Silicon-carbon bond synthesis

C. Eabom and R. W. Bott, Synthesis and reactions of the silicon-carbon bond, Organo-metallic Compounds of the Croup IV Elements, ed. A. G. MacDiarmid, Vol. 1, Part 1. Marcel Dekker, New York (1968). 

The reaction of tetramethylsilane with fluorine led to the isolation of several, partially fluorine-substituted tetramethylsilanes (see Tables VII-IX), and preservation of over 80% of the silicon-carbon bonds in the initial, tetramethylsilane reactant. The stability of many of the partially fluorinated germanes and silanes (some are stable to over 100°C) is very surprising, for the possibility of elimination of hydrogen fluoride is obvious. Indeed, before the first reported synthesis (12) of... 

Use of chromic acid-based oxidation reagents often results in silicon-carbon bond cleavage (Scheme 13)5, although such an oxidation has been used to produce a-cyclopropyl acyl silanes (vide infra, Section III.E)85. In an interesting development, this handicap of ready silicon-carbon bond cleavage during oxidation of a-hydroxy silanes has been cleverly utilized in an acyl silane synthesis by incorporation of two silicon moieties in the substrate (Scheme 14). 

This procedure consists of the synthesis of a precursor, methoxymethyl vinyl ether, an a-hydroxy enol ether, and the intramolecular hydrosilylatlon of the latter followed by oxidative cleavage of the silicon-carbon bonds. The first step, methoxymethylation of 2-bromoethanol, is based on Fujita s method.7 The second and third steps are modifications of results reported by McDougal and his co-workers. Dehydrobromination of 2-bromoethyl methoxymethyl ether to methoxymethyl vinyl ether was achieved most efficiently with potassium hydroxide pellets -9 rather than with potassium tert-butoxide as originally reported for dehydrobromination of the tetrahydropyranyl analog.10 Potassium tert-butoxide was effective for the dehydrobromination, but formed an adduct of tert-butyl alcohol with the vinyl ether as a by-product in substantial amounts. Methoxymethyl vinyl ether is lithiated efficiently with sec-butyllithium in THF and, somewhat less efficiently, with n-butyllithium in tetrahydrofuran. Since lithiation of simple vinyl ethers such as ethyl vinyl ether requires tert-butyllithium,11 metalation may be assisted by the methoxymethoxy group in the present case. 

A variety of reactions generate new silicon-carbon bonds, although most of them are not utilized in the systematic synthesis of organosilicon compounds. Representative and interesting examples are shown in the following. 

Synthesis of Silicon-functionalized Silanes by Cleavage of the Silicon-Carbon Bond... 

The silicon-carbon bond can also be cleaved by strong nucleophiles to give the corresponding carbanions, which may be utilized as the source of carbon nucleophiles in synthesis (equations 42 and 43). 

These two compounds 1 and 2 cannot always be considered as synthetic equivalents of diazomethane for example, they do not react with carboxylic acids to give esters. In fact, they play a very different role in synthesis, and it is demonstrated in this review that they are powerful synthetic building blocks. Since tin chemistry is often compared to that of silicon, a few results involving bis(trimethylstannyl)diazomethane 3 [4] will also be presented (Scheme 1). Juggling the relative steric bulk of the triisopropylsilyl versus the trimethylsilyl group, and the enhanced reactivity of tin-carbon compared to silicon-carbon bonds, the three diazomethane derivatives 1-3 are complementary synthons. Not only are they attractive precursors for the synthesis of stable compounds hitherto believed to be only transient intermediates, but also of new heterocycles or even of polymer crosslinking agents. 

Introducing the Triazole Rina. The final step of the synthesis was displacement of the carbon-bound chlorine with triazole salts. Once again, silicon made life easy for us, since it activates such hindered systems toward displacement. The corresponding all-carbon compounds react very sluggishly with triazole salts. Luckily, silicon-carbon bond cleavage is not observed, provided water or other oxygen nucleophiles are excluded. The displacement reaction is illustrated in Equation 5 for DPX-H6573. 

From a synthetic point of view, asymmetric synthesis at silicon in dihydrosilanes has proved to be useful. It provided the basis for a one-pot synthesis of Sommer s compound, R-(+)- or S-(-)-l-NpPhMeSiH, in 96% e.e. with a 51% chemical yield starting from 1 -NpPhSiH2 (83). The synthesis of a new optically active oxasilacycloalkane has also been realized in this way (84). Moreover, the stereochemistry of a cleavage reaction of a silicon-carbon bond has been determined using asymmetric synthesis (85). The configurations of the pertinent compounds could not be correlated in any other way. 

Organosilanes as Reducing Agents. The two principal categories of reductive chemistry of hydridosilanes are hydrosilylation and ionic reduction. Hydrosilylation is the catalyzed addition of a hydridosilane to a multiply bonded system. This chemistry is a principal technology in silicon—carbon bond formation. Ionic reduction by silanes, a class of chemistry more properly considered within the context of organic synthesis, is the subject of detailed reviews (132). 

In some of the syntheses described above, carbanion or carbanion-like intermediates are used to make silicon containing monomers or polymers. Our group has done extensive work in carbanion chemistry for monomer and polymer synthesis. This work describes the use of two dicarbanion species in the preparation of several different silicon containing monomers and polymers. Some of this work closely parallels the work described above in that similar reaction schemes are utilized, but the final polymers are all different (except one). The goal of this research was to prepare different types of dicarbanions and place those moieties in a polymer backbone via Si bonds. By selecting silicon-carbon bonds and/or siloxane bonds, we were able to substantially vary the properties of the final polymers. 

STEREOCHEMICAL ASPECT OF SILICON COMPOUNDS AND REACTION IN THE SYNTHESIS OF POLYMERS WITH SILICON-CARBON BONDS... 

Apart from problems of silicon-carbon bond cleavage, a second difficulty limiting the utility of this synthesis is that of obtaining the appropriate silylcarbinol. Whereas a-hydroxysilanes can be isolated from the reaction of silylmetallic reagents with aliphatic aldehydes 33,34), the initial adducts from reaction with aromatic aldehydes such as benzaldehyde rearrange to the isomeric silyl ethers too rapidly for any silylcarbinol to be isolated 33, 35). The mechanism of this reaction, summarized below, has been investigated in detail 30). 

The basis for silicone polymers was established by the pioneering work of Kipping (1901) on organosilicon chemistry, specifically on the synthesis and properties of chemical compo mds containing the silicon-carbon bond. 

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