Iron-Catalyzed Borylation Reactions of Alkynes: An Efficient Synthesis of E-Vinyl Boronates

Abstract

Iron nanoparticles or ferric chloride catalyze the monoborylation of alkynes using bis(pinacolato)diboron to provide E-vinyl boronates with high regio- and stereoselectivity in good to excellent yields. High catalytic activity was observed and the catalyst could be recovered simply by using an external magnetic field. The magnetic nanoparticles were recycled six times without any significant loss of catalytic activity.

Key words

alkynes - catalysis - hydroboration - iron - nanostructures

• References and Notes


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• 19 Typical Procedure: A mixture of alkyne (1 mmol), bis(pinacolato)diboron (1.2 mmol), Cs₂CO₃ (2 mmol) and np Fe₃O₄ or FeCl₃ (5 mol%) in acetone (5 mL) in a sealed tube was heated at 60 °C for 12 h. After completion of the reaction, EtOAc (1 mL) was added to the reaction mixture. An external magnet was applied to the reaction vial to accumulate catalyst, then the organic layer was decanted. This procedure was repeated two more times with EtOAc then twice with H₂O. The separated catalyst was then dried in an oven. For subsequent runs, fresh starting materials were added to the vial and the reaction was conducted as described above. The combined organic and aqueous phases were separated, and the aqueous layer was extracted with EtOAc twice more. The combined organic layers were washed with brine, dried over anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The crude product was purified by silica chromatography, eluting with hexane and acetone, to afford the pure product.
• 20 (E)-2-[2-(6-Methoxynaphthalen-2-yl)vinyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3c): White crystalline powder; IR (KBr): 473, 633, 814, 845, 895, 973, 1000, 1033, 1144, 1164, 1203, 1262, 1330, 1357, 1392, 1460, 1482, 1502, 1620, 2932, 2976, 3052, 3428 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 1.32 (s, 12 H, 4CH₃), 3.91 (s, 3 H, ArOCH₃), 6.22 (d, J = 18.1 Hz, 1 H, trans-CH=CH), 7.08–7.13 (m, 2 H, ArH), 7.51–7.56 (m, 1 H, ArH), 7.64–7.23 (m, 3 H, ArH), 7.77 (s, 1 H, ArH). ¹³C NMR (75 MHz, CDCl₃): δ = 24.7, 55.3, 83.2, 105.7, 118.4, 118.9, 123.9, 125.5, 126.5, 127.0, 127.8, 129.8, 134.9, 149.6, 158.1. MS (ESI): m/z = 311 [M]⁺. HRMS (ESI⁺): m/z [M]⁺ calcd for C₁₉H₂₄O₃B: 311.1813; found: 311.1820.
• 21 (E)-2-[3,5-Bis(trifluoromethyl)styryl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (3d): Light-yellow oil; IR (neat): 682, 705, 724, 846, 896, 968, 1137, 1174, 1279, 1332, 1377, 1464, 1513, 1624, 1716, 2856, 2928 cm^–1. ¹H NMR (300 MHz, CDCl₃): δ = 1.31 (s, 12 H, 4CH₃), 6.31 (d, J = 17.9 Hz, 1 H, trans-CH=CH), 7.41 (d, J = 18.9 Hz, 1 H, trans-CH=CH), 7.77 (s, 1 H, ArH), 7.88 (s, 2 H, ArH). ¹³C NMR (75 MHz, CDCl₃): δ = 24.7, 83.4, 119.5, 126.3, 126.7, 128.4, 131.0, 136.7, 145.8. MS (ESI): m/z = 391 [M + Na]⁺. HRMS (ESI⁺): m/z [M + H]⁺ calcd for C₁₆H₁₈BF₆O₂: 367.1299; found: 367.1298.