Stereospecificity centers

Such a species, which is unstable and is transformed into another species or solubilized after aging at 50 °C, would, in principle, have the requirements of a stereospecific center due to its limited rotational freedom. However, its attribution has been... 

The kinetic curve would then be the result of two curves, one representing the 1st order decay attributed to isospecific polymerization centers, and the other representing a stationary state attributed to the less stereospecific centers. This expression can be credited with taking into consideration a stationary state and, furthermore, it is in agreement with the inverse correlation between productivity and isotacticity of the polymer found experimentally. In fact, assuming Is to be the isotacticity of propylene produced by the isospecific centers, unstable with time, and IA the isotacticity of polypropylene produced by the less specific centers, stable with time, the total isotactic index IIt is given by the expression ... 

In TiCl3, on the other hand, AlEt2Cl activates only the predominantly stereo-specific surface sites, while AlEt3 can disrupt the crystalline lattice of the catalyst thus forming non-stereospecific centers. 

In binary catalysts two types of propagation centers can be kinetically identified stereospecific CJ and non stereospecific C. The aluminum alkyl causes the formation of such centers by means of irreversible alkylation reactions of the corresponding S and SA sites. Moreover, it brings about the reversible deactivation of the propagation species, which is preferential for the non-stereospecific centers. The external base, in equilibrium and competition with the organoaluminum, would reversibly poison the non-stereospecific centers and, to a much lower degree, also the stereospecific centers. In the ternary catalysts a further stereospecific center, would be present. This center is most likely, but not necessarily, donor associated. In this case the aluminum alkyl, besides deactivating the various active centers to different... 

Since for various catalytic systems only the relative content of different fractions changes (e.g. from 25 to 98.5% for a fraction insoluble in boiling n-heptane without changing their stereoregularity, the composition of catalytic systems influences the relative amount of isospecific and non-stereospecific centers. The reactivities of these centers (rate constants of propagation of isotactic and atactic polymers) for the titanium chloride-based catalysts are similar (Table 2 and Ref >). 

However, the comparative data on (Table 1) and the stereoregularity of polymer fractions (Table 6) for one- and two-component catalysts based on titanium chlorides indicate that the cocatalyst does not influence the reactivity and stereospecificity of the propagation centers. Its effect on the overall polymerization rate is apparently due to the change in the total number of active centers and the ratio of isospecific and non-stereospecific centers. 

When the active center is located on the catalyst surface cis-trans conversion of the alkyl may be hindered due to the steric effect of the surface. This results in the stabilization of the active centers (complex F) which have the non-equivalence sites for monomer coordination and can be stereospecific centers, If the ligand environment of the active center permits rapid conversion of the cis-alkyl group (complex F) to the trans-alkyl (complex C) the coordination sites become equivalent in this case, the active center will be nonstereospecific. 

Bukatov, G.D Zakharov, V.A. Propylene Ziegler-Natta polymerization numbers and propagation rate constants for stereospecific and non-stereospecific centers. Macromol. Chem. Phys. 2001, 202, 2003-2009. 

The statement "of the stereospecific centers" is vague. It is not clear which molecule this refers to. In addition, stereospecific refers to a reaction type. Does the author mean a stereogenic or a chiral site ... 

Further investigations showed that the means of improving enantioselectivity consists not in searches for new modifier structures but in elaboration of the stereochemical mechanism of reaction based on concepts of dual sites, the existence of chiral modified and non-stereospecific centers, on the surface of catalysts (see, Sachtlei , Klabunovskii Smith - )... 

Figure 10.3-40. The rating for the disconnection strategy carbon-heteroatom bonds is illustrated, Please focus on the nitrogen atom of the tertiary amino group. It is surrounded by three strategic bonds with different values. The low value of 9 for one ofthese bonds arises because this bond leads to a chiral center. Since its formation requires a stereospecific reaction the strategic weight of this bond has been devalued. In contrast to that, the value of the bond connecting the exocyclic rest has been increased to 85, which may be compared with its basic value as an amine bond.

Microorganisms and their enzymes have been used to functionalize nonactivated carbon atoms, to introduce centers of chirahty into optically inactive substrates, and to carry out optical resolutions of racemic mixtures (1,2,37—42). Their utifity results from the abiUty of the microbes to elaborate both constitutive and inducible enzymes that possess broad substrate specificities and also remarkable regio- and stereospecificities. 

Some of the newer compounds may contain both saturated and unsaturated rings, heteroatoms such as oxygen, nitrogen, or sulfur, and halogen substituents. Others, such as synthetic pyrethroids, may have one or more chiral centers, often needing stereospecific methods of synthesis or resolution of isomers (42). Table 4 Hsts examples of some more complex compounds. Stmctures are shown ia Eigure 1 (25). 

The occurrence of stereospecific polymerization in solution has been explained by the stetic restrictions of ligands bonded to the metal center. For example, the following stmcture has been postulated as an intermediate in solution catalysis (68) ... 

Not stereospecific racemization accompanies inversion when leaving group is located at a chirality center. (Section 8.10) Stereospecific 100% inversion of configuration at reaction site. Nucleophile attacks carbon from side opposite bond to leaving group. (Section 8.4)... 

These singlet and triplet state species exhibit the important differences in chemical behavior to be expected. The former species, with their analogy to carbonium ions, are powerful electrophiles and the relative rates of their reaction with a series of substrates increases with the availability of electrons at the reaction center their addition reactions with olefins are stereospecific. Triplet state species are expected to show the characteristics of radicals i.e., the relative rates of additions to olefins do not follow the same pattern as those of electrophilic species and the additions are not stereospecific. 

Mechanistically the observed stereospecificity can be rationalized by a concerted, pericyclic reaction. In a one-step cycloaddition reaction the dienophile 8 adds 1,4 to the diene 7 via a six-membered cyclic, aromatic transition state 9, where three r-bonds are broken and one jr- and two cr-bonds are formed. The arrangement of the substituents relative to each other at the stereogenic centers of the reactants is retained in the product 10, as a result of the stereospecific y -addition. 

Diels-Alder reaction and. 494-495 El reaction and, 392 E2 reaction and, 387-388 R.S configuration and, 297-300 S 1 reaction and, 374-375 S -2 reactions and, 363-364 Stereogenic center, 292 Stereoisomers, 111 kinds of, 310-311 number of, 302 properties of, 306 Stereospecilic, 228, 494 Stereospecific numbering, sn-glycerol 3-phosphate and, 1132 Steric hindrance, Sjvj2 reaction and, 365-366 Steric strain, 96... 

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