Clean and Simple Chemoselective Reduction of Imines to Amines Using Boric Acid-Activated Sodium Borohydride under Solvent-Free Conditions

Byung Tae Cho; Sang Kyu Kang

doi:10.1055/s-2004-829066

LETTER

Clean and Simple Chemoselective Reduction of Imines to Amines Using Boric Acid-Activated Sodium Borohydride under Solvent-Free Conditions

Byung Tae Cho*, Sang Kyu Kang
Department of Chemistry, Hallym University, Chuncheon, Kangwondo 200-702, Republic of Korea
Fax: +82(33)2518491; e-Mail: btcho@hallym.ac.kr;

Abstract

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Abstract

The first clean and highly effective solvent-free chemoselective reduction of functionalized aldimines and ketimines bearing easily reducible functional groups, such as ketone, carboxylic acid, ester, nitrile, amide, nitro, furyl and alkenyl groups, to the corresponding amines using boric acid-activated sodium borohydride is described.

Key words

imines - reductions - chemoselectivity - sodium borohydride - solvent-free reaction

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References

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General Procedure for Chemoselective Reduction of Aldimines and Ketimines. A mixture of imine derivatives 1 (10 mmol), NaBH₄ (10 mmol) and boric acid (10 mmol) was ground with an agate mortar and pestle for 0.5-1.0 h until TLC showed complete disappearance of the starting material. The mixture was quenched with a sat. aq solution of NaHCO₃, followed by filtration of the resultant suspension to give product amines 2. When the product was liquid, it was isolated from extraction with CH₂Cl₂ or Et₂O instead of filtration. N -Phenyl-4-acetylbenzylamine ( 2a): Yield: 98%; white solid; mp 74-75 °C. IR (KBr): 3379, 3024, 2994, 2838, 1663, 1602, 1507, 1273, 749, 691 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.58 (s, 3 H), 4.17 (br s, 1 H), 4.40 (s, 2 H), 6.58 (d, J = 8.80 Hz, 2 H), 6.70 (t, J = 7.29 Hz, 1 H), 7.14 (t, J = 7.56 Hz, 2 H), 7.43 (d, J = 7.98 Hz, 2 H), 7.90 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 27.0, 48.2, 113.1, 118.1, 121.1, 127.5, 128.9, 129.1, 129.5, 136.3, 145.4, 147.9, 197.8. Anal. Calcd for C₁₅H₁₅NO: C, 79.97; H, 6.71; N, 6.22. Found: C, 79.96; H, 6.73; N, 6.22. N -Phenyl-4-carboxybenzylamine ( 2b): Yield: 99%; white solid; mp 196-198 °C. IR (KBr): 3412, 3360, 3050, 2933, 2836, 1684, 1603, 1317, 1291, 760 cm^-1. ¹H NMR (300 MHz, DMSO-d ₆): δ = 3.64 (br s, 1 H), 4.31 (s, 2 H), 6.35 (br s, 1 H), 6.48 (t, J = 7.15 Hz, 1 H), 6.52 (d, J = 7.43 Hz, 2 H), 7.00 (t, J = 7.84 Hz, 2 H), 7.42 (d, J = 7.98 Hz, 2 H), 7.87 (d, J = 7.98 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 46.9, 112.9, 116.5, 127.7, 129.5, 130.0, 130.4, 146.2, 149.0, 168.2. Anal. Calcd for C₁₄H₁₃NO₂: C, 73.99; H, 5.77; N, 6.16. Found: C, 73.98; H, 5.75; N, 6.18. N -Phenyl-4-methoxycarbonylbenzylamine ( 2c): Yield: 98%; oil. IR (neat): 3415, 3024, 2951, 2840, 1698, 1616, 1521, 1123, 763, 698 cm^-1. ¹H NMR (300 MHz, CDCl₃):
δ = 3.87 (s, 3 H), 4.15 (br s, 1 H), 4.35 (s, 2 H), 6.56 (d, J = 7.70 Hz, 2 H), 6.69 (t, J = 7.43 Hz, 1 H), 7.13 (t, J = 7.84 Hz, 2 H), 7.38 (d, J = 7.98 Hz, 2 H), 7.96 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 48.3, 52.4, 113.1, 118.0, 127.3, 129.2, 129.5, 130.1, 145.2, 148.0, 167.1. Anal. Calcd for C₁₅H₁₅NO₂: C, 74.67; H, 6.27; N, 5.81. Found: C, 74.70; H, 6.37; N, 5.86. N -Methyl-4-methoxycarbonylbenzyl-amine ( 2d): Yield: 99%; white solid; mp 152-153 °C. IR (KBr): 3236, 3018, 1004, 2952, 1704, 1428, 1286, 1168, 894 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.42 (d, J = 5.78 Hz, 3 H), 3.59 (dd, J = 9.77, 13.62 Hz, 1 H), 3.93 (s, 3 H), 4.30 (dd, J = 2.89, 13.89 Hz, 1 H), 4.39 (br s, 1 H), 7.39 (d, J = 8.25 Hz, 2 H), 8.05 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 40.8, 60.7, 129.7, 130.5, 130.8, 166.6. Anal. Calcd for C₁₀H₁₃NO₂: C, 67.02; H, 7.31; N, 7.82. Found: C, 67.01; H, 7.33; N, 7.83. N - tert -Butyl-4-methoxycarbonyl-benzylamine ( 2e): Yield: 98%; white solid; mp 30-31 °C. IR (KBr): 3316, 3187, 2961, 2877, 1716, 1437, 1281, 1110, 1019, 769 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 1.18 (s, 9 H), 1.43 (s, 1 H), 3.77 (s, 2 H), 3.90 (s, 3 H), 7.40 (d, J = 7.98 Hz, 2 H), 7.96 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 29.5, 47.3, 51.2, 52.5, 128.3, 128.7, 129.9, 147.1, 167.3. Anal. Calcd for C₁₃H₁₉NO₂: C, 70.56; H, 8.65; N, 6.33. Found: C, 70.58; H, 8.67; N, 6.33. N -Phenyl-4-cyanobenzylamine ( 2f): Yield: 99%; mp 83-84 °C. IR (KBr): 3425, 3047, 2911, 2864, 2220, 1600, 1507, 1332, 1266, 818, 765, 697 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 4.20 (br s, 1 H), 4.40 (s, 2 H), 6.55 (d, J = 7.70 Hz, 2 H), 6.71 (t, J = 7.29 Hz, 1 H), 7.14 (t, J = 7.01 Hz, 2 H), 7.44 (d, J = 7.98 Hz, 2 H), 7.58 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 48.1, 111.1, 113.1, 118.3, 119.1, 127.9, 129.6, 132.6, 145.6, 147.6. Anal. Calcd for C₁₄H₁₂N₂: C, 80.74; H, 5.81; N, 13.45. Found: C, 80.76; H, 5.78; N, 13.46. N -Methyl-4-cyanobenzylamine ( 2g): Yield: 98%; white solid; mp 108-109 °C. IR (KBr): 3158, 3013, 2854, 2260, 1520, 1350, 1169, 853 cm^-1. ¹H NMR (300 MHz, CDCl₃):
δ = 2.43 (d, J = 5.50 Hz, 3 H), 3.64 (dd, J = 7.35, 13.75 Hz, 1 H), 4.26 (dd, J = 3.58, 13.75 Hz, 1 H), 4.68 (br s, 1 H), 7.47 (d, J = 8.25 Hz, 2 H), 7.70 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 41.0, 60.4, 112.9, 118.6, 130.6, 133.0, 139.1. Anal. Calcd for C₉H₁₀N₂: C, 73.94; H, 6.89; N, 19.16. Found: C, 73.99; H, 6.95; N, 19.17. N - n -Heptyl-4-cyanobenzylamine ( 2h): Yield: 98%, white solid; mp 63-64 °C. IR (KBr): 3193, 2955, 2926, 2856, 2228, 1457, 1160, 897, 854 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 0.86 (t, J = 6.60 Hz, 3 H), 1.13-1.27 (m, 6 H), 1.58-1.74 (m, 4 H), 2.61-2.67 (m, 2 H), 3.77 (dd, J =8.12, 13.34 Hz, 1 H), 4.08 (br s, 1 H), 4.17 (dd, J = 4.26, 13.34 Hz, 1 H), 7.46 (d, J = 8.25 Hz, 2 H), 7.69 (d, J = 8.53 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 14.5, 22.9, 26.6, 27.0, 29.2, 31.9, 54.3, 59.2, 112.9, 118.5, 130.6, 132.9, 139.4. Anal. Calcd for C₁₅H₂₂N₂: C, 78.21; H, 9.63; N, 12.16. Found: C, 78.23; H, 9.64; N, 12.17. N- 2-Furfuryl-4-cyanobenzyl-amine ( 2i): Yield: 97%; oil. IR (neat): 3203, 2962, 2927, 2852, 2230, 1504, 1167, 1010, 821, 739 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 3.72 (dd, J = 8.94, 14.99 Hz, 1 H), 3.83 (d, J = 5.50 Hz, 2 H), 3.87 (br s, 1 H), 4.10 (dd, J = 3.30, 14.30 Hz, 1 H), 6.30-6.34 (m, 2 H), 7.25 (d, J = 8.53 Hz, 2 H), 7.35 (m, 1 H), 7.55 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 51.9, 57.4, 111.3, 111.9, 112.8, 118.6, 130.8, 132.7, 139.0, 143.7, 147.5. Anal. Calcd for C₁₃H₁₂N₂O: C, 73.56; H, 5.70; N, 13.20. Found: C, 73.55; H, 5.69; N, 13.22. N -Phenyl-4-acetamido-benzylamine ( 2j): Yield: 98%; oil. IR (neat): 3426, 3301, 3193, 3050, 2931, 2840, 1662, 1601, 1511, 1315, 745, 689 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.12 (s, 3 H), 4.03 (br s, 1 H), 4.24 (s, 2 H), 6.58 (dd, J = 1.10, 8.53 Hz, 2 H), 6.68 (t, J = 7.30 Hz, 1 H), 7.13 (dd, J = 7.43, 8.53 Hz, 2 H), 7.25 (d, J = 8.25 Hz, 2 H), 7.42 (d, J = 8.25 Hz, 2 H), 7.76 (br s, 1 H). ¹³C NMR (75 MHz, CDCl₃): δ = 24.9, 48.1, 113.1, 117.8, 120.6, 128.2, 129.5, 135.6, 137.1, 148.2, 168.9. Anal. Calcd for C₁₅H₁₆N₂O: C, 74.97; H, 6.71; N, 11.66. Found: C, 74.95; H, 6.75; N, 11.68. N -Phenyl-4-nitrobenzyamine ( 2k): Yield: 99%; oil. IR (neat): 3423, 3052, 2923, 2843, 1614, 1506, 1334, 761, 701 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 4.27 (br s, 1 H), 4.43 (s, 2 H), 6.54 (d, J = 8.53 Hz, 2 H), 6.70 (t, J = 7.29 Hz, 1 H), 7.13 (t, J = 7.70 Hz, 2 H), 7.47 (d, J = 8.25 Hz, 2 H), 8.12 (d, J = 8.44 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 47.9, 113.1, 118.3, 124.1, 127.9, 129.6, 147.2, 147.5, 147.8. Anal. Calcd for C₁₃H₁₂N₂O₂: C, 68.41; H, 5.30; N, 12.27. Found: C, 68.43; H, 5.32; N, 12.25. N -Methyl-4-nitrobenzyamine ( 2l): Yield: 99%; yellow solid; mp 111-112 °C. IR (KBr): 3200, 3069, 2999, 2951, 1465, 1290, 1164, 894 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.47 (d, J = 5.78 Hz, 3 H), 3.72 (dd, J = 9.35, 13.75 Hz, 1 H), 4.30 (dd, J = 3.85, 13.75 Hz, 1 H), 4.71 (br s, 1 H), 7.54 (d, J = 8.53 Hz, 2 H), 8.25 (d, J = 8.80 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 41.1, 60.1, 124.4, 130.9, 140.9, 148.2. Anal. Calcd for C₈H₁₀N₂O₂: C, 57.82; H, 6.07; N, 16.86. Found: C, 57.84; H, 6.10; N, 16.87. N -Phenyl- trans -cinnamylamine ( 2m): Yield: 98%; oil. IR (neat): 3416, 3023, 2925, 2833, 1601, 1505, 1320, 1251, 747, 693 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 3.77 (br s, 1 H), 3.88 (dd, J = 1.22, 5.49 Hz, 2 H), 6.29 (m, 1 H), 6.55-6.75 (m, 4 H), 7.13-7.37 (m, 7 H). ¹³C NMR (75 MHz, CDCl₃): δ = 46.8, 113.7, 118.2, 127.0, 127.7, 128.2, 129.2, 129.9, 132.1, 137.5, 148.7. Anal. Calcd for C₁₅H₁₅N: C, 86.08; H, 7.22; N, 6.69. Found: C, 86.09; H, 6.67; N, 6.68. N -Phenyl-α-methyl-(4-cyanobenzyl)amine ( 2n): Yield: 98%; mp 99-101 °C. IR (KBr): 3373, 3029, 2962, 2864, 2228, 1605, 1512, 1497, 1324, 1259, 832, 754, 693 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 1.51 (d, J = 6.60 Hz, 3 H), 4.07 (br s, 1 H), 4.49 (q, J = 6.79 Hz, 1 H), 6.41 (d, J = 7.70 Hz, 2 H), 6.66 (t, J = 7.43 Hz, 1 H), 7.07 (dd, J = 7.56, 8.39 Hz, 2 H), 7.46 (d, J = 8.53 Hz, 2 H), 7.58 (d, J = 8.25 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 25.4, 53.7, 101.9, 113.4, 118.0, 119.2, 126.8, 129.4, 132.8, 146.7, 151.2. Anal. Calcd for C₁₅H₁₄N₂: C, 81.05; H, 6.35; N, 12.60. Found: C, 81.07; H, 6.37; N, 12.65. N -Phenyl-α-methyl-(4-nitro-benzyl)amine ( 2o): Yield: 97%; oil. IR (neat): 3409, 3052, 2971, 2927, 2869, 1614, 1588, 1450, 1013, 744, 702 cm^-1. ¹H NMR (200 MHz, CDCl₃): δ = 1.54 (d, J = 6.72 Hz, 3 H), 4.09 (br s, 1 H), 4.57 (q, J = 6.72 Hz, 1 H), 6.44 (d, J = 7.63 Hz, 2 H), 6.68 (t, J = 7.33 Hz, 1 H), 7.10 (t, J = 7.94 Hz, 2 H), 7.55 (d, J = 8.85 Hz, 2 H), 8.18 (d, J = 8.85 Hz, 2 H). ¹³C NMR (50 MHz, CDCl₃): δ = 25.6, 54.0, 114.0, 118.7, 124.8, 127.4, 129.9, 147.2, 147.8, 153.9. Anal. Calcd for C₁₄H₁₄N₂O₂: C, 69.41; H, 5.82; N, 11.56. Found: C, 69.43; H, 5.82; N, 11.55. N -Phenyl-2-(1′-cyclohexenyl)ethyl-amine ( 2p): Yield: 97%; oil. IR (neat): 3410, 3050, 2925, 2855, 1602, 1504, 1319, 749, 691 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 1.28 (d, J = 6.88 Hz, 3 H), 1.48-1.64 (m, 4 H), 1.94-1.99 (m, 4 H), 3.75 (q, J = 6.60 Hz, 1 H), 3.76 (m, 1 H), 5.66 (s, 1 H); 6.54 (d, J = 7.43 Hz, 2 H), 6.63 (t, J = 7.29 Hz, 1 H), 7.11 (dd, J = 7.15, 8.53 Hz, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 21.7, 23.1, 23.2, 24.7, 25.4, 55.1, 113.3, 117.0, 121.8, 129.2, 139.6, 147.9. Anal. Calcd for C₁₄H₁₉N: C, 83.53; H, 9.51; N, 6.96. Found: C, 83.52; H, 9.54; N, 6.98.