Aryl-Substituted Cyclopropyl Acetylenes as Sensitive Mechanistic Probes in the Gold-Catalyzed Hydration of Alkynes. Comparison to the Ag(I)‑, Hg(II)‑, and Fe(III)-Catalyzed Processes

The gold-catalyzed hydration of 2-phenyl- or 2,2-diphenylcyclopropyl acetylene, sensitive probes to trace the formation of vinyl carbocations, provides exclusively the corresponding cyclopropyl methyl ketones. On the other hand, in the Ag­(I)- or Fe­(III)-catalyzed hydration, a profound vinyl carbocationic character appears in the initially formed metal–alkyne complexes, as judged by the partial (Ag⁺) or exclusive (Fe³⁺) formation of allene-type rearrangement products. These findings provide clear evidence for subtle electronic differences in metal–alkyne complexes, including Au­(I or III), Ag­(I), Fe­(III), and Hg­(II).