Gold(I)-catalyzed divergence in the preparation of bicyclic enol esters: from exclusively [3C+2C]-cycloaddition reactions to exclusive formation of vinylcyclopropanes.

With the use of benzonitrile-stabilized Au(I) catalyst [Au(IPr)(NCPh)]SbF(6) (Ic; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), a spectrum of reactivity is observed for propargyl ester 4a with cyclic vinyl ethers, ranging from exclusively [3C+2C] cycloaddition reactions to exclusively cyclopropanation depending only on the structure of the substrate. Some initially formed cyclopropanation products rearrange into the corresponding formally [3C+2C] cycloaddition products after treatment with fresh Au(I) complex at 80 °C. Vinylcyclopropanes formed from dihydrofuran and dihydropyran resisted such rearrangement, even in the presence of fresh Au(I) catalyst at elevated temperature. This study addresses an important mechanistic question concerning whether the five-membered-ring products were produced by a direct [3C+2C] cycloaddition reaction or by a sequential cyclopropanation/ring-expansion reaction. A dual pathway is proposed for the Au(I)-catalyzed reactions between propargyl esters and cyclic vinyl ethers. The different behavior among vinyl cyclic ethers is attributed to the difference in the polarization of the π bond. Highly polarized bonds appear to undergo the cycloaddition reaction whereas less polar π-bonds produce cyclopropanes.