Synlett 2012(5): 747-750  
DOI: 10.1055/s-0031-1290365
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

Synthesis of Allenes via Nickel-Catalyzed Cross-Coupling Reaction of Propargylic Bromides with Grignard Reagents

Qinghan Li*a, Hanmou Gaub
a College of Chemistry and Environmental Proection Engineering, Southwest University for Nationalities, Chengdu, 610041, P. R. of China
Fax: +86(28)85524382; e-Mail: lqhchem@163.com;
b Department of Chemistry, National Chung Hsing University, 250 Kuo-Kuang Road, Taichung 402, Taiwan
Further Information

Publication History

Received 11 November 2011
Publication Date:
24 February 2012 (online)

Abstract

We describe a convenient method for the synthesis of terminal allenes from cross-coupling of propargylic bromide with Grignard reagent. The reaction of propargylic bromide with 1.2 equivalents of Grignard reagent mediated by Ni(acac)2 (2 mol%) and Ph3P (4 mol%) in THF may produce terminal allenes in good yields and high regioselectivities at room temperature.

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General Procedure for the Cross-Coupling of Propargylic Bromide with Grignard Reagent
Under an atmosphere of nitrogen, Ni(acac)2 (5.12 mg, 0.02 mmol), Ph3P (0.04 mmol), and THF (2 mL) were mixed in a Schlenk flask. Shortly afterwards, aryl or alkyl Grignard reagent (1.2 mmol) was added, and subsequently the proparglic bromide was added. Then the reaction mixture was stirred at r.t. for 4-6 h. After completion of the reaction, the mixture was diluted with H2O (15 mL) and extracted with Et2O (3 × 15 mL). The combined organic layers were dried over anhydrous Na2SO4, filtered, and evaporated in vacuum. The residue was subjected to flash column chromatography on silica gel (hexane or EtOAc-hexane, 100:1) to afford the corresponding allene products 3. Allenylbenzene (3aa):¹³e colorless oil; ¹H NMR (400 MHz, CDCl3): δ = 7.31-7.19 (m, 5 H), 6.17 (t, J = 6.8 Hz, 1 H), 5.16 (d, J = 6.8 Hz, 2 H). ¹³C {¹H} NMR (100 MHz, CDCl3): δ = 209.8, 133.9, 128.6, 126.9, 126.7, 93.9, 78.7. HRMS: m/z calcd for C9H8: 116.0626; found: 116.0637 [M]+.