Allenylidene to Indenylidene Rearrangement in Cationic p-Cymene Ruthenium(II) Complexes: Solvent, Counteranion, and Substituent Effects in the Key Step toward Catalytic Olefin Metathesis
Résumé
The reaction of [(η6-p-cymene)RuCl(PCy3)][OTf] with Ph2C(OH)C≡CH, at rt in benzene-d6 or in dichloromethane-d2, affords the allenylidene complex [(η6-p-cymene)RuCl(═C═C═CPh2)PCy3][OTf] ({4a}), which is used in situ as a catalyst for the ring-closing metathesis (RCM) reaction of diallyltosylamide. A kinetic study is performed by 1H NMR spectroscopy in benzene-d6 and dichloromethane-d2. The rates of the olefin metathesis process increase with the concentration of the ruthenium complex, but are independent of the olefin concentration, due to rate-determining transformation of the allenylidene complex. The TOF values are higher in benzene-d6 than in dichloromethane-d2. The spontaneous or thermally induced transformations of the allenylidene complexes [(η6-p-cymene)RuCl(═C═C═CAr2)PR3][X] (Ar = Ph, X = CF3SO3−(OTf) ({4a}), BF4− ({4b}), PF6− ({4c}), SbF6− ({4d}) ; X = OTf, Ar = p-MeOC6H4 ({4e}), Ar = p-ClC6H4 ({4f}), Ar = p-FC6H4 ({4g})), prepared in situ from the cationic precursors [(η6-p-cymene)RuCl(PCy3)][X] (X = OTf (8a), BF4− (8b), PF6− (8c), SbF6− (8d) and the propargylic alcohols (4-YC6H4)2C(OH)C≡CH (Y = H, OMe, Cl, F), are studied by UV−visible spectroscopy by following the disappearance of the allenylidene moiety Ru═C═C═CAr2. The allenylidene to indenylidene rearrangement involves attack by the ortho-carbon of the aryl rings to the electrophilic allenylidene Cα, to form an intermediate arenium ion, and H-transfer from aryl to Cβ. Faster rates are observed in benzene than in dichloromethane in all cases. The first-order rate constants, kobs, for complexes 4a−d (Ar = Ph) depend on the counteranion nature and decrease in the order BF4− > OTf PF6− > SbF6− in benzene, while they are insensitive to the counteranion nature in dichloromethane. Measurements performed at different temperatures afford the activation parameters ΔH‡ = 22 ± 2 kcal mol−1 and ΔS‡ = −1 ± 5 cal mol−1 K−1 for complex {4a} at 18−50 °C and ΔH‡ = 24 ± 1 kcal mol−1 and ΔS‡ = 1 ± 1 cal mol−1 K−1 for complex {4e} at 25−55 °C. The values of activation entropy, which do not support a rate-determining phosphine dissociation step, and the rate effects of the aryl p-substituents are consistent with an early transition state in the C−C bond forming step of the ring-closing process. Rate differences between benzene and dichloromethane are discussed in terms of counteranion interactions in the less polar aromatic solvent. The rates of the allenylidene transformation affect directly the rates of the RCM reaction, as indicated by parallel solvent, proton, counteranion, and aryl substituent effects in the two processes, thus establishing the role of allenylidene−indenylidene rearrangement in the acid-free olefin metathesis.