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Synlett 2009(20): 3352-3354  
DOI: 10.1055/s-0029-1218383
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Facile Preparation of CF3-Containing 1-Bromoallenes

Yohsuke Watanabe, Takashi Yamazaki*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan
Fax: +81(42)3887038; e-Mail: tyamazak@cc.tuat.ac.jp;
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Publikationsverlauf

Received 17 September 2009
Publikationsdatum:
18. November 2009 (online)

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Abstract

The novel synthetic method is described for preparation of not only 1-bromo- but 1,3-dibromo-1-trifluoromethylated allenes from the corresponding propargylic alcohols with a combination of CBr4 and Ph3P. Selection of the organic solvent employed enabled us to access to these two different allenes quite readily.

Key words

alkynes - allenes - fluorine - trifluoromethyl - bromination

    References and Notes

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8

Typical Procedure for the Preparation of the Bromoallene 3a To a solution of 1a (0.12 g, 0.60 mmol) and Ph3P (0.38 g, 1.4 mmol) in DMF (2.0 mL) was added CBr4 (0.24 g, 0.72 mmol) at r.t. The solution was stirred at that temperature for 2 h. The reaction mixture was quenched with H2O (20 mL) and extracted with hexane-EtOAc (1:1, 3 × 15 mL). Concentration by rotary evaporator after dried over Na2SO4 furnished a crude mixture that was purified by silica gel chromatography (hexane-EtOAc, 20:1) to afford 3a (0.14 g, 0.51 mmol, 85%) as a pale yellow oil. R f  = 0.79 (CH2Cl2-EtOAc, 1:1). ¹H NMR (300 MHz, CDCl3): δ = 6.71 (q, J = 2.7 Hz, 1 H), 7.34-7.44 (m, 5 H). ¹³C NMR (75.5 MHz, CDCl3): δ = 82.5 (q, J = 45.3 Hz), 106.8, 119.8 (q, J = 271.7 Hz), 128.5, 129.2, 129.4, 129.9, 202.6 (q, J = 3.1 Hz). ¹9F NMR (283 MHz, CDCl3): δ = -65.66 (s). IR (neat): 613, 653, 690, 720, 749, 819, 847, 897, 918, 951, 1003, 1029, 1045, 1075, 1119, 1267, 1293, 1313, 1397, 1459, 1496, 1597, 1736, 3034, 3066 cm-¹. HRMS-FAB: m/z calcd for C10H7F3Br [M + H]+: 262.9683; found: 262.9668.

9

Procedure for the Preparation of the Allenylcarbinols 7a and 7b To a solution of 3a (0.32 g, 1.20 mmol) and isobutyr-aldehyde (142 µL, 1.6 mmol) in Et2O (5 mL) was added BuLi (0.98 mL, 1.6 mmol, 1.6 M in hexane) at -105 ˚C. The solution was stirred at that temperature for 2 h. The reaction mixture was quenched with 1 M aq HCl solution (3 mL) and extracted with EtOAc (3 × 20 mL). Usual workup and purification by silica gel chromatography (hexane-EtOAc, 12:1) afforded 7a (0.087 g, 0.41 mol 34%) and 7b (0.14 g, 0.66 mmol 55%).
Compound 7a: colorless oil. R f  = 0.56 (hexane-EtOAc, 4:1). ¹H NMR (300 MHz, CDCl3): δ = 1.01 (d, J = 6.9 Hz, 6 H), 1.80 (br s, 1 H), 1.96 (oct, J = 6.6 Hz, 1 H), 4.12 (dd, J = 6.6, 1.5 Hz, 1 H), 6.73 (qd, J = 3.3, 1.5 Hz, 1 H), 7.26-7.38 (m, 5 H). ¹³C NMR (75.5 MHz, CDCl3): δ = 17.0, 19.5, 32.7, 73.8, 103.0, 106.0 (q, J = 32.3 Hz), 123.1 (q, J = 274.1 Hz), 127.4, 128.6, 129.0, 131.1, 204.4 (q, J = 4.4 Hz). ¹9F NMR (283 MHz, CDCl3): δ = -62.44 (s). IR (neat): 692, 721, 747, 828, 920, 1001, 1029, 1074, 1127, 1210, 1273, 1369, 1387, 1411, 1462, 2874, 2934, 2963, 3429 cm-¹. HRMS-FAB: m/z calcd for C14H16OF3 [M + H]+: 257.1153; found: 257.1176.
Compound 7b: colorless oil. R f  = 0.47 (hexane-EtOAc, 4:1). ¹H NMR (300 MHz, CDCl3): δ = 1.00 (d, J = 6.6 Hz,
3 H), 1.01 (d, J = 6.6 Hz, 3 H), 1.88 (br s, 1 H), 1.96 (oct, J = 6.6 Hz, 1 H), 4.12 (dd, J = 6.6, 1.2 Hz, 1 H), 6.78 (qd, J = 3.3, 1.5 Hz, 1 H), 7.27-7.39 (m, 5 H). ¹³C NMR (75.5 MHz, CDCl3): δ = 17.1, 19.6, 32.8, 73.6, 103.2, 106.1 (q, J = 32.2 Hz), 123.2 (q, J = 272.9 Hz), 127.6, 128.6, 129.0, 131.1, 204.1 (q, J = 4.3 Hz). ¹9F NMR (283 MHz, CDCl3):
δ = -62.52 (s). IR (neat): 692, 719, 747, 829, 920, 1000, 1029, 1074, 1123, 1210, 1274, 1369, 1388, 1411, 1462, 1715, 1961, 2876, 2933, 2967, 3036, 3410 cm-¹. HRMS-FAB: m/z calcd for C14H16OF3 [M + H]+: 257.1153; found: 257.1187.

10

Procedure for the Preparation of the 2,5-Dihydrofuran 8a To a solution of 7a (0.087 g, 0.41 mmol) in acetone (3 mL) was added AgNO3 (12 mg, 0.082 mmol) at r.t. The solution was protected from light and stirred at that temperature for 4 d. Concentration by rotary evaporator furnished a crude mixture that was purified by silica gel chromatography (hexane-EtOAc, 20:1) to afford 8a (0.066 g, 75%) as a pale yellow oil; R f  = 0.77 (hexane-EtOAc, 4:1). ¹H NMR (300 MHz, CDCl3): δ = 0.94 (d, J = 6.9 Hz, 3 H), 1.13 (d, J = 6.9 Hz, 3 H), 2.03 (septd, J = 6.6, 1.2 Hz, 1 H), 5.18 (dq, J = 6.6, 0.9 Hz, 1 H), 5.83-5.88 (m, 1 H), 6.45 (sext, J = 1.8 Hz, 1 H), 7.26-7.44 (m, 5 H). ¹³C NMR (75.5 MHz, CDCl3): δ = 14.5, 19.9, 32.0, 87.7 (q, J = 0.6 Hz), 89.3 (q, J = 0.6 Hz), 121.7 (q, J = 269.2 Hz), 126.4, 128.4, 128.7, 131.9 (q, J = 33.5 Hz), 135.8 (q, J = 4.4 Hz), 140.0. ¹9F NMR (283 MHz, CDCl3): δ = -64.13 (s). IR (neat): 698, 726, 760, 879, 917, 1009, 1051, 1068, 1126, 1158, 1242, 1265, 1281, 1334, 1360, 2876, 2935, 2970 cm-¹. HRMS-FAB: m/z calcd for C14H16OF3 [M]+: 256.1075; found: 256.1046.