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Synlett 2005(8): 1235-1238  
DOI: 10.1055/s-2005-865238
LETTER
© Georg Thieme Verlag Stuttgart · New York

Polymer-Supported β-Amino Thioesters as Catalysts for the Enantioselective Addition of Diethylzinc to Aldehydes

Heidi S. Eriksen, Sandra Cotes Oyaga, David C. Sherrington, Colin L. Gibson*
Department of Pure & Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow, G1 1XL, UK
Fax: +44(141)5484246; e-Mail: c.l.gibson@strath.ac.uk;
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Publikationsverlauf

Received 9 February 2005
Publikationsdatum:
21. April 2005 (online)

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Abstract

A series of novel polymer-supported thioesters were prepared and found to be effective catalysts for the enantioselective addition of diethylzinc to benzaldehyde. These catalysts gave 1-phenylpropanol in up to 86% ee. The catalysts were fully recyclable and could be used in subsequent additions with retention of the enantioselectivity and efficiency levels.

Key words

asymmetric catalysis - ligands - nucleophilic additions - polymers - supported catalysis

1

Present address: Departamento de Química y Biología, Universidad del Norte, Km 5 via Puerto Colombia, Barranquilla, Colombia.

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Typical Procedure for the Synthesis of the Polymer-Supported Thioesters 6 and 7.
To a stirred solution of [(2S)-1-(polystyrylmethyl)pyrrol-idinyl]methanol (0.5008 g) and triphenylphosphine (0.5885 g) in anhyd toluene (10 mL) under nitrogen was added at r.t. a solution of DEAD (0.3912 g) in toluene (1 mL). After 3-5 min thiobenzoic acid (0.3012 g) in toluene (1 mL) was added. The resulting mixture was stirred for a further 45 h at r.t. At the completion of this period the polymer was filtered and rinsed with toluene, DMF and EtOH. The polymer beads were then transferred to a soxhlet apparatus and washed with THF for 1 d, rinsed with acetone and finally soxhlet extracted with Et2O for 1 d. The beads were then dried in a drying pistol (40 °C, 0.05 mmHg) for 24 h to give 0.5639 g of polymer. IR (KBr): νmax = 1664 cm-1. Anal. Calcd: N, 2.08; S, 4.78. Found: N, 1.94; S, 3.82. Loading 1.19 mmol S/g and an 80% conversion based on sulfur microanalysis.

18

Typical Procedure for the Enantioselective Addition of Diethylzinc to Benzaldehyde Catalysed by Polymer-Supported Catalysts 6-8.
Diethylzinc (1.87 mL of a 1 M solution in hexane, 1.87 mmol) was added to a stirred solution of the catalyst (0.024 mmol, 0.048 mmol, or 0.096 mmol based on sulfur content) in anhyd toluene (5 mL) under a nitrogen atmosphere. After stirring at r.t. for 2 h the solution was cooled to -27 °C whereupon, freshly distilled benzaldehyde (0.937 mmol) was added and the resulting solution was stirred at 0 °C. After stirring for 19 h at this temperature additional diethylzinc was added (1.87 mL, 1.87 mmol) and the mixture stirred for a further 46 h at 0 °C. At the completion of this period there was no starting aldehyde, so HCl was added (1 M, 3 mL). The aqueous phase was extracted with CH2Cl2 (3 × 10 mL) and dried over Na2SO4. Evaporation of the solvent and silica column chromatography (hexane-EtOAc, 88:12) afforded (R)-(+)-1-phenyl-1-propanol as a colourless oil. 1H NMR (CDCl3): δ = 0.9 (t, 3 H, CH2CH 3, J = 7.4 Hz), 1.65-2.00 (m, 3 H, CH 2CH3, J = 7.4 Hz, OH), 4.60 (t, 1 H, CHOH, J = 6.6 Hz), 7.30-7.40 (m, 5 H, ArCH). The ee was determined by HPLC using a DAICEL chiralcel OB column with 3% i-PrOH in hexane (flowrate: 0.5 mL/min); t R = 25 min for the S-isomer and 31 min for the R-isomer.