Mechanism of Hydride Transfer from an NADH Model Compound to p-Benzoquinone Derivatives

Charge-transfer (CT) complexes formed between an NADH model compound, l-benzyl-l,4-dihydronicotinamide (BNAH), and a series of p-benzoquinone derivatives Q were isolated from benzene solutions of these reactants. Some isolated CT complexes exhibited long-wavelength absorption maxima in the range 670-735 nm, depending on the electron-acceptor ability of the quinone derivatives. Transient CT bands equivalent to the CT bands of the isolated complexes were observed also in the course of the hydride-transfer reactions from BNAH to Q in acetonitrile, suggesting that the CT complexes are intermediates for the hydride-transfer reactions. The rate constants k for the hydride-transfer reactions vary significantly with the redox potentials E°(Q/Q^-·) of pbenzoquinone derivatives and span a range of more than 10¹¹. The primary kinetic isotope effects k_H/k_D also show a large variation in the range 1.5-6.2, and a bell-shaped dependence of the k_H/k_D values on the E°(Q/Q_- ·) values has been obtained with a clear “Westheimer maximum”. Quantitative analyses for these correlations of the rate constants and the isotope effects with the redox potentials of p-benzoquinone derivatives have been presented on the basis of a sequential electron-proton-electron transfer mechanism where the radical ion pair [BNAH⁺·Q^-·] formed by the first electron transfer from BNAH to Q in the CT complex [BNAH-Q] is considered to be closer to a “transition state” than an “intermediate” for most p-benzoquinone derivatives used in this study.