Stable carbonium ions. Part IV. Torsional barriers in secondary and tertiary ferrocenylalkylium ions

Abstract

Ferrocenylalkylium ions are generated stereospecifically from 1-hydroxyalkylferrocenes in acidic solvents. Using spectroscopic techniques, restricted rotation has been observed for the exocyclic bond of the fulvene ligand of a number of these cations and the height of the torsional barrier has been measured for three systems. In trifluoroacetic acid, the barrier to rotation for a tertiary ferrocenylalkylium ion is lower than that for a secondary ferrocenylalkylium ion which is lower than that for a secondary ruthenocenylalkylium ion. The last cation is configurationally stable at 70°. Restricted ligand torsion has been found for a number of sterically encumbered ferrocenylalkylium ions.