The Persian Gulf calcined coral, Acropora species: An efficient catalyst for aldol condensation

Abstract

Background: Coral, a marine organism, is known for its unique solid, natural, and efficient catalyst in organic reactions. Calcium carbonate (CaCO3), the main component of marine coral, is an efficient, basic, and natural solid for catalytic aldol condensation reaction of aldehydes and ketones having α-hydrogens to produce β-hydroxy carbonyl compounds with good yields at room temperature. Objectives: The purpose of this research was to study using calcined coral as an efficient catalyst for aldol reaction at room temperature. Materials and Methods: Marine coral (Acropora sp.) was washed several times using methanol-ethyl acetate and deionized water and then dried in an oven at 80°C for 48 hours. The dried coral was ground to get fine powder. For preparation of calcined coral, its powder was put in an oven at 800°C for 2 hours. Calcined coral and sodium nitrate (1:1) were well stirred in sufficient water for about 5 hours. Then, the mixture was calcined in an oven at 800°C for 2 hours. In general experiment, the reaction was carried out by mixing aldehyde (150 mmol), ketone (50 mmol), 0.1 g of calcined coral/sodium nitrate, 5 mL water-ethanol (1:1) as a solvent. Finally, the mixture was stirred at room temperature for the appropriate time. Results: The best conditions for condensation reaction are using the calcined coral/sodium nitrate, as a catalyst in the presence of ethanol/water at room temperature. Aldol condensation reaction of nitro-substituted benzaldehydes with ketones has greater yields than benzaldehydes. Electron-withdrawing groups such as NO2 on benzaldehyde ring activate aldol condensation reaction. In comparison, 4-methoxybenzaldehyde reacts more slowly with ketones and decreases the yield of products. Conclusions: Calcined coral/sodium nitrate as a new, efficient, and natural heterogeneous catalyst used in aldol condensation reaction of ketones and aldehydes led to β-hydroxyketones in good to high yields in appropriate condition.