L-prolinate-Amberlite Adduct: An Efficient and Recyclable Organocatalyst for the Synthesis of Biscoumarin Derivatives

Background: L-proline as a famous and most applied organocatalyst contains a nucleophilic secondary amino group and a carboxylic acid moiety functioning as a Brønsted acid has been used to catalyze more and more essential transformations. Although the L-proline is commercially available at low cost, however the organocatalytic methods in general require high catalyst loading for the achievement of high conversions in reasonable reaction times. In this research work, L-prolinate- Amberlite is introduced as an efficient and reusable organocatalyst for the one-pot synthesis of biscoumarins. Methods: L-prolinate catalyst was supported on the surface of commercially available amberlite IRA- 900OH (mesh 16-50) using ion-pair immobilization. Results: Preparation of the heterogeneous polymer-supported L-prolinate catalyst by this procedure is facile and straightforward. The reaction times for the preparation of biscoumarin derivatives using [Amb] L-prolinate adduct are shorter than reported methods. The introduced catalyst is cheap, easy to handle, shorter reaction times and the catalyst does not have any corrosion property. More importantly, its bifunctional activity can efficiently catalyze this ring opening reaction by simultaneous activation of both electrophilic and nucleophilic sites during the reaction progress. Conclusion: This methodology made the organocatalyst mobile and flexible. This condition not only helped the supported catalyst to be more efficient than its non-supported form, but also made it to be easily recoverable with simple filtration. The catalyst can be used at least eight times without substantial reduction in its catalytic activity and very low leaching amount of organocatalyst into the reaction mixture was observed.