Agric. Chem. Biotechnol. 46(4), 156-159 (2003)
Article
Chiral Diphosphites Derived from (L)-Tartaric Acid:
New Effective Ligands for the Enantioselective Hydrogenation
Suk-Hun Kyung* and Chi-Hyun Kim
Department of Applied Biology and Chemistry, Konkuk University, Seoul 143-701, Korea Received November 3, 2003; Accepted December 15, 2003
New chiral diphosphites derived from (L)-tartaric acid (1),1, 2-ethanediol and (S)-2,2'-dihydroxy-1,1'- binaphthyl [(S)-BINOL], were synthesized, and applied as ligands in metal-catalyzed asymmetric hydrogenation reactions. The phosphite derived from (L)-tartaric acid and (S)-BINOL provided moderate ee value for dimethyl itaconate, high ee value for α-acetamino methyl cinnamate in Rh- catalyzed hydrogenation.
Key words: Diphosphite ligands, asymmetric hydrogenation
To obtain the new biologically active compounds, catalytic asymmetric synthesis using various transition-metal complexes that contain chiral ligands has been one of the efficient methods, in which the enantioselective hydrogenation of functionalized prochiral olefins plays an important role.1,2) Several chiral phosphines, phosphonites, and phosphites have been synthesized and successfuly applied as ligands in various homogeneous catalytic asymmetric syntheses, such as enantioselective hydrogenation3), hydroformylation4), allylic alkylation5), and conjugation addition of diethylzinc to α,β-unsaturated carbonyl compounds6,7). In addition to the phosphorus compounds descrived above, diamines8), amino alcohols9) and oxazolines,10) which are bidentates, have been often used as ligands, whereas the metals applied in the catalytic systems are mainly Rh, Ru, Cu, and Pd. Of these ligands the diphosphines have been well studied and widely applied as chiral ligand in enantioselective organic syntheses to other bidentate compounds. However, recent reports on the utility of diphosphonite11,12) and diphosphite ligands13) in asymmetric reactions have greatly increased the interest of chemists.
Reetz et al. have for the first time reported the successful enantioselective hydrogenation using diphosphonite ligands11). They have synthesized chiral diphosphonite ligands with chiral binaphthol (BINOL, 2,2'-dihydroxy-1,1-binaphthyl) and demonstrated that their corresponding Rh-complexes were excellent catalysts in the hydrogenation of dimethyl itaconate (12) and 2-acetamido methyl acrylate (14).
Recently Ruiz et al. synthsized new phosphine-phosphite and diphosphite ligands derived from carbohydrates, D- xylofuranose and D-glucose, and applied them in the Rh- catalyzed hydrogenation, where both excellent
enantioselectivities and activities were achieved14,15).
Nevertheless the demand for new highly efficient ligand systems derived from readily available simple chiral pools is still on the increase. In this study we synthesized chiral diphosphites derived from an achiral and two chiral back- bones such as 1,2-ethanediol (4) and L-tartaric acid (1), and (S)-BINOL (6), and applied them as ligand in Rh-catalyzed hydrogenation of prochiral olefins.
Materials and Methods
Materials. All reactions were carried out under argon (Ar) atmosphere using standard Schlenk techniques.
Chemicals were distilled or recrystallized prior to use.
Solvents were distilled from K-benzophenone. 1H-NMR spectra were recorded on Bruker DPX-400 spectrometer.
Chemical shifts and coupling constants (J) are given in ppm and in Hertz. GC analyses were carried out on Hewlet- Packard 5890 Series II. For the determination of enantiomeric excess of hydrongenated products chiral capillary β-DMX 225(30 m× 0.32 mm) column was used.
Methods. The representative synthetic procedure of C2- symmetric disphosphites ligands is shown in Scheme 1.
Chloroisopropyltartanophosphite (3). Synthesis was carried out as previously described12). To 100 ml Schlenk flask 20 ml of THF, 2.34 g diisopropyl-L-tartrate (1), and 2.79 ml triethylamine were charged under Ar atmosphere. The mixture was cooled down to −78oC. Subsequently 0.87 ml (10 mmol) of phosphorus trichloride (2) solution was slowly added to the mixture. After completely adding reagent 2, the reaction mixture was allowed to warm to room temperature.
The mixture was further stirred overnight. The ammonium salt was filtered off, followed by removal of solvent.
Fractional distillation of the resulting residue gave product (3), which was employed for next experimemts without purification or charaterization. Other chlorodialkylphosphite
*Corresponding author
Phone: +82-2-450-3758; Fax: +82-2-456-7183 E-mail: [email protected]
Chiral Diphosphites for the Enantioselective Hydrogenation 157
(7) from (S)-BINOL was prepared as described above.
Disphosphites (5, 8). A 100 ml nitrogen flask was charged with 20 ml THF, 0.7 g (5 mmol) of triethylamine, and 1.32 g (5 mmol) of chloro bis(diisopropyl-L-tartano)phosphite (3).
The reactions mixture was cooled down to −78oC and 0.16 g (2.5 mmol) of ethyleneglycol (4) in 5 ml THF was slowly added to the mixture with stirring. Upon completion of adding ethyleneglycol (4) the reaction mixture was allowed to warm to room temperature and stirred for 5 hours until formation of triethylammoniun salt cersed. The salt was then removed by filtration under Ar. Distillation of resulting residue, after removal of solvent by condensation, under reduced pressure gave relatively pure diphosphite (5), which was used for the preparing of chiral metal-complex (10). Other disphosphites
(8) was prepared as described above.
Metal-catalyzed hydrogenation of olefins. The hydrogenation of dimethyl itaconate (12) and methyl α- acetamino acrylate (14) was carried out according to the procedure described in literature11). After a 20 ml Schlenk flask was charged with 1 mmol of olefinic substrate, the whole apparatus was evacuated and refilled with Ar. 3 ml of solvent was added to the flask to dissolve substrate, followed by the addition of 0.002~0.005 mmol of catalyst. After Ar was removed from the reaction flask hydrogen (1atm) was introduced, and stirred further several hours. The reaction mixture was filtered off, and yield and enantiomeric excess (%
ee) was determined through gas chromatography.
Scheme 1.
158 Suk-Hun Kyung and Chi-Hyun Kim
Results and Discussion
Ligands synthesis. Scheme 1 shows the synthetic sequence of diphosphites ligands. Compounds 5 and 8 were synthesized in two steps from diols, 1, 2-ethanediol (4), (L)- tartaric acid (1), and (S)-binaphthol (6). The reactions of the diols with phosphorus trichloride (2) in mild conditions formed corresponding phosphorochloridite 3 and 7, which under further treatments with diols 4 and 1 resulted desired ligands 5 and 8 with good to excellent yield (85-92%).
1,2-Ethoxy bis (diisopropyl tartano)phosphite (5). Yield, 85%; colorless liquid. 1H-NMR(400 MHz, CDCl3): δ 1.33(m, 24H, CH-CH3), 4.94 (t, J = 12 Hz, CH-COO), 5.08 (m, 4H, CH-CH3)
Chloro-(S)-bis-β-naphthophospine (7). Yield, 80%; 1H- NMR (400 MHz, CDCl3): δ 7.43 (m, 3H, ArH), 7.68 (m, 3H, ArH), 8,04(d, 2H, ArH), 8.48(d, 2H, ArH), 9.10(d, 2H, ArH).
1,2-Diisopropyl-L-(+)-tartanobis(binaphtho)phosphite
(8). Yield, 92%; white solid. 1H-NMR (400 MHz, CDCl3): δ 1.09 (m, 6H, CH3), 1.23-1.34 (m, 6H, CH3), 7.17-7.43 (m, 16H, ArH), 7.88-7.97 (m, 8H, ArH).
Hydrogenation of functionalized prochiral olefins. The Rh(I) complexes, (10, 11) used as catalyst in the hydrogenation of olefins were synthesized in situ by reacting stoichiometric amounts of chiral diphosphites ligands 5 and 8 with [Rh(COD)2]BF4 (COD, cyclooctandienyl) in dichloromethane (Scheme 2).
In the first experiment we performed a Rh-catalyzed hydrogenation of dimethyl itaconate (12). Mole ratio of substrate vs catalyst (S/C) was kept at 1/200. Reactions were carried out at room temperature and 1 atm of hydrogen pressure.
Scheme 3 shows that, of the two kinds of catalytic systems, the one combined with diphosphite having achiral back-bone (1,2-ethanediol, 10) resulted almost complete conversion of olefin to the corresponding hydrogenated product (13), Scheme 2.
Scheme 3.
Chiral Diphosphites for the Enantioselective Hydrogenation 159
although unfortunately with a poor enantioselectivity (10%).
In contrast to the complex (10), the employment of the other catalytic system of Rh-diphosphite (11), which has (L)-tartaric acid as chiral back-bone, led to acceptable enantiomeric excess at 77% ee (Scheme 3).
For the next study of the asymmetric metal-catalyzed hydrogenation of prochiral olefins we chose α- methylacetaminocinnamate (14) as substrate (Scheme 4). A pronounced enhancement in enantioselectivity, when diphosphite (8) derived from (L)-tartaric acid was used as chiral ligand. The results show that catalytic systems of Rh- diphosphite ligand formed particularly from (S)-BINOL- tartaric acid are extremely effective in Rh-catalyzed asymmetric hydrogenation of functionalized prochiral olefins, especially of α-methylacetaminocinnamate (14).
In this preliminary investigation we synthesized diphosphites readily available from achiral or chiral diols, and applied them in asymmetric syntheses. Recently Dieguez et al.
reported that efficiency of a catalyst depends strongly on the nature of the solvent in the hydrogenation reaction with chiral diphosphite derived from D-glucose15). Therefore further investigation on the reaction conditions, including solvents and S/C for metal-catalyzed asymmetric reactions using ligands 5 and 8 is necessary.
Acknowledgments. This study was supported by Konkuk University in 2002.
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