Counteranion and Solvent Assistance in Ruthenium-Mediated Alkyne to Vinylidene Isomerizations

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2013-07Department
Ciencia de los Materiales e Ingeniería Metalúrgica y Química InorgánicaSource
Inorganic Chemistry 2013, 52, 8919−8932Abstract
The complex [Cp*RuCl(iPr2PNHPy)] (1) reacts with 1-alkynes HC≡CR (R = COOMe, C6H4CF3) in
dichloromethane furnishing the corresponding vinylidene complexes [Cp*Ru≡C≡CHR(iPr2PNHPy)]Cl (R = COOMe (2a-
Cl), C6H4CF3 (2b-Cl)), whereas reaction of 1 with NaBPh4 in MeOH followed by addition of HC≡CR (R = COOMe,
C6H4CF3) yields the metastable π-alkyne complexes [Cp*Ru(η2-HC≡CR)(iPr2PNHPy)][BPh4] (R = COOMe (3a-BPh4),
C6H4CF3 (3b-BPh4)). The transformation of 3a-BPh4/3b-BPh4 into their respective vinylidene isomers in dichloromethane is
very slow and requires hours to its completion. However, this process is accelerated by addition of LiCl in methanol solution.
Reaction of 1 with HC≡CR (R = COOMe, C6H4CF3) in MeOH goes through the intermediacy of the π-alkyne complexes
[Cp*Ru(η2-HC≡CR)(iPr2PNHPy)]Cl (R = COOMe (3a-Cl), C6H4CF3 (3b-Cl)), which rearrange to vinylidenes in minutes,
i.e., much faster than their counterparts containing the [BPh4]− anion. The kinetics of these isomerizations has been studied in
solution by NMR. With the help of DFT studies, these observations have been interpreted in terms of chloride- and methanolassisted
hydrogen migrations. Calculations suggest participation of a hydrido−alkynyl intermediate in the process, in which the
hydrogen atom can be transferred from the metal to the β-carbon by means of species with weak basic character acting as proton
shuttles.
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