Effects of base on oxidation of an NADH model compound by iron(III) complexes and tetracyanoethylene

Abstract

Effects of base on both an electron-transfer reaction from an NADH model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH) to [Fe(N–N)₃]³⁺(N–N = 2,2′-bipyridine and 1,10-phenanthroline) and a hydride transfer from BNAH to tetracyanoethylene (TCNE) in acetonitrile have been examined. The stroicheiometry of the electron transfer from BNAH to [Fe(N–N)₃]³⁺ in the absence of a base indicates that BNAH is a one-electron donor. In the presence of a base, however, BNAH acts as an apparent two-electron donor, when the two-electron transfer proceeds via a multistep process; a fast one-electron transfer from BNA˙ to [Fe(N–N)₃]³⁺. The rate constants for the proton transfer from BNAH⁺˙ to a series of pyridine derivatives have been determined. In the reduction of TCNE by BNAH, BNAH appears to be a two-electron donor in both the absence and presence of a base. Rates of the reduction of TCNE by BNAH increased with increasing base concentration, suggesting the involvement of BNAH⁺˙ as an intermediate in the hydride transfer from BNAH to TCNE. The kinetic analyses have led to the evaluation of the proton transfer rate constants for the deprotonation of BNAH⁺˙ with various bases, which accord with those obtained from the electron-transfer reactions of BNAH with [Fe(N–N)₃]³⁺ in the presence of bases. Based on the Brönsted plot of the proton transfer rate constants as well as the variation of the primary kinetic isotope effects k_H/k_D with the pk_a of the base, the pk_a value for BNAH⁺˙ has been evaluated as 3.6 ± 0.4.