Helical polymeric catalysts

Inspired by the helical structures of naturally occurring DNA, RNA, and polypeptides, the creation and application of helical chiral polymers have received extensive attention, recently evolving into a hot research topic but still challenging field [67]. Generally, helical chiral polymers, including the static helical polymers and the dynamic and responsive helical polymers, can be prepared via polymerization of chiral monomers, or with a chiral initiator or chiral catalyst in which either a left- or right-handed helical sense is prevalent. It is to be noted that helical chiral polymers as a novel class of recyclable ligands enable asymmetric induction for catalysis [68]. 

it is highly desirable to develop new helical polymers with properties not achievable with small molecules and nonhelical polymers. 

To improve the catalytic efficiency and selectivity, Suginome s group [73] optimized their catalysts by adding chiral side chains onto the helical polymer. Similarly, these helical chiral polymers were prepared via living block copolymerization with 

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With these catalysts, the cation complexes with the monomer so weakly that a solid surface and low polymerization temperatures are required to achieve sufficient orientation for stereospecificity. Braun, Herner and Kern (217) have shown that lower polymerization temperatures are required (in n-hexane diluent) to obtain isotactic polystyrene as the alkyl metal becomes more electropositive (RNa, —20° C. RK, —60° to —70° C. and RRb, —80° C.). They correlate isotacticity with the polymerization rate as a function of catalyst, temperature or solvent. However, with Alfin catalysts, stereospecific polymerization of styrene is unrelated to rate (226). A helical polymerization mechanism as proposed by Ham (229) and Szwarc (230) is also inadequate for explaining the temperature effects since the probability for adventitious formation of several successive isotactic placements should have been the same at constant temperature in the same solvent for all catalysts. 

It was already shown by means of statistical methods that randomly coiled isotactic polymers without any true asymmetric center have no optical activity, even if these polymers are obtained with chiral initiators or polymerization catalysts. But if the polymers have a righthanded or a left-handed helical configuration in solution, the molecular asymmetry of the polymer main chain may give rise to optical rotatory power. 

Another interesting aspect of this work is the relation between conformation and catalytic activity of the polymeric catalyst. The authors showed that the main chain of PLL— Cu(II) has a helical conformation and that the selectivity ratio in the oxidation of L versus D-DOPA is proportional to the a-helix content of the polymer. However, a highly helical catalyst containing a relatively low Cu " -content (see last entry of Table X) exibited almost no selectivity - which was taken to indicate that the a-helical structure may be necessary, but not sufficient for the selectivity. (Data of Table X alone are anyway not sufficient to demonstrate per se the importance of the a-helix in catalysis, since the various helical contents have been obtained by varying temperature, and this independent parameter could in principle affect die selectivity ratio). To further substantiate the importance of the helical conformation in catalysis, Nozawa and Hataro investigated the behaviour of poly (L-a-7-diaminobutyric acid). This forms a complex with cupper, but has no helical content, and it does not catalyze the oxidation of DOPA selectivity [42]. 

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. 

Polymerization of triphenylmethyl methacrylate in the presence of a chiral anion catalyst results in a polymer with a helical structure that can be coated onto macroporous silica [742,804). Enantioselectivity in this case results from insertion and fitting of the analyte into the helical cavity. Aromatic compounds and molecules with a rigid nonplanar structure are often well resolved on this phase. The triphenylmethyl methacrylate polymers are normally used with eluents containing methanol or mixtures of hexane and 2-propanol. The polymers are soluble in aromatic hydrocarbons, chlorinated hydrocarbons and tetrahydrofuran which, therefore, are not suitable eluents. 

Introducing chirality into polymers has distinctive advantages over the use of nonchiral or atactic polymers because it adds a higher level of complexity, allowing for the formation of hierarchically organized materials. This may have benefits in high-end applications such as nanostructured materials, biomaterials, and electronic materials. Synthetically, chiral polymers are typically accessed by two methods. Firstly, optically active monomers - often obtained from natural sources - are polymerized to afford chiral polymers. Secondly, chiral catalysts are applied that induce a preferred helicity or tacticity into the polymer backbone or activate preferably one of the enantiomers [59-64]. 

As discussed in Section 3.1.6.1., natural biopolymers are useful chiral selectors, some of which are readily available they are constructed from chiral subunits (monomers), for instance, from L-amino acids or D-glucose. If synthetic chiral polymers of similar type are to be synthesized, appropriate chiral starting materials and subunits, respectively, must be found. Chiral polymers with, for example, a helical structure as the chiral element, are built using a chiral catalyst as chirality inducing agent in the polymerization step. If the chirality is based on a chiral subunit, the chirality of the polymer is inherent, whereas if the polymer is constructed from chiral starting materials, chiral subunits are formed which lead to chirally substituted synthetic polymers that in addition may order or fold themselves to a supramolecular structure (cf. polysaccharides). 

Recently, we found that the polymerization of TrMA with chiral anionic catalysts gave an optically active polymer the chirality of which is caused by helicity (12). This is the first example of optically active vinyl polymer the activity of which arises only from the helicity. This article describes the detailed results of the polymerization of TrMA by chiral anionic catalysts in addition to a brief review on our earlier studies described above. 

Some of the polymers slowly change their helicity in solution. A chiral crown ether-potassium ferf-butoxide combined system was reported to cause polymerization of methyl, tert-butyl, and benzyl methacrylate to form isotactic polymers that had high rotation values (164). Detailed scrutiny, however, raised questions about the result (135, 165). At first, in the presence of the initiator, the oligomers exhibit considerable activity, but after removal of the catalyst, the optical activity decreases. This decrease may be attributed to unwinding of the helixes in the chain the helicity could be caused by the anchored catalyst. 

Another type of synthetic polymer-based chiral stationary phase is formed when chiral catalyst are used to initiate the polymerization. In the case of poly(methyl methacrylate) polymers, introduced by Okamoto, the chirality of the polymer arises from the helicity of the polymer and not from any inherent chirality of the individual monomeric subunits (109). Columns of this type (eg, Chiralpak OT) are available from Chiral Technologies, Inc., or J. T. Baker Inc. 

Isocyanide Polymers Bulky isocyanides give polymers having a 4 1 helical conformation (115) [154]. An optically active polyisocyanide was first obtained by chromatographic resolution of poly(f-butyl isocyanide) (poly-116) using optically active poly((S)-sec-butyl isocyanide) as a stationary phase and the polymer showing positive rotation was found to possess an M-helical conformation on the basis of CD spectral analysis [155,156]. Polymerization of bulky isocyanides with chiral catalysts also leads to optically active polymers. 

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