Two-site insertion mechanism

Two-Site Insertion Mechanism for the de Novo Synthesis of Polysaccharide... 

FIG. 3.—Two-site insertion mechanism for the synthesis of Leuc. mesenteroides B-512F dextran by dextransucrase. The symbols are the same as in Fig. 2. X orients the glucosyl units so that their 6-hydroxyl groups can attack at C-l of the apposed glucosyl unit. (Adapted from ref. 58 and reprinted by permission of Academic Press.)... 

Fig. 7.—Two-site insertion mechanism for the synthesis of Leuc. mesenteroides B-742 regular comb dextran by dextransucrase. The symbols are the same as in Figs. 3 and 5.

Even though the reaction scheme is complex, attempts have been made to model the reaction and develop kinetic equations. Mooser et al.lu developed kinetic equations for a limited case based on the primer mechanism. Reh et a/.145 developed kinetic equations for the synthesis of dextran and for the maltose acceptor reaction based on the two-site insertion mechanism. Likewise, Boker et al,146 developed a kinetic equation for the formation of the fructose acceptor product, leucrose, based on the two-site insertion mechanism that included an acceptor site. An equation was derived for the overall reaction rate that was a function of the consumption of sucrose by dextran formation and acceptor-product formation. 

Figure 10.13. Two-site insertion mechanisms involved in the biosynthesis of (A) L mesenteroides B-512F dextran by dextransucrase, (B) 5. mutans mutan by mutansucrase, and (C) L mesenteroides B-1355 alternan by altemansucrase. X and Y are nucleophiles at the active site of the enzymes X orients glucosyl units to make a-1 -> 6 linkages, and Y orients glucosyl units to make a-1 3 linkages, cxi is sucrose o is glucose and < is...

B Two-site insertion mechanism for the biosynthesis of Streptococcus mutans alternating comb dextran by dextransucrase... 

Figure IQ. 15. Two-site insertion mechanism for the biosynthesis of (A) L. mesenteroides B-742 regular comb dextran and for the biosynthesis of (B) S. mutans alternating comb dextran by the respective dextransucrases.

Once the vacant coordination site is occupied by a com-plexed monomer, the next productive step is the migratory insertion of the monomer into the metal-carbon bond. Two limiting insertion mechanisms are conceivable ... 

In the framework of the chain migratory insertion mechanism (Scheme 1.3), the stereospecific behavior of the model sites depends on the relationship between the two situations obtained by exchanging, in the coordination step, the relative positions of the growing chain and the incoming monomer. Depending on the local symmetry of the coordinated bridged n-ligand, these two situations can be as follows ... 

Site control versus chain-end control. Over the years two mechanisms have been put forward as being responsible for the stereo-control of the growing polymer chain firstly the site-control mechanism and secondly the chain-end control mechanism. In the site control mechanism the structure of the catalytic site determines the way the molecule of 1-alkene will insert (enantiomorphic site control). Obviously, the Cossee mechanism belongs to this class. As we have seen previously, propene is prochiral and a catalyst may attack either the re-face or the, v/-facc. If the catalyst itself is chiral as the one drawn in Figure 10.2, a diastereomeric complex forms and there may be a preference for the... 

The mechanical properties of PLA rely on the stereochemistry of insertion of the lactide monomer into the PLA chain, and the process can be controlled by the catalyst used. Therefore, PLAs with desired microstructures (isotactic, heterotactic, and S3mdiotactic) can be derived from the rac- and W50-Iactide depending on the stereoselectivity of the metal catalysts in the course of the polymerization (Scheme 15) [66]. Fundamentally, two different polymerization mechanisms can be distinguished (1) chain-end control (depending on stereochemistry of the monomer), and (2) enantiomorphic site control (depending on chirality of the catalyst). In reality, stereocontrolled lactide polymerization can be achieved with a catalyst containing sterically encumbered active sites however, both chain-end and site control mechanisms may contribute to the overall stereocontrol [154]. Homonuclear decoupled NMR analysis is considered to be the most conclusive characterization technique to identify the PLA tacticity [155]. Homonuclear... 

The /l-olefin isomerisation-polymerisations are composed of two distinct reactions, isomerisation to the respective a-olefin and a-olefin polymerisation via a 1,2-insertion mechanism, which involve different active sites ... 

However, the actual process of insertion could not be observed directly and has been the subject of speculation. Extensive studies of the reaction of the catalyst precursor with the cocatalyst showed that 14-electron species which have alkylzirconocenium ion character (Cp2Zr(polymeryl))+ are reasonable models of the catalytically active species (121). Each of these complexes has a vacant coordination site suitable for olefin approach. Basically two types of mechanisms for insertion have been proposed (122,123) ... 

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