Carbosilanes reactions

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

G. Fritz and E. Matern, "Carbosilanes. Syntheses and Reactions," Springer-Verlag, Berlin, 1986. 

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... 

Van Leeuwen et al. used several generations of carbosilane dendrimers with 4, 8, 24, and 36 diphenylphosphine end-groups (Figure 4.15) for the allylic alkylation reaction of allyl trifluoracetate with sodium diethyl 2-methylmalonate.[31]... 

Van Koten et al. reported on a negative dendritic effect in the Kharasch addition reaction. [3 9,40] A fast deactivation for the carbosilane dendrimer supported NCN pincer catalyst (Figures 4.28 and 4.29) was observed by comparison with a mononuclear analogue. This deactivation is expected to be caused by irreversible formation of inactive Ni(III) sites on the periphery of these dendrimers. 

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 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)... 

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... 

Pincer ligand Carbosilane Den- Ni Kharasch reaction 0.03 mol% 1-79 Ultrafiltration Negative dendritic 68, 70,... 

In the group of van Koten, dilithiated precursors for the peripheral functionalization of carbosilane dendrimers were generated by deprotonation of compounds 32, 34a and 34b using f-butyllithium. The reaction was effected in n-pentane at room temperature, using the appropriate amount of the alkyllithium base. Dilithiated compounds 33, 35a and 35b were almost quantitatively obtained, the para positions of the aromatic ring systems... 

Ethylene coordinates too strongly to the ruthenium center to act as a hydrogen acceptor in this system, but slightly larger olefins do show activity. However, reaction selectivity is reduced compared with f-butyl ethylene. Thus, the use of cis and trans 2-pentene as hydrogen acceptor leads to products consisting of 90% carbosilane and 10% hydrosilylated olefin, and 1-hexene yields a carbosilane/hydrosilylation ratio of 4 6. In comparison, no hydrosilylation products are observed for dehydrocoupling in the presence of cyclohexene, but carbosilane formation is accompanied by disproportionation to benzene and cyclohexane as a side reaction. 

Less usual carbosilane dendrimers were synthesised by reaction of acetyl-protected hydroxyethyl glycosides with chlorosilanes. Introduction of the carbohydrate component proceeds via alcoholysis of the chlorosilane with formation of silicon-oxygen bonds a carbosilane core unit is formed with carbosiloxane side arms [82]. Reverse assembly of carbosilanes with a carbohydrate core is also feasible [54]. 

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). 

Eggeling, E.B., Hovestad, N.J., Jastrzebski, J.T.B.H., Vogt, D. and van Koten, G. (2000) Phosphino carboxylic acid ester functionalized carbosilane dendrimers nanoscale ligands for the Pd-catalyzed hydrovinylation reaction in a membrane reactor. J. Org. Chem., 65, 8857. 

An interesting reaction of disilane is the transition metal-catalyzed insertion of unsaturated hydrocarbons. The palladium-mediated reaction of cyclotrisilane 26 with phenylacetylene to afford the seven-membered carbosilane 51 (Equation 3) indicates that this general reaction scheme is also applicable to strained cyclic trisilanes <20040M490>. 

Photolysis of the acyclic trisilane 57 produces 1,2,4-trisilacyclopentanes 42 in moderate yields (67-79%) (Equation 4). Marker experiments suggest that the reaction proceeds via a bimetallic silylene bridged dimer, which collapses to give the cyclic carbosilane <20020M503>. 

Phenolic substrates are not readily reduced to benzene derivatives, and this fact can be used to effect the transformation of methyl or benzyl ethers of phenols to their silyl ethers with elimination of CH4 or toluene as the only by-products. These studies include detailed mechanistic studies that support and corroborate our experiments and support the silane activation mechanism. The reaction is clean and high yielding enough to be applied towards the functionalization of the periphery of carbosilane dendrimers with perfluoroaryl borane moieties (Scheme 25).196... 

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