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

sigma-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, sigma-type CPCs are challenging to access and their synthetic utility remains largely unexplored. Recently, we introduced electrophilic cyclopropenyl-gold(III) species as equivalents of sigma-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 sigma-type CPC equivalents. With the same reagents, we have also developed a synergistic Au/Ag bimetallic catalytic cyclopropenyl cross-coupling system that enables 1,1 '-bicyclopropenyl derivatives to be forged 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. 1 Introduction 2 Transferring sigma-Type CPCs to Terminal Alkynes 3 Transferring sigma-Type CPCs to Vinylboronic Acids 4 Transferring sigma-Type CPCs to Terminal Cyclopropenes 5 Reaction Mechanism 6 Conclusions