Ultrasound in Li-Barbier Reactions

A very powerful new tool was applied in synthetic chemistry with ultrosonic irradiation of reaction mixtures [27, 28]. It is symbolized with the following icon )) above or below the reaction arrow. 

Sonication was successfully introduced in organometallic chemistry in 1980 [29] when it was applied to Li-Barbier reactions. In most cases these were completed in 10-15 minutes. Normally observed sidereactions such as enoliz-ation and reduction seemed to have been minimised. 

This reaction could be performed in wet technical grade solvent at room temperature although more of the halide then had to be used, presumably due to hydrolysis of intermediate organolithium compounds. 

The use of ultrasound allows the direct in situ formation of benzyllithium from benzyl bromide while normally Wurtz coupling predominates with this halide. (See e.g. p. 79 [15]). Condensation, e.g. with acetophenone gives the expected product in 95% yield. Also allylic and vinylic halides undergo clean 

Several other sonochemically catalyzed Li-Barbier reactions were published shortly after this first one. 

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