Dendrimers, silicon-containing, synthesis

Although the first publication on dendrimers appeared in 1978 [ 1 ] it is only in 1990 that the first report on phosphorus-containing dendrimers was published [2] i.e. after numerous papers devoted to the synthesis of organic dendrimers and practically at the same time than the description of a silicon containing dendrimer. 

Krska, S. W. Son, D. Y. Seyferth, D. Organosilicon Dendrimers. In Silicon-Containing Polymers. The Science and Technology of Their Synthesis and Applications-, Jones, R. G., Ando, W., Chojnowski, J., Eds. Kluwer Dordrecht, 2000 pp 615-641. 

Catalytic hydrosilylation provides a convenient route to silicon-containing den-drimers. Combinations of hydrosilylation with alkenylation (usually allylation and vinylation) and ethynylation give powerful protocols for the rapid and efficient synthesis of silicon-containing dendrimers (mostly carbosilanes and carbosilox-anes) [4]. This topic has recently been reviewed by Majoral [31] and Schlenk and Frey [32], At present carbosilane dendrimers are the most important class of Si-based dendrimers (cf. also Section 3.2.2). 

Modifications of organic polymers with organosilicon compounds were reviewed by McGrath and co-workers (32). Synthesis and properties of silicon-containing dendrimers including hydrosilylation were the subjects of the review by Majoral and Caminade (33). Functionalization of silsesquioxanes via hydrosilylation (34,35) and production of new composite materials (36) were also presented. 

The hydrosilylation processes of carbon-carbon multiple bonds, applied in the field of polymer chemistry and materials science, are described in part three (section 4) of this article. It includes functionalization (and cross-linking) of polysiloxanes and (poly)silsesquioxanes as well as hydrosilylation polymerization of difunctional organosilicon monomers besides the application of hydrosilylation in the synthesis of silicon-containing dendrimers and hybrid organic-inorganic materials. 

Functionalized (Poly)Silsesquioxanes and Organosilicon Den-drimers. One of the most rapidly developing branches of organosilicon chemistry is that of functionalized (poly)silsesquioxanes (34,36,370-372) and silicon-containing dendrimers (4,33,373-375). They are characterized by the presence of a large number of functional groups on the surface therefore, they can be very useful precursors for the synthesis of new nanomaterials. 

The group of Van Leeuwen has reported the synthesis of a series of functionalized diphenylphosphines using carbosilane dendrimers as supports. These were applied as ligands for palladium-catalyzed allylic substitution and amination, as well as for rhodium-catalyzed hydroformylation reactions [20,21,44,45]. Carbosilane dendrimers containing two and three carbon atoms between the silicon branching points were used as models in order to investigate the effect of compactness and flexibility of the dendritic ligands on the catalytic performance of their metal complexes. Peripherally phosphine-functionalized carbosilane dendrimers (with both monodentate... 

Figure 5.30. Illustration of a poly(amidoamine-organosilicon) (PAMAMOS) dendrimer, with two generations of each PAMAM and organosihcon units. Although the PAMAMOS represents a block copolymer, an unhmited number of other variations that contain a random copolymer array, or varying dendron subunits, may also be synthesized. Reproduced with permission from Dvornic, P. R. Owen, M. J. Synthesis and Properties of Silicones and Silicone-Modified Materials, ACS Symposium Series 838,2002,236.

Scheme 1. Synthesis of silicon, phosphorus -containing dendrimers...

Cuadrado and co-workers have also been active in the synthesis of den-drimers containing ferrocene and cobaltocene moieties (241-244). The synthesis of propylenimine-based dendrimers with up to five generations and 64 peripheral ferrocenyl moieties underwent reversible oxidation processes at.Bi = 0.59 V (241,242). The guest-host relationship of some low generation dendrimers with cy-clodextrins was examined (241). Silicon-based ferrocenyl dendrimers possessing electrochemical communication between the iron centers were also synthesized (243). 

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