Stereoselectivity achiral olefins

The asymmetric cyclisation of achiral olefinic organohthium reagents by a stereogenic alkah metal centre can be modulated by ( )-sparteine, which confers enantiofacial selectivity on the reaction such that the anionic cyclisation process discriminates between the enantiotopic faces of an unactivated C=C bond. Recently, modifications have been made to the well known hthium-ene cyclisation reaction whereby the subsequent expulsion of a thiophenoxide group yields a fused vinylcyclopropane. Moreover, allylic lithium oxyanion-induced reactivity and stereoselectivity in this intramolecular carbometallation has been demonstrated in the highly stereoselective synthesis of a natural bicyclo[3.1.0] hexane. ... 

The dissection of a molecular model into those components that are deemed to be essential for the understanding of the stereochemistry of the whole may be termed factorization (9). The first and most important step toward this goal was taken by van t Hoff and Le Bel when they introduced the concept of the asymmetric carbon atom (10a, 1 la) and discussed the achiral stereoisomerism of the olefins (10b,lib). We need such factorization not only for the enumeration and description of possible stereoisomers, important as these objectives are, but also, as we have seen, for the understanding of stereoselective reactions. More subtle differences also giving rise to differences in reactivity with chiral reagents, but referable to products of a different factorization, will be taken up in Sect. IX. 

As mentioned earlier, the cycloaddition of chiral nitrile oxides to achiral alkenes generally results in poor stereoselection. The cycloaddition of glyceronitrile oxide acetonide and 2-0-benzyl-glyceronitrile oxide to mono-, 1,1-di- and 1,2-disubsti-tuted olefins have been studied most extensively (18,23,121,207,215,221,225,234). 

It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis - achiral or chiral - has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6]. 

Fig. 11.15. Analysis of the overall stereoselectivity of a Still—Gennari olefination such as the one in Figure 11.13 simple diastereoselectivity of the formation of the alkoxide intermediate from the achiral phosphonate A and the achiral aldehyde B. For both reagents the terms "back face" and "front face" refer to the selected projection.

Despite the catalysts different stereoselectivity in a-olefin polymerization, all three favored the formation of the meio-hydrodimer. In case of the achiral catalyst Cp2ZrCl2 (1, Cp = cyclopentadienyl), this has to be attributed to chain-end control since the Cp ligands do not induce stereopreference. From the meso/rac ratio, a difference in the free energies of activation (AG g o - AGg g) of 1.5 kJ/mol at 30 °C has been calculated. 

Stereoselectivity in the insertion of olefins is brought about by a chiral environment of the coordination sphere. When the selectivity is predominantly governed by chirality of the catalyst itself, stereospecificity shows catalytic-site control , also called catalyst control or enantiomorphic-site control . Catalysts must have chiral structures (but need not be homochiral) in this case. Even though a catalyst is achiral, an inserted a-olefin can make a chiral center at the end of a polymer chain. When this chirality controls the stereoselectivity of the next monomer insertion, it is called chain-end control . 

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