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 3ac and vinylvinylidene 4ac tautomers. Deprotonation of these mixtures with K2CO3 allows the preparation of σ-enynyl derivatives 5ac, which can be selectively alkylated with MeOSO2CF3 to yield disubstituted vinylvinylidene complexes 6ac. 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 7ac are selectively obtained by demetalation of vinylvinylidenes 4ac, phosphonio-alkynyl complexes 9ac are exclusively formed in the second case as the result of the nucleophilic addition of PMe2Ph on the electrophilic Cγ atom of allenylidenes 3ac. 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.