Silica-supported l-proline organocatalysts for asymmetric aldolisation

New heterogenised silica-based organocatalysts have been prepared via the sol–gel process from two silylated derivatives of l-proline, featuring either a carbamate or an ether linker. Co-gelification with variable amounts of TEOS was performed with and without porogen to yield high surface area solids. These materials were evaluated as heterogeneous phase organocatalysts for the asymmetric aldol reaction of p-nitrobenzaldehyde with acetone.

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(2S,4R)-Dibenzyl 4-(3-(triethoxysilyl)propoxy)pyrrolidine-1,2-dicarboxylateC29H41NO8SiEe = 100%[α]D24=-24.5 (c 1.2, CHCl3)Source of chirality: trans-4-hydroxy-l-proline as starting materialAbsolute configuration: (2S,4R)

(2S,4R)-4-(3-(Triethoxysilyl)propoxy)pyrrolidine-2-carboxylic acidC14H29NO5SiEe = 100%[α]D24=-4.5 (c 0.5, CHCl3)Source of chirality: trans-4-hydroxy-l-proline as starting materialAbsolute configuration: (2S,4R)

(2S,4R)-4-Propoxypyrrolidine-2-carboxylic acidC8H15NO3Ee = 100%[α]D20=-30.2 (c 0.008, CHCl3)Source of chirality: trans-4-hydroxy-l-proline as starting materialAbsolute configuration: (2S,4R)

(2S,4R)-4-(Butylcarbamoyloxy)pyrrolidine-2-carboxylic acidC10H18N2O4Ee = 100%[α]D20=-11.8 (c 0.01, CHCl3)Source of chirality: trans-4-hydroxy-l-proline as starting materialAbsolute configuration: (2S,4R)

(2S,4R)-Dibenzyl 4-(allyloxy)pyrrolidine-1,2-dicarboxylateC23H25NO5Ee = 100%[α]D24=-38.8 (c 1.0, CHCl3)Source of chirality: trans-4-hydroxy-l-proline as starting materialAbsolute configuration: (2S,4R)