Commercial ferrocenium hexafluorophosphate ([FeCp 2 ]PF 6 ) and ferrocenium boronic acid hexafluoroantimonate ([FcB(OH) 2 ]SbF 6 ) 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 CH 2 Cl 2 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 [a] 7349 Accordingly, we decided to synthesize cyclopropyl-substituted propargylic alcohol substrates (4 in Scheme 1c) to investigate whether the reaction proceeds through a radical mechanism. Cyclopropyl-substituted radicals 6 (Scheme 1c) may ringopen to form alkenes, but carbocation 5 may also have this tendency (vide infra). As exemplified in Scheme 1c, ene-ynes 7 can form through rearrangement if a cyclopropyl-substituted propargylic alcohol 4 is employed. [35] The employment of cyclopropyl-substituted propargylic alcohols in reactions with alcohols to give conjugated, achiral enynes has been reported previously only four times, utilizing Yb(OTf ) 3 , [36a] triflic acid (TfOH), [36b] HAuCl 4[36c] and ruthenium complexes [36d] as catalysts. In these reports, the nucleophile was either the solvent [36b,36c] or employed in large excess. [36a,36d] Herein, we report ferrocenium-catalyzed substitution reactions with these substrates and isolated both ene-yne products and cyclopropylsubstituted products, depending on the substituent R in 4. Experimental evidence points toward an ionic mechanism through carbocation 5 (Scheme 1c). [37] Eur.Scheme 2. Formation of the cyclopropyl-substituted intermediate 21 and ring-opening.

Ferrocenium catalysis is a vibrant research area, and an increasing number of ferrocenium-catalyzed processes have been reported in the recent years. However, the ferrocenium cation is not very stable in solution, which may potentially hamper catalytic applications. In an effort to stabilize ferrocenium-type architectures by inserting a bridge between the cyclopentadienyl rings, we investigated two ferrocenophanium (or ansa-ferrocenium) cations with respect to their stability and catalytic activity in propargylic substitution reactions. One of the ferrocenophanium complexes was characterized by single crystal X-ray diffraction. Cyclic voltammetry experiments of the ferrocenophane parent compounds were performed in the absence and presence of alcohol nucleophiles, and the stability of the cations in solution was judged based on the reversibility of the electron transfer. The experiments revealed a moderate stabilizing effect of the bridge, albeit the effect is not very pronounced or straightforward. Catalytic propargylic substitution test reactions revealed decreased activity of the ferrocenophanium cations compared to the ferrocenium cation. It appears that the somewhat stabilized ferrocenophanium cations show decreased catalytic activity.