Silicon bond synthesis

Allylsilanes have been found to be more reactive than vinyl silanes toward electrophiles and they are being studied more intensively in recent years because they hold considerable promise in organic synthesis. In allylsilanes, the geometry of carbon-silicon bond can be more favourably be oriented for efficient stabilization of a developing positive change P to silicon. 

The chemistry of a-haloketones, a-haloaldehydes and a-haloimines Nitrones, nitronates and nitroxides Crown ethers and analogs Cyclopropane derived reactive intermediates Synthesis of carboxylic acids, esters and their derivatives The silicon-heteroatom bond Synthesis of lactones and lactams Syntheses of sulphones, sulphoxides and cyclic sulphides Patai s 1992 guide to the chemistry of functional groups—Saul Patai... 

The carbon-silicon bond is quite strong ( 75 kcal/mol), and trimethylsilyl groups are stable under many of the reaction conditions which are typically used in organic synthesis. Thus, much of the repertoire of synthetic organic chemistry can be used for elaboration of organosilanes.46 47 48... 

Substitution of the carbon-silicon bond in allyl- and vinylsilanes by an electrophile has served as a powerful tool in organic synthesis. Electrophiles ranging from proton, carbon and main group heteroatoms to certain transition metal species have been employed. A... 

Since allylsilane can be considered as a very soft nucleophile because of the involvement of o-7i conjugation between the -electrons of the double bond and the o-electrons in the carbon-silicon bond, the addition of allylsilanes to ,/3-unsaturated enone moiety occurs exclusively via 1,4-addition19,20. A typical Sakurai-Hosomi reaction is illustrated in the reaction of allylsilane 66 with enone 67 in the presence of TiCU to give 68 which is used for the synthesis of a cyclic enediyne (equation 45)108. A similar reaction has been used for the synthesis of ewf-herbasolide 70 from enone 69 (equation 46)109. Prenylsilane undergoes 1,4-addition with squaric acid chloride 71 followed by dechlorosilylation to give 72 as the predominant product (equation 47). Interestingly, other simple allylsilanes react in a 1,2-addition fashion to yield 73110. 

As described in Section II, Lewis acid catalyzed desilylative carbon-carbon bond formation with an electrophile has been shown to be very versatile in organic synthesis. Occasionally, depending on the nature of the substrates (e.g. the presence of appropriate functional groups), the carbon-silicon bond may remain intact. For example, treatment of 132 with a Lewis acid affords a mixture of cyclization products 133-135 (equation 113). The isolation of 133 indicates that the carbocation intermediate thus formed is trapped by the oxygen nucleophile before elimination of the silyl moiety occurs204. 

Cobalt-silicon bonds, in hydridocobalt complexes, 7, 5 Cobalt—tin bonds, in hydridocobalt complexes, 7, 5 Co-catalyst effects, in olefin polymerization, 4, 1111 (—)-Goccinine, via Alder-ene reactions, 10, 593 Co-condensation sites, in metal vapor synthesis technique,... 

Lambert et al. succeeded in developing second-generation dendritic polysilanes [71]. The repetitive steps of this divergent synthesis consist in cleavage of silicon-methyl bonds by trifluorosulphonic acid (CF3SO3H) and subsequent formation of silicon-silicon bonds (Fig. 4.39). Starting from tetrasilanes, up to second-generation starfish-like dendritic molecules with a molecular mass of 1832.9 g/mol could be prepared in this way [72]. 

The organopolysilanes are those compounds containing at least one silicon-silicon bond and one silicon-carbon linkage. This review is mainly concerned with the chemistry of aliphatic derivatives of polysilanes. Consideration of aromatic organopolysilanes is excluded from this review except as far as they are used as intermediates for synthesis and their properties correlate with the aliphatic silicon-silicon compounds, because the aromatic organopolysilanes have recently been well reviewed elsewhere (31,51, 73, 76a, 212). Physical properties of the polysilanes also are excluded from consideration except for spectral properties of ultraviolet absorption and nuclear magnetic resonance, since they are well summarized in earlier excellent reviews and texts (8, 34, 35, 51,131,132). 

However, the method is no longer satisfactorily applicable to synthesis of higher polysilane derivatives because extensive cleavage of the silicon-silicon bond occurs and a mixture of isomeric halopolysilane derivatives that are inseparable by distillation is formed. For example, the reaction of decamethyltetrasilane with sulfuric acid and treatment with ammonium hydrogen fluoride leads to the formation of several fluoro derivatives of... 

The stabilization of the silicon-silicon bond by negative substituents is also indicated in the reaction of one molar equivalent of bromine with octamethyltrisilane and with decamethyltetrasilane in chloroform at — 40° C, which proceeds entirely as indicated by the following equations, and constitutes a most convenient method for synthesis of the bromodisilane and -trisilane (Section II, A, l,d) (178). 

Ring expansion in conjunction with Tamao-type oxidation of carbon-silicon bonds provides access to 1,4-diols. The l-(l-iodoalkyl)-l-silacyclobutanes are available from 1-chlorosilacyclobutanes (addition of vinyl, Scheme 34) <1991TL6383>. The utility of silacyclopentanes formed by the ring expansion of SCB for the synthesis of diols has been reported <1992TL7031, 1995BCJ625>. 

To develop new methods for organic synthesis, Woerpel and coworkers exploited the inherent reactivity of di -fc/ f-butylsilacyclopropanes to create new carbon-carbon bonds in a stereoselective fashion (Scheme 7.7).62 They discovered that transition metal salts catalyze the insertion of carbonyl compounds into the strained carbon-silicon bond to form oxasilacyclopentanes. The regioselectivity of insertion could be controlled by the identity of the catalyst. Copper promoted the insertion of croto-naldehyde into the more substituted C-Si bond of 52 to afford oxasilacyclopentane 53,63 whereas zinc catalyzed the insertion of butyraldehyde into the less substituted bond of 52 to provide the complementary product, 54.64 Oxasilacyclopentanes (e.g., 55) could be transformed into useful synthetic intermediates through oxidation of the C-Si bond,65 66 which provided diol 56 with three contiguous stereocenters. 

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