Ruthenium(II) Complexes Containing Optically Active Hemilabile P,N,O-Tridentate Ligands. Synthesis and Evaluation in Catalytic Asymmetric Transfer Hydrogenation of Acetophenone by Propan-2-ol

Abstract

The trifunctional ligands (R)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane (2R), (S)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane (2S), and (S)-(phenyl(2-anisyl)phosphino)(2-pyridyl)methane (3) have been synthesized, as well as the corresponding RuCl2(PPh3)(L) complexes. The complexes RuCl2(PPh3)(2R) (5) and RuCl2(PPh3)(2S) (6) were isolated as mixtures of two isomers, 5a and 5b and 6a and 6b, respectively. In each of these isomers, the ligands 2 are η3-(P,N,O) bound. They differ by the position of the triphenylphosphine, which is either in a trans position relative to the pyridyl ring (5a or 6a) or in a trans position relative to the ether function (5b or 6b). Variable temperature NMR experiments have shown that the hemilabile character in solution of the ligands 2 is through their pyridyl arm in the isomers a or their ether arm in b. The complexes 6b and RuCl2(PPh3)(3) (9) were characterized by X-ray diffraction. The complexes 5, 6, and 9 are very active catalysts for the transfer hydrogenation of acetophenone by propan-2-ol in basic media, the higher activity being observed for 9 (turnovers frequency 48 900 h-1). The enantioselectivity is modest and dependent on the reactions conditions. The best result has been observed for 5, with an ee of 60%.

Synthesis and characterisation of new pyrazole–phosphinite ligands and their ruthenium(II) arene complexes

“…Ligands possessing both ÔsoftÕ and ÔhardÕ donor atoms coordinated to the same metal centre have been found suitable for catalytic purposes since the stability of intermediate species is favoured [1]. Recently, many efforts have been made to improve the catalytic activity of some complexes by using hemilabile ligands [3][4][5][6][7][8]. Among ÔsoftÕ donor atoms, phosphorus is the most common in homogeneous catalysis and, for this reason, it is found in many ligands combined with a variety of ÔhardÕ labile donor groups (i.e., N-or O-donor).…”

Synthesis of a New Potentially Hemilabile Ligand: 1-[2-(Diphenylphosphanyl)ethyl]-3,5-dimethylpyrazole, and Comparison of Its Bonding Properties with the Related 1-[2-(Ethylamino)ethyl]-3,5-dimethylpyrazole Ligand toward RhI

“…[1,2,3] Their interest in catalysis arises from the easy decoordination of the weak donor centre without the displacement of the ligand, which should confer a better stability to the reaction intermediates. Mixed ligands containing phosphorus along with oxygen, nitrogen or sulphur donor atoms have specifically been designed for use in catalytic reactions such as hydrogenation, [4,5,6] oligomerisation, [7] and hydroformylation of olefins. [8] Pyrazole-derived molecules are good candidates for the preparation of interesting N,NЈ or N,P mixed ligands, owing to their ease of synthesis and the possibility of electronic and steric modulation of their properties.…”

Stabilization of the labile metal configuration in halfsandwich complexes [CpRh(PN)Hal]X

“…2-Menthoxy-6-methylpyridine (1b) was prepared by substitution of the bromo substituent in 2-bromo-6-methylpyridine [16] for the anion of L-(À)-menthol. 2-Diphenylphosphanylmethylpyridine (2a) is a known compound [17][18][19]. For the synthesis of 6-diphenylphosphanylmethyl-2-menthoxypyridine (2b) the methyl group in a-position of the pyridine ring was deprotonated with BuLi and reacted with PPh 2 Cl to introduce the diphenylphosphanyl group.…”

“…1 H NMR spectra: Bruker AC 250 and ARX 400 and Bruker Avance 300 and 400 spectrometers. 31 [17][18][19] 2a and 2-(2-cyclopentadienyl-1-diphenylphosphanyl-2-methylprop-1-yl)pyridine [21] 3a are known compounds. [(CpRhCl) 2 (l-Cl) 2 [33] and [(Cp*RhCl) 2 (l-Cl) 2 ] [34] were prepared as published.…”