Gomberg-Bachmann pinacol synthesis

What is Gomberg-Bachmann pinacol synthesis?

The Gomberg-Bachmann pinacol synthesis, first reported in 1927 by Gomberg and Bachmann, is a reaction that involves the preparation of pinacols from aromatic aldehydes or ketones in the presence of magnesium and magnesium iodide.

Gomberg-Bachmann pinacol synthesis - general reaction scheme
Gomberg-Bachmann pinacol synthesis

R = alkyl, aryl
The reaction is characterized by the appearance of green and brown colors in the reaction solution, which disappear when exposed to air and reappear when air is removed, indicating the involvement of a radical mechanism. The ketone is reduced to a ketyl radical, which then combines to give pinacol after workup, resulting in almost quantitative yields.

This method is particularly effective for stable ketones, including xanthone and fluorenone, and can be used to prepare pinacols that have not been synthesized by alternative methods.

In the case of pyridyl ketones, two equivalents of magnesium iodide MgI2 are required for the completion of reduction, with one magnesium iodide complexing with the nitrogen atom of the pyridyl ring and the second magnesium iodide MgI2 reducing the ketone in combination with magnesium.

References

The Reducing Action of a Mixture of Magnesium Iodide (or Bromide) and Magnesium on Aromatic Ketones. Probable Formation of Magnesium Subiodide (or Subbromide)
M. Gomberg and W. E. Bachmann
Journal of the American Chemical Society 1927 49 (1), 236-257
DOI: 10.1021/ja01400a027

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