Asymmetric Catalytic Hydroboration of Enol Carbamates Enables an Enantioselective Matteson Homologation

Abstract

A rhodium-catalyzed asymmetric hydroboration of enol carbamates yields α-boryl carbamates with a good enantioselectivity. The enol carbamate starting materials can be prepared with moderate Z-selectivity by using a modified Juila olefination; they can then be used as the resulting E/Z mixtures, taking advantage of the faster reactivity of the major Z-isomers in the directed hydroboration. Optically active α-boryl carbamates participate in a Matteson-type homologation with Grignard reagents, in which the O-carbamate is substituted, with high conservation of optical activity, to provide enantioenriched secondary boronic esters.

Key words

hydroboration - Matteson homologation - Julia olefination - boronic esters - rhodium catalysis

• References and Notes

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• 20 (1Z/E)-3-(4-Methoxyphenyl)prop-1-en-1-yl Diisopropylcarbamate; Typical Procedure Under an argon atmosphere, a solution of sulfone 1 (2.09 g, 6.0 mmol, 1.50 equiv) in anhyd THF (40 mL) was cooled to –78 °C. A 1.0 M solution of LiHMDS in THF (6.8 mL, 6.8 mmol, 1.7 equiv) was added dropwise, maintaining the temperature below –60 °C, and the resulting mixture was stirred with dry-ice/acetone cooling for 40 min. A solution of 2-(4-methoxyphenyl)acetaldehyde (2a; 0.60 g, 4.0 mmol, 1.00 equiv) in anhyd THF (5.3 mL) was added dropwise, maintaining the temperature below –60 °C, and the resulting mixture was stirred at RT overnight. MeOH (3 mL) was added to quench the reaction, and the volatiles were removed under vacuum. The residue was purified by flash chromatography [silica gel, hexanes–Et₂O (90:10)] to give a yellow oil; yield: 0.714 g (61%) (Z/E = 80:20). ¹H NMR (400 MHz, CDCl₃): δ = 7.19–7.09 (m, 3 H, Z + 3 H, E), 6.89–6.79 (m, 2 H, Z + 2 H, E), 5.43 (dt, J = 12.4, 7.5 Hz, 1 H, E), 4.91 (td, J = 7.5, 6.4 Hz, 1 H, Z), 4.11 (br s, 1 H, Z + 1 H, E), 3.81 (br s, 1 H, Z + 1 H, E), 3.79 (s, 3 H, Z + 3 H, E), 3.46 (dd, J = 7.5, 1.7 Hz, 2 H, Z), 3.27 (dd, J = 7.4, 1.7 Hz, 2 H, E), 1.30–1.19 (br m, 12 H, Z + 12 H, E). ¹³C NMR (151 MHz, CDCl₃): δ = 158.05, 153.05, 137.26, 135.63, 132.65, 132.57, 129.48, 129.31, 113.97, 111.35, 109.82, 55.41, 46.98, 45.82, 33.02, 30.46, 21.70, 20.51. ESI MS: m/z [M + H]⁺ calcd for C₁₇H₂₆NO₃: 292.2; found: 292.2.
• 21 (1R)-3-(4-Methoxyphenyl)-1-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)propyl Diisopropylcarbamate (4a); Typical Procedure Enol carbamate 3a (102 mg, 0.35 mmol, 1.0 equiv) was weighed into a 2-dram screwcap vial, which was then transferred into a N₂-filled glove box. Inside the glove box, ligand L6b (10 mg, 21 μmol, 6 mol%) was weighed into a 1-dram screwcap vial. [Rh(cod)₂]OTf (8.5 mg, 17.5 μmol, 5 mol%) was weighed into a separate 20-mL screwcap vial equipped with a stirrer bar. Substrate 3a was dissolved in PhCF₃ (2.8.0 mL), and the resulting solution was transferred to the vial with the ligand (an additional 0.7 mL of PhCF₃ was used for rinsing). The solution of substrate 3a and ligand L6b was then transferred to the vial with the catalyst. HBpin (0.10 mL, 0.090 g, 0.70 mmol, 2.0 equiv) was added, and the mixture was briefly stirred. The reaction vial was sealed with a Teflon-coated screwcap and removed from the glove box, and the mixture was stirred at RT for 48 h. The mixture was then concentrated, and pinacol was removed by transferring the crude product into a 250 mL round-bottomed flask, dissolving the residue in MeOH (2 mL), adding H₂O (0.4–0.8 mL) until the solution became cloudy, and concentrating on a rotary evaporator. After 6–9 azeotropic evaporation cycles, the pinacol was removed completely. The final residue was purified by flash chromatography [silica gel, hexanes–EtOAc (70:30)] to give a pale-yellow oil; yield: 98 mg (67%). Chiral HPLC (CHIRALCELOZ-3, 0.75% iPrOH–hexane, 1.0 mL/min, λ = 220 nm): 98% ee; t _R = 7.1 and 11.7 min. ¹H NMR (400 MHz, CDCl₃): δ = 7.16–7.07 (m, 2 H), 6.87–6.77 (m, 2 H), 4.07 (hept, J = 6.9 Hz, 1 H), 3.82 (dd, J = 10.6, 3.9 Hz, 1 H), 3.78 (s, 3 H), 3.76 (hept, J = 6.9 Hz, 1 H), 2.79 (ddd, J = 14.7, 9.9, 5.3 Hz, 1 H), 2.65 (ddd, J = 13.8, 9.4, 6.6 Hz, 1 H), 2.03–1.93 (m, 1 H), 1.92–1.80 (m, 1 H), 1.28–1.20 (m, 12 H), 1.17 (s, 12 H). ¹³C NMR (101 MHz, CDCl₃): δ = 162.77, 157.66, 134.60, 129.42, 113.70, 79.74, 55.26, 48.44, 46.61, 33.61, 33.35, 25.30, 24.96, 20.62, 20.61, 20.35, 20.26 (The carbon attached to boron was not observed due to quadrupolar relaxation). ¹¹B NMR (128 MHz, CDCl₃): δ = 12.86. ESI MS: m/z [M + H]⁺ calcd for C₂₃H₃₉BNO₅: 420.3; found: 420.2.
• 22 CCDC 226136 contains the supplementary crystallographic data for compound 4d. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures
• 23 (1S)-1-Cyclohexyl-3-(4-methoxyphenyl)propan-1-ol (5a); Typical Procedure Freshly prepared CyMgCl (0.12 mmol, 1.4 equiv) was added dropwise over 2 min to a solution of 4a (35 mg, 0.083 mmol, 1.0 equiv, 98% ee) in anhyd Et₂O (1.0 mL) at –78 °C under argon, and the mixture was stirred under argon in a cold room (4 °C) for 2 h. Insoluble solids were removed by filtration through a Pasteur pipette with a silica gel plug (~1 mL of silica gel was used; an additional 2-mL portion of Et₂O was used for washing). The filtrate was concentrated under vacuum, and the crude secondary boronate was then redissolved in THF (1.0 mL), and the solution was cooled to 0 °C and treated with 3 M aq NaOH (0.17 mL) and 30% aq H₂O₂ (0.17 mL). The mixture was stirred at RT for 2 h, then cooled to 0 °C. Sat. aq Na₂S₂O₃ (1 mL) was carefully added to quench the reaction, and the product was extracted with EtOAc (3 × 2 mL). The combined organic layers were dried (Na₂SO₄), filtered, and concentrated. The residue was purified by flash chromatography [silica gel, hexanes–EtOAc (85:15)] to give a white solid; yield: 15 mg (71%). Chiral HPLC (CHIRALCEL OD-H, 3.5% iPrOH–hexane, 1.5 mL/min, λ = 220 nm): 95% ee, 97% es. ¹H NMR (400 MHz, CDCl₃): δ = 7.18–7.08 (m, 2 H), 6.88–6.79 (m, 2 H), 3.79 (s, 3 H), 3.38 (ddd, J = 8.9, 5.4, 3.2 Hz, 1 H), 2.77 (ddd, J = 13.8, 9.9, 5.4 Hz, 1 H), 2.59 (ddd, J = 13.8, 9.7, 6.7 Hz, 1 H), 1.88–1.58 (m, 6 H), 1.42–0.91 (m, 7 H). ¹³C NMR (151 MHz, CDCl₃): δ = 157.83, 134.53, 129.43, 113.92, 75.72, 55.39, 43.90, 36.26, 31.56, 29.30, 27.91, 26.65, 26.46, 26.32. ESI MS: m/z [M–OH]⁺ calcd for C₁₆H₂₃O: 231.17; found: 231.2.