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 Ag-I was used to generate reactive cationic Au-I catalysts, 2H-chromenes dimerize to form cyclobutane derivatives by a Ag-I-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 Pt-II-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.