Ionic catalytic dendrimer

This chapter will only deal with catalytic systems covalently attached to the support. Dendrimer [96-101], hyperbranched polymer [102, 103], or other polymer [100] encapsulated catalysts, micellar catalysis [104] and non-cova-lently bound catalysts (via ionic [105,106], fluorous, etc. intercations) are not being treated. Also catalysis with colloidal polymers [ 107,108] and biocatalysts, such as enzymes and RNA, will not be reviewed here. 

Dendrimers are branch-like polymers (see Chapter 5, Section 5.8) and have been used as nano-reactorsThese branched molecules have large voids in their structure that can incorporate a catalytic species into the inner nanoscale environment of the dendrimer. The dendrimer poly(propylene imine) (PPl) has been used as a nanoreactor for Heck reactions and alkyl aminations. 4-Dipheny Iphosphinobenzoic acid is fixed into the voids of the dendrimer via ionic contacts between the positive charges on protonated amine functional groups and the deprotonated benzoic acid and used to anchor the palladium catalyst (Figure 2.36). 

An elegant experiment by Crooks showed that the hydrophobic modification of PAMAM dendrimers by noncovalent interactions could also result in macromolecules that behave like inverted micelles (118). The spontaneous assembly between the fatty acids and amino periphery of the PAMAM dendrimer was driven by ionic interactions (Fig. 32). These dendrimers were shown to be capable of extracting hydrophilic dyes such as methyl orange fi om water into toluene. Similarly, these dendrimers were also shown to be excellent molecular containers for catalytically active metal nanoparticles. The inverted micellar nature of various dendrimers have been used by Crooks and others for the preparation of a variety of nanoparticles (119-122). A related macromolecule, but an architecture with less of a control, is a hyperbranched polymer. Hydrophobically modified hyperbranched polymers have also been shown to be capable of acting as inverted micelles (123,124). 

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