Solvent Free and Microwave-assisted Synthesis of Functionalized Trisubstituted Alkenes using Water Stable and Reusable Indium(III) Triflate as the Catalyst

Background: Trisubstituted alkenes are synthetically very important and recognised as versatile substrates for Diels-Alder reactions, Michael addition reactions and Morita-Baylis-Hilman reactions. Among the available procedures, Knoevenagel condensation is considered the most efficient method of synthesizing these alkenes. Numerous reaction conditions and catalysts have been reported till date. However, most of these procedures are being phased out to comply with the increasing demands for environmental protection. Clean, expeditious, sustainable and environmentally benign protocol for Knoevenagel condensation is required. Methods: Solvent free and microwave assisted greener synthetic strategy has been developed by making use of indium(III) triflate as the catalyst. Results: Under optimized reaction conditions, 25 sets of reactions using nucleophiles such as malonates, malonic acid and acetylacetone have been carried out and all of them have afforded the desired products in excellent yields. In condensation of two bulkier malonates to benzaldehydes, it was observed that the bulkiness of the malonates did not have much effect on the outcome of the reactions. The catalyst can be easily recovered quantitatively and reused. Conclusion: By making use of water stable and reusable indium(III) triflate, a simple, effective, convenient and environment- friendly protocol for the synthesis of trisubstituted alkenes has been developed. The yields are excellent and the catalyst could be recovered easily and reused. Solvent free condition, microwave irradiation, easy recovery and reusability of the catalyst are the significant features of this greener synthetic strategy.