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(eta(5)-C9H7)(PPh3)(2)] (1) reacts with ethisterone (2a), 17alpha-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 sigma-enynyl derivatives 5a-c, which can be selectively alkylated with MeOSO2-CF3 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-gamma atom of allenylidenes 3a-c. Ab initio molecular orbital calculations on the models [Ru{=C=C=C(H)CH3}(eta(5)-C5H5)(PH3)(2)](+) and [Ru{=C=C(H)CH=CH2}(eta(5)-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.