From Rhodium
Vanillin Demethylation with Pyridine and AlCl31
Anhydrous AlCl3 (9.7 g, 0.0724 mol) was suspended in a solution of 10g (0.0658 mol) of vanillin in 100 ml of methylene chloride in an apparatus
protected from atmospheric moisture. While stirring briskly and cooling to maintain the temperature at 30-35°C, 22.9g (0.290 mol) of pyridine was
added slowly. The reaction was vigorous; the resulting clear light orange solution of the reaction complex was heated to reflux (45°C) and maintained
at that temperature with stirring for 24 hours. The solution, which had darkened only slightly during the reflux period, was cooled to 25°C and the
product was hydrolyzed, while stirring and maintaining the temperature at 25-30°C, by the addition of dilute (15-20%) HCl until the mixture was
definitely acidic to congo red indicator. Of the two phases present at this time, the lower methylene chloride layer contained most of the small
amount of the unchanged vanillin, and essentially no protocatechuic acid, the latter was dissolved in the aqueous phase. Evaporation of the methylene
chloride yielded 0.8 g of vanillin. Extraction of the aqueous phase with ether followed by evaporation of the ether left 7.9 g (87%) of pale yellow
crystals of protocatechualdehyde melting at 153-154°C.
Using the cheaper triethylamine in place of pyridine lowered the yield to 61.5%
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