Influence of Counterions and Cyclopentadienyl Substituents on the Catalytic Activity of Ferrocenium Cations in Propargylic Substitution Reactions

Ferrocenium catalysis is a growing field of research. This study investigates the catalytic activity of different ferrocenium salts in propargylic substitution reactions to afford propargylic ethers. Four different ferrocenium catalysts were employed in the title reaction, which was monitored over time. The rate of the disappearance of the starting material can be fitted to a first order rate law and observed rate constants were determined. The catalyzed propargylic substitution reactions display a moderate but discernible dependence on the ferrocenium counterion. The lack of an induction period for the reaction indicates that the ferrocenium cation itself is catalytically active, and not just a decomposition product thereof, which would result in an induction period. The presence of a carboxylic acid substituent on one of the cyclopentadienyl rings enhances catalytic activity. The Meyer–Schuster rearrangement of the propargylic alcohol to the corresponding conjugated enone played only a minor role in the ferrocenium-catalyzed reactions. Catalyst decomposition moderately retards the reaction but does not suppress product formation, as demonstrated by experiments with aged FcBF4. In contrast, the presence of TEMPO as a radical scavenger completely inhibits product formation, while not causing detectable catalyst decomposition at room temperature. In turn, FeCl3 catalyzes both the propargylic substitution and the Meyer–Schuster rearrangement equally and decomposes the catalysis product over time. These findings reinforce the notion that strong Lewis acids readily promote the rearrangement of propargylic alcohols and that Lewis acidity plays a crucial role in finding a balance between the substitution reactions of propargylic alcohols and their rearrangement to unsaturated aldehydes.