Sequential O-Arylation/Lanthanide(III)-Catalyzed [3,3]-Sigmatropic Rearrangement of Bromo-Substituted Allylic Alcohols

Dedicated to the Cardinal Chemist, the inimitable Prof. Victor Snieckus, on the occasion of his 80th birthday

Abstract

Lanthanide(III)-catalyzed aryl-Claisen rearrangement of substrates bearing halo-substituted allyl groups, specifically 2-bromoallyl aryl ethers, afford ortho-2-bromoallylphenols. Aryl ether substrates were synthesized from brominated allylic alcohols via Mitsunobu reaction, Cu(II)-catalyzed arylation using potassium aryltrifluoroborate salts, or S_NAr reaction. Aryl-Claisen rearrangements proceeded in moderate to excellent yields using Eu(III) catalysis. The alkenylbromide functionality remains intact, illustrating the compatibility of synthetically important alkenylhalides during C–O/C–C σ-bond migration processes. Subsequent derivatization of the ortho-2-bromoallylphenol products through O-alkylation or C-arylation/alkenylation via Suzuki–Miyaura cross-coupling demonstrate the potential to access densely-functionalized molecules.

Key words

aryl-Claisen [3,3]-sigmatropic rearrangement - alkenylbromide - arylation - organotrifluoroborate salt - lanthanide catalysis - phenols - Suzuki–Miyaura reaction - Chan–Lam–Evans arylation

• References and Notes

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• 17 General Procedure for O-Aryl 2-Bromoallylic Ether Synthesis via Cu(II)-Catalyzed Arylation A suspension of ArBF₃K, Cu(OAc)₂·H₂O (10 mol%), DMAP (20 mol%), and powdered 4Å MS in CH₂Cl₂ was stirred at rt for 5 min. To this suspension was added alcohol 6. The mixture was stirred at rt for 60–72 h under an O₂ atmosphere. Subsequently, the mixture was filtered through a pad of Celite, and the filtrate was concentrated in vacuo. The resultant crude mixture was purified using flash column chromatography on silica gel to afford aryl ether 7. Example Reaction of 6a (0.250 g, 1.4 mmol) with 4-FC₆H₄BF₃K (0.601 g (95% purity), 2.8 mmol) using the general procedure afforded 7c (0.171 g, 45%) as a clear oil. Analytical Data for Compound 7c R_f = 0.43 (5% EtOAc/hexanes). IR (thin film): ν_max = 3073, 3051, 2945, 2934, 2866, 2834, 1647, 1601, 1505, 1439, 1368, 1240, 1202, 1090, 1057, 999, 974, 914, 826, 785, 725 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 7.00–6.94 (4 H, m), 6.38 (1 H, dd, J = 5.0, 3.0 Hz), 4.64–4.62 (1 H, m), 2.27–2.19 (1 H, m), 2.14–2.04 (1 H, m), 1.86–1.74 (2 H, m), 1.70–1.61 (1 H, m). ¹³C NMR (100 MHz, CDCl₃): δ = 158.0 (d, ¹ J _CF = 239.5 Hz), 154.4 (d, ⁴ J _CF = 2.5 Hz), 135.0, 121.2, 118.6 (d, ³ J _CF = 8.0 Hz), 116.1 (d, ² J _CF = 23.0 Hz), 78.0, 29.3, 28.0, 16.8. ¹⁹F NMR (376 MHz, CDCl₃): δ = –123.41 (m). LRMS (EI⁺): m/z (rel. intensity) = 272 (5), 270 (5) [M]⁺, 161 (14), 160 (93), 159 (17), 158 (94), 112 (78), 79 (100). HRMS (EI⁺): m/z calcd for C₁₂H₁₂OFBr [M]⁺: 270.0056; found: 270.0061.
• 18 General Procedure for the Aryl-Claisen Rearrangement of O-Aryl 2-Bromoallylic Ethers 7 to 8 A mixture of aryl ether 7 and Eu(fod)₃ (5 mol%) in PhMe was stirred at 120–130 °C in a sealed tube for 24 h under an atmosphere of argon. The mixture was directly purified (without removal of the solvent under reduced pressure) by flash column chromatography on silica gel (gradient: hexanes – 25% EtOAc/ hexanes) to afford phenol 8. Example Reaction of 7c (0.050 g, 0.18 mmol) using the general procedure afforded 8c (0.048 g, 96%) as a white solid. Data for Compound 8c Mp 52–53 °C (EtOAc/hexanes); R_f = 0.53 (25% EtOAc/hexanes). IR (thin film with CDCl₃): ν_max = 3468 (br), 2928, 2860, 1644, 1620, 1597, 1504, 1434, 1332, 1260, 1172 cm^–1. ¹H NMR (400 MHz, CDCl₃): δ = 6.89 (1 H, dd, J = 9.5, 3.0 Hz), 6.82 (1 H, ddd, J = 8.5, 8.0, 3.0 Hz), 6.70 (1 H, dd, J = 8.5, 4.5 Hz), 6.40 (1 H, ddd, J = 4.0, 4.0, 1.5 Hz), 4.81 (1 H, br s), 4.02–3.99 (1 H, m), 2.26–2.13 (2 H, m), 2.11–2.03 (1 H, m), 1.90–1.83 (1 H, m), 1.63–1.52 (2 H, m). ¹³C NMR (100 MHz, CDCl₃): δ = 157.4 (d, ¹ J _CF = 237.5 Hz), 149.3 (d, ⁴ J _CF = 2.5 Hz), 133.2, 131.0 (d, ³ J _CF = 6.5 Hz), 123.5, 116.6 (d, ³ J _CF = 8.0 Hz), 116.4 (d, ² J _CF = 24.0 Hz), 114.1 (d, ² J _CF = 23.0 Hz), 44.0, 31.1, 27.9, 18.0. ¹⁹F NMR (376 MHz, CDCl₃): δ = –123.9 (ddd, J _FH = 8.5, 8.5, 4.5 Hz). LRMS (EI⁺): m/z (rel. intensity) = 272 (5), 270 (6) [M]⁺, 191 (39), 163 (29), 149 (16), 133 (11), 125 (13), 109 (10), 86 (73), 84 (100). HRMS (EI⁺): m/z calcd for C₁₂H₁₂OBrF [M]⁺: 270.0056; found: 270.0059.
• 19 CCDC 1558909 contains the supplementary crystallographic data for this paper. The data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/getstructures.
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