An Efficient Deprotection of 2,6-Bis(trifluoromethyl)phenylboronic Esters via Catalytic Protodeboronation Using Tetrabutyl­ammonium Fluoride

Abstract

We herein describe an efficient deprotection of 2,6-bis(trifluoromethyl)phenylboronic esters, which serve as effective protective groups for 1,2- or 1,3-diols in various organic transformations, via protodeboronation by using a catalytic amount of tetrabutylammonium fluoride (TBAF).

Key words

protective groups - diols - deprotection - 2,6-bis(trifluoromethyl)phenyl boronic esters - tetrabutylammonium fluoride - protodeboronation - catalysis

• References and Notes

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• 18 TBAF (ca. 1 mol/L in tetrahydrofuran including maximum 10% of water) was purchased from Tokyo Chemical Industry Co., Ltd and used.
• 19 In this study, maximun 6.7 equiv of water are included when 120 mol% of TBAF were used.
• 20 The only byproduct of this reaction is 1,3-bis(trifluoromethyl)benzene (3) with low boiling point (b.p. 116 °C), which can be easily removed by evaporation during the workup procedure. Therefore, the desired diol at satisfactory level of purity was obtained by simple filtration of reaction mixture through a pad of basic amino silica gel eluting with EtOAc. See Supporting Information for experimental details.
• 21 For a report on the effect of water in reactivity of fluoride ion, see: Sun H, DiMagno SG. J. Am. Chem. Soc. 2005; 127: 2050
• 22 Deprotection of the corresponding phenylboronic ester only gave the small amount of diol 2a (16% conversion yield) after 24 h under the conditions using 10 mol% of TBAF in the presence of 3.0 equiv of water at room temperature. See the Supporting Information for details.

The Perrin and Lloyd-Jones groups independently reported that two ortho electron-withdrawing substituents on arylboronic acid accelerate base-catalyzed protodeboronation, see:
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• 24 The reaction in the presence of 3.0 equiv of water with 5 mol% of TBAF resulted in a remarkably decreased yield (53%).
• 25 Our attempt to deprotect under the optimized conditions using boronic ester derived from 4,6-dihydoroxyhexanoic acid failed, resulting in a nearly quantitative recovery of the starting material.
• 26 General Procedure for the Deprotection of the Boronic Esters with TBAF; Method A (Catalytic Conditions, Table [1], Entry 8) TBAF (0.20 M in THF, 100 μL, 0.0200 mmol, 10 mol%) and H₂O (6.0 M in THF, 100 μL, 0.600 mmol, 3.0 equiv) were added to a solution of 1a (89.2 mg, 0.200 mmol, 1.0 equiv) in dry THF (1.8 mL, total 0.10 M) at room temperature. After stirring for 2 h under reflux and cooling to room temperature, the reaction mixture was filtered through a short pad of amino silica gel (800 mg) eluting with EtOAc (20 mL), and the filtrate was concentrated under reduced pressure to give 2a (46.8 mg, 0.200 mmol, >99% yield) as a colorless oil. Analytical Data for 2a R_f = 0.13 (n-hexane/EtOAc, 4:1). ¹H NMR (400 MHz, CDCl₃): δ = 7.38–7.27 (m, 5 H), 4.53 (s, 2 H), 3.90–3.79 (m, 3 H), 3.57–3.49 (m, 2 H), 2.34 (br s, 2 H), 1.79–1.52 (m, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 137.9, 128.4, 127.8, 127.7, 73.1, 71.9, 70.5, 61.7, 38.3, 35.2, 26.2. IR (neat): ν = 3372, 2942, 2865, 1278, 1099 cm^–1. HRMS (ESI): m/z calcd for C₁₃H₂₀O₃Na [M + Na]⁺: 247.1310; found: 247.1311. Method B (Stoichiometric Conditions, Table [1], Entry 1) TBAF (1.0 M in THF, 0.24 mL, 0.240 mmol, 120 mol%) was added to a solution of 1a (89.2 mg, 0.200 mmol, 1.0 equiv) in dry THF (1.8 mL, total 0.10 M) at room temperature. After stirring for 2 h under reflux, cooling to room temperature, the reaction mixture was filtered through a short pad of amino silica gel (800 mg) eluting with EtOAc (20 mL), and the filtrate was concentrated under reduced pressure to give 2a (44.8 mg, 0.200 mmol, >99% yield) as a colorless oil.