Silica dendrimers

The control of the pore size of porous silica gel by the structure of the organic polymer has been accomplished by using the so-called starburst dendrimer as an... 

Atomic force microscopy (AFM) has been used to characterize dendrimers that have been adsorbed onto a surface such as silica. AFM involves moving a finely tipped stylus across a surface and monitoring the tip movements as it traces the surface topography. In studying adsorbed dendrimers, samples can be scanned repeatedly and in a variety of directions. When this is done, it is found that all the images are the same. True dendrimers form objects of only one size. 

Knecht, M.R. and Wright, D.W. (2004) Amine-terminated dendrimers as biomimetic templates for silica nanospheres formation. Langmuir, 20, 4728-4732. 

For some reviews, see (a) Majoral J-P (2007) Influence of cationic phosphorus dendrimers on the surfactant-induced synthesis of mesostructured nanoporous silica. New J Chem 31 1259-1263 (b) Puntoriero F, Nastasi F, Cavazzini M et al (2007) Coupling synthetic antenna and electron donor species a tetranuclear mixed-metal Os(II)-Ru(II) dendrimer containing six phenothiazine donor subunits at the periphery. Coord Chem Rev... 

The use of heterogeneous catalysts in this reaction has also been achieved palladium-montmorillonite clays [93] or palladium/activated carbon [94] in the presence of dppb transformed 2-allylphenols into lactones, the regiose-lectivity of the reaction being largely dependant on the nature of the support. Very recently, palladium complexes immobilized onto silica-supported (polyaminoamido)dendrimers were used as catalysts in the presence of dppb for the cyclocarbonylation of 2-allylphenols, 2-allylanilines, 2-vinylphenols, and 2-vinylanilines affording five-, six-, or seven-membered lactones and lactams. Good conversions are realized and the catalyst can be recycled 3-5 times [95]. 

Although beyond the scope of this book, a vast amount of work has been directed to supporting homogeneous catalysts on solid supports including silica, alumina and zeolites, and functionalized dendrimers and polymers [19]. These give rise to so-called solid-liquid biphasic catalysis and in cases where the substrate and product are both liquids or gases then co-solvents are not always required. In many ways solvent-free synthesis represents the ideal method but currently solvent-free methods can only be applied to a limited number of reactions [20],... 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

As prepared and purified G4-OH(Pt5o) DENs solutions (ca. 20 /xM) can also be used to prepare high surface area sol-gel Pt/silica catalysts. This process can be used to carefully control metal loadings without condensing the solution provided the loadings remain low (less than 0.2 % Pt). The low concentrations do not allow the dendrimer to serve as a porosity template. Eurther, low loadings combined with delayed introduction of the DENs solution to a preformed sol likely lead to the occlusion of metal nanoparticles within the oxide matrix. ... 

G. Larsen, E. Lotero, and M. Marquez, Use of polypropylene tetrahexacontaamine (DAB-Am-64) dendrimer as a single-molecule template to produce mesoporous silicas, ChertL Mater. 12, 1513-1515 (2000). 

G. Larsen and E. Lotero, Amine dendrimers as templates for amorphous silicas, J. Phys. ChertL B 104, 4840-4843 (2000). 

L. Beakley, R. Cheng, S. Yost, andB. D. Chandler, Immobilization of dilute dendrimer encapsulated Pt nanoparticles in a sol-gel silica matrix, Appl. Catal. A Gen. 292, 124-129 (2005). 

S. D. Deutsch, et al, Decomposition and activation of Pt-dendrimer nanocomposites on a silica support, Catal Lett. 97, 139-143 (2004). 

The deprotection step and subsequent purification are equally facile. Exposure of a TiPS-ethynyl-protected dendrimer to tetrabutylammonium fluoride in THE results in a quantitative reaction within 2 h. The tetrabutylammonium salts can be removed by siHca plug filtration rendering pure product, and the obtained yields are higher than 95%. If deprotection is effected instead with ammonium fluoride and catalytic amounts of tetrabutylammonium fluoride, even silica plug filtration is unnecessary. 

As shown in Tab. 11.5, multi-component catalyst (27) matches the activity of its corresponding monomer (4), promoting efficient RCM of (19) in just 15 minutes at 40 °C. The reaction mixture was passed through a short column in methylene chloride to isolate the desired product. Subsequent washing of the silica with diethyl ether led to quantitative recovery of the dendritic catalyst. 400 MHz NMR analysis revealed that 13% of the styrene ligands on the dendrimer were va-... 

Scheme 9 Synthesis of three generation melamine dendrimer on silica surfaces...

The monomeric catalyst fraction showed similar R[ values as the metathesis products, which complicated the chromatographic separation and recycling procedure. Immobilization of the ruthenium catalyst on a dendrimer was anticipated to facilitate the chromatographic separation. Indeed, the presence of multiple (polar) organometallic sites on the dendrimer periphery resulted in stronger adsorption interactions between the dendritic catalyst and the silica and thus a better separation from the product. Two types of dendritic catalysts were prepared in which... 

---