PAMAM dendrimers amidation reaction

Since mild activation conditions appear to be important, a number of solution activation conditions were tested. PAMAM dendrimers are comprised of amide bonds, so the favorable conditions for refro-Michael addition reactions, (low pH, high temperature and the presence of water) may be able to cleave these bonds. Table 1 shows a series of reaction tests using various acid/solvent combinations to activate the dendrimer amide bonds. Characterization of the solution-activated catalysts with Atomic Absorption spectroscopy, FTIR spectroscopy and FTIR spectroscopy of adsorbed CO indicated that the solution activation generally resulted in Pt loss. Appropriate choice of solvent and acid, particularly EtOH/HOAc, minimized the leaching. FTIR spectra of these samples indicate that a substantial portion of the dendrimer amide bonds was removed by solution activation (note the small y-axis value in Figure 4 relative... 

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

Desorption reaction in water at different pHs on polyelectrolyte-adsorbed PAMAM dendrimer SAM substrates was followed up with SEIRAS (Fig. 7) [24, 110]. The characteristic amide I and II bands of NaPGA, a C-OH band of NaHA and DNA, and a P=0 band of DNA decrease with increasing pH of immersion water. The desorption is almost done within initial 30 min but proceeds slowly with time till overnight and, in all cases, the desorption reaction runs down, when the adsorbed molecules decrease down to the amount adsorbed at each pH. Namely, it can be remarked that the adsorption/desorption processes are reversible. Moreover, the present dendrimer SAMs have an advantage to be reusable for the adsorption/desorption reactions. In the case of the desorption reaction at pH 9.08 on a DNA-adsorbed PAMAM dendrimer SAM substrates, DNA on a substrate prepared at pH 3.04 does not achieve the adsorption amount at the adsorption reaction at the same pH, owing to the... 

The preceding discussions have hopefully convinced readers that the PAMAM dendrimer template offers a variety of new opportunities for studying catalysis. These properties also present challenges for evaluating nanoparticle properties, particularly reaction kinetics. Nanoparticle surface geometric and electronic properties are extremely difficult to probe in solution, especially when the dendrimer inhibits access by various probe molecules. Further, the number of bonds between nanoparticle surfaces and dendrimer amine and amide groups is essentially unknown. In cases where the dendrimer may preferentially bind one metal over another, stoichiometries and activities are difficult to evaluate, thus making it extremely difficult to interpret catalysis results in terms of particle composition. 

The synthesis of radially layered poly(amidoamine-organosilicon) (PAMAMOS) copolymeric dendrimers starts from amine terminated PAMAM dendrimers, which, in turn, are obtained by a well-known excess-reagent divergent growth method that involves a reiterative sequence of (a) Michael addition reactions of methyl acrylate (MA) to primary amines, and (b) amidation of the resulting methyl ester intermediates with ethylenediamine (EDA), as shown in Reaction Scheme 1 (39-41). These PAMAM dendrimers are commercially obtained from Dendritech Inc., (Midland, MI) and they can be used for PAMAMOS preparation without any further purification. The synthesis then involves another Michael addition reaction, this time of a silylated acryl ester, such as (3-acryloxypropyl)dimethoxymethylsilane, as shown in Reaction Scheme 2 (4). 

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