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Synlett 2005(14): 2191-2194  
DOI: 10.1055/s-2005-871961
LETTER
© Georg Thieme Verlag Stuttgart · New York

The First Synthesis and Reaction of β-Ethoxy-α-fluoro-α-(phenyl­selanyl)ethene: Scandium or Lanthanum Triflate Catalyzed α-Fluoroformylalkenylation of Aldehydes

Fuminori Hatanaka, Miyuki Tsuchiya, Mitsuhiro Yoshimatsu*
Department of Chemistry, Faculty of Education, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan
Fax: +81(58)2932207; e-Mail: yoshimae@cc.gifu-u.ac.jp;
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Publication History

Received 17 May 2005
Publication Date:
13 July 2005 (online)

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Abstract

We describe the first preparation of the hitherto unknown β-ethoxy-α-fluoro-α-(phenylselanyl)ethenes and the successful Lewis acid catalyzed α-fluoroformylalkenylation of non-enolizable aldehyde acetals to provide α,β-unsaturated aldehydes or acetals exclusively.

Key words

(phenylselanyl)ethane - scandium triflate - lanthanum triflate - double bond formation - fluorine - α,β-unsaturated aldehydes

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Preparation of β-ethoxy-α-fluoro-α-(phenylselanyl)ethene(4). To a toluene (80 mL) solution of ethyl 2-fluoro-2-(phenylselanyl)acetate (1; 4.00 g, 15.3 mmol) was added DIBAL-H (18.4 mL, 1 M toluene solution, 18.4 mmol) at -78 °C under an Ar atmosphere. After the reaction mixture was stirred for 30 min, 1 M HCl solution (16 mL) was added to the mixture. The resulting mixture was stirred for 10 min and then poured into H2O (200 mL). The organic layer was separated and the aqueous layer was extracted with Et2O. The combined organic layer was dried over MgSO4. The solvent was removed under reduced pressure. To the residue in EtOH (23 mL) was added ethyl orthoformate (11.4 g, 76.6 mmol) and p-TsOH (0.26 g, 1.53 mmol). The mixture was stirred for 12 h at 20 °C and poured into a sat. soln of NaHCO3 (200 mL). Work-up and purification by column chromatography on silica gel eluting with EtOAc-hexane (1:50) gave 2-fluoro-2-(phenylselanyl)acetaldehyde diethyl acetal(3) (14.1 g, 91%) as a yellow oil. IR: 2978, 2929, 2884, 2362, 2344, 1579, 1478, 1372, 1340, 1301, 1118, 1067, 1023, 999, 891, 741, 691, 470 cm-1; 1H NMR: δ = 1.25 (3 H, t, J = 7 Hz, Me), 1.26 (3 H, t, J = 7 Hz, Me), 3.62-3.68 (2 H, m, OCH2), 3.76-3.82 (2 H, m, OCH2), 4.71 (1 H, dd, J = 4, 9 Hz, acetal H), 6.02 (1 H, dd, J = 4, 52 Hz, CHF), 7.24-7.33 (3 H, m, ArH), 7.64-7.66 (2 H, m, ArH); 19F NMR (referenced to CF3CO2H): δ = -87.05 (dd, J = 9, 53 Hz); MS: m/z = 292 (M+). Anal. Calcd for C12H17FO2Se: C, 49.49; H, 5.88. Found: C, 49.27; H, 5.79
An ethereal (5.0 mL) solution of 3 (3.0 g, 10.3 mmol)was added to lithium isopropylcyclohexylamide (prepared from isopropylcyclohexylamine (2.90 g, 20.6 mmol) and BuLi (6.0 mL, 15.4 mmol) in Et2O (60 mL)) at -60 °C. The reaction mixture was warmed to -40 °C and then stirred for 10 min. The mixture was poured into H2O (200 mL). The organic layer was separated and the aqueous layer was extracted with Et2O. The combined organic layers were dried over MgSO4. The solvent was removed under reduced pressure. The residue was purified by flash chromatography on silica gel eluting with EtOAc-hexane (1:100) to give 4 (2.02 g, 80%) as a yellow oil. IR: 3444, 2980, 2357, 1658, 1577, 1477, 1439, 1395, 1313, 1194, 1130, 1061, 1000, 902, 737, 689, 535, 515, 466, 441, 425, 406 cm-1; 1H NMR: δ = 1.33 (3 H, t, J = 7 Hz, Me), 3.97 (2 H, q, J = 7 Hz, OCH2), 6.24 (1 H, d, J = 19 Hz, olefinic H), 7.24-7.30 (3 H, m, ArH), 7.48 (2 H, d, J = 8 Hz, ArH); 19F NMR: δ = -39.67 (d, J = 18 Hz); MS: m/z = 246 (weak M+). Compound 4 was too volatile to measure the elemental analysis.

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A typical procedure for the preparation of 2-fluoro-4′-methoxycinnamaldehyde dimethyl acetal(6a): Lanthanum triflate (60 mg, 0.10 mmol) was added to a ClCH2CH2Cl (5.0 mL) solution of 4 (0.50 g, 2.04 mmol) and p-methoxybenz-aldehyde dimethyl acetal (0.80 g, 4.07 mmol) under an Ar atmosphere. The reaction mixture was refluxed for 10 min, cooled, and treated with Et3N (2 drops). Work-up afforded 2-fluoro-4′-methoxycinnamaldehyde dimethyl acetal (6a, 0.77 g, 85%).