Enantioselective Hydrosilylation of Aromatic Alkenes Catalyzed by Chiral Bis(oxazolinyl)phenyl–Rhodium Acetate Complexes

Abstract

Highly efficient and enantioselective hydrosilylation of aromatic alkenes catalyzed by the chiral rhodium acetate complexes with the bis(oxazolinyl)phenyl ligands has been reported that afforded chiral silane derivatives with up to 99% ee.

Key words

asymmetric hydrosilylation - alkene - bis(oxazolinyl)phenyl - rhodium - hydrosilane

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• 16 Typical Procedure for Hydrosilylation of 4a with (EtO)₃SiH (Table 1, Entry 2): To a mixture of the phebox-Rh complex 2a (5.4 mg, 0.010 mmol), 4-phenylstyrene (4a; 180 mg, 1.0 mmol) in toluene (1 mL) was added (EtO)₃SiH (3b; 220 μL, 1.2 mmol) at 30 °C and the mixture was stirred for 1 h. After concentration of the reaction mixture, the crude product was purified by silica gel column chromatography with hexane–EtOAc (100:1–50:1) as an eluent to give a mixture of 5a and 6a (342 mg, 0.99 mmol; 5a/6a = 78:22). To the mixture of 5a and 6a (342 mg, 0.99 mmol), KHCO₃ (300 mg, 3.0 mmol) and KF (174 mg, 3.0 mmol) in THF (1 mL) and MeOH (1 mL) was added H₂O₂ (30%, 1 mL) at 0 ºC. After being stirred for 12 h, the reaction was quenched by addition of sat. aq Na₂S₂O₃ (10 mL). The mixture was additionally stirred for 1 h and was extracted with EtOAc (3 × 6 mL). The extract was washed with aq NaCl (10 mL) and was dried over MgSO₄. After concentration, the residue was purified by column chromatography on silica gel with hexane–EtOAc (15:1–5:1) as an eluent to give 7a (81.2 mg, 0.41 mmol, 41%). HPLC Analysis :(S)-1-(para-Biphenyl)ethanol (7a): Daicel CHIRALPAK^® AD-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 14.8 min (S), t _R = 16.0 min (R); 98% ee. (S)-1-(4′-Methylphenyl)ethanol (7b): Daicel CHIRALCEL^® OJ-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 12.4 min (major), t _R = 14.4 min (minor); 98% ee. (S)-1-(4′-Methoxyphenyl)ethanol (7c): Daicel CHIRALPAK^® AS-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 22.1 min (minor), t _R = 27.8 min (major); 97% ee. (S)-1-(4′-Chlorophenyl)ethanol (7d): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (90:10); flow rate: 1.0 mL/min; λ = 215 nm; t _R = 5.2 min (major), t _R = 5.7 min (minor); 97% ee. (S)-1-(4′-Bromophenyl)ethanol (7e): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (95:5); flow rate: 0.5 mL/min; λ = 254 nm; t _R = 15.9 min (major), t _R = 18.5 min (minor); 96% ee. (S)-(–)-1-(4′-Trifluoromethylphenyl)ethanol (7f): Daicel CHIRALCEL^® OJ-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 8.5 min (major), t _R = 9.2 min (minor); 98% ee. Methyl 4-(1-Hydroxyethyl)benzoate (7g): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 19.5 min (major), t _R = 24.0 min (minor); 98% ee. (S)-1-(3′-Methoxyphenyl)ethanol (7h): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 18.4 min (major), t _R = 28.0 min (minor); 98% ee. (S)-1-(3′-Chlorophenyl)ethanol (7i): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 9.9 min (major), t _R = 13.5 min (minor); 98% ee. (S)-1-(2′-Methoxyphenyl)ethanol (6j): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 10.6 min (major), t _R = 18.6 min (minor); 49% ee. (S)-1-(2′-Chlorophenyl)ethanol (6k): Daicel CHIRALCEL^® OJ-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 9.7 min (major), t _R = 10.3 min (minor); 91% ee. (S)-1-Phenylpropanol (10): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 7.9 min (major), t _R = 9.7 min (minor); 98% ee. (R)-2-Tetralol (12): Daicel CHIRALCEL^® OJ-H; hexane–2-propanol (95:5); flow rate: 0.8 mL/min; λ = 254 nm; t _R = 12.5 min (major), t _R = 13.4 min (minor); 91% ee. (S)-1,2-Dihydroacenaphthylen-1-ol (14): Daicel CHIRALCEL^® OB-H; hexane–2-propanol (90:10); flow rate: 1.0 mL/min; λ = 215 nm; t _R = 7.2 min (minor), t _R = 10.0 min (major); 83% ee.