Rearrangements and dimerizations of congested ferrocenyl allyl alcohols

Starting with the α,β-unsaturated ketones tigloylferrocene (1) and tetramethylcyclopent-2-enone, the corresponding ferrocenylallyl alcohols were prepared. Under acidic (aqueous) conditions, in order to eliminate water for the generation of olefinic derivatives, competitive allylic rearrangements are predominant. The in situ generated allyl cations undergo subsequent electrophilic additions. The parent tigloylferrocene-derived alcohols 2 and 4 preferentially form dimers of which one distinct stereoisomer, 6, was separated and fully characterized. In the case of tetramethylferrocenylcyclopentenol, a stabilized divinyl cation is generated by allylic rearrangements involving hydride shifts and a CC scission step, (retro-reaction of a Nazarov-type cyclization), followed by a protic H2O readdition/elimination sequence, which affords 4-ferrocenyl-4-hydroxy-2,3,5,6-tetramethyloxane (8) as the major product. Based on isolated intermediate allylic alcohols, the respective mechanisms of formation of the final products are postulated. X-ray structure determinations of the dimer 6, the isomorphic ferrocenyloxanes 8a, 8b, and 1,1′-ditigloylferrocene (9) are presented. The unique crystal packing of 9 (eclipsed conformation due to a lowered spatial extent of the molecule) is examined. A supplementary 57Fe Mössbauer study revealed a comparatively low quadrupole splitting (QS) interaction and a very small temperature dependence of the QS in 9.