Unlocking σ-Type Cyclopropenium Cation Transfer Reactions by an Iodine(III)-Based Umpolung Strategy

σ-Type cyclopropenium cations (CPCs), which are formally generated by removing one substituent on the alkene of cyclopropenes, represent promising intermediates for forging functionalized cyclopropenes. However, σ-type CPCs are challenging to access and their synthetic utility remain largelyunexplored. Recently, we introduced electrophilic cyclopropenyl-gold(III) species as equivalents of σ-type CPCs, which can then react with terminal alkynes or vinylboronic acids to afford alkynyl- or alkenyl-cyclopropenes.The merging of redox gold catalysis and a new class of hypervalent iodine reagents—the cyclopropenyl benziodoxoles (CpBXs)—plays a central role in generating the σ-type CPC equivalents. With the same reagents, we have also developed a synergistic Au/Ag bimetallic catalytic cyclopropenyl cross-coupling system that enables to forge 1,1'-bicyclopropenyl derivatives in an efficient and modular manner. Our strategy provides access to previously inaccessible, yet highly useful, functionalized cyclopropenes, thereby significantly advancing both cyclopropene and hypervalent iodine chemistry.