Aldol-condensation, polymer-analogous

Hyperbranchedpolytriallylsilanes functionalized with NCN [C H3X(CH2NMe2)2-2,6] ligands were reported by van Koten and Frey [19]. The soluble supports were used as ligands for the Pd-catalyzed aldol condensation of benzaldehyde and methyl isocyanate. Activities similar to that of parent NCN-Pd complexes were observed (Scheme 2). The hyperbranched polymeric systems showed similar properties to those of analogous dendritic compounds, indicating that structural perfection is not always required. The polymers were purified by means of dialysis, showing a potential application in continuous-flow membrane reactors. 

Few other asymmetrie reactions have been performed using insoluble or soluble polymer-supported ligands. The first example is a Mukaiyama-aldol condensation between silyl ketene acetal and different aldehydes using polymeric Box analog of 99 as chiral ligands and Cu(OTf)2 as metal soiu ce in water (Scheme 147) [216]. When using benzaldehyde as substrate, yields were very low (12-34%) and ee were moderate (40-62%) whatever the polymer-supported Box. The same level of enantioseleetivity was observed with other aldehydes while the yield was better with all the ligand/Cu complexes used. 

Efforts to prepare ladder-type polymers in a multistep process were first undertaken in the late 1930s. Marvel and coworkers [1,2] described the first attempt at the polymer-analogous cyclization of poly(methyl vinyl ketone) (Scheme 2, aldol condensation of the acetyl side groups). However, a degree of conversion (cyclization) only up to 86% is possible on purely statistical grounds. 

---