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DOI: 10.1055/s-2004-834791
Asymmetric Conjugate Reduction of α,β-Unsaturated Esters with Chiral Rhodium(bisoxazolinylphenyl) Catalysts
Publication History
Publication Date:
20 October 2004 (online)
Abstract
Chiral rhodium(bisoxazolinylphenyl) complexes reduced α,β-unsaturated esters in high enantioselectivity up to 97-98% ee in the combination of alkoxyhydrosilanes.
Key words
rhodium - conjugate reduction - unsaturated ester - hydrosilylation - asymmetric catalysis
- Non-chiral Cu-H and hydrosilylation:
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Synthesis of the Ligand Precursor and the Complexes. The starting compound, 1,3-bis[(4S)-4-isopropyl-2-oxazolin-2-yl]benzene was prepared by reaction of isophthaloyl chloride (5.0 mmol), l-valinol (10.0 mmol), and Et3N (75 mmol) in CH2Cl2 (40 mL) for ca 1 h followed by oxazoline formation by addition of MsCl (11 mmol). The mixture was stirred for 5 h. After alkaline treatment and concentration, the desired ligand was obtained in 87% by silica gel chromatography with hexane-EtOAc. For an alternative route with isophthalonitrile, see ref. 6b. The complex 2 was easily prepared by C-H bond activation method. The mixture of RhCl3(H2O)3 (2.4 mmol) and the bis(oxazolinyl)benzene (2.0 mmol) in MeOH (40 mL) was heated at 50 °C for 6 h. The concentrated residue was purified by silica gel chromatography with EtOAc and hexane to give 2 in 64% yield; see ref. [5a] [d] for spectroscopic data. The complex 1 was prepared by reaction of 2 (0.5 mmol) and silver acetate (2.0 mmol) in CH2Cl2 (28 mL) at r.t. The mixture was vigorously stirred for 18 h. The concentrated residue was purified by silica gel chromatography with EtOAc and MeOH to give 1 in 88% yield as yellowish orange solids, mp 235 °C (dec.). 1H NMR (300 MHz, CDCl3): δ = 0.75 (d, J = 7.2 Hz, 6 H), 0.97 (d, J = 7.2 Hz, 6 H), 1.71 (s, 6 H), 2.50 (m, 2 H), 4.38 (m, 2 H), 4.60-4.80 (m, 4 H), 5.08 (b, 2 H), 7.30 (t, J = 7.2 Hz, 1 H), 7.43 (d, J = 7.2 Hz, 2 H). 13C NMR (75 MHz, CDCl3): δ = 14.79, 19.13, 23.42, 29.50, 67.72, 71.27, 122.9, 127.5, 131.5, 171.3, 182.3 (d, J Rh-C = 25.7 Hz). IR (disk): ν = 1615, 1395 cm-1. The structure of 1 was also determined by elemental analysis and X-ray analysis of a single crystal, which will be disclosed elsewhere.
- 6b
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For preparation of 11, see:
Tanaka K.Fu GC. J. Org. Chem. 2001, 66: 8177 - 8b For the relation between absolute configuration and optical rotation of 13, see:
Ahlbrecht H.Enders D.Santowski L.Zimmermann G. Chem. Ber. 1989, 122: 1995 - 8c
The ee of (3S)-(-)-13 was determined by GC with Alltech Chiraldex G-TA (105 °C, 80 kPa).
References
Typical Reaction (Run 1 of Table 1). To a mixture of ethyl (E)-3-phenyl-2-butenoate (4, 190 mg, 1.00 mmol) and the catalyst 1 (5.4 mg, 0.01 mmol) in 1 mL of toluene was slowly added diethoxymethylsilane (201 mg, 1.50 mmol) at 60 °C. The mixture was stirred for 1 h and then treated with 1 N HCl (1 mL). After extraction and concentration, the residue was purified by silica gel chromatography (EtOAc-hexane) to give the reduction product (R)-(-)-5 of 184 mg (96%) as colorless oil. For characterization, see ref. 3a,4c. The unsaturated esters were synthesized by Horner-Emmons reaction; see ref. 3a.