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DOI: 10.1055/sos-SD-025-00443

Schall, A.; Reiser, O.Science of Synthesis, (200725643.

The ortho formylation of phenols with formaldehyde in the presence of a variety of metal salt catalysts, e.g. based on tin, titanium, iron, chromium, or zirconium, has been described in the patent literature, but the reaction conditions necessary, such as high pressure, are not generally amenable for laboratory practice. Aryloxymagnesium bromides undergo formylation with formaldehyde; however, stoichiometric amounts of hexamethylphosphoric triamide need to be employed, posing a limitation especially for large-scale applications.[‌137‌] A breakthrough was made with the discovery that ortho formylation using paraformaldehyde and magnesium bis(phenoxides), prepared from phenols 43 and magnesium methoxide, is possible (Scheme 24).[‌138‌] After initial hydroxymethylation of the phenoxide, coordination of a second formaldehyde molecule makes the redox conversion into the salicylaldehyde 44 with the loss of methanol possible. The latter has to be removed by distillation in the course of the reaction because it hampers the progress of the formylation.

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