Siloxane dendrimers

The data presented in Figure 8 graphically illustrate the tremendous and rapid growth in interest in FOSS chemistry, especially for patented applications. This looks set to continue with current applications in areas as diverse as dendrimers, composite materials, polymers, optical materials, liquid crystal materials, atom scavengers, and cosmetics, and, no doubt, many new areas to come. These many applications derive from the symmetrical nature of the FOSS cores which comprise relatively rigid, near-tetrahedral vertices connected by more flexible siloxane bonds. The compounds are usually thermally and chemically stable and can be modified by conventional synthetic methods and are amenable to the usual characterization techniques. The recent commercial availability of a wide range of simple monomers on a multigram scale will help to advance research in the area more rapidly. 

Scheme 3 Siloxane dendrimer. Reprinted with permission from Majoral, J.-P. Caminade, A.-M. Chem. Rev. 1999, 99, 845-880. Copyright 1999 American Chemical Society.

Siloxanes, prepared in 1989 as representatives of silicon-based dendritic molecules ( silicodendrimers ), were the first dendrimers to contain heteroatoms other than the usual ones (N, O, S, halogens) [68]. As with the phosphodendri-mers (Section 4.1.10), their readily modifiable architecture and their pronounced thermostability hold promise of applications, for example, in the form of carbo-silanes as liquid-crystalline materials and catalyst supports. They can be subdivided into a number of basic types and their properties are presented below with the aid of characteristic representatives ... 

Hydrosilylation of the protected allyl-glycoside 1 with the carbosilane 2 (by means of Silopren , a platinum-siloxane complex from Bayer AG) led via Si-C bond formation to a glycosidic carbosilane dendrimer (Fig. 4.42) [82]. 

Fig. 4.49 Synthesis of fourth-generation siloxane dendrimers (according to Muzfarov et al.)...

Morikawa, Kakimoto, and Imai 101 employed divergent methodology for the construction of a series of siloxane-based dendrimers possessing dimethylamino, phenyl, benzyl, or hydroxy peripheral moieties. Sequential tier addition included transforming phenyl... 

Ester-based cascades (e.g., 107) have been prepared[77 80i by using 5-(tert-butyldime-thylsiloxy)isophthaloyl dichloride (108), which was synthesized in high yield from 5-hydroxy-isophthalic acid (Scheme 5.26). The dendron wedges were prepared by treatment of siloxane 108 with phenol to give bis(aryl ester) 109, which was hydrolyzed, or desilylated (HC1, acetone), to generate a new phenolic terminus. Treatment of this free phenolic moiety with monomer 108, followed by hydrolysis, afforded the next tier (110). Repetition of the sequence followed by reaction of the free focal phenols with a triacyl chloride core, (e.g., 86), afforded the fourth tier dendrimer 107 of the polyester aryl series. It was noted that the choice of base (N, A-dimethylaniline) used in the final esterification was critical, since with pyridine bases (pyridine or 4-(dimethylamino)pyridine) facile transesterification resulting in branch fragmentation occurred. 

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... 

The utilization of allyl alcohol as the nucleophilic reagent to prepare dendritic carbo-siloxanes was also reported by Lang and Brilning134,135. Using a second generation allyl-terminated carbosiloxane dendrimer, they were able to attach titanium moieties via a simple hydrosilylation reaction (equation 12)134. Lang and coworkers were also successful in binding cobalt moieties to ethynyl-terminated carbosiloxane dendrimers (e.g. equation 13)136,137. 

The first dendrimers of this type were reported by Rebrov and coworkers in 1989154. Reaction of MeSiC with (EtO MeSiONa followed by reaction with SOCI2 gave the SiCl-terminated first generation. Repetition of the (EtO MeSiONa and SOCI2 sequences led to formation of siloxane dendrimers up to the fourth generation (Scheme 14). 

Masamune and coworkers reported a divergent stepwise synthesis of siloxane dendrimers as shown in Scheme 15155. The third generation dendrimer was the largest obtained. Characterization techniques included 1H, 13C and 29Si NMR spectroscopy, mass spectroscopy and size-exclusion chromatography. 

Another stepwise approach to siloxane dendrimers was described by Morikawa and coworkers156 and is outlined in Scheme 16. Using the key building block, A, the researchers were able to construct dendrimers up to the third generation in 32% yield after purification. The dendrimers were characterized with NMR spectroscopy and intrinsic viscosity measurements, which indicated globular geometries. 

These methods were used extensively for structure verification of dendrimers prepared by the divergent initiator core method such as Starburst PAMAM [124] poly(ether) [82], and poly(ethylenimine) dendrimers [2], as well as poly(siloxane), poly(phosphonium), poly (ary lalkyl)ether, poly(arylene) and poly(arylester) dendrimers. In many cases, small-molecule model systems were used for process optimization, defect identification, and stoichiometry studies. 

We would like to report the synthesis of a star-shape poly(vinylmethyl-c6>-dimethyl)siloxane polymers functionalized in their exterior, which makes them especially suitable for application as catalytic supports. Similarly to catalysts bound to periphery-functionalized dendrimers [3] they offer regularly distributed and available catalytic sites. 

There have been very few families of dendrimers built up around a pure siloxane core. One such dendrimer consisted of a dendritic methylsesquioxane matrix (Nq = 3, Mb = 2, Fig. 21) to which six terminal cholesteryl groups were attached via undecylene spacers [178]. This G1 compound exhibited a very broad temperature SmA phase (G -1.5 SmA 120 I). The molecules arrange in a single layer smectic phase with complete overlap of the cholesteryl meso-gens separated by siloxane layers. 

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