Carbosilane dendrimers, synthesis

Carbosilane Dendrimer Synthesis. is also used as a building block for the preparation of carbosilane dendrimers. The preparation of these dendrimers follows the method of van der Made, which involves alternating cycles of platinum-catalyzed hydrosilylation of a terminal alkene with a chlorosilane and alkylation of the resulting chlorosilane with allylmagnesium bromide. The first step in the preparation of the dendrimeric structure involves the hydrosilylation of with dichloromethylsilane in the presence of platinum on carbon as the catalyst. This process introduces a methyldichlorosubstituent on each of the four vinyl groups to afford 7 in excellent yield (eq 5). 

Figure 4.15. Synthesis of phosphine-functionalized carbosilane dendrimers.[31]...

A stochiometric approach was applied by Van Koten and co-workers [29], who used chiral carbosilane dendrimers as soluble supports in the in situ ester enolate-imine condensation in the synthesis of /Mactams (e.g. 19, Scheme 20). The formation of the /Mactam products proceeded with high trans selectivity, and with the same level of stereoinduction as was earlier established in reactions without the dendritic supports, (i.e. the use of the enantiopure dendritic support did not affect the enantioselectivity of the C-C bond formation). After the reaction, the dendrimer species could be separated from the product by precipitation or GPC techniques and reused again. 

The different carbosilane dendrimer supports (generation 0, 1 R=H, Me) were then used for the synthesis of the / -lactam (13). As shown in Scheme 7.2, the first step was again an immobilization of a carboxylic acid via ester bond formation. Treatment with LDA and ZnCl2 yielded in situ the corresponding zinc ester enolate (11) which reacts with N-(trimethylsilyl)phenylimine (12) to form the final four membered lactam ring (13). The last reaction step includes several intermediates. The last one is a supported /9-amino ester which undergoes spontaneous... 

The synthesis of carbosilane dendrimers with peripheral acetylenedicobalt hexa-carbonyl substituents has been reported Seyferth, D. Kugita, T. Organometallics 1995,14,5362. 

A combination of hydrosilylation-ethynylation and hydrosilylation-reduction is successfully applied to the syntheses of carbosilane dendrimers bearing 4, 8 and 12 end-ethynyl groups as well as their Co2(CO)6 complexes263. The synthesis of a second generation dendrimer 268 possessing twelve Co2(CO)6(HC=C—) units is shown in Scheme 29 as an example. The structure of 268 is confirmed by X-ray crystallographic analysis263. 

Reports of carbosilane dendrimers were published from 1990 onwards by the working groups of van der Made, van Leeuwen [76], Roovers [77], Muzafarov [78], and Seyferth [79]. Synthesis was generally accomplished in two steps ... 

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... 

Olefin metathesis has become a very important reaction in polymer chemistry and natural product synthesis [47-49]. Garber et al. have used the physical properties of dendrimers in order to improve the separation between the dendritic metathesis catalyst and products on silica gel column chromatography [50]. The Van Koten group has reported on the synthesis of different generations of carbosilane dendrimers functionalized with ruthenium metathesis catalysts [51]. 

Scheme 4.23. Synthesis of ferrocene terminated carbosilane dendrimers.

Kim and Kwon reported the divergent synthesis of a dendritic carbosiloxane from a cyclic siloxane tetramer core130 using the standard carbosilane dendrimer synthetic approach. Allyl alcohol was employed as the nucleophilic reagent and trichlorosilane... 

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). 

Carbosilane dendrimers containing titanium and zirconium complexes on their periphery have been prepared and used in olefin polymerization reactions.Generally, the best synthetic route to these materials involves the synthesis of generations of suitably functionalized dendrimers to which the metal is added in the final step giving products such as 276. Routes involving the incorporation of pre-metallated building blocks gave only low yields of the desired dendrimers. 

The efficiency of the linking reactions of polychlorosi-lanes with poly(dienyl)lithium compounds has been documented by synthesis of well-defined, narrow molecular weight distribution, 18-armed star-branched polyisoprenes, polybutadienes, and butadiene end-capped polystyrenes by linking reactions with a decaoctachlorosilane [(SiCl)ig] [256, 257]. The linking reactions of poly(butadienyl)lithium (Mjj = 5.3-89.6 x 10 g/mol) with carbosilane dendrimers with up to 128 Si-Cl bonds have been reported to proceed... 

In our strategy for the synthesis of carbosilane dendrimers, hydrosilylation and allylation reaction sequences were chosen starting with octa(vinyldimethylsiloxy)octasilsesquioxane (Q lVIg ) as core molecule (Scheme 1). 

The multifunctional initiator approach was also used for the synthesis of stars by ROMP. Carbosilane dendrimers bearing mthenium complexes on their surface (Scheme 59) were prepared and used to synthesize 4- and 8-arm polynorbomene stars. End-group analysis by NMR revealed that well-defined stmctures were obtained. 

In 1992, the synthesis of a carbosilane dendrimer was reported (96). Starting from tetrachlorosilane, allylations of Si—Cl groups by allyl Grignard reagents and hydrosilations of the allyl groups with trichlorosilane were alternately repeated to give a dendrimer composed of silylenetrimethylene units up to the fifth generation (G5) (eq. 17) (96). 

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