A Second Job for Electron Upconversion: Single-Electron Activation of Leaving-Group Departure

Electron upconversion, the promotion of a single electron into a higher-energy orbital as the direct consequence of an exergonic chemical step, has been studied mainly as a way to generate transient “super-reductants” that hand off a high-energy electron to an external acceptor. Here, we explore a different application of this phenomenon. Using density functional theory calculations in combination with a survey of enzyme-catalyzed processes, we show that the same upconverted radical anions can act intramolecularly to expel otherwise recalcitrant leaving groups, accomplishing transformations that are formally two-electron, heterolytic eliminations. Taking redox dehydratases as the starting inspiration, we compare the energetics of hydroxide elimination from a ketyl radical anion against the classical, stereoelectronically favorable enolate route. Elimination through the upconverted ketyl is thermodynamically preferred by ~10 kcal/mol, and although this preference diminishes, it does not disappear even for a remote, non-activated γ-hydroxyl group. The orbital picture is simple: occupation of a high-energy antibonding orbital is relieved when electron density flows into the σ* orbital of the scissile bond, so that bond cleavage demotes the electron into a lower-energy, non-bonding orbital. Thus, upconversion is not only a route to strong reductants; it is a general activation mode for bond cleavage. The same logic applies to the C–O, C–N, and C–S eliminations carried out by diol dehydratases, 4-hydroxybutyryl-CoA dehydratase, certain glycyl radical enzymes, and ribonucleotide reductase under mild, metal-sparing, anaerobic conditions, and it suggests design principles for synthetic eliminations that avoid strong acids, strong bases, and pre-activated substrates.