Activation of Propargylic Alcohols Derived from Hormonal Steroids by the Indenyl−Ruthenium(II) Complex [RuCl(η⁵-C₉H₇)(PPh₃)₂]:  Experimental and Theoretical Evidence of an Allenylidene−Vinylvinylidene Equilibrium

The indenyl−ruthenium(II) complex [RuCl(η⁵-C₉H₇)(PPh₃)₂] (1) reacts with ethisterone (2a), 17α-ethynylestradiol (2b), and mestranol (2c), in methanol and in the presence of NaPF₆, to afford equilibrium mixtures containing the corresponding allenylidene 3a−c and vinylvinylidene 4a−c tautomers. Deprotonation of these mixtures with K₂CO₃ allows the preparation of σ-enynyl derivatives 5a−c, which can be selectively alkylated with MeOSO₂CF₃ to yield disubstituted vinylvinylidene complexes 6a−c. Displacement of these equilibriums can also be accomplished by treatment of the reaction mixtures with acetonitrile or PMe₂Ph. Thus, while in the first case terminal 1,3-enynes 7a−c are selectively obtained by demetalation of vinylvinylidenes 4a−c, phosphonio-alkynyl complexes 9a−c are exclusively formed in the second case as the result of the nucleophilic addition of PMe₂Ph on the electrophilic C_γ atom of allenylidenes 3a−c. Ab initio molecular orbital calculations on the models [Ru{CCC(H)CH₃}(η⁵-C₅H₅)(PH₃)₂]⁺ and [Ru{CC(H)CHCH₂}(η⁵-C₅H₅)(PH₃)₂]⁺ show that the vinylvinylidene tautomer is only 2.1 kcal/mol more stable than the allenylidene. The spontaneous tautomerization process between both complexes, which involves a [1,3]-hydrogen sigmatropic rearrangement, requires an activation energy of 66.5 kcal/mol.