Intramolecular reactions of alkynes with furans and electron rich arenes catalyzed by PtCl2: the role of platinum carbenes as intermediates.

5-(2-Furyl)-1-alkynes react, with PtCl(2) as catalyst, to give phenols. On the basis of DFT calculations, a cyclopropyl platinacarbene complex was found as the key intermediate in the process. The cyclopropane and dihydrofuran rings of this intermediate open to form a carbonyl compound, which reacts with the platinum carbene to form an oxepin, which is in equilibrium with an arene oxide. When the reaction is carried out in the presence of water, dicarbonyl compounds are obtained, which support the proposed mechanism. Other cyclizations of alkynes with furans or electron-rich arenes give products of apparent Friedel-Crafts-type reactions, although these processes could also proceed by pathways involving the formation of cyclopropyl platinum carbenes.