Photoreduction of 4-substituted nitrobenzenes by amines

Abstract

The reduction of several nitrobenzenes bearing electron-donor and electron-withdrawing substituents in the 4-position by triethylamine in acetonitrile was studied by cyclic voltammetry, EPR spectroscopy, and steady-state photolysis. The electrochemical reduction of nitro compounds showed a reversible cathodic wave corresponding to one electron transfer, and forms the nitrobenzene radical anion. The reduction potentials were well correlated with the hyperfine nitrogen coupling constants of the radical anion and with the donor-acceptor characteristics of the 4-substituent. The photoreduction of nitro compounds in the presence of triethylamine gave the nitrobenzene radical anion. EPR spin trapping experiments showed that the alpha-aminoethyl radical is formed only in the photolysis of nitrobenzenes with electron-donating substituents. These compounds in the presence of the amine were efficient photoinitiators of the methyl methacrylate polymerization. The polymerization rate increases with the amine concentration reaching a constant value. Similar behaviour was observed for the photobleaching quantum yield. These results indicate that the photobleaching and the reaction that leads to radicals able to add to the monomer come from the interaction of the triplet state of the nitro compound with the amine followed by proton transfer within the geminate ion pair giving the neutral radicals. In the absence of monomer, nitrosobenzene appeared to be the primary product. The polymerization in the presence of electron-withdrawing substituted nitrobenzenes was negligible. This is in agreement with the lack of alpha-aminoalkyl radical formation.