Iron-Catalyzed Silylation of Alcohols by Transfer Hydrosilylation with Silyl Formates

Published as part of the Cluster Silicon in Synthesis and Catalysis

Abstract

An iron catalyst is shown for the first time to promote transfer hydrosilylation with silyl formates and is utilized for the silylation of alcohols. Attractive features of this protocol include the use of an earth-abundant transition-metal catalyst, mild reaction conditions, and the release of gases as the only byproducts (H₂ and CO₂).

Key words

silylation - iron catalysis - silanes - alcohols - silyl formates - hydrosilanes - CO₂

• References and Notes

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• 31 General Procedure for the Synthesis of Silylformates (2)In a glovebox, a flame-dried round-bottom flask equipped with a Solv-seal connection and a J-Young valve was charged with sodium formate (42 mmol, 1.4 equiv) and suspended in Et₂O (30 mL, c 1 M). To the resulting suspension was added the silyl chloride 1 (30 mmol, 1 equiv). The flask was then sealed, brought out of the glovebox, and immersed in a pre-heated oil bath at 90 °C (oil temperature). Unless otherwise stated, the reactions were generally complete within 15 h. The mixture was then filtered over a sintered glass funnel and the solvent removed by distillation under vacuum at 0 °C. No other purification was needed. The formation of silyl formate was confirmed by ¹H, ¹³C NMR, and elemental analysis.Analytical Data for 2e (Representative Example) ¹H NMR (200 MHz, d₈ -THF): δ = 8.18 (s, 1 H), 7.71–7.56 (m, 4 H), 7.38 (m, 6 H), 0.85 (s, 3 H). ¹³C NMR (50 MHz, d₈ -THF): δ = 160.88, 135.09, 134.47, 130.99, 128.53, –2.64. Anal. Calcd (%) for C₁₄H₁₄O₂Si (243.3 g mol^–1): C, 69.39; H, 5.82. Found: C, 69.03; H, 5.67.General Procedure for the Transfer Hydrosilylation of Alcohols with Silylformates 5–21In a glovebox, a flame-dried flask equipped with a J-Young valve was charged with Fe(OAc)₂ (1.7 mg , 0.01 mmol, 2 mol%) and P(C₂H₄PPh₂)₃ (4, 6.7 mg , 0.01 mmol, 2 mol%) followed by CH₂Cl₂ (2 mL, c 0.25 M). To the resulting white suspension were sequentially added the alcohol (0.5 mmol, 1 equiv) and the silyl formate 2 (0.6 mmol, 1.2 equiv per hydroxyl group) reagents. The flask was then sealed, brought out of the glovebox, and immersed in a pre-heated oil bath at 90 °C (oil temperature). A purple coloration appeared when heated. At this temperature, all the reactions were generally complete within 1.5 h with silyl formates 2. The coloration observed turned from purple to bright orange. Yields of silyl ethers were determined after isolation and purification by column chromatography on silica gel.Analytical Data for 16b (Representative Example) ¹H NMR (200 MHz, d₈ -THF): δ = 5.59 (s, 1 H), 3.58 (m, 2 H), 1.89 (m, 14 H), 1.20 (s, 3 H), 1.12–0.78 (m, 4 H), 0.75 (s, 3 H), 0.06 (s, 9 H). ¹³C NMR (50 MHz, d₈ -THF): δ = 195.30, 167.91, 122.75, 80.52, 53.42, 49.26, 41.92, 37.52, 35.86, 35.00, 34.79, 32.75, 31.49, 30.85, 29.81, 22.38, 19.69, 15.74, 9.80, –1.61. ESI-HRMS: m/z [M + H]⁺ calcd for C₁₄H₂₅O₂Si⁺: 361.2557; found: 361.2557.