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.