Tetrafluoroboric Acid Adsorbed on Silica Gel as a Reusable Heterogeneous Dual-Purpose Catalyst for Conversion of Aldehydes/Ketones into Acetals/Ketals and Back Again

Abstract

Aldehydes and ketones can be protected as acetals and ketals by treatment with trimethyl orthoformate (TMOF) or triethyl orthoformate (TEOF) under the catalytic influence of tetrafluorobo- ric acid adsorbed on silica gel (HBF4-SiO2). In the case of aldehydes or ketones with highly electrophilic carbonyl group, the reactions are carried out under solvent-free conditions. Aryl alkyl ketones, aryl styryl ketones, aldehydes with weakly electrophilic carbonyl groups, and aldehydes with substituents that can coordinate with the catalyst require the presence of the corresponding alcohol as sol- vent. For substrates that can be converted into acetals under neat conditions, the acetal formation takes place at a faster rate when the alcohol is used as the solvent. The catalyst can be recovered and re- used/recycled four times (after reactivation after each use) without any significant decrease in its catalytic efficiency. The parent alde- hydes/ketones are regenerated from the corresponding acetals/ ketals in high yields by the treatment with water-alcohol in the pres- ence of HBF4-SiO2 at room temperature for short times. Excellent selectivity was observed during inter- and intramolecular competi- tion studies involving carbonyl substrates with varying electronic and steric environments. Selective acetal formation of benzalde- hyde takes place in the presence of 4-(dimethylamino)benzalde- hyde, thiophene-2-carboxaldehyde, 1-naphthaldehyde, 9- anthraldehyde, or acetophenone, but 3-nitrobenzaldehyde under- goes selective acetal formation in preference to benzaldehyde. In the case of 4-acetylbenzaldehyde, exclusive acetal formation of the aldehyde carbonyl group occurs.