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Aldol Reaction in Water Catalyzed with Polymer-Supported ProlineContributor(s):Yasuhiro Uozumi, Takao Osako
Institute of Chemical Research of Catalonia, Tarragona and Universitat de Barcelona, Spain
Toward an Artificial Aldolase
Org. Lett. 2008, 10: 337-340
19 March 2008 (online)
aldol reaction - prolines - polymer support - 1,2,3-triazole linker
Polystyrene-support prolines (1-5) were prepared by the cycloaddition of the corresponding azides and alkynes. Prolines 1-5 catalyzed the aldol reaction of cyclohexanone with benzaldehyde in water to give the aldol product 6 in traces to 74% yield with 40-98% ee. When proline 5 was used for the reaction, a gel-like single phase was formed in water. Water-swollen proline 5 showed higher catalytic activity for the aldol reaction of cyclohexanone and 4-nitrobenzaldehyde (99% yield and 96% ee) than anhydrous 5 (80% yield and 89% ee). The aldol reaction of cyclic ketones with various aldehydes was also catalyzed by 5 in water at room temperature to afford the corresponding products in 16-99% yield and 94-99% ee.
In the reaction of cyclohexanone and benzaldehyde, the water-swollen 5 was superior in reaction rate and stereoselectivity to the other polymeric prolines 1-4. TGA analysis of the water-swollen 5 showed up to 24% water content in weight. Theoretical calculation suggested that water molecules formed a hydrogen-bonding network between the triazole and the amino acid group in 5. High yields, enantioselectivities, and diastereoselectivities were achieved in the reaction of cyclic ketones and aromatic aldehydes with 5 in water except for the reaction of 4-methoxybenzaldehyde. The catalyst was recycled and reused at least five times without loss of catalytic activity.