Carbosilane complexes

The anionic ring-opening copolymerization of hexamethyl-cyclotrisiloxane (Dj) with 2,4,6-trivinyl-2,4,6-trimethyl-cyclotrisiloxane (Vj) was performed under dry Nj atmosphere in a glass ampoule equipped with a teflon stopcock. All other reactions, except the preparation of Pt(0)-[poly(vinylmethyl-co-dimethyl)siloxane]-carbosilane complexes, were performed using standard Schlenk s or syringe techniques under an atmosphere of argon. 

Exemplary Procedure of Formation of Pt(0)-[poly(vinylmethyl-co-dimethyl) siloxaneJ-Carbosilane Complexes... 

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

One of the main applications of dendrimers is in catalysis allowing easy recycling of the homogeneous catalyst by means of nanofiltration. Carbosilane dendrimers functionalized with diphenylphosphine groups at the periphery have been synthesized and characterized. Palladium complexes of these dendrimers have been used as catalysts in the allylic alkylation reaction. These dendrimeric catalysts can be used in a continuous process using a membrane reactor.509... 

Seyferth and coworkers [181] introduced ethynyl groups onto the periphery of carbosilane dendrimers by displacement of chloride from the terminal silicon groups. They further treated these ethynyl terminated silicon dendrimers with Co2(CO)8 to afford the corresponding acetylenedicobalt hexacarbonyl dendritic complexes. 

The last example of a dendritic effect discussed in this chapter is the use of core-functionalized dendritic mono- and diphosphine rhodium complexes by Van Leeuwen el al. [45] Carbosilane dendrimers were functionalized in the core with Xantphos, bis(diphenylphosphino)ferrocene (dppf) and triphenylphosphine (Figures 4.22, 4.32 and 4.33). 

Attachment of dendritic wedges of either the carbosilane or benzylphenyl ether type to the para-hydroxy aryl site in [2,6-(ArN=CMe)2C5H3N (1 R = Me, Ar = 2-Me-4-OHC6H3), has been shown to proceed in good yield [162], Complexation with iron(II) chloride allows access to dendrimer-supported precatalyst 42 (Scheme 13). Using MAO as a co-catalyst, it was shown that 42 are active in the oligomerisation of ethylene the activity of these new catalysts is not, however, related to the type of dendritic wedge employed. 

The research group of Van Leeuwen reported the use of carbosilane de-ndrimers appended with peripherial diphenylphosphino end groups (i.e. 25, Scheme 26) [37]. After in situ complexation with allylpalladium chloride, the resultant metallodendrimer 25 was used as catalyst in the allylic alkylation of sodium diethyl malonate with allyl trifluoroacetate in a continuous flow reactor. Unlike in the batch reaction, in which a very high activity of the dendrimer catalyst and quantitative conversion of the substrate was observed, a rapid decrease in space time yield of the product was noted inside the membrane reactor. The authors concluded that this can most probably be ascribed to catalyst decomposition. The product flow (i.e. outside the membrane reactor)... 

Fig. 7.21 [Gl] Carbosilan dendrimer with pincer ligands (Ni"-complex).

A planar arrangement (297) for a cluster of four Li atoms, consisting of two equilateral triangles, is found by XRD for the solid complex 298. Each Li atom is coordinated to methylene groups of both types (from n-BuLi and the metallated carbosilane) and to N and O atoms of the substituent in the carbosilane. Further characterization of the solid 298 can be made by Li and CP/MAS NMR spectroscopies, and in solution by H, Li, and Si NMR spectroscopies. The combination of both organolithium compounds in 298 is found to form a more effective reagent than each of them alone °. 

De Groot et al. (18) prepared phosphine-functionalized carbosilane dendrimers of different generations (4, 8, 24, and 36 phosphine end groups) and used their palladium complexes as catalysts for the allylic alkylation of allyl trifluoroacetate with diethyl sodio-2-methylmalonate. 

Rhodium complexes of the phosphine-functionalized carbosilane dendrimers are active for the hydroformylation of alkenes. The influence of the flexibility of the dendritic backbone on the catalytic performance was characterized by comparing dendritic ligands 84a-84c (conditions toluene, 80°C, 20 bar CO/H2) 49). 

Although the chemistry of the initial thermal transformation is obviously quite complex, it was determined that considerable carbon insertion into the Si-Si bonds occurs, resulting in an intermediate carbosilane which can be drawn into fibers. At this point, brief oxidation results in the formation of a surface oxide which imparts dimensional stability, and subsequent heating to 1300°C produces silicon carbide fibers (3,5). 

Presumably, carbosilane formation in the catalytic cycle occurs via an analogous intermediate silene complex in which one of the two hydrides is replaced by a SiMe3 group. Subsequent migration of the silyl group to the carbon of the silene would produce a new carbosilyl ligand. 

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]. 

Terminal palladium-complexed, phosphane-functionalised carbosilane dendrimers have been used as potential catalysts in membrane reactors [87]. 

Fig. 6.29 Dendritic carbosilane-nickel complexes with decreasing catalytic activity owing to mixed complex formation...

The decrease in catalytic activity of the nickel-containing carbosilane dendri-mer shown in Fig. 6.28 was attributed to the formation of mixed complexes with nickel in both oxidation states II and III on the dendrimer surface, which competes with the reaction with substrate radicals occurring in Kharash reactions (Fig. 6.29). 

Compared to polymers, dendrimer architectures offer favourable conditions for fixation of catalytically active moieties thanks to their monodispersity, variability, structural regularity of the molecular scaffold, and numerous functionalisation possibilities. Catalytic units can be fixed - multiply if required - on the periphery, in the core of a dendrimer, or at the focal point of a dendron. If the dendrimers are suitably functionalised at the periphery, appropriate metal complexes can be directly attached to the surface of the molecule. In contrast, dendrimers functionalised in the core or at the focal point shield the catalytically active site through their shell structure in a targeted manner, for example to attain substrate selectivity in the case of reactants of different sizes [1]. The corresponding concepts of exodendral and endodendral fixation of catalysts were inttoduced in the context of functionalistion of carbosilane, polyether, and polyester dendrimers [2]. Exodendral fixation refers to attachment of the catalytic units to the... 

Van Koterfs group used a chemically inert, lipophilic carbosilane dendrimer scaffold as support material for fixation of up to 12 transition metal complex fragments. The covalently fixed fragments with nickel as catalytic site acceler-... 

Their advantage over other types of dendrimers is their straightforward synthesis and, most importantly, their chemical and thermal stabilities. Two distinct steps characterize their synthesis a) an alkenylation reaction of a chlorosilane compound with an alkenyl Grignard reagent, and b) a Pt-cata-lyzed hydrosilylation reaction of a peripheral alkenyl moiety with an appropriate hydrosilane species. Scheme 2 shows the synthesis of catalysts Go-1 and Gi-1 via this methodology. In this case, the carbosilane synthesis was followed by the introduction of diamino-bromo-aryl groupings as the precursor for the arylnickel catalysts at the dendrimer periphery. The nickel centers of the so-called NCN-pincer nickel complexes were introduced by multiple oxidative addition reactions with Ni(PPh3)4. 

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