Activation of Propargylic Alcohols Derived from Hormonal Steroids by the Indenyl−Ruthenium(II) Complex [RuCl(η<sup>5</sup>-C<sub>9</sub>H<sub>7</sub>)(PPh<sub>3</sub>)<sub>2</sub>]:  Experimental and Theoretical Evidence of an Allenylidene−Vinylvinylidene Equilibrium

The indenyl−ruthenium(II) complex [RuCl(η<sup>5</sup>-C<sub>9</sub>H<sub>7</sub>)(PPh<sub>3</sub>)<sub>2</sub>] (<b>1</b>) reacts with ethisterone (<b>2a</b>), 17α-ethynylestradiol (<b>2b</b>), and mestranol (<b>2c</b>), in methanol and in the presence of NaPF<sub>6</sub>, to afford equilibrium mixtures containing the corresponding allenylidene <b>3a</b>−<b>c</b> and vinylvinylidene <b>4a</b>−<b>c</b> tautomers. Deprotonation of these mixtures with K<sub>2</sub>CO<sub>3</sub> allows the preparation of σ-enynyl derivatives <b>5a</b>−<b>c</b>, which can be selectively alkylated with MeOSO<sub>2</sub>CF<sub>3</sub> to yield disubstituted vinylvinylidene complexes <b>6a</b>−<b>c</b>. Displacement of these equilibriums can also be accomplished by treatment of the reaction mixtures with acetonitrile or PMe<sub>2</sub>Ph. Thus, while in the first case terminal 1,3-enynes <b>7a</b>−<b>c</b> are selectively obtained by demetalation of vinylvinylidenes <b>4a</b>−<b>c</b>, phosphonio-alkynyl complexes <b>9a</b>−<b>c</b> are exclusively formed in the second case as the result of the nucleophilic addition of PMe<sub>2</sub>Ph on the electrophilic C<sub>γ</sub> atom of allenylidenes <b>3a</b>−<b>c</b>. Ab initio molecular orbital calculations on the models [Ru{CCC(H)CH<sub>3</sub>}(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(PH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> and [Ru{CC(H)CHCH<sub>2</sub>}(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)(PH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> 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.