A Solvent-Free Reaction for Silyl Enol Ethers Synthesis

Abstract

Silyl enol ethers are extremely useful nucleophilic intermediates for chemical transformations because they are synthetically versatile substrates for a wide range of C–C bond-forming reactions. Here, we present a new, mild, and solvent-free procedure for the synthesis of silyl enol ethers that employs a catalytic amount of solid-supported base and an equimolar amount of N,O-(bistrimethylsilyl)acetamide as a silylating agent.

Key words

silyl enol ethers - catalyst support - green chemistry - heterogeneous catalysis - solvent-free conditions

• References and Notes

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• 11 Silyl Enol Ethers 3 and 7a–o; General Procedure The appropriate aromatic ketone (1.0 mmol), BSA (244 µL, 1.0 mmol), and PS-BEMP (10 mol%, 45 mg or 5 mol%, 22 mg) were added to a vial, and the mixture was stirred at 60 °C for 5 h. The catalyst was filtered off, the mixture was washed with H₂O (3 × 2 mL), and the aqueous phase was extracted with hexane (3 × 2 mL). The organic phases were combined, washed with brine (2 mL), dried (Na₂SO₄), filtered, and concentrated in vacuo to eliminate the acetamide byproduct. The crude mixture was then purified by flash chromatography [silica gel, hexane–Et₂O (99:1)] to remove any residual reagent. {[(Z)-2-Bromo-1-(3-nitrophenyl)vinyl]oxy}(trimeth­yl)silane (7k) Yellow oil; yield: 309 mg (98%). ¹H NMR (200 MHz, CDCl₃): δ = 0.25 (s, 9 H), 6.15 (s, 1 H), 7.52 (t, J = 8 Hz, 1 H), 7.78 (t, J = 4 Hz, 1 H), 8.16 (d, J = 12 Hz, 1 H), 8.31 (s, 1 H). ¹³C NMR (400 MHz, CDCl₃): δ = 1.18, 91.33, 120.76, 123.71, 129.96, 131.48, 138.92, 148.78, 151.65. GC/EIMS: m/z (%) = 315 (18) [M⁺], 300 (43), 139 (43), 89 (25), 73 (100), 63 (9). Anal. calcd for C₁₁H₁₄BrNO₃Si: C, 41.78; H, 4.46; N, 4.43. Found: C, 41.75; H, 4.60; N, 4.32. Silyl enol ethers 7p–s, 7v, and 7x: General Procedure The appropriate aliphatic ketone or aldehyde (1.0 mmol), BSA (244 µL, 1.0 mmol), and PS-BEMP (10 mol%, 45 mg) were added to a vial, and the mixture was stirred at 60 °C for the appropriate time. The products were purified by distillation under pressure. (3,6-Dihydro-2H-pyran-4-yloxy)(trimethyl)silane (7s) Colorless oil; yield: 133 mg (77%). ¹H NMR (200 MHz, CDCl₃): δ = 0.26 (s, 9 H), 2.18–2.22 (m, 2 H), 3.86 (t, J = 5.59 Hz, 2 H), 4.19 (q, J = 2.53, 5.23 Hz, 2 H), 4.87–4.90 (m, 1 H).