Magnesium-Assisted Photoinduced Polarity-Mismatched Coupling: Access to Thioester-Decorated Oxindoles and Fused...

The polarity of radical intermediates dictates their innate reactivity, making polarity-matched radical couplings inherently favored, while polarity-mismatched variants remain a formidable challenge. Here, we demonstrate a redox-neutral, Mg²⁺-driven strategy that enables efficient coupling of electrophilic radicals with electron-deficient alkenes without the need for alkene activation or polarity inversion. Crucially, Mg²⁺ plays a nontraditional mechanistic role: rather than functioning as classical Lewis acidic activator, it acts as a spatial organizer and aligns the transient radical partners to override their intrinsic polarity-mismatch. Using this approach, we have developed a step-economical, three-component tandem synthesis of thioester-functionalized oxindoles using feedstock aldehydes, elemental sulfur as a sustainable and readily available sulfur source, thereby avoiding prefunctionalized sulfur reagents and multiple steps. The protocol was further applied for the hetero-annulation of 1,7-enynes utilizing elemental sulfur for the synthesis of polycyclic compounds, where acyl radical functions as an effective sulfur atom carrier. In addition to several mechanistic studies, theoretical calculations were conducted to support our hypothesis and clarify the mechanism. DFT calculations revealed that redox-neutral Mg²⁺ lowers the radical addition free energy barrier by simultaneously coordinating to both reacting partners, steering them into a spatial arrangement that promotes out-of-plane orbital overlap, essential for efficient C-S bond formation.