Explanation of “Silver Effects” in Gold(I)-Catalyzed Hydroalkoxylation of Alkynes
journal contributionposted on 02.10.2015, 00:00 by Alexander Zhdanko, Martin E. Maier
An extensive experimental NMR study of gold-catalyzed hydroalkoxylation was conducted to explain the influence of a silver salt additive on the gold-catalyzed process (silver effect). Addition of silver salt may have no effect or a negative or positive effect on gold-catalyzed hydroalkoxylation. However, silver was shown to be essentially innocent (plays no role) with regard to the mechanism of the catalytic process itself. The effect occurs only if silver induces variations in the fraction of in-cycle organogold intermediates and H+. This is associated with the formation of the argento vinyl gold species G, which was shown to be an off-cycle intermediate. This species is formed by trapping the vinyl gold species B with Ag+; in the same way, the diaurated species D (another possible off-cycle intermediate) is formed by trapping B with LAu+. The argento vinyl gold species G1 was extensively characterized in solution by various NMR techniques at different temperatures. Furthermore, bringing together our results and the topical research of others, we introduced classification of silver effects to eliminate the confusion around the interpretation of erratic effects. Reactions that are separately catalyzed either by silver or by gold were beyond the scope of this study (for those reactions a “true” silver effect would take place).