Deprotection of Acetals and Ketals in a Colloidal Suspension Generated by ­Sodium Tetrakis(3,5-trifluoromethylphenyl)borate in Water

Abstract

Deprotection of acetals and ketals can be achieved by ­using sodium tetrakis(3,5-trifluoromethylphenyl)borate (NaBAr^F ₄) as the catalyst in water at 30 °C. For example, a quantitative con­version of 2-phenyl-1,3-dioxolane into benzaldehyde was accomplished within five minutes by using this sodium salt (0.1 mol%) in water.

Key words

acetals - phase-transfer catalysis - hydrolysis - sodium - protecting groups

References and Notes

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• Recent reviews:
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• Acetals 1, 6, 7, 8, 9, 10, 11, 15, 17, 18 and 19 were synthesized according to the literature methods. Other acetals were obtained from commercial sources:
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13

Typical Procedure for Hydrolysis: A mixture of NaBAr^F ₄×2H₂O (5 ¥ 10^-3 mmol) [11] and acetal (5.0 mmol) in H₂O (10 mL) was placed in a 25-mL flask. The resulting suspension was stirred vigorously at 30 °C for a certain period as shown in Table [2] . After the completion of the reaction, the reaction mixture was extracted with CH₂Cl₂ (2 ¥ 5 mL). The extracts were dried and filtered through a short column of silica gel to remove the sodium salt. Upon concentration, the desired carbonyl products were obtained in pure form. For all products reported in Table [2] , full ¹H and ¹³C NMR data were compared with those of the pure sample obtained from commercial sources, or with reported data. [25]

14

Spectral Data for the Hydrolyzed Products:
1,2- O -Isopropylidene-α-d-glucofuranose: [18] mp 154-158 °C (dec.).¹H NMR (400 MHz, D₂O): δ = 5.90 (d, J = 4 Hz, 1 H), 4.58 (d, J = 4 Hz, 1 H), 4.20 (s, 1 H), 3.97 (d, J = 8 Hz, 1 H), 3.78 (m, 1 H), 3.68 (d, J = 12 Hz, 1 H), 3.97 (d, J = 8 Hz, 1 H), 3.52 (dd, J = 6 Hz, 1 H), 1.39 (s, 3 H), 1.24 (s, 3 H). ¹³C NMR (100 MHz, D₂O): δ = 112.5 (C), 104.6 (CH), 84.3 (CH), 79.6 (CH), 78.6 (CH), 68.2 (CH), 63.4 (CH₂), 25.4 (CH₃), 25.0 (CH₃).
4-Propylbenzaldehyde: [21] ¹H NMR (400 MHz, CDCl₃): δ = 9.98 (s, 1 H), 7.81 (d, J = 6.8 Hz, 2 H), 7.34 (d, J = 6.8 Hz, 2 H), 2.68 (t, J = 7 Hz, 2 H), 1.68 (m, 2 H), 0.96 (t, J = 7 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 191.64 (CH), 150.1 (C), 134.4 (C), 129.9 (CH), 129.2 (CH), 39.0 (CH₂), 26.2 (CH₂), 14.2 (CH₃).
4-( tert -Butyldimethylsilyloxymethyl)benzaldehyde: [22] ¹H NMR (300 MHz, CDCl₃): δ = 10.01 (s, 1 H), 7.85 (d, J = 8 Hz, 2 H), 7.49 (d, J = 8 Hz, 2 H), 4.82 (s, 2 H), 0.96 (s, 9 H), 0.13 (s, 6 H). ¹³C NMR (100 MHz, CDCl₃): δ = 191.6 (CH), 148.6 (C), 135.3 (C), 129.9 (CH), 126.3 (CH), 65.0 (CH₂), 26.7 (CH₃), 19.3 (C), -4.3 (CH₃).
4-Hydroxymethylbenzaldehyde: [23] ¹H NMR (400 MHz, CDCl₃): δ = 9.96 (s, 1 H), 7.84 (d, J = 6.8 Hz, 2 H), 7.50 (d, J = 6.8 Hz, 2 H), 4.78 (s, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 191.7 (CH), 147.7 (C), 135.6 (C), 130.1 (CH), 127.0 (CH), 65.1 (CH₂).
3-Ethoxypropionaldehyde: [24] ¹H NMR (400 MHz, CDCl₃): δ = 9.78 (t, J = 2 Hz, 1 H), 3.76 (t, J = 6.4 Hz, 2 H), 3.50 (q, J = 6.8 Hz, 2 H), 2.67 (td, J = 1.6, 6 Hz, 2 H), 1.20 (t, J = 6.4 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 200.9 (CH), 67.1 (CH₂), 64.6 (CH₂), 64.6 (CH₂), 44.6 (CH₂), 16.0 (CH₃).
Methyl 4-Formylbenzoate: [25] ¹H NMR (400 MHz, CDCl₃): δ = 10.09 (s, 1 H), 8.18 (d, J = 8 Hz, 2 H), 7.94 (d, J = 8 Hz, 2 H), 3.95 (s, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 191.6 (CH), 166.0 (C), 139.1 (C), 135.1 (C), 130.2 (CH), 129.5 (CH), 52.6 (CH₃).

16

Preparation of 20: A solution of 17 (0.32 g, 2 mmol) and Et₃N (0.38 mL, 2.8 mmol) in anhyd CH₂Cl₂ (5 mL) was cooled in an ice-water bath under dried nitrogen. Chlorotrimethylsilane (0.31 mL, 2.4 mmol) was slowly added to the above solution with stirring. After the addition, the resulting mixture was heated to reflux for 6 h. The mixture was filtered to remove the ammonium salt and the filtrate was evaporated under reduced pressure to remove the excess amine and chlorotrimethylsilane. The residue was dissolved in CH₂Cl₂ (10 mL) and washed with sat. NaHCO₃. The organic extract was dried over MgSO₄ and concentrated to give the desired compound 20 (0.46 g, 100%). ¹H NMR (400 MHz, CDCl₃): δ = 3.64 (d, J = 12 Hz, 2 H), 3.55 (s, 2 H), 3.52 (d, J = 12 Hz, 2 H), 1.41 (s, 3 H), 1.37 (s, 3 H), 0.79 (s, 3 H), 0.09 (s, 9 H). ¹³C NMR (100 MHz, CDCl₃): δ = 97.5 (C), 66.2 (CH₂), 65.2 (CH₂), 35.0 (C), 27.0 (CH₃), 21.2 (CH₃), 18.3 (CH₃), -0.1 (CH₃). Anal. Calcd for C₁₁H₂₄O₃Si: C, 56.85; H, 10.41. Found: C, 56.77; H, 10.21.