A Facile Tandem Protocol for the Crossed Aldol Condensation in SiO2.OSO3H Ionic Liquid under Solventless Condition

Objectives: Removing organic solvents in chemical synthesis is important in the drive towards benign chemical technologies. Organic solvents are high on the list of toxic compounds due to the problems in containing volatile compounds and the sheer large volume of them used in industry. Some advantages of utilizing solventless reactions are that the compounds are often sufficiently pure to avoid extensive purification using chromatography, the reactions can be rapid, and often reaching substantial completion in several minutes compared to hours in organic solvents, and the energy usage can be much lower. Among organic reactions, aldol condensations are important and excellent tools in organic synthesis, providing a good way to form carbon–carbon bonds. The main objectives of this paper are to carryout crossed - aldol condensation reactions with dicyclohexylketones with different aromatics aldehydes in the presence of silica sulphuric acid [SiO2.OSO3H] as an ionic liquid catalyst under solvent free condition to afford the corresponding α, β - unsaturated crossed - aldol products in excellent yields and to recover and reuse the catalyst for subsequent use.
Study design: Green chemical reaction using silica-sulphuric acid as a catalyst under solventless condition.
Place and Duration of Study: Department of Chemistry, Sir Theagaraya College, Chennai-21, Tamilnadu, India, between June 2009 and July 2010.
Methodology: The ionic liquid SiO2.OSO3H was prepared according to the reported procedure in an eco-friendly way by the reaction of silica gel and chlorosulphonic acid .It is interesting to note that the reaction is easy, clean and not requiring any work-up procedure. Using this ionic liquid, crossed-aldol condensation of dicyclohexyl ketones with aromatic aldehydes were carried out. After complete conversion of the ketones as indicated by the TLC, the mixture was cooled to room temperature. Dichloromethane (20 - 30 ml) was added and heated for a few minutes, the reagent was concentrated and the solid residue was recrystallized from ethanol to afford the pure product. The catalyst was recycled by washing the solid reagent remained on the filter by EtOAC (20 ml) followed by drying in an oven (50oC) for 2hr. Products were characterized by spectral analyses such as UV-Visible, FT-IR,1H NMR and 13C NMR.
Results: The reactions were completed with excellent yields (90 - 95%) within 2 - 5 hr. at the temperature range of 80-85oC. The reaction does not require any additional catalyst because the ionic liquid itself acts as a catalyst and solvent as well in these experiments, the products were isolated and the remaining catalyst was washed and reloaded with fresh substrates benzaldehyde and dicyclohexyl ketones. Under these conditions, no self-condition of the starting materials was observed. When similar reactions were carried out in sulphuric acid (conventional acid catalyst), a mixture of products was obtained in poor yields One of the greatest advantages of the reagent SiO2.OSO3H is that it is recoverable and reusable for several times without potential loss in its catalytic activity.
Conclusion: The conventional esterification reactions (without ionic liquids) suffer from many problems like poor efficiency, side reactions, consumption of huge environmentally hazardous chemicals, etc., The use of ionic liquids for these reactions eradicate the aforementioned problems encountered in the conventional procedures.