Stereospecific sites

Section 3 will deal with catalytic systems whose stereospecificity is mainly controlled by the chirality of the environment of the transition metal, independently of the possible chirality of the growing chain (chiral site stereocontrol). In particular, in Section 3.1 the chirality and stereospecificity of homogeneous catalytic systems based on metallocenes of different symmetries and in different experimental conditions will be reviewed. In Section 3.2 the chirality of model catalytic sites, which have been supposed for isospecific first-generation TiCl3-based and high-yield MgC -supported catalysts, is described. In Section 3.3 we will present a comparison between model catalytic sites proposed for heterogeneous and homogeneous stereospecific site-controlled catalysts. 

Using a similar catalyst, Kashiwa 83 n8> also noticed an increase of the isotactic production rate after the addition of EB, although the total productivity was lowered. Moreover, he found that the molecular weight of the isotactic polymer increased, while the number of the isospecific sites did not change, and hence concluded that the effect of the Lewis base was to increase the propagation rate constant (kp) of the stereospecific sites. 

The increase in isotacticity seems to be essentially connected to the decrease of the initial rate, as practically no change in the isotacticity index with polymerization time was detected. Moreover, while the atactic productivity decreases monotonically with the EB/TEA ratio in both systems, the isotactic productivity has a more complex behavior with the binary catalyst it remains almost unchanged up to EB/TEA s 0.25 and then falls, whereas with the ternary catalyst it increases up to EB/TEA 0.2 and then rapidly drops. On the grounds of these results, Spitz suggested that the reversible adsorption on the catalytic surface of the TEA EB complex (which is supposed to be very fast) changes the non specific centers into stereospedfic, though less active, centers, while the slower adsorption of free EB reversibly poisons both types of sites. The differences between the binary and the ternary catalysts would arise mainly from the presence, in the latter, of a larger number of potential stereospecific sites. 

Even though many chemically diverse compounds have demonstrated activity, studies indicate certain structural commonalities. The interaction of these drugs with AR seems to be at a common stereospecific site in the enzyme. The molecular requirements appear to be a planar structure with two hydrophobic (aromatic) areas and a carbonyl moiety susceptible to (reversible) nucleophilic attack. An inhibitor site proposes a charge-transfer complex formation between the carbonyl and nucleophile residue. 

The properties of the glucose transport system may be summarized as follows. The process involves combination of the sugar with a stereospecific site on the membrane and is distinct from diffusion. The process is reversible, and free sugar is liberated on either side of the membrane. Transport... 

It is necessary in the mathematical equation to take into account both stereospecific sites of concentration and CJ on which homopropagation takes place and non stereospecific sites C on which cross propagation occurs. 

The simplest explanation is that kR = ks for stereospecific sites and that the election results from different concentrations of R and S type sites. For the amorphous part, one could consider the limiting case for which cross propagation occurs on all sites with different rates of addition of R and S leading to a kinetic choice. More probably as proposed by Tsuruta [5], there is an existence of a whole spectrum of sites having different degrees of R and S nature. 

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