Stable carbocations. Part 14. Ferrocenyl-substituted allenyl ↔ propynyl cations

Abstract

Whereas 3-ferrocenylprop-2-yn-1-ol undergoes protonation of the triple bond in trifluoroacetic acid to give a ferrocenylvinyl cation, the secondary alcohol 1,3-diferrocenylprop-2-yn-1-ol and the tertiary 3-ferrocenyl-1,1-dimethyl- and -diphenylpropynols undergo dehydroxylation giving mesomeric allenyl ↔ propynyl cations which are rapidly converted into trifluoroacetoxyallylium ions by solvent addition. Under similar conditions, the secondary alcohol 1-ferrocenyl-3-phenylprop-2-yn-1-ol gives the stable 1-ferrocenyl-3-phenylprop-2-ynylium ion, which can also be generated inefficiently in dichloromethane solution by hydride abstraction from 3-ferrocenyl-1-phenylpropyne. The (ferrocenylethynyl)tropylium cation has been prepared as a stable salt. The products formed on quenching solutions of these cations have been identified.