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.
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