Electrons and photons are essential chemical "currencies" commonly traded in chemical transformations. The many applications of photon upconversion, i.e., conversion of low energy photons into high energy photons, raises the question about the possibility of "electron upconversion". In this review, we illustrate how reduction potential can be increased by using the free energy of exergonic chemical reactions. The electron (reductant) upconversion can produce up to ~20-25 kcal/mol of additional redox potential, creating powerful reductants under mild conditions. We will present the two common types of electron-upconverting systems - dissociative (based on unimolecular fragmentations) and associative (based on bimolecular formation of three-electron bonds). The possible utility of reductant upconversion encompasses redox chain reactions in electrocatalytic processes, photoredox cascades, design of peroxide-based medicines, firefly luminescence, and reductive repair of DNA photodamage.
N-oxyphthalimides are stable and easily accessible compounds that can produce oxygen radicals upon 1-electron reduction. We present a systematic study of electrochemical properties of N-oxyphthalimide derivatives (PI-ORs) in DMF by cyclic voltammetry. In all cases, electron transfer to the substrate leads to decomposition of the intermediate radical anion via the N-O bond cleavage. In the case of benzyloxyphthalimide or its derivatives containing electrondonating substituents, reductive electron transfer induces the chain decomposition of the substrate to phthalimide (PI) radical-anion and the corresponding carbonyl compound. The PI radical-anion product is a powerful reductant that can transfer an electron to the reactant PI-OR, thus establishing a catalytic cycle for reductive N-O scission. This self-catalytic process is reflected in a considerable decrease in the reduction current for the substrate (<1e -/molecule). By contrast, reductive fragmentations of benzyl derivatives containing electronwithdrawing substituents in the aromatic ring or at the benzylic position, as well as tosyl and alkyl derivatives, occur via a 1-electron mechanism. A sequence of N-O and C-C scissions was engineered to support the intermediacy of O-centered radicals in these processes.
Twenty six peroxides belonging to bridged 1,2,4,5-tetraoxanes, bridged 1,2,4-trioxolanes (ozonides), and tricyclic monoperoxides were evaluated for their in vitro antimalarial activity against Plasmodium falciparum (3D7) and for their cytotoxic activities against immortalized human normal fibroblast (CCD19Lu), liver (LO ), and lung (BEAS-2B) cell lines as well as human liver (HepG2) and lung (A549) cancer-cell lines. Synthetic ozonides were shown to have the highest cytotoxicity on HepG2 (IC =0.19-0.59 μm), and some of these compounds selectively targeted liver cancer (selectivity index values for compounds 13 a and 14 a are 20 and 28, respectively) at levels that, in some cases, were higher than those of paclitaxel, artemisinin, and artesunic acid. In contrast some ozonides showed only moderate antimalarial activity against the chloroquine-sensitive 3D7 strain of P. falciparum (IC from 2.76 to 24.2 μm; 12 b, IC =2.76 μm; 13 a, IC =20.14 μm; 14 a, IC =6.32 μm). These results suggest that these derivatives have divergent mechanisms of action against cancer cells and malaria-infected cells. A cyclic voltammetry study of the peroxides was performed, but most of the compounds did not show direct correlation in oxidative capacity-activity. Our findings offer a new source of antimalarial and anticancer agents through structural modification of peroxide compounds.
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