(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-)-chiraldphos, 4, reacted with [Ru(η 6 -arene)Cl 2 ] 2 (arene = p-cymene, benzene or hexamethylbenzene), in dichloromethane or tetrahydrofuran solution, affording the corresponding [Ru(η 6 -arene)-(P-N*)Cl 2 ] 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(η 6 -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(η 6 -p-MeC 6 H 4 Pr i )-(S-phglyphos)Cl]BF 4 . Complexes [Ru(η 6 -arene)(P-N*)Cl]Cl were also obtained by adding small amounts of methanol to solutions of [Ru(η 6 -arene)(P-N*)Cl 2 ] in chloroform. A kinetic study, in CDCl 3 solution containing variable amounts of methanol, on the chelation process in the neutral species [Ru(η 6 -arene)(P-N*)Cl 2 ] 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 Ϫ by a molecule of methanol. A reaction mechanism is proposed.

Asymmetric Induction and Configurational Stability at the Metal Centre in Half‐Sandwich (η6‐p‐Cymene)ruthenium(II) and (η5‐C5Me5)rhodium(III) Complexes Containing Chiral N‐N* Ligands with Different Rigidity and Flexibility

“…The complexity and number of atoms in the considered structures prevents the use of ab initio methods for all the diastereomers of interest, therefore we performed density calculations on molecules containing the (S a )-2 ligand but with a biphenyl moiety instead of a binaphthyl one. We have verified in a previous work [4] that the difference between real binaphthyl species and the biphenyl-optimised model in density functional calculations is not significant. The optimised structure of the cationic diastereomer (S a ,R Rh )-[Rh(η 5 -C 5 Me 5 )(S a -2)Cl] + was found to be in good agreement with the crystal structure of (S a ,R Rh )-[Rh(η 5 -C 5 Me 5 )(S a -2)Cl]PF 6 (10a), thereby indicating the validity of the performed simplification on the chiral backbone (Figure 12).…”

“…In fact, experimental data show that, in some cases, increasing the bulk of the incoming ligand also increases the diastereomeric excess; this supports an associative Ia-type interchange mechanism. [18] We have found previously [4] that the kinetics, under pseudo-first-order conditions, of the chelation process in the neutral species [Ru(η 6 -arene)(η 1 -P-N*)Cl 2 ] [P-N* = (β-aminoalkyl)phosphanes; arene = benzene, hexamethylbenzene, p-cymene], in CDCl 3 solution containing variable amounts of methanol, or of alcohols with different hydrogen-bonding properties, follows a first-order course, and the k obs values are linearly correlated to the nucleophile (methanol) concentration, with no significant intercept. These kinetic results led us to propose a mechanism in which the rate-determining step of the process is solvolysis of the starting complex in methanol.…”

“…Subsequent fast closure of the chelate ring by coordination of the nitrogen atom to the ruthenium(II) centre determines the stereoselectivity of the process. [4] Since a similar mechanism, in which the chloride dissociation step is assisted by the methanol solvent, could be active in the reactions studied here, we carried out a kinetic measurement of the nucleophilic substitution reactions of Figure 10). This linear correlation loses validity at low methanol concentrations, and the intercept becomes Ͼ 0, thereby indicating competition between CHCl 3 and CH 3 OH; the kinetic law becomes more complicated in these low methanol concentration ranges.…”

“…Control of the conformational stability at the metal centre is therefore an important area of research. We have reported [4,5] previously on the synthesis of [Ru(η 6 -p-cymene)(η 1 -P-N*)Cl 2 ] [where P-N* is a chiral (β-aminoalkyl)phosphane, (β-aminoalkyl)phosphonito, (8-quinolinyl)phosphonito or (8- …”

Rhodium complexes with a new chiral amino-phosphinite ligand and their behavior as pre-catalysts in the hydroformylation of styrene

“…P,N-ligands can coordinate to metal centre via j 1 -P or j 2 -P,N [3]; these coordination modes can change during reaction route, with noteworthy involvements when the complex is used as pre-catalyst. For example, the behavior of amino-phosphine as hemilabile ligands, with change of coordination to metal from j 2 -P,N to j 1 -P in the catalysis conditions, allows the dangling nitrogen atom to act as ''proton messenger" in the catalytic process [4].…”