(η6-Arene)ruthenium(II) complexes containing enantiomerically pure (β-aminoalkyl)phosphines or a (β-aminoalkyl) phosphinite: synthesis, stereochemical and kinetic studies

Abstract

The chiral P–N* ligands [(S)-2-(dimethylamino)-3-phenylpropyl]diphenylphosphine, (S)-phephos, 1, [(S)-2-(dimethylamino)-2-phenylethyl]diphenylphosphine, (S)-phglyphos, 2, [(S)-2-(dimethylamino)-3-methylbutyl]diphenylphosphine, (S)-valphos, 3, and [(+)-(2S,3R)-4-(dimethylamino)-3-methyl]-2-diphenylphosphinoxy-1,2-diphenylbutane, (+)-(2S,3R)-chiraldphos, 4, reacted with [Ru(η⁶-arene)Cl₂]₂ (arene = p-cymene, benzene or hexamethylbenzene), in dichloromethane or tetrahydrofuran solution, affording the corresponding [Ru(η⁶-arene)(P–N*)Cl₂] complexes, 5, in which the P–N* acts as a monodentate P-bonded ligand. In methanol the same reactions easily afforded the corresponding chelate complexes [Ru(η⁶-arene)(P–N*)Cl]Cl. Using ligands 1–3, when the arene is p-cymene, 90∶10 diastereomeric mixtures of the cationic complexes have been obtained while only one diastereomer was formed in the corresponding reactions when the arene is benzene or hexamethylbenzene. The determination of the absolute configuration of the major products as R_Ru,S_C diastereoisomers was made from the crystal structure and CD spectra comparison of the complex (R_Ru,S_C)-[Ru(η⁶-p-MeC₆H₄Prⁱ)(S-phglyphos)Cl]BF₄. Complexes [Ru(η⁶-arene)(P–N*)Cl]Cl were also obtained by adding small amounts of methanol to solutions of [Ru(η⁶-arene)(P–N*)Cl₂] in chloroform. A kinetic study, in CDCl₃ solution containing variable amounts of methanol, on the chelation process in the neutral species [Ru(η⁶-arene)(P–N*)Cl₂] showed first-order behaviour of the k_obs values with the nucleophile (methanol) concentration. The pseudo-first-order rate constants are ascribed to replacement of Cl⁻ by a molecule of methanol. A reaction mechanism is proposed.