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Reactivity of Indenyl−Ruthenium(II) Vinylidene Complexes:  Selective Synthesis of Alkenyl−Phosphonio Derivatives via Nucleophilic Addition of Triphenylphosphine on Their η2-Alkyne Tautomers. Theoretical Study of the η1-Vinylidene−η2-Alkyne Tautomerization

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journal contribution
posted on 02.11.2001, 00:00 by Victorio Cadierno, M. Pilar Gamasa, José Gimeno, Covadonga González-Bernardo, Enrique Pérez-Carreño, Santiago García-Granda
The activation of terminal alkynes with the halide derivatives [RuX(η5-1,2,3-R3C9H4)(CO)(PR3)] (R = Me, X = Br, PR3 = PPh3 (1), PiPr3 (3); R = H, X = I, PR3 = PiPr3 (2)) and AgBF4 affords, in dichloromethane at room temperature, equilibrium mixtures containing the corresponding η1-vinylidene and η2-alkyne tautomers [Ru{=CC(H)R‘}(η5-1,2,3-R3C9H4)(CO)(PR3)][BF4] (4ac) and [Ru(η2-HC⋮CR‘)(η5-1,2,3-R3C9H4)(CO)(PR3)][BF4] (5ac), respectively. The reaction of 1 with AgBF4 and phenylacetylene has been studied by variable-temperature 31P{1H} and 1H NMR spectroscopy:  at low temperature (−60 °C) the vinylidene complex [Ru{CC(H)Ph}(η5-1,2,3-Me3C9H4)(CO)(PPh3)][BF4] (4a) is initially observed which upon warming (14 °C) forms an equilibrium with the π-alkyne derivative [Ru(η2-HC⋮CPh)(η5-1,2,3-Me3C9H4)(CO)(PPh3)][BF4] (5a). Treatment of this mixture with KOtBu, in dichloromethane at room temperature selectively yields the neutral σ-alkynyl derivative [Ru(C⋮CPh)(η5-1,2,3-Me3C9H4)(CO)(PPh3)] (6) by displacement of the aforementioned equilibrium via deprotonation of the acidic vinylidene proton in 4a, while the addition of PPh3 to this mixture stereoselectively affords the cationic alkenyl−phosphonio complex (E)-[Ru{C(H)C(PPh3)Ph}(η5-1,2,3-Me3C9H4)(CO)(PPh3)][BF4] [(E)-7] via the nucleophilic attack of PPh3 on the coordinated π-alkyne in 5a. In a similar fashion, compounds (E)-[Ru{C(H)C(PPh3)R‘}(η5-1,2,3-R3C9H4)(CO)(PR3)][BF4] (R = Me, PR3 = PPh3, R‘ = 1-cyclooctenyl [(E)-13]; R = H, PR3 = PiPr3, R‘ = Ph [(E)-8], 1-cyclooctenyl [(E)-14] can be selectively obtained by addition of PPh3 to the corresponding reaction mixture. The monosubstituted alkenyl−vinylidene complexes [Ru{CC(H)CHCRR‘}(η5-C9H7)(PPh3)2][BF4] (R = R‘ = Ph (11a); R = H, R‘ = (η5-C5H4)Fe(η5-C5H5) [(E)-11b], 4-OMe-C6H4 [(Z)-11c]) also react with triphenylphosphine, but in refluxing methanol, to afford the alkenyl−phosphonio derivatives (EE)-[Ru{C(H)C(PPh3)CHCRR‘}(η5-C9H7)(PPh3)2][BF4] (12ac) stereoselectively. The process also proceeds via an initial η1-vinylidene−η2-alkyne tautomerization followed by the nucleophilic attack of PPh3 on the coordinated π-alkyne. The crystal structures of (E)-[Ru{C(H)C(PPh3)Ph}(η5-1,2,3-Me3C9H4)(CO)(PPh3)][BF4] ((E)-7) and (EE)-[Ru{C(H)C(PPh3)CHCH(η5-C5H4)Fe(η5-C5H5)}(η5-C9H7)(PPh3)2][BF4] ((EE)-12b) have been determined by X-ray diffraction methods. Ab initio molecular orbital calculations on the η1-vinylidene to η2-alkyne tautomerization on the models [Ru(η5-C9H7)(PH3)L(C2H2)]+ (L = CO, PH3) are also reported.