Synthesis of α,β-Alkynyl Esters and Unsymmetrical Maleate Esters Catalyzed by Pd/C; An Efficient Phosphine-Free Catalytic System for Oxidative Alkoxycarbonylation of Terminal Alkynes

Abstract

Pd/C-catalyzed oxidative alkoxycarbonylation of terminal alkynes using alcohols in the presence of tetrabutylammonium iodide under CO/O₂ (5:1 atm) has been investigated. The desired α,β-alkynyl esters and unsymmetrical maleate esters are formed in good to excellent yields under different reaction conditions. The present protocol eliminates the use of phosphine ligands and has straightforward catalyst recovery. The catalyst was recycled up to six times without significant loss of catalytic activity.

Key words

alkynes - carbonylation - heterogeneous catalysis - palladium - alcohols

• References and Notes

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• 12 Oxidative Alkoxycarbonylation of Terminal Alkynes; General Procedure: To a 100 mL stainless steel autoclave, the alk-1-yne (1 mmol), the alcohol (0.5 mL), 10% Pd/C (10 mol%), TBAI (0.6 mmol), and 1,4-dioxane (10 mL) were added. The autoclave was closed and pressurized with oxygen (1 atm) and CO (5 atm) without flushing. The reaction mixture was stirred (550 rpm) and heated at 80 °C for 8 h. The reactor was then cooled to r.t. and the pressure released. The reaction mixture was filtered and the filtrate was washed with saturated sodium thiosulphate (3 × 4 mL), dried over Na₂SO₄, filtered and the solvent evaporated. The crude product was purified by column chromatography. Ethyl 3-[4(Trifluoromethyl)phenyl]propiolate (3j): ¹³ Yield: 83%; colorless liquid; IR (neat): 2253, 1711 cm^–1; GCMS (EI, 70 eV): m/z (%) = 242 (7) [M⁺], 223 (3), 197 (100), 170 (67), 147 (7); ¹H NMR (300 MHz, CDCl₃): δ = 7.71–7.59 (m, 4 H, ArCH), 4.32 (q, J = 6.9 Hz, 2 H, CH ₂CH₃), 1.46 (t, J = 6.9 Hz, 3 H, CH₂CH ₃). Ethyl 3-(Pyren-2-yl)propiolate (3l): Yield: 85%; yellowish solid; IR (KBr): 2205, 1704 cm^–1; ¹H NMR (400 MHz, CDCl₃): δ = 8.57 (d, J = 9.1 Hz, 1 H, ArCH), 8.27–8.04 (m, 8 H, ArCH), 4.40 (q, J = 7.1 Hz, 2 H, CH ₂CH₃), 1.44 (t, J = 7.1 Hz, 3 H, CH₂CH ₃); ¹³C NMR (75 MHz, CDCl₃): δ = 154.30, 133.26, 132.63, 130.69, 130.63, 130.48, 129.14, 129.02, 126.74, 126.23, 126.13, 125.99, 124.53, 124.11, 123.76, 123.53, 113.08, 85.94, 85.59, 62.05, 14.17; HRMS (ESI): m/z calcd. for [C₂₁H₁₄O₂Na]⁺: 321.0886; found: 321.0879. Diethyl 2-(Pyridin-3-yl)maleate (4g): Yield: 89%; yellowish liquid; IR (neat): 1723, 1629 cm^–1; ¹H NMR (300 MHz, CDCl₃): δ = 8.76 (d, J = 1.8 Hz, 1 H, ArCH), 8.67–8.65 (m, 1 H, ArCH), 7.86–7.82 (m, 1 H, ArCH), 7.41–7.37 (m, 1 H, ArCH), 6.35 (s, 1 H, COCH=CCO), 4.43 (q, J = 6.9 Hz, 2 H, CH₃CH ₂O), 4.26 (q, J = 6.9 Hz, 2 H, CH₃CH ₂O), 1.38 (t, J = 6.9 Hz, 3 H, CH ₃CH₂O), 1.33 (t, J = 6.9 Hz, 3 H, CH ₃CH₂O); ¹³C NMR (75 MHz, CDCl₃): δ = 166.92, 164.44, 150.60, 147.40, 145.12, 134.60, 130.11, 123.92, 120.14, 62.32, 61.38, 14.22, 14.06; GCMS (EI, 70 eV): m/z (%) = 249 (13) [M⁺], 220 (78), 204 (55), 192 (16), 176 (91), 148 (100), 132 (25), 120 (18), 104 (56), 76 (33), 50 (24); HRMS (ESI): m/z calcd. for [C₁₃H₁₅O₄N + H]⁺: 250.1074; found: 250.1070.
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