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.
![Intramolecular hydroarylation of alkynes catalyzed by platinum or gold: mechanism and endo selectivity](https://www.iciq.org/wp-content/uploads/2014/03/Intramolecular-hydroarylation_eng_noticies_articles_2005_45.gif)