Dendrimer effect

Maliakal AJ, Turro NJ, Bosman AW, Cornel J, Meijer EW. Relaxivity studies on dinitroxide and polynitroxyl functionalized dendrimers effect of electron exchange and structure on paramagnetic relaxation enhancement. J Phys Chem A 2003 107 8467-8475. 

Abstract Enantioselection in a stoichiometric or catalytic reaction is governed by small increments of free enthalpy of activation, and such transformations are thus in principle suited to assessing dendrimer effects which result from the immobilization of molecular catalysts. Chiral dendrimer catalysts, which possess a high level of structural regularity, molecular monodispersity and well-defined catalytic sites, have been generated either by attachment of achiral complexes to chiral dendrimer structures or by immobilization of chiral catalysts to non-chiral dendrimers. As monodispersed macromolecular supports they provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers, and have been successfully employed in continuous-flow membrane reactors. The combination of an efficient control over the environment of the active sites of multi-functional catalysts and their immobilization on an insoluble macromolecular support has resulted in the synthesis of catalytic dendronized polymers. In these, the catalysts are attached in a well-defined way to the dendritic sections, thus ensuring a well-defined microenvironment which is similar to that of the soluble molecular species or at least closely related to the dendrimer catalysts themselves. 

As has been emphasized at the beginning of this overview of asymmetric den-drimer catalysis, the kinetically controlled stereoselection depends on very small increments of free activation enthalpy. It is therefore an excellent sensitive probe for dendrimer effects and will continue to be studied in this fundamental context. As mono dispersed macromolecules, chiral dendrimer catalysts provide ideal model systems for less regularly structured but commercially more viable supports such as hyperbranched polymers. 

The aim of catalyst recycling and the exploitation of possible constructive interactions between catalytic sites underlie the rapidly growing field of stereoselective dendrimer catalysis. Since enantioselection is governed by small increments in the free enthalpy of activation, such transformations are particularly suited to assessing dendrimer effects , which result from the immobilization of catalysts. 

Scheme 7.14. Construction of homochiral dendrimers effected using (2ft,3A1)-tartaric acid-based building blocks.

M. Enomoto, T. Aida, Self-Assembly of a Copper-Ligating Dendrimer That Provides a New Non-Heme Metalloprotein Mimic Dendrimer Effects on Stability of the Bis(mu-oxo)dicopper(lll) Core , J. Am. Chem. Soc., 121,874 (1999)... 

In this chapter, we attempt to summarize the recently developed chiral dendrimer catalysts with their chiral catalytically active species located either at the core or at the periphery of the dendritic macromolecular supports. The discussion will also be focused on dendrimer effects and the development of new methodologies for the recovery and reuse of chiral dendrimer catalysts, with special emphasis on their applications in enantioselective synthesis. The published data have been classified according to the type of reachon in each of the following three sections. 

In 2000, Breinbauer and Jacobsen reported the use of dendrimer-supported Co(salen) complexes for the asymmetric ring opening of epoxides and demonstrated the first example of a positive dendrimer effect in asymmetric catalysis [95], A series of dendrimers with up to 16 catalytic sites at the periphery were synthesized by covalently attaching Co-salen to the commercially available PAMAM dendrimers with NH2-terminals (Figure 4.30). 

When using (rac)-vinylcyclohexane epoxide as a standard substrate (Scheme 4.24), the dendrimer catalysts exhibited significantly higher catalytic activities in the hydrolytic kinetic resolution of terminal epoxides as compared to the monomer or the dimer catalysts. Among the dendrimer catalysts, the first-generation dendrimer catalyst gave the best results and the efficiency of the catalyst on a per-metal basis was in the following order 4-Co(salen)-PAMAM > 8-Co(salen)-PAMAM > 16-Co(salen)-PAMAM. This dendrimer effect was considered to arise from the restricted conformation imposed by the dendrimer structure, which enhanced the... 

Recently, Soai et al. reported the synthesis of series of chiral dendrimer amino alcohol ligands based on PAMAM, hydrocarbon and carbosilane dendritic backbones (Figure 4.31) [99-102]. These chiral dendrimers were used as catalysts for the enantioselective addition of dialkylzincs to aldehydes and N-diphenylphosphi-nylimines (Scheme 4.25). The molecular structures of the dendrimer supports were shown to have a significant influence on the catalytic properties. The negative dendrimer effect for the PAMAM-bound catalysts was considered due to the fact that the nitrogen and oxygen atoms on the dendrimer skeleton could coordinate to zinc. 

In addition to the antennae effect, dendrimers are also effective insulators and exhibit the shell effect. In providing a dense shell around the incorporated chromophores, dendrimers effectively prevent aggregation which leads to non-emissive excimers and self-quenching that occurs when chromophores with small Stokes shifts are within short distances of one another. This shell effect allows for increased photoluminescence efficiency of the enclosed chromophore, which is important for optoelectronic devices. 

Li, W.S., D.L. Jiang, and T. Aida. 2004. Photoluminescence properties of discrete conjugated wires wrapped within dendrimeric envelopes Dendrimer effects on tr-electronic conjugation. Angew Chem 43 2943. 

Choi, M.-S., T. Aida, H. Luo, Y. Araki, and O. Ito (2003). Fullerene-terminated dendritic multiporphyrin arrays Dendrimer effects on photoinduced charge separation. Angew Chem. Int. Ed. 42(34), 4060-4063. 

In order to elucidate the dendrimer effects, the two-photon ionization process (308 and 266 nm) of stilbene dendrimers having a stilbene core and benzyl ether-type dendrons has been investigated in an acetonitrile 1,2-dichloroethane mixture [59]. 

Camerup AM, Ainalem M-L, Alfiedsson V, NylanderT (2011) Condensation of DNA using poly(amido amine) dendrimers effect of salt concentratitm on aggregate morphology. Soft Matter 7 760-768... 

This superior ee was assigned to the positive dendrimer effect of the PAMAM dendrimer support. The high local concentrations of the active sites were confined to the surface of the PAMAM dendrimer through a phenoxy group, which resulted in the cooperative interaction between neighboring catalytic sites. 

The precipitation of CaCOs in the absence of any additives imder comparable conation was continued. Crystalline products were immediately isolated after incubation for 3 and 20 min by centrifugation. In the case of the product after an incubation period of 3 min, vaterite and calcite coexisted. The size of the vaterite particle was 4.2 1.6p,m (Fig. 11a). In the case of the product after 20 min, the size of the vaterite particle was 10 2.3 p,m (Fig. 11b). The size of the spherical vaterite increased with an increase in incubation time. These results suggest that the anionic PAMAM dendrimers effectively modified the vaterite surface and inhibited fmther growth of the vaterite particles. The higher concentration of the PAMAM dendrimer adsorbed on the vaterite particles of CaCOs more completely and smaller-sized vaterite was produced compared with that in the lower concentration of the PAMAM dendrimer. These results suggest that the PAMAM dendrimers act as effective stabilizers for the metastable vaterite phase, and... 

We have also evidenced a clear dendrimer effect on the dimerization process involving the reduced viologen units. The dendritic structure of compounds 89 ", ... 

Helms B, Frechet JMJ (2006) The dendrimer effect in homogeneous catalysis. Adv Synth Catal 348 1125-1148... 

---