H-Bonding Electron Donor–Acceptor (EDA) Complexes and Energy Transfer Synergy for Photocatalytic Synthesis of α-Substituted Vinyl Sulfones

This study develops a novel photocatalytic strategy that utilizes energy transfer (EnT) to excite an electron donor-acceptor (EDA) complex for radical generation, offering new insights into this underexplored pathway in radical chemistry. Innovatively, sodium dithionite (Na₂S₂O₄) was employed as a sustainable SO₂ source and electron donor to form a unique EDA complex with electron-deficient allene derivatives via hydrogen bonding. Driven by the EnT process, this complex successfully mediated a three-component radical cascade reaction involving haloalkyl alkenes, Na₂S₂O₄, and electron-deficient allenes, enabling the highly regioselective synthesis of α-substituted vinyl sulfones. Combined experimental characterization and theoretical calculations confirmed the structural stability of the hydrogen-bonded EDA complex and its electron transfer mechanism. Mechanistic studies further established the critical role of the energy transfer pathway in the photoactivation of the EDA species. Notably, Quantum Theory of Atoms in Molecules (QTAIM) analysis revealed that the C═C═C-H···O═S hydrogen bond is a closed-shell interaction with partial covalent character, providing deeper electronic-level understanding of the hydrogen bonding mechanism.