Synthesis organosilicon compounds used

The preparation of new asymmetric organosilicon compounds using the principles of asymmetric synthesis so widely employed in organic chemistry (65) has also been envisaged. Kinetic resolution, as depicted in Scheme 8, is of general... 

Organosilicon compounds are widely used in our daily life as oil, grease, rubbers, cosmetics, medicinal chemicals, etc. However, these compounds are not naturally occurring substances but artificially produced ones (for reviews of organosilicon chemistry, see [59-64]). Hydrosilylation reactions catalyzed by a transition-metal catalyst are one of the most powerful tools for the synthesis of organosilicon compounds. Reaction of an unsaturated C-C bond such as alkynes or alkenes with hydrosilane affords a vinyl- or alkylsilane, respectively (Scheme 16). 

So far only a few examples of the uses of organosilicon compounds in cross-couplings have been published. Noteworthy is the smooth reaction with a sterically hindered substrate (87).295 The synthesis of the alkaloid nitidine included a cross-coupling step using an alkenylsilane (88),296 while the syntheses of some antitumor agents involved the alkenylation of unprotected iodouracyls using alkenyl-silicon species.297... 

Recently organosilicon compounds are being used for the synthesis of olefines by elimination reactions both in acidic and basic conditions. Thus P hydroxysilanes give defines. These reactions have been shown to be highly stereospecific. The acid catalysed elimination taking place by an anti pathway and the base induced elimination taking place by a syn pathway. 

Organosilicon compounds are widely used in organic synthesis. The understanding of the structure and properties of the intermediates involved in their reactions is a prerequisite for further development and optimization of useful synthetic transformations involving silicon substituted compounds. Trialky lsilyl-substituted carbocations are particularly... 

Hydrosilylation, the addition of a silicon-hydrogen bond to multiple bonds, is a valuable laboratory and industrial process in the synthesis of organosilicon compounds. The addition to carbon-carbon multiple bonds can be accomplished as a radical process initiated by ultraviolet (UV) light, y irradiation, or peroxides. Since the discovery in the 1950s that chloroplatinic acid is a good catalyst to promote the addition, metal-catalyzed transformations have become the commonly used hydrosi-... 

Two new mechanisms have been suggested to account for the observation. Dehydrogenative silylation has become a useful method for the synthesis of unsaturated organosilicon compounds.611,614... 

Stereoselective enzymatic hydrolyses of esters represent a further type of biotransformation that has been used for the synthesis of optically active organosilicon compounds. The first example of this particular type of bioconversion is illustrated in Scheme 15. Starting from the racemic (l-acetoxyethyl)silane rac-11, the optically active (l-hydroxyethyl)silane (5)-41 was obtained by a kinetic racemate resolution using porcine liver esterase (PLE E.C. 3.1.1.1) as the biocatalyst7. The silane (5)-41 (isolated with an enantiomeric purity of 60% ee bioconversion not optimized) is the antipode of compound (R)-41 which was obtained by an enantioselective microbial reduction of the acetylsilane 40 (see Scheme 8). 

Enantioselective enzymatic amide hydrolyses can also be applied for the preparation of optically active organosilicon compounds. The first example of this is the kinetic resolution of the racemic [l-(phenylacetamido)ethyl] silane rac-84 using immobilized penicillin G acylase (PGA E.C. 3.5.1.11) from Escherichia coli as the biocatalyst (Scheme 18)69. (R)-selective hydrolysis of rac-84 yielded the corresponding (l-aminoethyl)silane (R)-85 which was obtained on a preparative scale in 40% yield (relative to rac-84). The enantiomeric purity of the biotransformation product was 92% ee. This method has not yet been used for the synthesis of optically active silicon compounds with the silicon atom as the center of chirality. 

In view of the importance currently attached to the synthesis of homochiral organic molecules, examples which illustrate the use of organoboron and organosilicon compounds in this area are included where appropriate. 

Synthesis of saturated 5-membered heterocycles using organosilicon compounds 92YZ147. 

As in other preparative methods for organosilicon compounds, the direct synthesis produces a mixture of methylchlorosilanes rather than the single compound shown in equation 3. Besides dimethyl-dichlorosilane, the mixture usually contains silicon tetrachloride, tri-chlorosilane, methyltrichlorosilane, methyldichlorosilane, trimethyl-chlorosilane, and even silicon tetramethyl. Under proper conditions, dimethyldichlorosilane is the principal product. Of the other compounds, methyltrichlorosilane usually is next in abundance this substance finds use in the cross-linked methyl silicone resins, or it can be methylated further by the Grignard method to increase the yield of dimethyldichlorosilane. There is no way of recycling it in the direct process, and so supplemental operations are required for the conversion. The interconversion of this and the other minor products of the direct synthesis, involving the exchange of methyl and chlorine groups as desired, has been a special study in itself.10... 

A large number of different a,(o-bis[(trifluoromethyl)sulfonyloxy]-substituted organosilicon compounds can be obtained by relatively simple methods from the corresponding amino-, allyl-, or phenylsilanes. Moreover, it is remarkable that these silyl triflate derivatives are often easily formed, when the synthesis of the corresponding chloro- or bromosilanes is difficult or does not appear to have been attempted. Eq. 2 and Eq. 3 show selected examples of this synthesis [10-12]. The products were prepared in high purities and yields. The resulting triflates should be used for the polycondensation without further purification, because they often cannot be destilled without decomposition. 

Synthesis of Other Silicon Compounds. The substitution route to organosilicon compounds obviously also opens up new avenues to alkyl silicates. For example, as indicated previously, the substitution route is a good avenue to the isomeric alkyl silicates [5.5.1] ( 2H50)ioSie07 and [5.3.3] ( 2H50)ioSie07. The availability of this substitution route for alkyl silicates is of considerable importance, because some of the routes that have been used to make complex alkyl silicates are not good in terms of yield or convenience (22). 

The first observation of penta- and hexaeoordinate silicon compounds was reported at the beginning of the 19th century by Gay-Lussac [87] and Davy [88], Subsequent investigation of hypercoordination in silicon compounds stimulated widespread use of nucleophilic activation and catalysis in the application of organosilicon compounds as reactive species in organic synthesis. Synthetic application for silicon-fluorine bond formation can be found in several reviews over the last two decades, and this section focuses on recent advances in the use of hypervalent organosilicon compounds in selective organic synthesis, in particular, selective carbon-carbon bond formation [89]. 

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