Asymmetric Catalysis by Layred Smectites Multi-Modified with Sugars

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Hectorite-supported rhodium (I) -chitosan complex (Rh-Chito^+/NaHT) was prepared by the intercalation of [Rh (Chito-OP) (COD) ] ^+ (Rh-Chito^+) (Chito-OP=chitosan phosphite) into sodium hectorites (NaHT) in DMF/H_2O.Asymmetric hydrogenation of alpha, beta-unsaturated carboxylates such as itaconates containing various chain lengths of alcohol groups and chemoselective hydrogenation of citronellal were studied on the intercalation compound.1 Characterization : XRD measurement showed that the basal spacing of Rh-Chito^+/NaHT was 2.02nm. The clearance space (d_<001>-0.96nm) of Rh-Chito^+/NaHT was 1.06nm which suggested that 2/1 helical structure of Rh-Chito^+ was formed in the interlayr rather than 8/5 helical structure. TEM and FT-IR measurement confirm the formation of monolayr of the rhodium complex in the interlayr of hectorite.2 Hydrogenation of itaconates and citronellal : The main hydrogenation product for dimethyl itaconates was dimethyl (R) -methylsuccinate. Small amount of dimethyl mesaconate was formed as an isomerization product during the hydrogenation. Catalytic activity remarkably depended on the reaction solvents. In MeOH,the reaction did not proceed. Asymmetric selectivity of the hetrogenized Rh catalyst was higher than that of Rh-Chito^+ (homogeneous complex). Highest selectivity (16.7% e.e (R) ) was obtained when EtOH/DMF (30 : 1) was used as a reaction solvent. Probably the controlled conformation of helical structure in the interlayr contributes the higher asymmetric selectivity for Rh-Chito^+/NaHT.Chemoselectivity was observed for the hydrogenation of citronellal. Rh-Chito^+/NaHT and Rh-Chito+ predominantly hydrogenated C=O group of citronellal. This catalytic behavior is remarkably different from [Rh (PPh_3) _2 (CCD) ] PF_6 which predominantly hydrogenated C=C bond.