C-Si Bond Formations

Me3SiCl reacts with phosphinomethanides I (R=Me) with at least one hydrogen as carbanion substituent (X = Y=H X=H, Y=SiMe3, PMej) via Si-C bond formation to give heteroelement substituted phosphinomethanides [4]. With fully C-heteroatom-substituted I, the reaction depends on the nature of X and Y, as shown by Eqs. (l)-(3) ... 

Also, the Si-C bond formation is not favorable for steric reasons. Therefore, a unique rearrangement takes place to give 18 [9], probably again via a ir-complex IV, comparable to III. The molecular structure of 18 is shown in Figure 2. 

A unique system for catalytic silaboration of allenes, in which a catalytic amount of organic halide is used as a crucial additive, has been reported (Equation (86)).232 In the presence of Pd2(dba)3 (5 mol%) with 3-iodo-2-methyl-2-cyclohexen-l-one (10mol%), reactions of terminal allenes with a silylborane afford /3-silylallylboranes in good yields with excellent regioselectivity. It is worth noting that the addition takes place at the terminal C=C bond in contrast to the above-mentioned palladium-catalyzed silaboration. The alkenyl iodide can be replaced with iodine or trimethylsilyl iodide. The key reaction intermediate seems to be silylpalladium(n) iodide, which promotes the insertion of allenes with Si-C bond formation at the central -carbon. 

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]. 

Depending on the substituents R, X, and Y, the reactivity of the ligand is tunable, e.g., silyl or phosphino C-substituents reduce the carbanion nucleophilicity. For instance, Me3SiCl reacts with LiCH2PMc2 via Si-C bond formation 4 [1]. In contrast, Me3SiCl reacts with the fully heteroelement substituted 2 under Si-P bond formation 5 [2]. 

In the first substitution step 3 reacts under Si-C bond formation, generating the tetraheteroatom substituted methane moiety. For sterical reasons in the second substitution step only "P"-coordination can be achieved. The preference of "C"-coordination in the first step can be confirmed by the 1 1 reaction of PhSiCl3with3(Eq. 4). 

These species are useful reagents for the Si-C bond formation in organic synthesis. 

We have now succeeded in the development of two new methods for the preparation of organofunctional silanes, both involving electroreductive coupling. The first synthesis concept comprises the anionic cleavage of electrochemically formed disilanes and subsequent reaction with organic halides, whereas the second even more general route involves the direct electrochemical Si-C bond formation. 

Scheme 2. Isolated organofunctional silanes synthesized electroreductive Si-C bond formation.

Direct electrochemical Si-C bond formation appears to be a very simple and elegant method for the synthesis of organofimctlonal silanes, thus being very interesting both in itself and not least for its potential industrial applications. 

The reaction of SiCU with two equivalents of the not completely heteroatom substitituted lithium ti /phosphinomethanide Li[CH(PMe2)2] leads to Si-C bond formation [3],... 

As described in Eq. 3, SiCU reacts with Li[C(PMe2)(SiMe3)2] 2-TMEDA under formation of a disubstitution product. An analogous reaction sequence (monosubstitution Si-C bond formation, 7 disubstitution Si-P bond formation, 8) can be observed in the reaction of PhSiCls with Li[C(PMe2)(SiMe3)2] 2 TMEDA. On storage of pure 8 at ambient temperature conversion to a novel heterocycle 13, which is obtained as very air-sensitive colorless crystals, is observed. 

Previous sections have shown that catalysis by solid acids has received much attention due to its importance in petroleum refining and petrochemical processes. Conversely, relatively few studies have focused on catalysis by bases, even if acid and base are paired concepts. Base catalysts, however, play a decisive role in several reactions essential for fine-chemical syntheses [248-251]. Solid-base catalysts have many advantages over liquid bases. Examples of successfijl reactions include isomerization, aldol condensation, Knoevenagel condensation, Michael condensation, oxidation and Si—C bond formation. Various reviews have discussed catalysis by solid bases [248-255]. 

The use of these materials in a range of reactions [isomerization of alkenes and alkynes, C—C bond formation, aldol condensation, Knoevenagel condensation, nitroaldol reactions, Michael addition, conjugate addition of alcohols, nucleophilic addition of phenylacetylene, nucleophilic ring opening of epoxides, oxidation reactions, Si—C bond formation, Pudovik reaction (P—C bond formation) and synthesis ofheterocycles] have been discussed in detail by Ono [248], as well as in the other cited reviews. We will thus discuss here only selected examples. 

Electrolyses of several chlorosilanes with PhCl indicate that competition between Si-Si and Si-C bond formation is dependent not only on the reduction potentials of the starting silanes, but also on steric or kinetic effects on the electrode surface. Although the reduction potentials of phenylated chlorosilanes lie very close to those of methylated ones [3], Si-C bond coupling was successful with MesSiCl and Me2SiCl2 (Table 1) only. 

The X Si-silicates 5 and 6 were prepared alternatively (method b) by reaction of the silane Ph(MeO)2SiCH2NMe2 (14) with two molar equivalents of acetohydroxamic acid and benzohydroxamic acid, respectively (Scheme 3). This method involves cleavage of two Si-O bonds and one Si-C bond (formation of two molar equivalents of MeOH and one molar equivalent of benzene). 

Nature forms various element-silicon-carbon bonds could be identified. Although the Si-C bond is thermodynamically less stable than the Si-O bond, it is kinetically more stable in aqueous biological systems. The challenge in this extraordinary and important field is the development of a biotechnological process for enzymatic Si-C bond formation for the large-scale production of organosilicon compounds The bio-route to silicones ... 

The mechanistic scheme presents the conventional oxidative addition— reductive elimination steps to explain the hydrosilylation. The oxidative addition of trisubstituted silanes HSiRs to a metal alkene complex (usually with d and d ° configuration) is followed by migratory insertion of alkene into the M—H bond, and the resulting metal(silyl)-(alkyl) complex undergoes reductive elimination by the Si—C bond formation and regeneration of metal alkene complex in excess of alkene. As the facile reductive elimination of silylalkane from [alkenyl-M]-SiRs species has not been well established in stoichiometric reaction, a modified Chalk-Harrod mechanism has been proposed to explain the formation of unsaturated (vinylsilane) organosilicon product, involving the alkene insertion into the metal-silyl bond followed by C—H reductive elimination (Scheme 2) (38). 

Thermal functionalization methods make up the majority of methods reported in the literature for Si-C bond formation on porSi (Tables 1,2,3,4,5, and 6). The procedures are quite straightforward samples can be placed in small flask or vial (Fig. 2a), immersed in or coated with the reactant, and heated (if required). If a vial is used, the cap should be lined with material that is inert to the vapors from the liquid. For electrochemical functionalization methods (Table 7), use of the same etching cell setup used originally to prepare the porSi works well (Fig. 2b). The electrolyte/reactant and electrode (usually Pt) are placed within the well above the porSi wafer. The wafer sits on a rectangular aluminum electrode, which in Fig. 2b was cut from a weighing dish. 

Not long after transition-metal catalyzed hydrosilation took root in the 1950s as a new and powerful tool for Si-C bond formation, based on parallels drawn from TM-catalyzed hydrogenation and on the intrinsic redox properties of these metals. Chalk and Harrod proposed a simple and yet very logical catalytic cycle for TM-catalyzed hydrosilation (Scheme 1). The key steps of this cycle are (1) oxidative addition of Si-H to the metal center, (2) coordination of the olefin or other unsaturated species to the metal, (3) insertion of this usually-t/ -coordinated molecule into the M-H bond and finally, (4) reductive elimination of the Si-C pair to regenerate the catalytically active metal center. 

Figure 17 presents resin-linker cores known to be compatible with C - Si bond formation and cleavage. The synthesis of those linkers is more difficult than that for the alcohol linkers because chloro-silyl resins, the often-used precursors of Si - C bond formation, are quite unstable on sohd supports. Some groups, among them Hone et al. [341], Elhnan et al. [344], Moore et al. [345], Showalter et al. [349] and BSuerle et al. [350], avoided this problem and coupled the sihcon-containing hnker in a solution phase to the compounds that have to be immobilized. 

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