Redox Upconversion and Electrocatalytic Cycles in Activation of Si–Si Bonds: Diverging Reactivity in Hole- and Electron-Catalyzed Transformations

Electron transfer is a powerful tool for promoting chemical reactions. A subset of redox-mediated transformations are "electroneutral" reactions, where the reactant and product are in the same oxidation state. In such processes, electrons (or holes) can serve as genuine catalysts if the products have higher reduction (or oxidation) potential than the reactants, i.e., in the presence of electron (or hole) upconversion. This work explores the differences between electron and hole catalysis in redox-activated transformations of the same substrate. We show that for redox upconversion to occur in both the reductive and oxidative modes, the reduced and oxidized reactions must follow different paths. The chosen substrate, 1,2-disila-3,5-cyclohexadiene 1, combines an electron-rich π-system with a Si–Si bond and can undergo Si–Si bond scission in both the oxidative and reductive regimes. Single-electron oxidation with a molecular oxidant leads to the formation of a radical cation that undergoes an intramolecular rearrangement with the elimination of dimethylsilylene and the formation of a radical cation of silole, 2. The latter can oxidize another molecule of neutral reactant 1, a step that closes the hole catalytic cycle. The unique utility of the radical-cationic reactivity mode is illustrated by the lack of hole upconversion under electrochemical conditions, where the radical-cationic reaction path is aborted by further oxidation at the electrode. In contrast, the radical anion formed from one-electron reduction of 1 is unreactive. This persistent species can be formed reversibly in THF, both chemically and electrochemically. Its reactivity can be unlocked by the addition of stoichiometric amounts of water. Although the subsequent reaction also involves Si–Si bond cleavage, it follows a path that is different from that of the radical-cationic version. Interestingly, electrochemical experiments clearly confirm that this process is also a chain reaction that requires a catalytic amount of electrons (0.3 equiv) for the complete conversion of the substrate. Computations identify a possible mechanistic pathway to the observed products and support the importance of reductant upconversion in this electron-catalyzed process. Hence, this work identified the first molecular system that can undergo true electron-catalyzed and hole-catalyzed processes and confirmed that these processes lead to different products.