CH/π Attraction: The Origin of Enantioselectivity in Transfer Hydrogenation of Aromatic Carbonyl Compounds Catalyzed by Chiralη6-Arene-Ruthenium(II) Complexes (original) (raw)
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Chemistry - A European Journal, 2003
A library of novel dipeptideanalogue ligands based on the combination of tert-butoxycarbonyl(N-Boc)-protected a-amino acids and chiral vicinal amino alcohols were prepared. These highly modular ligands were combined with [{RuCl 2 (p-cymene)} 2 ] and the resulting metal complexes were screened as catalysts for the enantioselective reduction of acetophenone under transfer hydrogenation conditions using 2-propanol as the hydrogen donor. Excellent enantioselectivity of 1-phenyl-ethanol (up to 98 % ee) was achieved with several of the novel catalysts. Although most of the ligands contained two stereocenters, it was demonstrated that the absolute configuration of the product alcohol was determined by the configuration of the amino acid part of the ligand. Employing ligands based on l-amino acids generated S-configured products, and catalysts based on damino acids favored the formation of the R-configured alcohol. The combination N-Boc-l-alanine and (R)-phenylglycinol (Boc-l-Ab) or its enantiomer (N-Boc-d-alanine and (S)-phenylglycinol, Boc-d-Aa) proved to be the best ligands for the reduction process. Transfer hydrogenation of a number of aryl alkyl ketones were evaluated and excellent enantioselectivity, up to 96 % ee, was obtained.
Advanced Synthesis & Catalysis, 2003
This account describes the development and application of Noyori×s type catalysts based on ruthenium-arene complexes and simple chiral bamino alcohols derived from ephedrine, for the asymmetric transfer hydrogenation of 2-propanol to carbonyl substrates. The influence of key parameters of the catalyst system has been studied systematically, resulting in particular in the design of the novel ligand (4-biphenylmethyl)norephedrine. Thanks to the latter, the catalytic precursors and true active species could be isolated for the first time, enabling a complete structural description of the catalytic cycle and of probable deactivation pathways. Highly effective applications of those catalysts systems, i.e., the asymmetric reductions of simple aryl ketones and aryl b-keto esters, the synthesis of chiral phthalides and syn-b,d-dihydroxy esters, are described.
Transfer hydrogenation reactions of photoactivatable N, N'-chelated ruthenium (II) arene complexes
We show that the reaction of Ru II arene chlorido complexes of the type [(η 6 -arene)Ru(N,N')Cl] + arene = p-cymene (pcym), hexamethylbenzene (hmb), indane (ind), N,N' = bipyrimidine (bpm) and 1,10-phenanthroline (phen) with excess sodium formate generates a very stable formate adduct through spontaneous hydrolysis of the Ru-Cl bond at 310 K and pH* = 7.0. The formate adducts are also produced when Ru II arene pyridine complexes of the type [(η 6 -arene)Ru(N,N')(Py)] 2+ (where Py = pyridine), are irradiated with UVA (λ irr = 300-400 nm) or visible light (λ irr = 400-660 nm) under the same conditions. The Ru II arene formato adducts do not catalyse the reduction of acetone through transfer hydrogenation. However, all the complexes (except complex 2 which contains phen as the chelating ligand) can catalyse the regioselective reduction of NAD + in the presence of formate (25 mol equiv) in aqueous solution to form 1,4-NADH. The catalytic activity is dependent on the nature of the chelating ligand. Most interestingly, the regioselective reduction of NAD + to 1,4-NADH can be also specifically triggered by photoactivating a Ru II arene Py complex.
Amino alcohol coordination in ruthenium(II)-catalysed asymmetric transfer hydrogenation of ketones
European Journal of Inorganic Chemistry, 1999
Keywords: Asymmetric catalysis / Transfer hydrogenation / Ruthenium / Amino alcohol ligands / X-ray structure The nature of ruthenium-amino alcohol precursors in the aminoethane (1) coordinates through two nitrogen atoms, was structurally characterised by X-ray diffraction (8). -catalytic cycle of asymmetric hydrogen transfer reactions was studied using two C 2 -symmetrical tetradentate ligands (1 and Based on the results of this study a series of new amino alcohol ligands was synthesised from easily available starting 2) that were synthesised from (nor)ephedrine. The structure of the catalyst precursor was examined through catalysis and materials. Optimisation of the amino alcohol ligand structure resulted in the most effective chiral amino alcohol ligand (5) NMR experiments. It was shown that the active catalyst contains one ligand per metal, which coordinates in a so far that is capable of reducing acetophenone at 0°C with up to 97% ee in the Ru II -catalysed transfer hydrogenation. didentate N,O fashion (9). In addition, a Ru II Cl 2 complex, in which N,NЈ-bis(2-hydroxy-1-methyl-2-phenylethyl)-1,2-di-From the work of Noyori and co-workers it is clear that [c] DSM Research, in the transfer hydrogenation of ketones using ru-Eur.
Angewandte Chemie-international Edition, 2007
The development of more efficient asymmetric catalysts for organic transformations is a topic of great interest for both industrial applications and academic research. [1] The choice of the chiral ligand for transition-metal complexes is a key factor in attaining a high level of asymmetric induction. Complexes containing two appropriate chiral ligands have been successfully employed to increase the level of enantioselectivity in catalytic reactions (the matched-ligands approach). This method is relatively tedious and requires the isolation of a library of enantiomerically pure ligands which need to be correctly assembled. A particularly successful example is the trans-[RuCl 2 (PP)diamine] (PP = diphosphane) system in which the correct combination of chiral diphosphane and diamine ligands leads to high enantioselectivity in the catalytic hydrogenation of ketones. [2] To overcome the problem of using two precious chiral ligands, different strategies have been developed. Efficient asymmetric catalytic systems have been obtained by reaction of a racemic metal complex, prepared from a racemic ligand, with a suitable chiral auxiliary, leading to deactivation (chiral poisoning) or activation of one metal enantiomer species. [3] Alternatively, efficient chiral catalysts have been prepared from a chirally flexible (tropos) ligand in combination with a rigid one. The ligand 1-(pyridin-2-yl)methanamine (Pyme; Scheme 1) has been used in the recently developed asymmetric transfer hydrogenation and hydrogenation complexes [RuCl 2 (PP)Pyme]. [5] The presence of this ligand has been proven to accelerate [5a, 6] dramatically the transfer hydrogenation [7] of ketones relative to the most active systems reported to date. Therefore, fast and highly enantioselective catalytic systems are expected when chiral diphosphanes are matched with an appropriate chiral Pyme ligand. Particularly attractive are the chiral 1-substituted-1-(pyridin-2-yl)methanamines (RPyme, R = alkyl; Scheme 1) that display a stereogenic carbon center bound to the active NH 2 function.
Journal of Organometallic Chemistry, 2000
The new complexes (R Ru S C , S Ru S C )-[(h 6 -pCym)Ru(L-Aze)Cl] (6a, b), (R Ru S C , S Ru S C )-[(h 6 -pCym)Ru(L-Pip)Cl] (7a, b), ()-[{(h 6 -pCym)Ru(L-Pip)} 3 ](BF 4 ) 3 (9a, b) (L-Aze= L-2-azetidinecarboxylate, L-Pip =L-2-piperidinecarboxylate) were prepared, characterized and used, together with the known [{(h 6 -pCym)Ru(L-Pro)} 3 ](BF 4 ) 3 , 5 and [{(h 6 -pCym)Ru(L-Ala)} 3 ](BF 4 ) 3 , 10 (L-Pro= L-prolinate, L-Ala = L-alaninate), in hydride transfer reduction of acetophenone, a series of substituted acetophenones and several other ketones with moderate to high conversions and enantioselectivities up to 86% e.e.
Tetrahedron: Asymmetry, 2017
Metal-catalyzed asymmetric transfer hydrogenation is a powerful and practical method for the reduction of ketones to produce the corresponding secondary alcohols, which are valuable building blocks in the pharmaceutical, perfume, and agrochemical industries. Hence, a series of novel chiral b-amino alcohols were synthesized by chiral amines with regioselective ring opening of (S)-propylene oxide or reaction with (S)-(+)-2-hydroxypropyl p-toluenesulfonate by a straightforward method. The chiral ruthenium catalytic systems generated from [Ru(arene)(l-Cl)Cl] 2 complexes and chiral phosphinite ligands based on amino alcohol derivatives were employed in asymmetric transfer hydrogenation of ketones to give the corresponding optically active alcohols; (2S)-1-{[(2S)-2-[(diphenylphosphanyl)oxy]propyl][(1R)-1-phenylethyl]amino}propan-2-yldiphenylphosphinitobis[dichol-oro(g 6-benzene)ruthenium(II)] acts an excellent catalyst in the reduction of a-naphthyl methyl ketone, giving the corresponding alcohol with up to 99% ee. The substituents on the backbone of the ligands were found to have a remarkable effect on both the conversion and enantioselectivity of the catalysts. Furthermore, this transfer hydrogenation is characterized by low reversibility under these conditions.