Ferrocenium Cations as Catalysts for the Etherification of Cyclopropyl‐Substituted Propargylic Alcohols: Ene‐yne Formation and Mechanistic Insights

Commercial ferrocenium hexafluorophosphate ([FeCp2]PF6) and ferrocenium boronic acid hexafluoroantimonate ([FcB(OH)2]SbF6) were found to be efficient catalysts for the etherification of terminal, tertiary, cyclopropyl‐substituted propargylic alcohols through nucleophilic substitution with primary and secondary alcohols. The alcohol nucleophiles and the propargylic alcohols were employed in a nearly equimolar amount and no further additives were required. After 2 h reaction time at 40 °C in CH2Cl2 and 3 to 5 mol‐% catalyst load, aromatic, cyclopropyl‐substituted propargylic alcohols gave rearranged, conjugated ene‐yne products as single isomers in 35 to 73 % isolated yields. Cyclopropyl‐substituted propargylic alcohols bearing a thienyl substituent gave the corresponding cyclopropyl‐substituted propargylic ethers in 27 to 56 % isolated yields (45 °C, 2 h reaction time), where the cyclopropyl unit did not rearrange. Cyclobutyl‐substituted propargylic alcohols gave the corresponding propargyl ether substituted products in 40 to 55 % isolated yields (40 to 45 °C, around 16 h reaction time), and no rearrangement of the cyclobutyl unit was observed. Only minor amounts of side products were observed in the reaction mixtures. Experimental evidence points toward an ionic mechanism since the more electron‐rich thienyl‐substituted substrates reacted faster. Ferrocenium or ferrocenium boronic acid cations (Fc+) catalyze propargylic substitution reactions utilizing alcohol nucleophiles. Rearrangement to the corresponding ene‐ynes takes place with a cyclopropyl substituent. No cyclopropyl ring‐opening is observed with a thienyl substituent. An ionic mechanism is suggested for the reactions.