Polyaniline-Supported Sulfuric Acid Salt as a Powerful Catalyst for the Protection and Deprotection of Carbonyl Compounds

Abstract

Structurally different carbonyl compounds were converted into their corresponding cyclic acetals using polyaniline-sulfate salt as catalyst in dry toluene in excellent yield. In turn, useful deacetalization in aqueous medium was demonstrated. Chemoselective protection of carbonyl compounds was also demonstrated. The advantages of the polyaniline-sulfate salt are ease of preparation and handling, stability, reusability and activity.

Key words

protection - deprotection - chemoselectivity - poly­aniline-sulfate salt - carbonyl compounds

Reference

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• 14a

  Preparation of Polyaniline-Sulfate Salt: In a typical experiment, benzoyl peroxide (24.2 g, 0.1 M) was dissolved in 300 mL acetone. Then 200 mL aqueous solution containing 10 g of sodium lauryl sulfate (0.034 M) was added into the above solution slowly. To this solution, 465 mL aqueous solution containing 27 mL of (1.0 N) sulfuric acid and 9.3 g aniline (0.1 M) was added dropwise over 15-20 min and the mixture was stirred at 40 °C for 8 h. The precipitated polyaniline salt was filtered, washed with distilled H₂O, followed by MeOH and acetone. The sample was dried at 100 °C to a constant weight.

• 14b

  Physical Data of Polyaniline-Sulfate Salt: Yield: 82.1 % with respect to the amount of aniline used. Conductivity (0.04 S/cm), Sulfuric acid group present in polyaniline salt (30 %), Pellet density (1.15 g/cm³), Particle size (0.3-75 µm), Elemental analysis: C, 52.5%; H, 3.3%; N, 10.7%; S, 7.4 %.

• General Experimental Procedures:
• 15a

  For protection: In a 50 mL round-bottom flask was placed p-methylbenzaldehyde (1 g, 8 mmol), 1,2-ethanediol (0.75 g, 12 mmol), 200 mg activated polyaniline-sulfate salt (20 wt% with respect to p-methylbenzaldehyde) and 25 mL of toluene. This was refluxed for 45 min with a Dean-Stark apparatus for azeotropic removal of H₂O; reaction being monitored by TLC until the starting material disappeared. The reaction mixture was cooled and filtered to remove the catalyst. The filtrate was washed with H₂O and the organic phase separated, dried over Na₂SO₄, filtered and concentrated in vacuum. The crude mixture was purified by preparative column chromatography (1:99 EtOAc:hexane) affording the cyclic acetal of p-methylbenzaldehyde (Table [1] ).

• 15b

  For Deprotection: The cyclic acetal of p-methylbenzaldehyde (1 g, 6 mmol), 200 mg activated polyaniline-sulfate salt (20 wt% with respect to cyclic acetal of p-methylbenzaldehyde) and 25 mL of water in a 50 mL round bottom flask were refluxed for 30 min, and the reaction was monitored by TLC. The reaction mixture was cooled and filtered to remove the catalyst. The filtrate was extracted with CHCl₃ and the organic extract dried over Na₂SO₄, filtered and concentrated in vacuum. The resultant sample was purified by preparative column chromatography (1:99 EtOAc:hexane) affording the parent p-methylbenz-aldehyde (Table [2] ).