Sodium dispersion-mediated reductive dimerization of benzylic halides for symmetrical bibenzyls: Column-free applications to natural products

Bubwoong Kang*, Tatsuro Imamura, Tetsuya Satoh

*Department of Applied Chemistry in Bioscience, Graduate School of Agricultural Science, Kobe University, 1-1 Rokkodai, Nada, Kobe, 657-8501, Japan, Email: kangbpegasus.kobe-u.ac.jp

B. Kang, T. Imamura, T. Satoh, Tetrahedron Green Chem., 2024, 55, 100052.

DOI: 10.1016/j.tgchem.2024.100052

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Abstract

Symmetrical bibenzyls can be prepared in 1 h at r.t. by a reductive dimerization of benzylic halides using sodium dispersion (SD). SD, a reagent consisting of sodium particles dispersed in mineral oil is a safer but more reactive source of sodium than sodium lump. The SD-derived mineral oil in the crude product can be readily removed without the need for column chromatography.

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Materials

Sodium dispersion (SD) with its product name of SD Super FineTM (sodium 25wt% dispersion in mineral oil) was purchased from Tokyo Chemical Industry Co., Ltd. (Product Number: D5792). The average size of SD particles is approximately 10 μm.

Typical procedure for reductive dimerization of benzylic halides

To a stirring solution of benzylic halides (2.00 mmol, 1.00 eq.) in dry THF (2.00 mL, 1.00 M) was dropwisely added SD (450 µL, 4.4 mmol, 2.2 eq.) under an argon atmosphere at r.t.. After stirring for 1 h, the reaction was quenched with H₂O (10 mL), evaporated to remove THF, and extracted with CHCl₃ (10 mL×3). The combined CHCl₃ extract was washed with brine (30 mL), dried over Na₂SO₄, and evaporated. The resulting crude product was purified by silica gel (SiO₂ 10 g) column chromatography, and/or PTLC using hexane/EtOAc to give the desired bibenzyls. Even though the weight of each crude product was relatively high (typically around 0.5 g), the entire amount was successfully applied to a TLC plate for PTLC, resulting in a purified product. For modified protocols see supplementary data.

Column-free synthetic protocol for 4,4′-dimethoxybibenzyl

To a stirring solution of 4-methoxybenzyl chloride (1.36 mL, 10.0 mmol, 1.00 eq.) in dry THF (10.0 mL, 1.00 M) was dropwisely added SD (2.23 mL, 22 mmol, 2.2 eq.) at r.t.. After stirring for 1 h, the reaction was quenched with H₂O (20 mL) in an ice-water bath, evaporated to remove THF, and extracted with CHCl₃ (20 mL×3). The combined CHCl₃ extract was washed with brine (60 mL), dried over Na₂SO₄, and evaporated. The resulting crude product was washed with hexane, and then acetonitrile to give the title compound as a pale yellow solid. Yield: 891 mg (74%).

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Key Words

Wurtz reaction, sodium, benzylation

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Note: This abstract is published under a Creative Commons license

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ID: JXX-Y2024