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

The cyclization of differently substituted aryl alkynes with Pt^II or Au^I catalysts proceeds by endo-dig pathways. When AgI was used to generate reactive cationic Au^I 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.