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

Abstract 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 for the first time that carbodication species generated under mild electrochemical or photocatalytic conditions display remarkable electrophilic reactivity toward primary amines and water. This enables a novel intermolecular umpolung [4+1] cycloaddition, proceeding via a cis-olefin-bridged dicationic intermediate, energetically favored over its trans-isomer, yielding a new 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 nBu₄PF₆ electrolyte stabilizes the transition state, promoting nucleophilic addition.