A New Approach to 4-Aryl-1,3-butanediols by Cobalt-Catalyzed Sequential Radical Cyclization-Arylation Reaction of Silicon-Tethered 6-Iodo-1-hexene Derivatives

Abstract

Treatment of 6-iodo-4-oxa-3-sila-1-hexene derivatives with arylmagnesium bromide in the presence of a catalytic amount of a cobalt-diamine complex in THF afforded the corresponding benzyl-substituted oxasilacyclopentanes in good yield. The products were converted to 4-aryl-1,3-diols after Tamao-Fleming oxidation.

Key words

1,3-diols - cobalt - arylation - radical reaction - silicon

References and Notes

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9

General Procedure for Sequential Cyclization-Cross-Coupling Reaction. Anhyd CoCl₂ (3.2 mg, 0.025 mmol) was placed in a 20-mL reaction flask and was heated with a hair dryer in vacuo for 2 min. After the color of the cobalt salt became blue, anhyd THF (2 mL) and racemic 1 (20 mg, 0.12 mmol) were sequentially added under argon. The mixture was stirred for 3 min and 6-halo-4-oxa-3-sila-1-hexene derivative 2 (155 mg, 0.5 mmol) was added. Phenylmagnesium bromide (1.0 M THF solution, 0.75 mL, 0.75 mmol) was then added over 5 s to the reaction mixture at 25 °C. While the Grignard reagent was being added, the mixture turned brown. After being stirred for 15 min at 25 °C, the reaction mixture was poured into sat. NH₄Cl solution. The products were extracted with hexane (2 × 20 mL). The combined organic layer was dried over Na₂SO₄ and concentrated to provide a yellow oil. The ¹H NMR analysis with dibromomethane as an internal standard indicated the formation of the desired oxasilacyclopentane 3a in 93% yield. Then, KF (58 mg, 1.0 mmol) and KHCO₃ (100 mg, 1.0 mmol) were dissolved in MeOH-THF (5 mL, 1:1 mixture). The crude product and 30% aq H₂O₂ (0.52 mL) were successively added. After being stirred at r.t. for 12 h, the reaction mixture was poured into sat. Na₂S₂O₃ solution. The product was extracted with EtOAc (2 × 20 mL). The combined organic layer was dried over Na₂SO₄ and concentrated. Purification by silica gel column chromatography (hexane-EtOAc = 2:1) provided the diol 4a (81 mg, 0.37 mmol) in 74% isolated yield.
Diol 4a (1:1 mixture of diastereomers): white solid; mp 69-72 °C. IR (nujol): 3345, 2924, 973, 743 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 1.21-1.32 (m, 1 H), 1.32-1.52 (m, 3.5 H), 1.56-1.87 (m, 4.5 H), 2.23-2.29 (br s, 2 H), 2.71-2.94 (m, 2 H), 3.86 (m, 0.5 H), 4.04-4.08 (m, 1 H), 4.40 (m, 0.5 H), 7.20-7.35 (m, 5 H). ¹³C NMR (125.7 MHz, CDCl₃): δ = 18.63, 19.97, 20.35, 25.00, 25.83, 25.88, 33.23, 33.87, 41.38, 41.97, 44.23, 44.77, 67.40, 72.43, 76.59, 77.62, 126.66, 126.75, 128.82, 128.88, 129.44, 129.50, 138.82, 138.83.
A Typical Procedure for Heck-Type Reaction. The CoCl₂ (6.5 mg, 0.05 mmol) was placed in a 20-mL reaction flask and was heated with a hair dryer in vacuo for 2 min. Then, DPPB (26 mg, 0.06 mmol) and anhyd THF (0.1 mL) were added under argon. After the mixture being stirred for about 5 min at r.t., trimethylsilylmethylmagnesium chloride (1.7 M THF solution, 1.2 mL, 2.0 mmol) and 2 (155 mg, 0.5 mmol) were sequentially added dropwise to the reaction mixture at 0 °C. The resulting mixture was heated at reflux for 10 min. After the mixture was cooled to r.t., the reaction mixture was poured into sat. NH₄Cl solution. The products were extracted with EtOAc (2 × 20 mL). The combined organic layer was dried and concentrated in vacuo. Chromatographic purification on silica gel afforded the alcohol 19 (87 mg, 0.32 mmol) in 65% isolated yield.
Alcohol 19: oil. IR (neat): 3567, 1447, 1249, 1051, 837 cm^-1. ¹H NMR (300 MHz, C₆D₆): δ = -0.24 (s, 2 H), 0.05 (s, 9 H), 0.11 (s, 3 H), 0.12 (s, 3 H), 1.16-1.42 (m, 6 H), 1.74-1.95 (m, 2 H), 2.08 (m, 1 H), 2.37 (dm, J = 12.3 Hz, 1 H), 3.86 (m, 1 H), 5.45 (d, J = 2.7 Hz, 1 H), 5.64 (dd, J = 2.7, 1.2 Hz, 1 H). ¹³C NMR (125.7 MHz, C₆D₆): δ = 0.22, 0.48, 1.87, 2.86, 20.40, 24.98, 27.11, 33.18, 47.22, 66.98, 125.68, 156.10. Anal. Calcd for C₁₄H₃₀Si₂O: C, 62.15; H, 11.18. Found: C, 62.39; H, 11.44.