Mechanism of 1,3-Hydrogen Migration in a Gold Bicyclo[3.2.0]heptene Complex: The Role of Brønsted Acid in the Gold-Catalyzed Cycloisomerization of 7-Aryl-1,6-Enynes

Abstract

The gold 7-phenylbicyclo[3.2.0]hept-1(7)-ene complex 1 is generated as an intermediate in the gold-catalyzed cycloisomerization of 7-phenyl 1,6-enyne 2 to 7-phenylbicyclo[3.2.0]hept-1(7)-ene 3 . We have investigated the kinetics of the isomerization of 1 via formal 1,3-hydrogen migration to form the corresponding gold 6-phenylbicyclo[3.2.0]hept-6-ene complex 4 to obtain direct experimental information regarding the mechanism by which 1 evolves into products. These experiments were in accord with conversion of 1 to 4 through a hidden Brønsted acid catalyzed pathway that occurred outside the coordination sphere of gold. Similarly, in situ spectroscopic analysis of the catalytic cycloisomerization of enyne 2 was consistent with a mechanism involving sequential gold-catalyzed cycloaddition followed by Brønsted acid catalyzed 1,3-hydrogen migration outside the coordination sphere of gold.