Silica-Supported Tetramethylguanidine: An Efficient Solid Base for Aldol-Type Coupling of Aldehydes with Ethyl Diazoacetate

Abstract

Silica-supported tetramethylguanidine catalyst was prepared and effectively used in the aldol-type coupling of aldehydes with ethyl diazoacetate to afford the corresponding α-diazo-β-hydroxy esters in good to excellent yields. The catalyst was quantitatively recovered from the reaction by a simple filtration and reused for a number of cycles with almost consistent activity.

Key words

silica - tetramethylguanidine - aldol - α-hydroxy-β-di­azoesters - reusable catalyst

References and Notes

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15

Preparation of Silica-Based Catalysts; SiO ₂ -TMG: 3-Chloropropyl-modified silica, 1 (5 g, 2.5% loading, Aldrich) was dispersed in dry hexane (50 mL) under nitrogen and then hexamethyldisilazane (HMDS, 5 mL) was added to the dispersion and refluxed for 8 h, cooled to r.t., filtered and washed with copious amount of hexane to get the trimethylsilylated chloropropyl modified silica 2. Trimethylsilylated chloropropyl modified silica, 2 (1 g) was stirred with tetramethylguanidine (TMG, 2 mmol) and Et₃N (1 mmol) in MeOH (20 mL) under nitrogen at 110 °C for 24 h. Afterwards the solid was separated by filtration and once again washed with toluene using Sohxlet for 24 h to afford SiO₂-TMG (loading of TMG: 0.16 mmol/g, calculated from elemental analysis). SiO₂-IMZ was prepared in a similar fashion using imidazole (loading of IMZ: 0.14 mmol/g, calculated from elemental analysis). SiO₂-NH₂ (loading 1 mmol/g) was purchased from Aldrich.

16

General Procedure for Aldol-Type Coupling of Aldehydes with EDA: A mixture of aldehyde (0.5 mmol), EDA (1.2 equiv), and SiO₂-TMG (50 mg) in DMSO (1 mL) was stirred in a round-bottom flask at r.t. After completion of the reaction, as monitored by TLC, the mixture was diluted with EtOAc (10 mL) and the catalyst was filtered off. The filtrate was washed with sat. aq NH₄Cl solution, followed by brine. The organic layer was dried over anhydrous Na₂SO₄ and the solvent was evaporated under reduced pressure to get the crude product. The crude product was purified by chromatography on silica gel using hexane-EtOAc mixture as eluent to afford the corresponding α-diazo-β-hydroxy esters. Spectroscopic data of the new compounds are as follows:
2-Diazo-3-hydroxy-3-(3-phenoxyphenyl)propionic Acid Ethyl Ester (Table [3] , entry 7): FTIR (neat): 3441, 2982, 2099, 1680, 1586, 1483, 1291, 1243, 1113, 756 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 7.20-7.37 (m, 3 H), 7.00-7.15 (m, 3 H), 6.84-6.99 (m, 3 H), 5.82 (s, 1 H), 4.23 (q, J = 6.8 Hz, 2 H), 3.72 (br s, 1 H), 1.28 (t, J = 6.8 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 166.2, 157.7, 157.0, 141.0, 130.0, 129.8, 123.4, 120.5, 118.9, 118.5, 116.2, 68.4, 61.2, 14.37 (one quaternary C was not observed). MS-ESI: m/z = 335 [M + Na]. Anal Calcd for C₁₇H₁₆N₂O₄: C, 65.38; H, 5.16; N, 8.97. Found: C, 65.32; H, 5.19; N, 8.94.
3-(4-Benzyloxy-3-methoxyphenyl)-2-diazo-3-hydroxy-
propionic Acid Ethyl Ester (Table [3] , entry 8): FTIR (KBr): 3446, 2932, 2104, 1674, 1513, 1394, 1229, 1114, 740 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 7.19-7.50 (m, 5 H), 6.95 (s, 1 H), 6.83 (s, 2 H), 5.79 (s, 1 H), 5.13 (s, 2 H), 4.29 (q,
J = 6.8 Hz, 2 H), 3.91 (s, 3 H), 3.1 (br s, 1 H), 1.34 (t, J = 6.8 Hz, 3 H). ¹³C NMR (100 MHz, CDCl₃): δ = 166.4, 150.0, 148.2, 137.0, 131.8, 128.5, 127.7, 127.2, 117.9, 114.1, 109.4, 71.0, 68.5, 61.1, 56.1, 14.4 (one quaternary C was not observed). MS-ESI: m/z = 357 [M + 1]. Anal. Calcd for C₁₉H₂₀N₂O₅: C, 64.04; H, 5.66; N, 7.86. Found: C, 64.01; H, 5.68; N, 7.90.