Polymeric catalyst-substrate complex

Figure 11. Illustration of Equation 1 for the calculation of the increase in number of intermediate chain conformations accompanying deformation and activation of the polymeric catalyst-substrate complexes...

Anyhow, our study has demonstrated the benefit of "strained" polymeric catalyst-substrate complexes, a phenomenon well-known in enzymology (26) and once indicated by the term "entatic state" (16). 

Until recently, catalysts were discussed in connection with the start of polymerization but today we know that the compounds initiating chain reactions do not satisfy the definition of a catalyst. A catalyst and a substrate form a transition complex which is decomposed to the product and the regenerated catalyst. The path to the product over the catalyst—substrate complex crosses a lower energy barrier (has a lower activation energy) than... 

The ability of transition-metal complexes to activate substrates such as alkenes and dihydrogen with respect to low-barrier bond rearrangements underlies a large number of important catalytic transformations, such as hydrogenation and hydroformy-lation of alkenes. However, activation alone is insufficient if it is indiscriminate. In this section we examine a particularly important class of alkene-polymerization catalysts that exhibit exquisite control of reaction stereoselectivity and regioselec-tivity as well as extraordinary catalytic power, the foundation for modern industries based on inexpensive tailored polymers. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Some catalysts suffer a different type of alkyne poisoning. Chlorotris(triphenylphosphine)rhodium(I) is an effective terminal alkyne polymerization catalyst. When this complex is used in the reduction of these alkynes, it gradually loses its activity because of the competing polymerization reaction. Even initially the rate of alkyne hydrogenation is much slower than that of the corresponding alkene because of the greater binding constant of the former substrate. 

While rare-earth metals are proven powerful olefin polymerization catalysts [21-24], there are only limited reports on controlled olefin oligomerizations or selective olefin dimerizations utilizing these elements [204,207,208], An ansa-scandocene [207] and the bis(indenyl)yttrium complex 41 (Fig. 25) [204] were reported to produce head-to-tail dimers from monosubstimted aliphatic alkenes (57). Complex 41 produces predominantly the tail-to tail adduct with styrene. The codimerization of an aliphatic alkene (including substrates containing various functionalities) with styrene affords tran -tail-to-tail dimers, apparently as a result of 1,2-insertion of the a-olefin followed by 2,1-insertion of styrene directed by the phenyl group (58). 

Polymer-supported chiral (salen)Mn complexes 131 were also used in other asymmetric epoxidation reactions (Scheme 3.37). For example, cis-P-methylstyrene 132 was efficiently epoxidized with uj-CPBA/NMO in the presence of the polymeric catalyst [73]. For most of the tested substrates, the enantioselectivities... 

Takaya and Nozaki invented an unsymmetrical phosphin-phosphite ligand, (R,S)-BINAPHOS, which was used in the Rh(l)-catalyzed asymmetric hydroformylation of a wide range of prochiral olefins, with excellent enantioselectivities [120, 155]. A highly crosslinked PS-supported fR,S)-BINAPHOS(257)-Rh(I) complex was prepared and applied to the same reaction (Scheme 3.83) [156]. Using the polymeric catalyst, the asymmetric hydroformylation of olefins was performed in the absence of organic solvents. The reaction of cis-2-butene, a gaseous substrate, provided (S -methylbutanal with 100% regioselectivity and 82% ee upon treatment with II, and CO in a batchwise reactor equipped with a fixed bed. 

A ten- to twenty-fold concentration excess of pol3nneric imidazole residues over that of substrate molecules was usually employed. This allowed a pseudo first-order presentation of the kinetic data.. In many cases, curvature in the plots of In versus time was found. Observation of complex kinetics in hydrolysis of functional groups on polymer chains is not uncommon. (30,31) Letsinger and Klaus(32) have observed some phenomena in the study of synthetic polymeric catalysts and substrates. In treating the data, they used the empirical relation... 

Mechanisms A and B represent the general basic catalysis. Cooperative interaction between two neutral imidazole molecules enhances the nucleophilic attack on the substrate. Mechanism C is a variety of the general acid catalysis caused by the nucleophilic attack. To prove the polyfunctional character of the catalysis, let us compare activation parameters of polymers with their low-molecular weight analogues. For instance, for a polymeric catalyst the change in enthalpy (AH) is 15.5 kJ/mole, whereas for imidazole this value is 29.4kJ/mole [23]. Additional entropy is obtained from the formation of a transition-state complex in which catalytic and reactive groups are oriented with respect to each other. Besides, with the transition from a low-molecular... 

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