A mechanistic dichotomy in scandium ion-promoted hydride transfer of an NADH analogue: Delicate balance between one-step hydride-transfer and electron-transfer pathways

The rate constant (k_H) of hydride transfer from an NADH analogue, 9,10-dihydro-10-methylacridine (AcrH₂), to 1-(p-tolylsulfinyl)-2,5-benzoquinone (TolSQ) increases with increasing Sc ³⁺ concentration ([Sc³⁺]) to reach a constant value, when all TolSQ molecules form the TolSQ-Sc³⁺ complex. When AcrH ₂ is replaced by the dideuterated compound (AcrD₂), however, the rate constant (k_D) increases linearly with an increase in [Sc³⁺] without exhibiting a saturation behavior. In such a case, the primary kinetic deuterium isotope effect (k_H/k_D) decreases with increasing [Sc³⁺]. On the other hand, the rate constant of Sc³⁺-promoted electron transfer from tris(2- phenylpyridine)iridium [Ir(ppy)₃] to TolSQ also increases linearly with increasing [Sc³⁺] at high concentrations of Sc³⁺ due to formation of a 1:2 complex between TolSQ^•- and Sc ³⁺, [TolSQ^•- (Sc³⁺)₂], which was detected by ESR. The significant difference with regard to dependence of the rate constant of hydride transfer on [Sc³⁺] between AcrH₂ and AcrD² in comparison with that of Sc³⁺-promoted electron transfer indicates that the reaction pathway is changed from one-step hydride transfer from AcrH₂ to the TolSQ-Sc³⁺ complex to Sc³⁺-promoted electron transfer from AcrD₂ to the TolSQ-Sc³⁺ complex, followed by proton and electron transfer. Such a change between two reaction pathways, which are employed simultaneously, is also observed by simple changes of temperature and concentration of Sc³⁺.