Photosensitized electron transfer promoted reductive activation of carbon-selenium bonds to generate carbon-centered radicals: application for unimolecular group transfer radical reactions

Abstract

The investigation presented in this paper explores the mechanistic aspects and synthetic potentials of photosensitized electron transfer (PET) promoted reductive activation of organoselenium substrates. PET activation of substrates 1-5 is achieved through a photosystem comprised of light-absorbing 1,5-dimethoxynaphthalene (DMN) as electron donor and ascorbic acid as co-oxidant. The fluorescence quenching of ¹DMN^∗ by organoselenium compounds 1-5, correlation of fluorescence quenching rate constant with the reduction potentials of 1-5, and the dependence of photodissociation quantum yields of 1-5 on their concentration suggests the occurrence of electron-transfer (ET) processes between ¹DMN^∗ and 1-5. Steady state photolysis of organoselenium substrates (R₂CHSePh) in the presence of ¹DMN^∗ and ascorbic acid leads to the cleavage of the -C-Se- bond to produce a carbon-centered radical and PhSe- species via the intermediacy of R₂CH-SePh^-·. The mechanistic interpretation for the reductive activation of -C-Se- bonds and the synthetic utility of observed cleavage pattern is extended for the unimolecular group transfer radical sequences.