Mechanism of photoreduction of diethyl benzylidene malonates by NAD(P)H model and comparison with thermal reaction

Abstract

Photoreduction of diethyl benzylidene malonates by NAD(P)H model has been investigated. The reduction of the carbon–carbon double bond occurs and is enhanced by electron-withdrawing groups on the benzene ring. Deuteriation experiments in [²H₄]methanol showed that the deuterium which originated from the solvent was mainly incorporated at the β-carbon. The mechanism of photoreduction is discussed in terms of sequential electron-hydrogen atom transfer from NAD(P)H model to the substrate in the solvent cage. On the other hand, thermal reduction of NAD(P)H model with the substrates in methan-[²H]-ol and/or [²H₄]methanol gave the reduced products with deuterium incorporated at the α-carbon. Reduction of diethyl benzylidene malonates with [4,4-²H₂]-1-benzyl-1,4-dihydronicotinamide (BNAH) led to the direct transfer of deuterium from C-4 to the β-carbon. These results have been interpreted as support for the direct hydride transfer mechanism.