Carbosilanes methylation

A first-generation carbosilane dendrimer consisting of 16 thiophene rings could be isolated in 19% yield, albeit together with various by-products in comparable amounts [85]. The synthetic building block methyl tris(2-thienyl)silyl ether was first prepared from tetramethoxysilane and an excess of thienyl-lithium (Fig. 4.44). 

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... 

Recently, Reek et al. published the synthesis of a 9H,9 H- [4,4 ]bicarbazole-3,3r-diol (BICOL)-based chiral monodentate phosphoramidite ligand, which was functionalized with two different third-generation carbosilane dendritic wedges (Fig. 26) [57]. As reference reaction in the catalytic study, the rhodium-catalyzed asymmetric hydrogenation of Z-methyl-a-acetamido-cinnamate was chosen. Using a ligand-to-rhodium ratio of 2.2 led to enantio-selectivities which were comparable to the results obtained using the parent BINOL-derived monodentate phosphoramidite MonoPhos. 

Although not siloxane based, organosilicon surfactants have also been made from per-methylated carbosilanes containing an Si-C-Si structure. The simplest version of this is the trimethyl silylated alkyl polyether discussed by Klein [22] and Wagner [11, 23]. These surfactants are more hydrolytically stable wetting agents than the trisiloxanes. 

The reaction shown in Eq. (4) (X=halogen, OR Me=MgCl, Li, Na) was used to synthesize linear fully methylated carbosilanes (me = CH3). By the reaction... 

In addition to efforts in the synthesis of the bicyclic carbosilanes, attempts have been made to prepare a Si-methylated 1,3,5,7,9,11-hexasila-perhydro-phenaline 39... 

Recent studies on the direct reaction of elemental silicon with alkyl chlorides such as methyl chloride, activated alkyl chlorides, polychloro-methanes, (chloromethyl)silanes, (dichloromethyl)silanes, etc. are summarized in this review. In the direct reaction of elemental silicon with activated alkyl chlorides and polychloromethanes, the decomposition of the reactants can be suppressed and the production of polymeric carbosilanes reduced by adding hydrogen chloride to the reactants. These reactions provide a variety of new organosilicon compounds containing Si-H and Si- Cl functionalities, which should find considerable application in the silicone industry. 

Homopolymers and copolymers containing carbosiloxane and carbosilane units have been produced that bear latent reactive sites along the chain [184]. Reactive carbosiloxane and unreactive carbosilane homopolymers were first prepared in order to ensure catalyst monomer compatibility and to set end points for copolymer properties. Carbosiloxane homo- and copolymers were synthesized with latent reactivity dispersed throughout the polymer chain in the form of methyl silyl ethers (Scheme 21). It is well known that Si-OMe bonds, although inert during metathesis, can react with atmospheric moisture creating stable Si-O-Si bonds and methanol [185]. 

The formation of tri- and especially tetrasilanes which are already branched (tertiary Si-units) as the first reaction products (described elsewhere [4]) suggests the appearance of intermediate silylene species which could enter in insertion reactions of Si-Si as well as Si-Cl bonds. The tri- and tetrasilanes undergo thermal crosslinking reactions at reaction temperatures of 165-250 °C. In addition dehydrochlorination reactions initiated by acid H-abstraction of methyl groups cause the formation of carbosilane (methylene) units in the polymer framework. Table 1 shows the gross compositions of poly(methylchlorosilanes) which are determined by the reaction temperature. 

The same trends are observed for the change from siloxanes to carbosilanes (7—>12). The higher surface tension of the surprisingly polar carbosilane is caused by an increased portion Obviously methyl groups attached to flexible siloxane backbones are energetically distinctly different from those bonded to rigid structures. Expectedly a significant portion occurs at the interface solid/liquid for compound 12... 

Products formed by pyrolyzing Si(CH3)4, which are mostly Si-methyl carbosilanes, are not very reactive. In many investigations there is a need for compounds with the same basic structure but with more reactive groups attached to silicon (e.g., Si—Cl). Such compounds may be prepared by pyrolyzing the three methyl chlorosilanes (2If). Thermal decomposition of these substances is sufficiently rapid at 700°C and a preparative study of the pyrolysis products can be made by the streaming method used for Si(CH3)4. 

All of the compounds formed in the pyrolysis of (CH3)3SiCl, (CH3)2SiCl2, and CH3SiCl3 with boiling points up to about 250°C could be detected by gas chromatographic methods (84, 35) and their relative amounts determined (36). The greatest number of compounds was produced from (CHj) SiCl. Table V shows the carbosilanes obtained from the methyl chlorosilanes, with their structural formulas. 

SiCl - combining carbosilane with 4 Si atoms from CHjSiClj Methylation product ----------------------... 

The carbosilanes described in Section II,A, 1 and 2 are formed from Si(CH3)4 and the methyl chlorosilanes by gas phase thermal decomposition at 700°C. Under these conditions it is reasonable to assume that radical mechanisms are operative. Kinetic measurements of the thermal decomposition of Si(CH3)4 in a static system have shown (68) that, on heating to 700°C in the gas phase for several hours, hydrogen and methane are formed while Si and C are deposited. The primary step in the decomposition is assumed to be ... 

Similar mechanistic considerations may be applied to the formation of SiCl-containing carbosilanes from the three methyl chlorosilanes in the gas phase at 700°C. The 1,3-disilapropanes containing SiCl groups, shown in Table V, are key substances in considering the mechanism of this reaction. As in the case of Si(CH3)4, cleavage of both the Si—C and the C—H... 

Figures 2 and 4-7 show the percentage abundance of the masses referred to the most abundant species observed. This is, for example, 73 in the case of Si(CH3)4 and [(CH3)3Si—CH2]2Si(CH3)2, 145 (molecular mass — 15) for [(CH3)3Si]2CH2, and 201 (molecular mass — 103) for the Si-methylated compound [(CH3)3Si—CH2—Si(CH3)2]2CH2. The mass numbers and percentage abundances of the ions from the linear compounds are summarized in Fig. 2. The ions which occur result, as a rule, from cleavage of the Si—C bond. Both CH2Si(CH3)3 and CH3 groups are split off and this will account for the greater part of the observed masses. For the mass spectra from carbosilanes of the general formula Si C3n+iH8n+4, the most abundant masses of the ions formed may be represented by ...

Figures 5 and 6 show the mass spectra of linear and cyclic carbosilanes containing SiH groups. The most striking difference between carbosilanes with SiH and SiCH3 groups is that, as the hydride content increases, the mass spectrum becomes richer in lines. With methylated compounds, ions are formed almost exclusively by cleavage of Si—C bonds (mass lines single, apart from isotope effects, with a minimum separation of 14 mass units). On the other hand, ions are formed from molecules with Si—H bonds by...

In spite of the difficulties discussed above, the spectra of the cyclo-carbosilanes may be used in solving structural problems such as those associated with position isomerism in unsymmetrical methyl-substituted rings. This type of analytical application of nuclear magnetic resonance spectroscopy is particularly valuable for the carbosilanes, as the possibilities of establishing structures by chemical means are very restricted. The carbosilanes are not reactive and, unlike carbon compounds, undergo few reactions which yield information concerning their structures. In fact the structures of a number of compounds were first established with the aid of nuclear resonance. 

In this review the authors have traced the development of the chemistry of compounds containing in their structures alternate Si and C atoms, although in all probability the literature coverage is not complete at all points. The article does not represent an area of research which has been fully explored indeed, in many places the subject is still in its infancy. Thus the high molecular weight compounds formed in the pyrolysis of Si(CH3)4 and the methyl chlorosilanes have scarcely been examined, and the possibilities of further reactions of carbosilanes substituted on the bridge carbon atom are also not completely known. In part the text reports only the experimental material available at the moment and discusses its possible interpretation. Further research will have to provide the final answers. It seems certain in any case that the subject will develop far more in the years ahead. 

These cyclometalation compounds are van Koten s carbosilane pincer nickel dendrimer 9.54 [109, 111], pincer SCS palladium dendrimer 9.55 [110, 112], azobenzene platinum chloro-bridged liquid crytal 9.56 [113, 114], and N,N-dimethylnaphthylene palladium resolving agent 9.57 [117-119] as shown in Fig. 9.10, and photosensitizers for hydrogen production, bis(2-phenylpyridine-4-methyl,4 -fluoride) 9.59 and other photosensitizers with bis(2-phenylpyridine) derivatives 9.60 as shown in Fig. 9.11 [120,121]. 

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