Substituent effect on the reaction pathway of Au(I)-catalyzed ene-yne cycloisomerization

The Au(I)-catalyzed ene-yne cycloisomerization pathway which is highly dependent on the substrate structures is described. The steric and electronic effects of substrate substituents could be important factors inducing the formation of the desired and undesired products. Comparison between the Au(I)-catalyzed reactions of substrates bearing the substituted phenyl moiety with and without methoxy group suggests that the methoxy group would direct the formation of undesired products through a cationic intermediate. This intermediate could be formed via aryl moiety attack presumably induced by the methoxy group at the para position. The use of nitrile groups instead of ester groups in the substrate effectively leads to the preferential formation of the desired product. This is probably because 1,3-diaxial interactions in the transition state of the Au(I)-catalyzed ene-yne cycloisomerization would be reduced by the use of the relatively small nitrile group. This steric effect is different from the electronic effect of the methoxy group of the aryl moiety, which makes the undesired reaction pathway favorable.