Photoreduction of Alkyl Halides by an NADH Model Compound. An Electron-Transfer Chain Mechanism

An NADH model compound, 1-benzyl-1,4-dihydronicotinamide (BNAH), reduces alkyl halides such as benzyl bromide, p-cyanobenzyl bromide, and methyl iodide to yield the corresponding alkanes when only the absorption band of BNAH (λ_max 350 nm) is irradiated in the presence of pyridine. The quantum yields for the photoreduction of alkyl halides by BNAH exceed unity under the appropriate conditions. An electron-transfer chain mechanism has been presented on the basis of the inhibitory effect of a radical scavenger isopentyl nitrite and an electron acceptor p-dinitrobenzene on the rate of the photochemical reactions as well as the detailed analysis of the kinetics. The chain length and the propagation rate constant of the chain process have been determined. The rate constants of the electron-transfer reactions from the excited state of BNAH to alkyl halides determined from the fluorescence quenching of BNAH by alkyl halides have been consistent with those obtained from the kinetic analysis of the photochemical reactions. It is demonstrated that an electron transfer from the excited state of BNAH to alkyl halides is solely responsible for the photoinitiation step.