Helical polymeric catalysts reactions

Recently, Yashima et al. showed that optically active helical polymers bearing cinchona alkaloid derivatives as the pendant group efficiently catalyze the reaction of nitromethane with electron-poor aromatic aldehydes (up to 87% yield, 94% ee) [14]. Interestingly, these polymers exhibited a higher enantioselectivity than the monomeric chiral units on their own, demonstrating the important role of the induced helical chirality of these polymeric catalysts. 

When conducting the ROMP of norbornene or cyclooctadiene in miniemulsions [82], two approaches were followed (i) addition of a catalyst solution to a miniemulsion of the monomer and (ii) addition of the monomer to a miniemulsion of Grubbs catalyst in water. With the first approach it was possible to synthesize stable latexes with a high conversion, whereas for the second approach particles of >400 nm were created, without coagulum, but with 100% conversion. Subsequently, a water-soluble ruthenium carbene complex [poly(ethylene oxide)-based catalyst] was prepared and used in the direct miniemulsion ROMP of norbornene [83], whereby particles of 200-250 nm were produced. The catalytic polymerization of norbornene in direct miniemulsion was also carried out in the presence of an oil-soluble catalyst generated in situ, or with a water-soluble catalyst [84] the reaction was faster when using the oil-soluble catalyst. Helical-substituted polyacetylene could be efficiently polymerized in direct miniemulsion to yield a latex with particles that ranged between 60 and 400 nm in size, and which displayed an intense circular dichroism [85] that increased as the particle size decreased. The films were prepared from dried miniemulsion latexes that had been mixed with poly(vinyl alcohol) (PVA) in order to conserve the optical activity. 

Helical polyacetylene was synthesized in asymmetric reaction field consisting of chiral nematic LC. The chiral nematic LC was prepared by adding a chiroptical binaphthyl derivative as a chiral dopant to a mixture of two nematic LCs. Acetylene polymerizations were carried out using the catalyst Ti(0-n-Bu)4— EtsAl dissolved in the chiral nematic LC solvent. The polyacetylene film consisted of clockwise or counterclockwise helical structure of fibrils in SEM. Cotton effect was observed in the region of ir — ir transition of the polyacetylene chain in CD spectrum. The high-electrical conductivities of 1.5 1.8 x... 

This builds an asymmetric reaction field for Ziegler-Natta catalyst which was used for the interfacial polymerization of acetylene. Bundles of PA chains formed fibrils in a PA film, resulting in a hierarchical helical structure. Further, the winding direction could be precisely controlled by selecting the chirality of the chiral dopant. The direction of the twist in the fibril bundles has been found to be opposite to that of the N -LC reaction field. 

The first use of chiral helical polymers bearing no chiral side chains for chiral reaction induction was realized by Reggelin et at. in 2002 [69]. Two poly(methyl methacrylate)-based chiral polymers (40) was prepared by hehx-sense selective anionic polymerization of sterically congested methacrylates with a chiral base mixture as initiator. The pyridine moieties in helical polymers allowed various metal coordinations [70] or formation of ionic pairs [71]. Their complexes with palladium precursor were found to be active catalysts for the allyHc substitution reaction of l,3-diphenylprop-2-enyl acetate (Figure 4.36). Although the ee values were only moderate (<33%), this research opened up a new area for asymmetric catalysis with unnatural helical chiral polymers. 

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