Chiral active centers, olefins

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Stereoelective polymerization (type 3) requires the presence of a chiral catalyst with an excess of active centers of a given configuration or with a differential reactivity of the centers that catalyze polymerization of one or the other of the two enantiomers (299). With regard to racemic a-olefins, the best results were obtained with 3,7-dimethyloctene in the presence of TiCl, + Zn[(S)-2-methylbutyl]2 as catalyst (309). The resulting polymer is dextrorotatory, la o = -1-16.1, and the residual monomer is levorotatory, a o = —0.63, (310). These values indicate a rather ihodest degree of stereoelectivity. 

Thus, recent experimental data and calculationsshow the important role of the end of the growing polymer chain on the stereospecificity of the olefin addition. This role is due to the influence of the carbon atom (Cp, C ) of the main chain on the isospecificity of the olefin addition and not to the effect of the substituent of the last monomeric unit as in the case of the syndiospecific addition. The growing polymer chain is one of,the ligand which, together with other ligands of titanium ions, determines the chirality of the active center. 

In 2002, Hoveyda and coworkers introduced an alternative concept to install chirality in ruthenium olefin metathesis complexes through a Ci-symmetric bidentate NHC ligand, bearing binaphtholate moieties (Figure 11.33). The NHCs in this type of complexes lacked backbone substitution and it was chelation that prevented free rotation of the ligand [118]. In this case, chiral information installed within the A-substituent was transferred directly to the ruthenium center. Unfortunately, these complexes were found to be less active because of the reduced Lewis acidity at the metal center, mainly due to the exchange of Cl... 

Fractionation of the polymer by solvent extraction, optical rotation ([a]o ) and IR absorption (D763/D732) of the fractions indicated that the more soluble polymer fractions consisted, as expected, of copolymer of (S)-monomers, whereas the last fraction was based on poly[(R)DMO)]. In the presence of the above high yield system, activated by Al(i-Bu)3 and p-methyl toluate (MT) as external base, DM0 and (S)3MP (3.8/1 molar ratio) gave results very similar to the conventional catalysL This indicates that the chiral recognition of active centers capable of polymerizing these sterically hindered branched olefins remains substantially the same even in the presence of the support (MgCl2) and the Lewis base. 

A 7r-bond can react with various active species, such as the electrophile oxene and its isoelec-tronic species (nitrenes and carbenes) and radicals. A 7r-bond can also react with a nucleophile, when it is conjugated with an electron-withdrawing group. In these reactions O, N, or C atom(s) are transferred from the active species to the olefins, forming two tr-bonds, such as C—O, C—N, and C—C, at the expense of the 7r-bond. If the 7r-bond is prochiral, chiral center(s) are... 

Hetero Diels-Alder reactions are very useful for constructing heterocyclic compounds, and many important chiral molecules have thus been synthesized. Although the retro Diels-Alder reaction does not itself involve the asymmetric formation of chiral centers, this reaction can still be used as an important tool in organic synthesis, especially in the synthesis of some thermodynamically less stable compounds. The temporarily formed Diels-Alder adduct can be considered as a protected active olefin moiety. Cyclopentadiene dimer was initially used, but it proved difficult to carry out the pyrrolytic process. Pentamethyl cyclopentadiene was then used, and it was found that a retro Diels-Alder reaction could easily be carried out under mild conditions. 

The mechanistic study on the hydrophosphination of activated olefins, in conjunction with rapid inversion of the configuration at the phosphorus center, was elaborated to develop asymmetric hydrophosphination catalyzed by a chiral phosphine platinum complex although the % ee is not excitingly high yet (Scheme 9) [15]. 

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