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.

Catalyst decomposition has a negative effect on catalytic activity, and knowledge of decomposition pathways can assist with catalyst development. Ferrocenium cations have been employed as catalysts in a number of organic transformations, and we investigated the stability of a number of ferrocenium salts in solution. The observed rate decomposition constants for [Fc]Cl, [Fc]PF6, [Fc]BF4, [Fc]CSA [Fc = ferrocenium, CSA = camphor-10-sulfonate (β)], [AcFc]SbF6, (AcFc = acetylated ferrocene), and [FcB(OH)2]SbF6 [FcB(OH)2 = ferrocenylboronic acid] were determined in CH2Cl2 solution by time-resolved UV-vis spectroscopy. The rate decomposition constants depended on the nature of the counterion, with [Fc]Cl being the most stable complex in solution. The decomposition rate constants dropped by roughly an order of magnitude in most cases when the experiments were performed in nitrogenated solvent, demonstrating that the decomposition is mainly an oxidative process. The cosolvent HFIP (1,1,1,3,3,3-hexafluoropropan-2-ol) slowed the decomposition of the ferrocenium cations as well. Many catalytic or stoichiometric reactions of ferrocenium cations are performed with alcohols; we determined that hexan-1-ol is decomposed over the course of 16 hours, but not oxidized in the presence of a ferrocenium cation. Finally, the different ferrocenium cations were employed in a test reaction to determine catalytic activity. The nucleophilic substitution of hydroxyl groups in a tertiary propargylic alcohol by an alcohol is catalyzed by all complexes, and, again, a counterion dependency of the catalytic activity was observed. Also, HFIP increases the catalytic activity of the ferrocenium cations. The research has importance in the development of ferrocenium-based catalyst systems, because changes in the counterion as well as the architecture of the ferrocenium cation have an influence on stability and catalytic activity.

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