Intramolecular Hydroarylation of Alkynes Catalyzed by Platinum or Gold: Mechanism and endo Selectivity

The cyclization of differently substituted aryl alkynes with PtII or AuI catalysts proceeds by endo‐dig pathways. When AgI was used to generate reactive cationic AuI catalysts, 2H‐chromenes dimerize to form cyclobutane derivatives by a AgI‐catalyzed process. A DFT study on the cyclization mechanism shows a kinetic and thermodynamic preference for 6‐endo‐dig versus 5‐exo‐dig cyclizations in PtII‐catalyzed processes. Calculations indicate that although Friedel–Crafts and the cyclopropanation processes via metal cyclopropyl carbenes show very similar activation energies, platinum cyclopropyl carbenes are the stationary points with the lowest energy. Metal cyclopropyl carbenes are probable intermediates in the intramolecular hydroarylation reaction catalyzed by electrophilic transition metals (see scheme).