Metal cation-exchanged montmorillonite catalyzed protection of aromatic aldehydes with Ac2O

The protection of aromatic aldehydes with acetic anhydride was investigated in the presence of 26 metal cation-exchanged montmorillonites (Mn+-MMT) and commercial acid-activated K10 and KSF montmorillonites (Südchemie Moosburg, Germany). The acid sites of all catalysts were investigated by infrared spectroscopy and thermal analysis using pyridine as a molecular probe. The yield of reaction is roughly related to the strength of acid sites and depends also on the substitution on the benzene ring of the aldehydes. The order of activity of selected cation-exchanged forms was Al>K10∼KSF>Zn>Na which correlates well with the polarizing power of these cations and resulting acidity. Al3+-MMT is revealed to be an effective catalyst for protection of aromatic aldehydes with Ac2O and produces almost solely 1,1-diacetates. Relatively high yield of products observed for “moderately acid” cations, such as Zn2+, can be explained by the fact that protection of aldehydes requires medium-strength acid sites.