Catalysis on Photoinduced Electron Transfer

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We have found that Sc^<3+> ion is highly effective for the reduction of benzaldehyde and its derivatives by an NADH analogue at room temperature. The apparent reactivity is shown to be comparable with the enzyme. The catalytic mechanism is discussed based on the comparison of the catalysis of Sc^<3+> on the electron transfer reactions of carbonyl compounds. The hydride transfer reaction from 10-methyl-9,10-dihydroacridine (AcrH_2) to p-benzoquinone (Q) is catalyzed by various metal ions such as Mg^<2+> and rare-earth metal ions in acetonitrile (MeCN). Among triflate salts of metal ions examined, Sc (OTf) _3 was the most effective. The observed second-order rate constant increases with an increase in [Sc^<3+>] at low concentrations, changing to second order dependence at high concentrations. Essentially the same catalytic behavior of Sc^<3+> was observed in the electron transfer reduction of Q by CoTPP (TPP=tetraphenylporphyrin dianion) in MeCN.The second-order dependence of the rate constant with respect to [Sc^<3+>] is ascribed to formation of a 1 : 2 complex between Q^<・-> and Sc^<3+>. The formation of Q^<・->-2Sc^<3+> is confirmed by the ESR spectrum which exhibits the super hyperfine interaction with two equivalent Sc nuclei. Thus, the hydride transfer from AcrH_2 to Q may proceed via Sc^<3+>-catalyzed electron transfer from AcrH_2 to Q.When AcrH_2 is replaced by 1-benzyl-1,4-dihydronicotinamide (BNAH) which is a stronger electron donor than AcrH_2, benzaldehyde and its derivatives are reduced by BNAH efficiently in the presence of Sc^<3+> in MeCN at room temperature to yield the corresponding alcohols. The Sc^<3+> also catalyzes photoinduced electron transfer from the excited state of Ru (bpy) _3^<2+> to benzaldehydes in MeCN.Catalysis of metal ions on various thermal and photoinduced electron transfer reactions has been examined in detail.