Imidazole groups, polymeric catalysts

One way around this problem is to use imidazole as a catalyst.41,42 The ATP reacts with the amino group of the amino acid forming the phos-phoroamidate which does not polymerize. In the presence of imidazole (Im) the phosphoroamidate of the amino acid is converted to the imida-zolide (ImPA). 

The peptide-based isocyanides 50a-e were successfully polymerized by Ni(II) catalysts under an inert atmosphere [63-66]. An alanine-derived isocyanide, whose isocyano group was labeled using 13C and 15N, was prepared and successfully polymerized for structure elucidation [67]. Several solvent systems were used in the polymerization of the peptide-based isocyanides, depending upon the solubility of the isocyanides. As mentioned earlier, the use of alcohols as a solvent or co-solvent can accelerate the polymerization. The protective groups on the ester, hydroxy, and imidazole groups were removed after polymerization by treatment with aqueous NaOH to yield poly(isocyanide)s bearing unprotected peptide side chains. 

Esterolytic Reactions of Active Esters Using Heterogeneous Polymeric Catalysts Containing Imidazole Groups... 

The hydrophobic effect can be described in a broad sense as a type of non-specific apolar bonding between catalyst and substrate. These hydrophobic interactions, especially in an aqueous environment, have been predominant in determining the efficiency of the catalysts.(8,10,12) Toward this end, considerable effort on our part has been directed toward the preparation of polymeric catalysts that contain pendant imidazole groups and apolar bonding sites that are soluble in highly aqueous solvent systems. 

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... 

Cooperative interactions between catalytically active imidazole groups and other functional groups has been well documented (6). The total rate equation for the polymeric catalyst can be expressed by the following equation ... 

A similar concept was used in the development of artificial chymotrypsin mimics [54]. The esterase-site was modeled by using the phosphonate template 75 as a stable transition state analogue (Scheme 13.19). The catalytic triad of the active site of chymotrypsin - that is, serine, histidine and aspartic acid (carboxy-late anion) - was mimicked by imidazole, phenolic hydroxy and carboxyl groups, respectively. The catalytically active MIP catalyst 76 was prepared using free radical polymerization, in the presence of the phosphonate template 75, methacrylic acid, ethylene glycol dimethacrylate and AIBN. The template removal conditions had a decisive influence on the efficiency of the polymer-mediated catalysis, and best results were obtained with aqueous Na2CC>3. 

ADMET polymers containing imidazole functionalities, both in the main chain and as pendant groups, have also been synthesized (Figure 13.16) [115]. Initial polymerization attempts produced only oligomers, presumably due to contamination by trace quantities of A/ -alkyl-imidazole, which had been previously reported to cause catalyst deactivation [43]. This problem was circumvented by the addition of phosphoric acid, which had been previously shown to restore catalytic activity in ROMP by protonation of the impurity [116]. In an alternative approach, the ionomer moiety was installed after polymerization [117]. 

Polymeric imidazole catalysts are the synthetic prototype of esterolytic enzymes because, like the enzymes, they have binding centers for the substrate as well as for the catalytic groups. In case of the homopolymer, polyvinylimidazole, the partially protonated groups can act as binding sites, provided the substrate is negatively charged. Thus at pH 7.5, when... 

The high selectivity for dimer formation observed with trialkylphosphines can be explained by a stepwise mechanismThe general base catalysis of oligomerization of isocyanates involves the dimeric species as intermediates in the formation of the more stable isocyanate trimers (isocyanurates). Steric effects also play a role in the dimerization of isocyanates because o-tolyl isocyanate does not dimerize. This fact is utilized in the selective dimerization of 2,4-TDI 7, using a polymeric imidazole catalyst. In this reaction only the non-hindered isocyanate group participates to form the [2-1-2] cyclodimer 8. 

The addition polymerization reaction involves polymerizing through the intermolecular reaction of the epoxy groups driven by an ionic reaction initiated by one of several types of catalyst which are present in the adhesive formulation, such as dicyandiamide, certain aromatic amines, BFs-amine complexes or imidazole complexes. The polymerization reaction typically only... 

Examples of catalysts are shown on the first line, whereas all the other compounds are coreactants including dicyandiamide, ureas, imidazoles, aliphatic polyamines, cycloaliphatic polyamides, and cycloaliphatic dicarboxylic acid anhydrides. As all the corresponding reaction mechanisms have been previously disclosed in detail," the following presentation is limited to the initial reaction steps leading to the active species involved in the polymerization or polycondensation processes. These primary attacks are enlightened in Fig. 12.6, which displays only one epoxy group reacting with catalysts or coreactants. 

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