Electrochemical and photocatalytic generation of cis-olefin-bridged carbodication for umpolung [4+1] cycloaddition

Carbodications, bearing two positive charges on carbon atoms, are emerging as powerful superelectrophiles in organic synthesis. Traditionally formed through single-electron oxidation of overcrowded electron-rich alkenes or acid-mediated dehydration of diols, these species show high reactivity toward carbon-based nucleophiles. However, their reactivities with heteroatom nucleophiles like amines and water remain underexplored, largely due to difficulties in controlling oxidation and undesirable acid-base interactions. Here, we report that carbodication species, generated under mild electrochemical or photocatalytic conditions in the presence of heteroatom nucleophiles, display remarkable electrophilic reactivity toward primary amines and water. This enables an intermolecular umpolung [4 + 1] cycloaddition, proceeding via a cis-olefin-bridged dicationic intermediate, energetically favored over its trans-isomer, yielding a class of thioxanthene-based dispirocycles with 2,5-dihydro-1H-pyrrole or 2,5-dihydrofuran central cores. Theoretical studies of electrochemical cycloaddition with aniline suggest that hydrogen bonding between aniline and the fluorine atom of the ⁿBu₄NPF₆ electrolyte stabilizes the transition state, promoting nucleophilic addition.