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Synlett 2009(13): 2105-2108  
DOI: 10.1055/s-0029-1217701
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Chemoselective Isomerization of Secondary-Type Propargylic Alcohols to Propargylic/Allenic Bromides, and Brominated Dienes with Appel-Type Reaction Conditions

Norio Sakai*, Tsukasa Maruyama, Takeo Konakahara
Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science (RIKADAI), Noda, Chiba 278-8510, Japan
Fax: +81(4)71239890; e-Mail: sakachem@rs.noda.tus.ac.jp;
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Publication History

Received 7 May 2009
Publication Date:
16 July 2009 (online)

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Abstract

Herein is described the chemoselective isomerization of secondary-type propargylic alcohols to allenic bromides, propargylic bromides and brominated dienes under Appel-type reaction conditions containing Ph3P, CBr4 and additives.

Key words

Appel reaction - propargylic alcohol - allene - diene - isomerization

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General Procedure for the Synthesis of Allene Derivatives 2: CBr4 (331 mg, 1.0 mmol), Ph3P (262 mg, 1.0 mmol), propargylic alcohol 1 (0.50 mmol), i-Pr2NEt (129 mg, 1.0 mmol), P(n-Bu)3 (10 mg, 0.05 mmol), and a freshly distilled toluene (1 mL) were successively added into a screw-capped vial, and the vial was sealed with a cap containing a PTFE septum. The reaction mixture was stirred at 100 ˚C, and monitored by TLC until the propargylic alcohol 1 was consumed. To quench the reaction, H2O (2 mL) was added to the mixture. The mixture was extracted with CH2Cl2 (3 ×), and the combined organic extracts were dried over Na2SO4, filtered, and then evaporated under reduced pressure. The crude product was purified by silica gel chromatography(hexane) to produce the allene derivative 2, and if necessary, was further purified by a recycling preparative HPLC equipped with a GPC column (chloroform as an eluent). Spectral data for selected compound: 1-(3-Bromo-1,2-nonadien-1-yl)benzene (2a): pale yellow oil. ¹H NMR (300 MHz, CDCl3): δ = 0.86 (t, 3 H, J = 7.2 Hz), 1.24-1.38 (m, 6 H), 1.50 (quint, 2 H, J = 7.2 Hz), 2.52 (td, 2 H, J = 7.2, 3.0 Hz), 6.19 (t, 1 H, J = 3.0 Hz), 7.23-7.27 (m, 1 H), 7.32-7.33 (m, 4 H). ¹³C NMR (75 MHz, CDCl3): δ = 13.9, 22.5, 27.9, 28.2, 31.4, 38.0, 96.1, 100.3, 127.7, 128.1, 128.7, 133.0, 199.8. MS (FAB): m/z (%) = 281 (100) [M+], 279 (40) [M+]. HRMS (FAB): m/z calcd for C15H20Br: 279.0748; found: 279.0726.

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General Procedure for the Synthesis of Diene Derivatives 3: The same procedure as above without i-Pr2NEt gave the diene derivative 3. However, formation of a quite small amount of the (1Z,3E)-diene along with the (1E,3E)-diene separable by column chromatography was observed by NMR. Spectral data for selected compound: [(1E,3E)-3-Bromo-1,3-nonadien-1-yl]benzene (3a): pale brown oil. ¹H NMR (300 MHz, CDCl3): δ = 0.88 (t, 3 H, J = 7.2 Hz), 1.27 (m, 3 H), 1.35 (m, 1 H), 1.47 (m, 2 H), 2.35 (q, 2 H, J = 7.2 Hz), 6.08 (t, 1 H, J = 7.2 Hz), 6.73 (d, 1 H, J = 15.0 Hz), 6.89 (d, 1 H, J = 15.0 Hz), 7.23 (m, 1 H), 7.30 (m, 2 H), 7.42 (m, 2 H). ¹³C NMR (75 MHz, CDCl3): δ = 13.9, 22.4, 28.1, 31.4, 31.7, 126.7, 126.9, 127.7, 127.9, 128.6, 128.7, 132.1, 135.3. MS (EI): m/z = 279 [M+]. HRMS (FAB): m/z calcd for C15H20Br: 279.0748; found: 279.0728. Stereochemistry (1E,3E) of the isolated compound was determined by
the chemical shift and coupling constant of the related compound. Specific peaks derived from (1Z,3E)-diene 3a were observed by ¹H NMR. ¹H NMR: δ = 5.87 (t, 1 H, J = 7.2 Hz), 6.76 (d, 2 H, J = 15.0 Hz), 6.94 (d, 1 H, J = 15.0 Hz). Other peaks overlapped with those of the (1E,3E)-diene.

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We have no clear cause for the low yield of diene derivative 3; however, formation of several complex products, which were probably derived from the starting propargylic alcohol, was observed by an NMR measurement.