Ruthenium(II)-Catalyzed Hydration of Terminal Alkynes in PEG-400

Abstract

The ruthenium(II)-catalyzed hydration of terminal alkynes in PEG-400 to yield methyl ketones through Markovnikov addition of water across alkyne is reported.

Key words

PEG-400 - ruthenium - methyl ketones - terminal alkynes

• References and Notes

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• 17 General Procedure for Hydration of Terminal Alkynes in PEG-400: To a solution of alkyne (1.0 mmol) in PEG-400/H₂O (4:1) was added [Ru(p-cymene)Cl₂]₂ (0.01 mmol), and the mixture stirred at room temperature. Upon completion of the reaction (monitoring by TLC), the reaction mixture was diluted with Et₂O (10 mL), stirred for 10 min, and allowed to stand in an ice-salt bath to solidify the PEG-400. The organic layer was decanted, dried over anhydrous Na₂SO₄, filtered, and concentrated under reduced pressure. The residue obtained was purified by silica gel flash column chromatography (ethyl acetate–petroleum ether) to give the pure product. 4-(Benzyloxy)butan-2-one (2a): Prepared according to the general procedure and purified by flash chromatography (EtOAc–hexanes, 10%). Yield: 63%; green liquid. ¹H NMR (400 MHz, CDCl₃): δ = 7.56–7.02 (m, 5 H), 4.51 (s, 2 H), 3.73 (t, J = 6.3 Hz, 2 H), 2.71 (t, J = 6.3 Hz, 2 H), 2.17 (s, 3 H). ¹³C NMR (126 MHz, CDCl₃): δ = 207.2, 138.1, 128.4, 127.7, 73.3, 65.3, 43.8, 30.5. IR (neat): 2925, 2855, 1721, 1458, 1275, 1110, 707 cm^–1. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₁H₁₅O₂: 179.1066; found: 179.1065. 4-Oxopentanoic Acid (2g): Prepared according to the general procedure and purified by flash chromatography (EtOAc–hexanes, 10%). Yield: 92%; colorless oil. ¹H NMR (300 MHz, CDCl₃): δ = 9.25 (s, 1 H), 2.75 (t, J = 6.4 Hz, 2 H), 2.61 (t, J = 6.4 Hz, 2 H), 2.19 (s, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 207.1, 178.5, 37.7, 29.8, 27.8. IR (neat): 2923, 2854, 1708, 1404, 1367, 1165, 555 cm^–1. HRMS (ESI): m/z [M + H]⁺ calcd for C₅H₉O₃: 117.0546; found: 117.0557. 4-Methyl-4-(3-oxobutoxy)cyclohexa-2,5-dienone (2j): Prepared according to the general procedure and purified by flash chromatography (EtOAc–hexanes, 50%). Yield: 78%; dark-green oil. ¹H NMR (300 MHz, CDCl₃): δ = 6.72 (d, J = 10.2 Hz, 2 H), 6.23 (d, J = 10.2 Hz, 2 H), 3.50 (t, J = 6.0 Hz, 2 H), 2.57 (t, J = 6.0 Hz, 2 H), 2.11 (s, 3 H), 1.33 (s, 3 H). ¹³C NMR (75 MHz, CDCl₃): δ = 205.4, 184.1, 150.8, 129.2, 71.5, 59.5, 42.8, 29.6, 25.4. IR (neat): 2927, 1712, 1669, 1389, 1176, 1087, 863, 701 cm^–1. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₁H₁₅O₃: 195.1015; found: 195.1019. 2-Methyl-2-(2-oxopropyl)cyclopentane-1,3-dione (2k): Prepared according to the general procedure and purified by flash chromatography (EtOAc–hexanes, 50%). Yield: 81%; colorless oil. ¹H NMR (400 MHz, CDCl₃): δ = 3.10 (s, 2 H), 2.92–2.71 (m, 4 H), 2.00 (s, 3 H), 0.98 (s, 3 H). ¹³C NMR (101 MHz, CDCl₃): δ = 216.4, 206.3, 52.4, 51.7, 34.7, 28.2, 19.3. IR (neat): 2926, 2856, 1718, 1455, 1390, 1183, 1074, 802, 548 cm^–1. HRMS (ESI): m/z [M + H]⁺ calcd for C₉H₁₃O₃: 169.0859; found: 169.0855.