Polymers containing chiral complexes

Polymers containing chiral groups are useful for resolving racemic mixtures into the individual enantiomers [Kiniwa et al., 1987 Mathur et al., 1980 Wulff et al., 1980]. For example, the copper(II) complex of XXXXIII (either the R- or S-enantiomer) resolves racemates of amino acids [Sugden et al., 1980], The separation is based on the formation of a pair of diastereomeric complexes from the reaction of the polymer reagent with the two enantiomers. One of the enantiomers is complexed more strongly than the other and this achieves separation of the enantiomers. 

Asymmetric ECH with [Rh(L)2(Cl)2]+ complexes containing chiral polypyridyl ligands has been attempted, in homogeneous media (L = (7)-(12)) and at carbon electrodes coated with polymer films prepared by electropolymerization of [Rh(13)2(Cl)2]+ -61 62 The latter catalytic system gave the best results in terms of turnover number (up to 4,750) and enantiomeric excess, (ee) when applied to the hydrogenation of acetophenone (ee 18%) and 2-butanone (ee 10%).62 Polymeric materials derived from the complexes [RhI(bpy)(COD)]+ 36 and [Pd(bpy)2]2+33have also been applied to the ECH reaction. 

One additional factor that may add complexity in pyrolysate analysis is the presence of several diastereoisomers for the same compound. Many polymers contain in their backbone asymmetric atoms (usually carbons). A single asymmetric carbon generates two enantiomers, and these are not separated using regular chromatographic columns [17]. The number of stereoisomers of a compound with n chiral centers in the molecule... 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

Linear polystyrene has also been used to support asymmetric hydrogenation catalysts containing chiral diphosphine rhodium(I) complexes (50). Asymmetric hydrogenations of itaconic acid were carried out, forming (R)-2-raethylbutanedioic acid with e.e. s ranging from 20-37%. None of the polymer-bound catalysts were more effective than (-)-DIOP-RhCl and the observed e.e. s were found to be dependent on the molecular weight of the polymer chain, its raicrostructure and solubility. 

Helical coordination polymers containing large chiral cavities or charmels are interesting for stereospecific syntheses, separation of enantiomers and stereoselective catalysis. Crystals of 39 are obtained from a solution of nickel(II) acetate and benzoic acid in methanol which is covered with a solution of 4,4 -bipyridine in the presence of benzene or nitrobenzene [107]. Each helix turn contains three complex units and the chain consists of Ni(II) (with binding of benzoate) and the bipyridine. Because each helix is shifted against the next, large cavities are formed containing, for example, benzene or nitrobenzene. 

Parrinello G, Stille JK (1987) Asymmetric hydroformylation catalyzed by homogeneous and polymer-supported platinum complexes containing chiral phosphine ligands. J Am Chem Soc 109 7122-7127... 

---