Effects of magnesium(II) ion on hydride-transfer reactions from an NADH model compound to p-benzoquinone derivatives. The quantitative evaluation based on the reaction mechanism

Abstract

The effects of Mg2+ ion on hydride-transfer reactions from an NADH model compound, 1 -benzyl-l,4dihydronicotinamide (BNAH), t o a series of p-benzoquinone derivatives (Q) as well as on the redox potentials of BNAH and Q in acetonitrile have been examined. The Mg2+ ion shows both accelerating and retarding effects on the hydride-transfer reactions depending on the p -benzoquinone derivative and the Mg2+ concentration. Such dual effects of the Mg2+ ion have been well correlated with the change of the redox potentials of BNAH and Q in the presence of Mg2+ ion since it has been found that there is a simple correlation between the logarithm of the rate constant and the difference of the redox potentials between BNAH and Q in the absence and presence of Mg2+ ion. It is shown that a proposed reaction mechanism involving electron transfer from BNAH to Q followed by proton transfer from BNAH +' t o Q-' in the rate-determining step of the hydride-equivalent transfer provides a quantitative evaluation of the single and unified correlation between the logarithm of the rate constant and the difference between the redox potentials of BNAH and Q in the absence and presence of Mg2+ ion. The effect of Mg2+ ion on the primary kinetic isotope effects on hydride transfer from BNAH t o Q is also shown to be consistent with the proposed reaction mechanism. Moreover, a ternary complex involving BNAH, Mg2+ ion, and Q as a reaction intermediate prior t o electron transfer from BNAH to Q has been detected for the first time.

A 2,2′‐Bipyridyl‐Bridged Tetrathiafulvalene‐quinone Dyad: Intramolecular Electron‐Transfer Promoted by Metal Ions

“…However, the radical anion of quinone unit could not be detected for the fast disproportionation of the radical anion of quinone (Q •-) into the corresponding Q 2-and neutral Q in the presence of metal ion at room temperature. 8 Nevertheless, the emergence of ESR signal confirms that electron transfer takes place for dyad 1 in the presence of Sc 3 ＋ . Accordingly, both absorption and ESR spectroscopic investigations manifest that electron transfer occurs between TTF and quinone units of dyad 1 in the presence of Sc 3＋ .…”

Metal‐Ion‐Promoted Electron Transfer between Tetrathiafulvalene and Quinone Units Within a Calix[4]arene Framework and Tuning through Coordination of the Neighboring Crown Ether with a Sodium Cation

“…According to previous studies, the radical anion of quinone (Q ⋅− ) can bind metal ions;13, 18a moreover, in some cases, metal ions can facilitate the disproportionation of the radical anion of quinone (Q ⋅− ) into corresponding neutral (Q) and dianion (Q 2− ) species that may bind metal ions strongly 13. 18c Therefore, the disappearance of ESR signals due to the radical anion of the quinone unit in 1 is understandable. Nevertheless, the emergence of an ESR signal for TTF ⋅+ further confirms that electron transfer takes place within 1 in the presence of Zn 2+ .…”

Effects of Lewis Acids on Photoredox Catalysis

“…FlH 2 ‐Mg 2+ undergoes a reaction with O 2 to produce H 2 O 2 accompanied by the regeneration of Fl‐Mg 2+ (Scheme ) . The oxidation of FlH 2 ‐Mg 2+ by O 2 is also accelerated by the binding of Mg 2+ to O 2 .− , which facilitates the electron transfer from FlH 2 ‐Mg 2+ to O 2 . Hydride transfers from nicotinamide adenine dinucleotide (NADH) analogues to p ‐benzoquinone derivatives have also been reported to be accelerated by binding Mg 2+ to semiquinone radical anions, which then facilitates the electron transfer from NADH analogues to p ‐benzoquinone derivatives .…”