6. This paper uses contemporary computational methods to determine the reaction pathways involved; depending on the substrate, the aryl radical underwent (i) SRN1 onto the enolate anion of the acetylarene, (ii) aryl-aryl bond formation, (iii) tandem hydrogen atom abstraction followed by SRN1 cyclisation and even (iv) ArC-N cleavage. The influence of the solvent was investigated. In this paper it is shown that the solvent influences which reactive species are present in the reaction mixture, and whether each species acts as an electron donor or an electron acceptor in the radical initiation or propagation steps. The main initiation step is a single electron transfer from the enolate anion 7 of the DKP additive in benzene, but in DMSO the initiation can occur from the enolate anion of the substrate itself. Using computational techniques a deeper understanding of the radical pathways involved has been obtained, which shows how we can use solvent to preferentially access products arising from either SRN1 or aryl-aryl bond formation pathways.

KOtBu: A Privileged Reagent for Electron Transfer Reactions?

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.