Diiodoindium(III) cation, InI₂⁺, a potent yneophile. Generation and application to cationic cyclization by selective π-activation of C≡C.

The removal of the iodide ion from indium triiodide by means of reactive Ag(I) salts leads to the formation of the highly reactive ligandless cation InI2(+), which is unusual in having two vacant low-lying p-orbitals. This bivalent Lewis acidity leads to an especially high affinity for the two orthogonal π-bonds of carbon-carbon triple bonds. Consequently, the double-coordinating InI2(+) is an especially effective reagent for the selective activation of C≡C and the catalytic initiation of cationic cyclization processes. A number of such reactions are described to demonstrate synthetic utility.