Asymmetric selection

Extensive studies of stereoselective polymerization of epoxides were carried out by Tsuruta et al.21 s. Copolymerization of a racemic mixture of propylene oxide with a diethylzinc-methanol catalyst yielded a crystalline polymer, which was resolved into optically active polymers216 217. Asymmetric selective polymerization of d-propylene oxide from a racemic mixture occurs with asymmetric catalysts such as diethyzinc- (+) bomeol218. This reaction is explained by the asymmetric adsorption of monomers onto the enantiomorphic catalyst site219. Furukawa220 compared the selectivities of asymmetric catalysts composed of diethylzinc amino acid combinations and attributed the selectivity to the bulkiness of the substituents in the amino acid. With propylene sulfide, excellent asymmetric selective polymerization was observed with a catalyst consisting of diethylzinc and a tertiary-butyl substituted a-glycol221,222. ... 

The routine SavGo 1. m is very basic. It does not include the beginning and the end of the curve, and it does not allow asymmetric selection of data points for the polynomial fitting. Both these features could easily be implemented. We leave it to the reader to improve the function accordingly. In its present form it cannot be used for non-equidistant x-values. F would have to be recalculated within the loop but the vector xl used to generate F would still have to be centred around zero. 

A stereoelective (252, 299, 300) or asymmetric selective (298) or enantioasymmetric (301) polymerization where one of the enantiomers polymerizes in a preferential way in an ideal case 50% of the monomer is converted into a pure optically active polymer while the remainder is recovered as nonreacted compound also in the pure enantiomeric form (e.g., R -t- nS). ... 

Hatano et al.l39 found that the poly(L-lysine)-Cu(II) complex exerted asymmetrically selective catalysis on the hydrolysis of phenylalanine ester, whereas Cu ions and bis(bipyridyl)Cu had no catalytic activity. The great stability of the intermediate PLL-Cu complex with the D-ester was considered responsible for the catalytic activity. 

From the copolymer composition dependence of the molar ratio of the d- and L-enantiomeric units of 15e in the copolymer, the rate of reaction of the growing chain end of 21 with the D-enantiomer of 15e was estimated to be about four times faster than that with the L-enantiomer. Such asymmetric selection is mainly ascribable to the steric and electronic interactions between the asymmetric environment created by the bulky terminal unit of 21 and the rigid bicyclic monomer having three asymmetric centers and a polar bulky bromine substituent (Scheme 6, [23]). 

In addition to this we have several examples of which the polymer conformation of the polymeric complex leads the asymmetrical selectivity Hydrogenation reactions of 1-methylcinnamic acid and 1-acetamidocinnamic acid by several poly(L-amino acid)-Pd complexes are observed (142-144). Poly(L-valine) (/3-form) and poly(/3-benzyl-L-aspartate) (a-helix, sinistral) give dextrorotative products, and poly(L-leucine) and poly( 3-benzyl-L-aspartate) (a-helix, dextral) do levo-rotatory products. Also, optical active poly-/3-hydroxyl esters-Raney Ni catalyst (145) and Ion-exchange resin modified by optical active amino acid-metal complex (146,147) are observed in asymmetrically selective hydrogenations. 

The catalyst systems are also effective as initiators for ring-opening polymerization of olefin sulfides (57, 58, 224, 232, 284, 285, 297, 303, 548). Asymmetric selection polymerization has also been performed with olefin sulfides (14, 156, 430, 470-472). 

Y. Okamato, K. Suzuki, T. Kitayama, H. Yuki, H. Kageyama, K, Miki, N. Tanaka, and N. Kasai, Kinetic resolution of racemic methylbenzyl methacrylate Asymmetric selective polymerization catalysed by Grignard reagent-(-)-sparteine derivative complexes, /. Am, Chem, Soc., 104 4618 (1982). 

It has been known for some time that NCAs of racemic a-amino acids can be converted to optically active poly a-amino acids with the aid of optically active alcohols or combinations of such alcohols with metal alkyls as initiators. Such processes have been termed asymmetric-selective or stereo-selective . Some of these investigations will be described subsequently. Biihrer and Elias [63] have recently made a comprehensive investigation of the kinetics of polymerization of DL-leucine NCA initiated by a series of optically active primary and... 

Asymmetric selectivity in copolymerization of the enantiomorphs of alanine NCA initiated by organoaluminium derivatives has also been reported [47, 49, 51]. The authors state that polyalanine obtained from an L-rich monomer feed with the aid of AlEt3 is, in the initial stages of reaction, enriched in the D isomer, but that with extensive conversion the major enantiomorph in the monomer feed is preferentially selected. This surprising result was not observed when di-isobutyl aluminium... 

Another type of activation of aluminum alkyl was found in the asymmetric-selective polymerization of epichlorohydrin (ECH) with an optically active cobalt-salen type complex [Co (II)]. The structure of the salen-type cobalt complex was shown previously (13, 14). In a benzene solution of the binary system consisting of [Co (II)] and AlEt, no evolution of ethane or ethylene was observed at room temperature. The NMR signals of the methyl protons for AlEt shifted down field on mixing with [Co (II)]. These observations together with a circular dichroism study indicated that AlEt and [Co (II)] formed a molecular complex in benzene, none of Al-Et bonds being cleaved by this complexation. 

If Ziegler catalysts obtained from a transition metal halide and an optically active alkylmetal are used64,65, the auxiliary is incorporated and the polymer contains chiral chain ends. With this result, however, it is not clear whether asymmetric selection arises from control by the growing chain or by a chiral catalytic site. Chiral auxiliaries which are not incorporated into the chain end, such as (- )-a-pinene, also lead to optically active polymer and recovered starting material66,67. 

Asymmetric allylation and crotylation, synthetically equivalent to the aldol reaction, have been extensively studied and have become a very useful procedure for preparation of propionate units. Among various chiral ligands on boron-developed, isopinocampheyl- and tartrate-derived reagents, 51 and 52, which were developed by Brown et al. [18] and Roush et al. [19], respectively, are the most commonly used (Scheme 7). Reaction of aldehyde with (Sl-Sla or 52a gave anu -adduct 54, while that using (Z)-51b or 52b afforded syn-adduct 53 with high asymmetric selectivity. 

Representative examples are shown in Scheme 9. The Sharpless AE of geraniol (57) with (+)-diethyl tartrate (DET) gave a-epoxide 58 with 95% ee. In a double asymmetric induction, epoxidation of allylic alcohol 59 with (—)- and (+)-DET provided a- and P-epoxides, 60 and 61, in ratios of 40 1 and 1 14, respectively [23]. It is noteworthy that high asymmetric selectivity was induced even in the mismatched case. The Sharpless AE is also effective for the kinetic resolution of racemic allylic alcohols. In the reaction of 62 with 0.6 equiv. of t-BuOOH and... 

An interesting variation of the catalysis by poly(amino acids) was reported by Hatano et al. (141). Several metal ions and their complexes are known to be effective catalysts for the hydrolysis of amino acid esters, in wdiich coordination of substrates to the metal ion renders nucleophilic attack by H2O more facile 142). Hatano et al. found ttat D-phenylalanine methyl ester 20 was catalytically hydrolyzed by poly-L-lysine-Cu(II) complex 3 to 4 times more efficiently than the L-iso-mer. The rate difference arose from the lower enthalpy of activation for the D-sub-strate. At die same time, the circular dichroism data indicated that formation of the catalj t-substrate complex was more favorable with the D-ester than with the L-ester (74J). Ap endy, poly-L-lysine helped form an asymmetrically selective binding site. 

Niwa, S., Imai, S., Orito, Y. (1982) Asymmetric hydrogenation of ethyl benzoylformate using the platinum-carbon catalyst modified with cin-chonidine-influence of preparation of the catalyst on the asymmetric selectivity, J. Chem. Soc. Jpn. Nippon Kagaku Kaishi) Nl, 137-138. 

Introduction of relatively weak functional groups, such as carbonyl, hydroxyl, nitro, amide, etc., in the nanochannels of PCPs would affect the monomer alignment, which may lead to precision control of stereoselectivity and regioselectivity of the resulting polymers. In particular, PCPs with either helical or chiral structures on the pore surface are of intense interest in chemistry and such porous solids are potentially useful to find applications in enantioselective sorption/separation and catalysis [34, 38 0, 42, 45]. Of considerable interest is the use of the chiral channels to affect asymmetric polymerizations such as asymmetric selective polymerization of racemic monomers as well as asymmetric polymerization of prochiral monomers, which may give helical polymer conformations. 

K. Hatton, K. Takahashi, M. Uematsu, N. Sakai, Multiple interactions between host cyclodextrin and guest compound assisting asymmetrically selective reduction with NaBH4 in aqueous media, Chem. Lett., 1990, 19, 1463-1466 K. Hattori, K. Takahashi, N. Sakai, Enantioface differentiating reduction of keto-acid in the presence of 6-deoxy-6-amino-j3-cyclodextrin with NaBHi in aqueous media. Bull. Chem. Soc. Jpn., 1992,65,2690-2696 K. Hattori, K. Takahashi, Asymmetric reduction of prochiral inclusion complex in aqueous media, Supramol. Chem., 1993, 2, 209-213. 

The asymmetric selectivity arises from the preferential formation of (S)-elective center at the beginning followed by the formation of (R)-elective center after the consumption of most of the (S)-monomer. The copolymerization of the (RS)-mono-mer and methyl methacrylate by this complex yielded a highly isotactic copolymer in which the (S>monomer predominantly incorporated over the (R)-monomer. On the other hand, in the copolymerization with a,a-dimethylbenzyl methacrylate only the homopolymer of a-methylbenzyl methacrylate was obtained with the same as-i mmetric selectivity as in the homopolymerization of this monomer. The results indicate that the steric interaction between the methyl group at the a- rosition of benzyl ester and the (-)-sparteine moiety of the catalyst plays an important role in the stereoelection of the polymerization. 

T. Tsuruta, Stereoselective and asymmetric-selective (or stereoelective) polymerizations, J. Polym. Sci. 0 6, 119 (1972). 

Chirality transfer polymerizations leading to an excess of one stereoisomeric unit asymmetric selective, asymmetric transforming, or stereoelective polymerization. 

Matsuura, K. Inoue, S. Tsuruta, T. Asymmetric-selective polymerization of DL-propylene oxide with triethylaluminum-V-carboxy-L(-E)-alanine anhydride system. Makmmol. Chem. 1965, 86, 316-319. 

The catalytic asymmetric /-selective Diels-Alder annulation of a, -unsaturated /-butyrolactams with enones provided a synthesis of, y-functionalized bridged bi-or tri-cyclic dihydropyranopyrrolidin-2-ones in one step (up to 98% yield, >20 1 dr, and 99% ee) The inverse-electron-demand aza-Diels-Alder cycloaddition 0 of A-aryl-a,/0-unsaturated ketimines with enecarbamates in the presence of chiral bifunctional phosphoric acids produced 4,5,6-trisubstituted 1,4,5,6-tetrahydropyridines having three contiguous stereogenic centres in up to 84% yield, 95 5 dr, and 95% 5-Alkenylthiazoles react as in-out dienes with e-poor dienophiles in polar 44-2- 0 cycloaddition reactions. The cycloadditions are site selective. The mechanism is thought to lie between a concerted but highly asynchronous process and a stepwise process. 

A "stereoelective" (17) or "asymmetric-selective" (3) polymerization is a process in which a single stereoisomer oT a mixture is polymerized giving macromolecules containing one type of configurational base units. For example an optically active catalyst will choose one enantiomer from a racemic mixture and form a macromolecule containing only one type of enantiomeric units. Such an ideal reaction should stop at 50 % yield after consumption of the corresponding stereoisomer. 

This kind of polymerization, which has been called "stereoelective (91) or "asymmetric selective (92), corresponds to a kinetic resolution of a racemic monomer. 

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