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Transfer Hydrogenation Reactions Catalyzed by Chiral Half-Sandwich Ruthenium Complexes Derived from Proline

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Transfer Hydrogenation Reactions Catalyzed by Chiral Half-Sandwich Ruthenium Complexes Derived from Proline J. Chem. Sci. Vol. 128, No. 9, September 2016, pp. 1405–1415. c Indian Academy of Sciences. DOI 10.1007/s12039-016-1151-8 Transfer hydrogenation reactions catalyzed by chiral half-sandwich Ruthenium complexes derived from Proline ARUN KUMAR PANDIA KUMAR and ASHOKA G SAMUELSON∗ Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560 012, India e-mail: [email protected] MS received 5 May 2016; revised 5 July 2016; accepted 5 July 2016 Abstract. Chiral ruthenium half-sandwich complexes were prepared using a chelating diamine made from proline with a phenyl, ethyl, or benzyl group, instead of hydrogen on one of the coordinating arms. Three of these complexes were obtained as single diastereoisomers and their configuration identified by X-ray crystal- lography. The complexes are recyclable catalysts for the reduction of ketones to chiral alcohols in water. A ruthenium hydride species is identified as the active species by NMR spectroscopy and isotopic labelling exper- iments. Maximum enantio-selectivity was attained when a phenyl group was directly attached to the primary amine on the diamine ligand derived from proline. Keywords. Asymmetric transfer hydrogenation; Proline diamine ligands; Half-sandwich ruthenium complexes; Chiral alcohols; Ruthenium hydride. 1. Introduction very high enantiomeric excesses under mild conditions. In the case of acetophenone as a substrate, formation Asymmetric reduction of carbonyl compounds is an of (S)-1-phenylethanol in 97% ee and 95% yield was important reaction1 as it leads to chiral secondary alco- observed.14 Noyori and co-workers identified the origin hols, a key functional group found in several fine chem- of enantioselectivity19 in Noyori-Ikariya type half- icals, agrochemicals2 and pharmaceuticals.3 7 In nature, sandwich complexes with C-H··· π interactions between oxidoreductases catalyze transfer hydrogenation of car- the arene ring and a wide range of substrates.20,21 bonyl compounds to give stereospecific alcohols using Few studies have tested the hypothesis by comparing ··· alcohol dehydrogenases such as horse liver dehydroge- very similar ligands with and without C-H π interac- nase with cofactors like NADH or NADPH.8,9 In the tions arising from the chiral ligand and substrate for laboratory, transfer hydrogenation reactions are conve- enhancing enantioselectivity.22 niently carried out with a wide range of catalysts which Although the use of green solvents like water is pre- are convenient alternatives to high pressure hydrogen- ferable,23 solubility problems force one to use isopro- 14,24 ation which demands special vessels and hydrogen panol or a mixture of formic acid and triethyl 15 gas.10,11 As early as in 1970, the research groups of amine (HCOOH:Et3N) for transfer hydrogenation. Ohkubo and Sinou suggested the use of [RuCl2(PPh3)3] Several ligands modelled after the Noyori catalyst as a catalyst to carry out asymmetric transfer hydro- have been developed for the asymmetric transfer genation (ATH) in the presence of either a chiral hydro- hydrogenation reactions in water based on sulfonated 25,26 gen donor or a chiral monophosphine ligand.12,13 But diphenylethylenediamine (TsDPEN) and sulfona- 27,28 only in 1995, a breakthrough was made by Noyori and ted-cyclohexyldiamine (Ts-CYDN) ligands using 29 coworkers using a half-sandwich ruthenium catalyst sodium formate as a hydrogen source. A recent report containing a chiral N-sulfonylated 1,2-diamine ligand by Denizalti and co-workers on the asymmetric transfer for ATH of ketones and imines.14 18 The important fea- hydrogenation of ketones in water, but with added SDS, ture of this catalyst is that the ligand and the metal employing in situ generated ruthenium proline amide 30 play an active role in the transfer of two hydrogen and diamine complexes prompts us to report in this atoms, one of which is attached to the N of the lig- work, a closely related series of chiral half-sandwich S) and and hence protic in nature and the other attached ruthenium(II) complexes containing ( -N-substituted- to Ru and hence considered hydridic. The bifunctional 2-aminomethylpyrrolidine ligands, their synthesis and nature of the catalyst leads to chiral alcohols with use as catalysts for the asymmetric transfer hydrogena- tion of ketones in water. We have characterized the ∗For correspondence complexes completely through X-ray crystallography 1405 1406 Arun Kumar Pandia Kumar and Ashoka G Samuelson 1 and shown how silver nitrate can activate sluggish cat- L2 : Yield: 49%; H NMR (400 MHz, CDCl3): δ 3.15 alysts and how the reaction can be carried out without (m, 1H), 2.84 (m, 2H), 2.62-2.42 (m, 4H), 1.81 (m, 1H), using sodium dodecyl sulphate (SDS) as an additive 1.66 (m, 2H), 1.26 (m, 1H), 1.04 (t, 3H, J = 7.2 Hz); 13 1 which they found detrimental to the enantioselectivity. C{ H}NMR (100 MHz, CDCl3):δ 58.7, 55.7, 46.9, Further, we have shown the formation of a hydride 44.8, 30.2, 26.2, 15.7 1 intermediate through isotopic labelling and spectros- L3: Yield: 61%; H NMR (400 MHz, CDCl3): δ 7.23 copy. Our systematic study by varying the substituent (m, 1H), 7.31 (m, 4H), 3.79 (s, 2H), 3.22 (m, 1H), 2.88 on the chiral ligand suggests that phenyl groups on the (m, 2H), 2.49 (m, 2H), 1.87 (m, 1H), 1.69 (m, 2H), 1.31 13 proline ring has a beneficial effect in the enantiose- (m, 1H); C NMR (100 MHz, CDCl3): δ 140.9, 128.8, lectivity in the reduction of a variety of ketones and 128.6, 127.3, 58.8, 55.1, 54.7, 46.9, 30.2, 26.2 21 1 supports the earlier hypothesis of Noyori. L4: Yield: 36%; H NMR (400 MHz, CDCl3): δ 3.06 (m, 1H), 2.88 (m, 2H), 2.56 (m, 2H), 1.77 (m, 3H), 1.31 13 δ 2. Experimental (m, 1H); C NMR (100 MHz, CDCl3): 61.5, 47.5, 46.9, 29.6, 26.2. 2.1 Materials and methods 2.3 General procedure for the synthesis of chiral All reactions and manipulations were routinely per- half-sandwich ruthenium proline diamine complexes formed under a nitrogen atmosphere using standard Schlenk techniques in oven-dried glassware. L-Proline, To a solution of [Ru(p-cymene)Cl2]2 (0.286 mmol) in benzylchloroformate, benzylamine, L-prolinamide, ani- dry isopropanol (10 mL) was added (S)-N-substituted- line were obtained from Sigma-Aldrich U.S.A. All 2-aminomethylpyrrolidine ligand (0.573 mmol) and tri- ketone substrates were obtained from Sigma-Aldrich ethylamine (1.15 mmol). The resulting reaction mixture U.S.A. Ethyl amine was distilled from 70% aqueous waskeptat80◦C for 1 h. The solvent was removed solution and stored over potassium hydroxide pellets under vacuum, and ethanol was then added and kept for ◦ at −20 C. Tetrahydrofuran was distilled over sodium/ crystallization under nitrogen atmosphere. The yellow benzophenone. Triethylamine was distilled first over crystals formed were filtered and dried. 1 KOH and then over LiAlH4. Analytical thin layer chro- C1: Yield: 80%; H NMR (400 MHz, CDCl3): δ matography (TLC) was performed using precoated silica 9.20 (d, 1H, NH, J = 10.0 Hz), 7.98 (d, 2H, J = gel plates supplied by Merck (60 F254). Subsequent to 7.76 Hz), 7.59 (q, 1H, NH, J = 7.84 Hz), 7.32 (t, 2H, elution, plates were visualized using UV radiation (254 J = 7.52 Hz), 7.20 (t, 1H, J = 7.36 Hz), 5.70 (d, 1H, nm). Preparative TLC was done using ChemLabs Silica J = 5.76 Hz), 5.66 (d, 1H, J = 5.76 Hz), 5.46 (d, gel GF 254. Nuclear magnetic resonance spectra were 1H, J = 5.84 Hz), 5.44 (d, 1H, J = 5.8 Hz), 4.05 (m, recorded on a Bruker AMX 400 spectrometer operat- 1H), 3.55 (m, 1H), 3.38 (m, 1H), 3.18 (m, 1H), 2.70 ing at 400 MHz for 1H, 61.3 MHz for 2H NMR, 100 (m, 1H), 2.36 (m, 1H), 2.25 (s, 3H), 1.97 (m, 3H), MHz for 13C NMR. ESI-HRMS measurements were 1.35 (m, 1H), 0.98 (d, 6H, J = 6.8 Hz); Anal.Calcd performed on an Agilent 6538 UHD accurate-mass Q- for C21H30Cl2N2Ru: C 52.28; H 6.27; N 5.81, Found: TOF LC/MS instrument. Elemental analyses were per- C 51.85; H 6.02; N 5.62; ESI-HRMS: m/z calcu- + + formed by using a Thermo Finnigan Flash EA 1112 lated for [C21H30N2RuCl] : 447.1140 [M-Cl] ; found: CHNS analyzer. HPLC analysis was performed using a 447.1143. 1 Chiralcel OD column and Chiralcel OJ-H column on a C2: Yield: 75%; H NMR (400 MHz, CDCl3): δ 8.26 Merck Hitachi Lachrome HPLC device. (q, 1H, NH, J = 7.40 Hz), 6.95 (m, 1H, NH), 6.28 (d, 1H, J = 5.64 Hz), 5.86 (d, 1H, J = 5.68 Hz), 5.66 (d, 2.2 General procedure for the synthesis of 1H, J = 5.76 Hz), 5.26 (d, 1H, J = 5.96 Hz), 3.79 (m, (S)-N-substituted-2-aminomethylpyrrolidine ligands 1H), 3.29 (m, 3H), 3.03 (m, 2H), 2.72 (m, 1H), 2.45 (s, 3H), 2.08 (q, 1H, J = 12.12 Hz), 1.96 (m, 2H), 1.78 (m, Literature procedures were used for the preparation of 2H), 1.36 (t, 3H, J = 7.32 Hz), 1.28 (dd, 6H, J = 6.88, 38 39 39 39 L1, L2, L3, and L4. 6.92 Hz); Anal. Calcd for C17H30Cl2N2Ru: C 47.00; H 1 L1: Yield: 66%; H NMR (CDCl3): δ 7.17 (t, 2H, 6.96; N 6.45, Found: C 46.32; H 6.57; N 6.17; ESI- + J = 7.44 Hz), 6.69 (t, 1H, J = 7.32 Hz), 6.63 (d, 2H, J = HRMS: m/z calculated for [C17H30N2RuCl] : 399.1140 7.84 Hz), 3.38 (m, 1H), 3.16 (dd, 1H, J = 4.6, 4.64 Hz), [M-Cl]+; found: 399.1137.
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