Catalysis on Electron Transfer and the Mechanistic Insight into Redox Reactions

Abstract

Abstract Both thermal and photoinduced electron transfer reactions from one-electron reductants to various organic oxidants such as carbonyl compounds are catalyzed by the presence of acids and metal ions. The acceleration of rates of electron transfer is ascribed to the thermodynamic stabilization of products of electron transfer by the protonation or complexation with metal ions, which results in the positive shift of the reduction potentials of oxidants. In the case of Mg2+-catalyzed electron transfer reduction of p-benzoquinone derivatives, the formation of 1 : 1 and 1 : 2 complexes between semiquinone radical anion and Mg2+ is confirmed by the transient absorption spectra as well as by the ESR spectra. From the spectral change at different Mg2+ concentrations is determined the formation constant (K2) of the 1 : 2 complex. The K2 values of various semiquinone anions determined from the direct detection of the Mg2+ complexes agree well with those estimated from the kinetic analysis on the Mg2+-catalyzed electron transfer reactions. The direct spectroscopic detection of complexes between the semiquinone radical anions and Mg2+, combined with the kinetic analysis of the catalytic effects of Mg2+ on not only the electron transfer reactions but also on various reactions involving p-benzoquinone derivatives such as Mg2+-catalyzed Diels–Alder reactions of anthracenes with p-benzoquinone derivatives, provides a confirmative basis for delineating the catalytic mechanisms of Mg2+ on various types of reactions. A remarkable increase in the reactivity of the photoexcited states of carbonyl compounds in the electron transfer reactions is also found by the complexation with Mg2+. The photoexcitation of the Mg2+ complex results in strong fluorescence in contrast with the nonfluorescent excited states of uncomplexed carbonyl compounds. Thus, various redox reactions of carbonyl compounds via photoinduced electron transfer are made possible by the catalysis of metal ions which form complexes with carbonyl compounds. The fundamental concept and the scope of catalysis on electron transfer are reviewed in this account to provide some insight into the mechanistic viability.