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.

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) 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.

Potassium hydride behaves uniquely and differently than sodium hydride toward aryl halides. Its reactions with a range of haloarenes, including designed 2,6-dialkylhaloarenes, were studied in THF and in benzene. In THF, evidence supports concerted nucleophilic aromatic substitution, CS N Ar, and the mechanism originally proposed by Pierre et al. is now validated through DFT studies. In benzene, besides this pathway, strong evidence for single electron transfer chemistry is reported. Experimental observations and DFT studies lead us to propose organic super electron donor generation to initiate BHAS (base-promoted homolytic aromatic substitution) cycles. Organic donor formation originates from deprotonation of benzene by KH; attack on benzene by the resulting phenylpotassium generates phenylcyclohexadienylpotassium that can undergo (i) deprotonation to form an organic super electron donor or (ii) hydride loss to afford biphenyl. Until now, BHAS reactions have been triggered by reaction of a base, MOtBu (M = K, Na), with many different types of organic additive, all containing heteroatoms (N or O or S) that enhance their acidity and place them within range of MOtBu as a base. This paper shows that with the stronger base, KH, even a hydrocarbon (benzene) can be converted into an electron-donating initiator.

Many reactions have been discovered that lead to coupling of haloarenes to arenes using potassium tert-butoxide as the base, and one of a variety of organic compounds as an additive. The organic additive reacts with the base to form a strong organic electron donor in situ that initiates a base-induced homolytic aromatic substitution (BHAS) coupling reaction, by converting the haloarene into an aryl radical. This brief report presents an overview of the wide range of organic additives that can be used, and the organic electron donors that they form.