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 tetrafluoroboric 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 solvent. 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 reused/recycled four times (after reactivation after each use) without any significant decrease in its catalytic efficiency. The parent aldehydes/ketones are regenerated from the corresponding acetals/­ketals in high yields by the treatment with water-alcohol in the presence of HBF₄-SiO₂ at room temperature for short times. Excellent selectivity was observed during inter- and intramolecular competition studies involving carbonyl substrates with varying electronic and steric environments. Selective acetal formation of benzaldehyde takes place in the presence of 4-(dimethylamino)benzaldehyde, thiophene-2-carboxaldehyde, 1-naphthaldehyde, 9-anthraldehyde, or acetophenone, but 3-nitrobenzaldehyde undergoes selective acetal formation in preference to benzaldehyde. In the case of 4-acetylbenzaldehyde, exclusive acetal formation of the aldehyde carbonyl group occurs.

Key words

acetals - aldehydes - ketones - protection/deprotection - heterogeneous catalyst

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10

HBF₄-SiO₂ is not commercially available, and was invented by Chakraborti and Gulhane (see also ref. 9b).

12

The aldehyde proton of benzaldehyde, 3-nitrobenzaldehyde, 4-(dimethylamino)benzaldehyde, thiophene-2-carboxalde-hyde, 1-naphthaldehyde, and anthracene-9-carbaldehyde appeared at δ = 9.99, 10.15, 9.72, 9.93, 10.31, and 11.55, respectively. The corresponding proton of the dimethyl acetals of these aldehydes appeared at δ = 5.38, 5.48, 5.31, 5.63, 5.89, and 6.53, respectively. The aldehyde proton of 4-acetylbenzaldehyde appeared at δ = 10.11, which was shifted to δ = 5.44 in 1-[4-(dimethoxymethyl)phenyl]ethan-1-one.¹⁸