Radical-Hydroboration-Involved One-Pot Synthesis of Boron-Handled Glycol Derivatives

Published as part of the Cluster Radicals – by Young Chinese Organic Chemists

Abstract

A one-pot two-step protocol for the direct synthesis of boron-handled glycol derivatives is reported. The procedure starts by an NHC–boryl-radical-promoted regioselective hydroboration of glycol-protected cinnamaldehydes. After that, the reaction mixture is treated with pinacol in the presence of HCl, leading to the direct formation of pinacol boronate handled glycol monoalkyl ethers. In this acid-triggered conversion, a reductive ring-opening of glycol-derived acetal moiety takes place, during which an NHC–borane unit serves as the hydride source.

Key words

boryl radical - hydroboration - radical borylation - reduction - glycol monoalkylethers

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• 12 General Procedure for the Preparation of Boron-Substituted Ethylene Glycol 3a A solution of 1a (95.7 mg, 0.422 mmol), NHC–BH₃ (51.4 mg, 0.467 mmol), DTBP (30 μL, 0.163 mmol), and octadecanethiol (62 mg, 0.216 mmol) in toluene (5 mL) was stirred at 120 °C for 12 h under nitrogen atmosphere. After evaporation of solvent, the crude residue was treated with pinacol (77.5 mg, 0.655 mmol) and 2.0 M HCl (0.4 mL, 0.800 mmol) in THF (10 mL) for 2 h at room temperature. The reaction mixture was quenched with water and extracted with ethyl acetate. The organic layer was dried over Na₂SO₄ and concentrated in vacuo. The residue was purified by flash column chromatography (silica gel; petroleum ether–ethyl acetate = 3:1) to give product 3a (125.4 mg, 0.352 mmol) in 83% yield. Pale yellow liquid. ¹H NMR (400 MHz, CDCl₃): δ = 1.19 (6 H, s), 1.20 (6 H, s), 1.81 (1 H, dddd, J = 6.8, 6.8, 7.6, 8.0 Hz), 2.72 (1 H, br), 2.89 (1 H, dd, J = 7.6, 13.6 Hz), 3.00 (1 H, dd, J = 8.0, 13.6 Hz), 3.48–3.56 (4 H, m), 3.65–3.72 (2 H, m), 7.35–7.45 (3 H, m), 7.64 (1 H, s), 7.74–7.76 (2 H, m), 7.79 (1 H, dd, J = 1.6, 6.0 Hz). ¹³C NMR (100 MHz, CDCl₃): δ = 24.6, 24.7, 33.5, 61.5, 71.2, 71.6, 83.4, 125.0, 125.8, 126.9, 127.3, 127.5, 127.6, 127.7, 131.9, 133.4, 139.0. ¹¹B NMR (128.4 MHz, CDCl₃): δ = 33.7 (1 B, s). ESI-HRMS: m/z calcd for C₂₁H₂₉ ¹¹BNaO₄ [M + Na]⁺: 379.2057; found: 379.2065.