Silicon-based reagents

Morita, K. Kuroda, Y. Okamoto, and H. Sakurai, Chem. Lett., 1981, 921. 

Further applications of MesSiCN reported this year include the SnCU-cata-lysed cyanation of tertiary alkyl chlorides, avoiding the more usual elimination of HCl in such cases,and the exchange of an alkoxy for a cyano group in acetals and orthoesters/ The related system NaCN-Me3SiCl-NaI converts primary, secondary, and tertiary alcohols into the corresponding nitriles in high yield.  

Trimethylsilyl triflate can be prepared in situ from allyltrimethylsilane and trifluoromethanesulphonic acid the reagent so obtained has been used for the preparation of trimethylsilyl derivatives of ketones, alcohols, thiols, and carboxylic acids.The reagent also activates benzyl and allyl ethers, which then react with disulphides to give 5-benzyl or -allyl sulphonium species. 

Three approaches to the problem of preparing t-butyldimethylsilyl derivatives have appeared this year, two of which make use of silyl enol ether derivatives. Both (135) and (136, R = CH3, OCHa) give silylated products under mild 

Utimoto, Y. Wakabayashi, Y. Shishiyama, M. Inoue, and H. Nozaki, Tetrahedron Lett., 1981, 22,4279. 

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Silicon-based reagents are well suited to the construction of C-0 and C-C bonds systems. The ready availability of acceptors which contain fluorine as a potential leaving group makes such reagent/substrate combinations especially attractive. 

Brook MA (2000) Replacing H with Si silicon-based reagents. In Sihcon in organic, organometaUic and polymer chemistry. Wiley, New York, pp 189-255... 

Silicon-based Reagents. Cyclopropyl esters and ketones (including vinylogous cyclopropyl ketones)are cleaved by trimethylsilyl iodide to give ring-opened iodides, and in the case of cyclopropyl esters the initially formed products recyclize to y-butyrolactones on treatment with base (Scheme 26). The reagent... 

Ottosson H, Steel PG (2006) Silylenes, silenes, and disilenes novel silicon-based reagents for organic synthesis Chem Eur J 12 1576... 

The solid bases CaO and hydroxyapatite catalyze the hydrosilylation of benz-aldehyde by triethoxysilane at 90° in yields of 59% and 72% within one and two hours, respectively.323,324 These reductions also very likely involve activation by valence expansion of the silicon hydride reagent. 

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

Silicon-based Lewis acids have been known for some time, and the related chemistry in catalysis has recently been reviewed [24]. Most examples in the literature are mainly based on achiral species and will be discussed only briefly in this section. In general, a broad variety of reactions can be catalyzed with compounds like MejSiOTf, MejSiNTf or MOjSiClO. One advantage over some metal Lewis acids is that they are compatible with many carbon nucleophiles like silyl enol ethers, allyl organometallic reagents and cuprates. 

Micro flow control devices open new possibilities for the miniaturization of conventional chemical and biochemical analysis systems. The micro total analysis system (pTAS) including microfabricated detectors (e.g. silicon based chemical sensors, optical sensors), micro flow control devices and control/detec-tion circuits is a practical micro electro mechanical system (MEMS). pTAS realize very small necessary sample volume, fast response and the reduction of reagents which is very useful in chemical and medical analysis. Two approaches of monolithic and hybrid integration of these devices have been studied. Monolithic and hybrid types of flow injection analysis (FIA) systems were already demonstrated [4, 5]. The combination of the partly integrated components and discrete components is useful in many cases [6]. To fabricate such systems, bonding and assembling methods play very important roles [7]. 

Trimethyl(trifluoromethyl)silane can be converted into tributyl(trifluoromethyl)tin (40) in quantitative yield by treatment with bis(tributyltin) oxide in tetrahydrofuran under fluoride ion initiation. Tributyl(trifluoromethyl)tin readily trifluoromethylates disilyl sulfides at room temperature giving the corresponding trifluoromethylated di- and trialkylsilancs 41 in high yields. This method provides a new approach for the preparation of silicon-based trifluoro-methylating reagents via transtrifluoromethylation of compounds containing Si — S bonds. 

Protection of yie-diols. Several laboratories have recently reported migration of t-butyldimethylsilyl protective groups of ribonucleosides, Thus the 2 - and 3 -0-l-butyldimethylsilyl derivatives of ribonucleosides can rearrange in the presence of base (pyridine) by an intramolecular reaction. Obviously care must be exercised in the use of silicon-containing reagents for partial protection of polyols. 

In the presence of a fluoride ion generating reagent such as tris(diethylamino)sulfonium difluorotri-methylsilicate (TASF, 99) and a palladium catalyst, vinyltrimethylsilane couples with vinyl and aryl iodides." For substituted vinylsilanes, the fluorodimethylsilyl group is particularly effective in the coupling reaction, which takes place with retention of configuration of both substrates (Scheme 17). Silicon-based C—C bond formation is as useful as that which employs organoborons " for the synthesis of stereo-defined conjugated dienes. 

The same reaction conditions were applied to the coupling reaction of Trimethyl(vinyl)silane [Eq.(3)] with limited success [3], Thus, the mechanism of these silicon-based reactions appears to be different from those of the cross-coupling reactions using the main-group metal reagents. 

Summary A silicon-based technique of using RFSi(CH3)3 reagents is quite useful for introducing fluorinated substituents into perfluoroolefins and perfluoroimines, especially for constructing the highly branched derivatives of perfluoroolefins and perfluoroimines, which are otherwise difficult to synthesize. The scope and limitations of these reactions are described, and comparison of the reactivities of the perfluoroolefin and the perfluoroimine with RFSi(CH3)3 are made. 

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