Accessing elusive σ-type cyclopropenium cation equivalents through redox gold catalysis

Cyclopropenes are the smallest unsaturated carbocycles. Removing one substituent from cyclopropenes leads to cyclopropenium cations (C3 + systems, CPCs). Stable aromatic pi-type CPCs were discovered by Breslow in 1957 by removing a substituent on the aliphatic position. In contrast, sigma-type CPCs-formally accessed by removing one substituent on the alkene-are unstable and relatively unexplored. Here we introduce electrophilic cyclopropenyl-gold(III) species as equivalents of sigma-type CPCs, which can then react with terminal alkynes and vinylboronic acids. With catalyst loadings as low as 2 mol%, the synthesis of highly functionalized alkynyl- or alkenyl-cyclopropenes proceeded under mild conditions. A class of hypervalent iodine reagents-the cyclopropenyl benziodoxoles (CpBXs)-enabled the direct oxidation of gold(I) to gold(III) with concomitant transfer of a cyclopropenyl group. This protocol was general, tolerant to numerous functional groups and could be used for the late-stage modification of complex natural products, bioactive molecules and pharmaceuticals.