Phosphoric Acid Bridged Cobalt Bis(dicarbollide) Ion as a Highly Efficient Catalyst for the Organocatalytic Hydrogenation of Ketimines

Abstract

The high-yield reduction of aromatic ketimines into amines by using a novel catalyst based on a metallacarborane structure, 8,8′-μ-phosphate[(1,2-dicarba-closo-undecaborane)-3,3′-cobalt(-1)(1′,2′-dicarba-closo-undecaborane)] acid, is described.

Key words

metallacarboranes - hydrogenation - catalysis - reduction - imines - amines

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• 24 Preparation of 4a–k; General Procedure: All the reaction flasks were dried by flame, and all reactions were carried out under N₂. All the solvents were distilled under nitrogen and stored over 4 Å MS prior to use. Thin-layer chromatography was performed on Merck 60 F₂₅₄ silica gel plates and visualization was accomplished by irradiation with UV light or by treatment with a solution of phosphomolybdic acid solution followed by heating. Ketimine 3a–k (13.0 mg, 0.05 mmol), 1(HNEt₃ ⁺)(H⁺) (1.1 mg, 0.0025 mmol), and Hantzsch ester (2; 0.07 mmol, 1.4 equiv) were mixed in benzene (1 mL) in a flame-dried test tube containing dried molecular sieves (5 Å, 50 mg) under a nitrogen atmosphere at 50 °C for 20 h (not optimized). The resulting mixture was filtered through Celite (washed with CH₂Cl₂) and then evaporated under reduced pressure. The residue was purified by preparative silica gel thin-layer chromatography (hexane–EtOAc, 5:1 v/v).
• 25 NMR characteristics of 4a–k: NMR spectra were recorded with a Unity Inova-400 instrument (Varian Ltd., 400 MHz for ¹H, 100 MHz for ¹³C) using CDCl₃ as solvent. Chemical shifts (δ) for ¹H were referenced to tetramethylsilane (δ = 0.00 ppm) as an internal standard. Chemical shifts (δ) for ¹³C were referenced to the solvent signal (CDCl₃; δ = 77.00 ppm). N-(4-Methoxyphenyl)-1-(4-chlorophenyl)ethylamine (4a): ²⁷ Yield: 94%; pale-yellow oil.¹H NMR (400 MHz, CDCl₃): δ = 1.47 (d, J = 6.8 Hz, 3 H), 3.69 (s, 3 H), 4.37 (q, J = 6.8 Hz, 1 H), 6.42–6.46 (m, 2 H), 6.66–6.71 (m, 2 H), 7.24–7.32 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.1, 53.8, 55.7, 114.6, 114.7, 127.3, 128.7, 132.3, 141.1, 144.0, 152.1. N-(4-Methoxyphenyl)-1-phenylethylamine (4b): ²⁷ Yield: 94%; pale-yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.50 (d, J = 6.8 Hz, 3 H), 3.69 (s, 3 H), 4.41 (q, J = 6.8 Hz, 1 H), 6.45–6.50 (m, 2 H), 6.66–6.71 (m, 2 H), 7.19–7.24 (m, 1 H), 7.29–7.38 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.0, 54.4, 55.7, 114.7, 114.7, 125.9, 126.8, 128.6, 141.3, 145.3, 152.0. N-(4-Methoxyphenyl)-1-(4-methylphenyl)ethylamine (4c): ²⁷ Yield: 96%; colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.47 (d, J = 6.8 Hz, 3 H), 2.32 (s, 3 H), 3.69 (s, 3 H), 4.37 (q, J = 6.8 Hz, 1 H), 6.45–6.50 (m, 2 H), 6.66–6.71 (m, 2 H), 7.11 (d, J = 8.0 Hz, 2 H), 7.24 (d, J = 8.0 Hz, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.0, 25.1, 53.9, 55.7, 114.5, 114.7, 125.8, 129.2, 136.3, 141.6, 142.4, 151.8. N-(4-Methoxyphenyl)-1-(4-methoxyphenyl)ethylamine (4d): ²⁷ Yield: 75%; colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.47 (d, J = 6.8 Hz, 3 H), 3.69 (s, 3 H), 3.78 (s, 3 H), 4.36 (q, J = 6.8 Hz, 1 H), 6.46–6.51 (m, 2 H), 6.67–6.71 (m, 2 H), 6.83–6.87 (m, 2 H), 7.25–7.29 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.0, 53.6, 55.2, 55.7, 113.9, 114.6, 114.7, 126.9, 137.4, 141.6, 151.8, 158.4. N-(4-Methoxyphenyl)-1-naphthaylethylamine (4e): ²⁷ Yield: 88%; off-white sticky oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.57 (d, J = 6.8 Hz, 3 H), 3.67 (s, 3 H), 4.56 (q, J = 6.8 Hz, 1 H), 6.50–6.56 (m, 2 H), 6.64–6.70 (m, 2 H), 7.40–7.47 (m, 2 H), 7.48–7.52 (m, 1 H), 7.78–7.82 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.1, 54.5, 54.5, 55.7, 114.8, 124.3, 124.4, 125.5, 125.9, 127.6, 127.8, 128.4, 132.7, 133.6, 141.4, 142.9, 152.0. N-Phenyl-1-(4-chlorophenyl)ethylamine (4f): ²⁸ Yield: 97%; colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.46 (d, J = 6.7 Hz, 3 H), 4.38 (q, J = 2.2 Hz, 1 H), 6.45 (d, J = 7.6 Hz, 2 H), 6.63 (t, J = 7.4 Hz, 1 H), 7.06–7.12 (m, 2 H), 7.21–7.30 (m, 4 H), 7.48–7.52 (m, 1 H), 7.78–7.82 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 25.4, 53.2, 113.4, 117.3, 129.0, 129.3, 132.6, 144.1, 147.2. N-(Phenyl)-1-phenylethylamine (4g): ²⁷ Yield: quant.; colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.51 (d, J = 6.8 Hz, 3 H), 4.10 (br s, 1 H), 4.47 (q, J = 6.8 Hz, 1 H), 6.50 (d, J = 8.4 Hz, 2 H), 6.62–6.68 (m, 1 H), 7.06–7.12 (m, 2 H), 7.19–7.26 (m, 1 H), 7.28–7.40 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 24.9, 53.5, 113.3, 117.2, 125.8, 126.8, 128.6, 129.1, 145.1, 147.2. N-Benzyl-4-methoxybenzenamine (4h): ^29a,b Yield: 98%; colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 3.74 (s, 3 H), 4.28 (s, 2 H), 6.60 (d, J = 7.6 Hz, 2 H), 6.77 (d, J = 7.6 Hz, 2 H), 7.24–7.38 (m, 5 H). ¹³C NMR (100 MHz, CDCl₃): δ = 49.5, 56.0, 114.3, 115.1, 127.4, 127.8, 128.8, 139.9, 142.6, 152.4. N-Benzyl-4-chlorobenzenamine (4i): ³⁰ Yield: 95%; white solid. ¹H NMR (400 MHz, CDCl₃): δ = 4.14 (br s, 1 H), 4.31 (s, 2 H), 6.54–6.59 (m, 2 H), 7.10–7.15 (m, 2 H), 7.27–7.34 (m, 1 H), 7.34–7.39 (m, 4 H). ¹³C NMR (100 MHz, CDCl₃): δ = 48.3, 113.9, 122.1, 127.3, 127.4, 128.7, 129.0, 138.8, 146.5. N-(3,4,5-Trimethoxyphenyl)-1-phenylethylamine (4j): ³¹ Yield: 99%; pale-yellow oil. ¹H NMR (400 MHz, CDCl₃): δ = 1.51 (d, J = 6.8 Hz, 3 H), 3.67 (s, 6 H), 3.71 (s, 3 H), 4.03 (br s, 1 H), 4.42 (q, J = 6.8 Hz, 1 H), 5.75 (s, 2 H), 7.19–7.25 (m, 1 H), 7.29–7.35 (m, 2 H), 7.35–7.39 (m, 2 H). ¹³C NMR (100 MHz, CDCl₃): δ = 24.9, 54.1, 55.6, 60.9, 90.8, 125.7, 126.9, 128.6, 129.7, 144.0, 145.3, 153.6. Methyl (4-Chlorophenylamino)phenylacetate (4k): ³² Yield: 78%; colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 3.73 (s, 3 H), 5.03 (s, 1 H), 6.47 (d, J = 8.8 Hz, 2 H), 7.06 (d, J = 8.8 Hz, 2 H), 7.31–7.37 (m, 3 H), 7.45–7.47 (m, 2 H).¹³C NMR (100 MHz, CDCl₃): δ = 52.9, 60.7, 114.5, 122.8, 127.2, 128.5, 128.9, 129.1, 137.7, 144.3, 172.0.
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