Recoverable silica-gel supported binam-prolinamides as organocatalysts for the enantioselective solvent-free intra- and intermolecular aldol reaction

Silica-gel supported binam-derived prolinamides are efficient organocatalysts for the direct intramolecular and intermolecular aldol reaction under solvent-free conditions using conventional magnetic stirring. These organocatalysts in combination with benzoic acid showed similar results to those obtained under similar homogeneous reaction conditions using an organocatalyst of related structure. For the intermolecular process, the aldol products were obtained at room temperature and using only 2 equiv of the ketone with high yields, regio-, diastereo- and enantioselectivities. Under these reaction conditions, also the cross aldol reaction between aldehydes is possible. The recovered catalyst can be reused up to nine times providing similar results. More interestingly, these heterogeneous organocatalysts can be used in the intramolecular aldol reaction allowing the synthesis of the Wieland–Miescher and ketone analogues with up to 92% ee, with its reused being possible up to five times without detrimental on the obtained results.

This research was supported by the Ministerio de Ciencia e Innovación (MICINN: Projects CTQ2007-62771/BQU, CTQ2010-20387, CTQ2008-05409-C02-01, CTQ2011-22649 and Consolider Ingenio 2010 CSD2007-00006), FEDER, the Generalitat Valenciana (Project PROMETEO/2009/039), the DURSI-Generalitat de Catalunya (2009SGR-1441), the University of Alicante and the EU (ORCA action CM0905). A.B.-C. thanks the Spanish MICINN for a predoctoral fellowship (FPU AP2009-3601).