An Efficient Organocatalyzed Interconversion of Silyl Ethers to Tosylates Using DBU and p-Toluenesulfonyl Fluoride

Abstract

A mild and efficient interconversion from silyl ethers to sulfonates esters is reported with good yields. This silyl-sulfonyl exchange proceeds readily in acetonitrile at room temperature in the presence of p-toluenesulfonyl fluoride and a catalytic amount of 1,8-diazabicyclo[5.4.0]undec-7ene (DBU). This method can be used with trimethysilyl (TMS), triethylsilyl (TES) and tert-butyldimethylsilyl (TBDMS) ethers.

Key words

organocatalysis - sulfonate ester - silyl ether - DBU - p-toluenesulfonyl fluoride

References and Notes

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• 1b Kim J.-G. Jang DO. Synlett  2007,  2501 
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• 1d Comagic S. Schirrmacher R. Synthesis  2004,  883 
• Selected recent references:
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General Procedure for the Preparation of Silyl Ethers 1a-m
To a solution of 3-phenylpropanol (139 mg, 1 mmol) and Et₃N (120 mg, 1.1 mmol) in CH₂Cl₂ (5 mL) was added TMSCl (115 mg, 1.05 mmol). The mixture was stirred at r.t. overnight. The solvent was removed under reduced pressure and the residue was diluted with pentane (10 mL). Simple filtration through a short pad of Celite^® provided silyl ether 1a in 93% yield which could be used in the silyl ether exchange reaction without further purification.

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DBU (Sigma-Aldrich) and p-toluenesulfonyl fluoride 97% (Acros Organics) were used as received.

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General Procedure for the Interconversion of Silyl Ethers 1a-m
To a solution of TMS ether 1a (209 mg, 1 mmol) and TsF (179 mg, 1 mmol) in MeCN (2 mL) was added DBU (30 µL, 0.2 mmol). The mixture was stirred for 4 h at r.t. followed by addition of H₂O (4 mL). The aqueous phase was extracted with EtOAc (3 × 10 mL). The combined organic layers were washed with H₂O (10 mL) and brine (10 mL). After drying (MgSO₄) and concentration under vacuum, the residue was chromatographed on SiO₂ (cyclohexane-Et₂O, 9:1) affording tosylate 2a (278 mg, 96% yield).

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Spectral Data for Tosylate 2h
¹H NMR (300 MHz, CDCl₃): δ = 7.77 (d, 2 H, J = 8.3 Hz), 7.51 (d, 1 H, J = 7.5 Hz), 7.36 (d, 2 H, 8.3 Hz), 7.29 (t, 1 H, J = 7.5 Hz), 7.21 (t, 1 H, J = 7.5 Hz), 6.86 (d, 1 H, J = 7.5 Hz), 4.58 (s, 2 H), 2.61 (s, 1 H), 2.48 (s, 3 H). ¹³C (75 MHz, CDCl₃): δ = 146.9, 145.9, 134.6, 132.2, 130.3, 130.0, 128.8, 128.5, 127.6, 122.3, 59.6, 21.8. IR (KBr): ν_max = 3392, 1597, 1487, 1453, 1371, 1192, 1179, 1156, 1089, 1040.
Spectral Data for Tosylate 2d
¹H NMR (300 MHz, CDCl₃): δ = 7.66 (d, 2 H, J = 8.3 Hz), 7.35-7.17 (m, 7 H), 5.20-5.18 (m, 1 H), 4.93-4.89 (m, 1 H), 4.28-4.15 (m, 2 H), 2.43 (s, 3 H), 1.41 (s, 9 H). ¹³C (75 MHz, CDCl₃): δ = 155.0, 145.0, 137.8, 132.4, 129.9, 128.8, 128.0, 127.9, 126.6, 80.1, 71.6, 28.3, 21.7. IR (KBr): ν_max = 3383, 1690, 1525, 1361, 1172, 1097, 1052, 964. Mp 123-124 °C.
Spectral Data for Tosylate 2i
¹H NMR (300 MHz, CDCl₃): δ = 9.13 (d, 1 H, J = 6.0 Hz), 8.22 (t, 1 H, J = 7.7 Hz), 7.80 (d, 1 H, J = 7.7 Hz), 7.72 (t, 1 H, J = 6.9 Hz), 7.59 (d, 2 H, J = 8.1 Hz), 7.04 (d, 2 H, J = 8.1 Hz), 4.96 (t, 2 H, J = 7.7 Hz), 3.44 (t, 2 H, J = 7.7 Hz), 2.43-2.35 (m, 2 H), 2.27 (s, 3 H). ¹³C (75 MHz, CDCl₃): δ = 158.4, 144.9, 144.1, 142.4, 139.2, 128.7, 126.0, 125.9, 124.6, 59.4, 32.5, 21.7, 21.4. IR (KBr): ν_max = 3434, 1628, 1505, 1190, 1129, 1039, 1012, 812, 691, 571. Mp 120-121 °C. All other synthesized compounds are in accordance with the literature data.

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Polystyrene-bound DBU (1.15 mmol/g loading, 1% cross-linked with DVB) was purchased from Aldrich.