Silicon compounds, trimethylsilyl ethers

Silicone GE SE-52 300 support material used for trimethylsilyl ethers of polyhydroxy compounds, polyamines, etc. 

The other side of the coin is that the S 2 reaction at carbon is not much affected by partial positive [ charge (5+) on the carbon a tom. The Sn2 reaction at silicon is affected by the charge on silicon. The r most electrophilic silicon compounds are the silyl triflates and it is estimated that they react some 108-109 times faster with oxygen nucleophiles than do silyl chlorides. Trimethylsilyl triflate is, in fact, an excellent Lewis acid and can be used to form acetals or silyl enol ethers from carbonyl compounds, and to react these two together in aldol-style reactions. In all three reactions the triflate attacks an oxygen atom. 

The hypervalent silicon derivative 100, formed from the interaction of the more basic secondary amine group of the catalyst 99 with MesSiCN (Scheme 3.28), was assumed by the authors to produce the transition state 101 by interaction with the carbonyl compound (since the nucleophilicity of the cyano group of 100 is enhanced by electron donation from the pentavalent silicon) immediate silyla-tion gives the corresponding cyanohydrin trimethylsilyl ether 98 and restores the catalyst. 

Nucleophilic addition of the trifluoromethyl group to aldehydes, ketones and other carbonyl compounds leads primarily to the corresponding trimethylsilyl ether this must subsequently be hydrolyzed to the alcohol. Because typical reaction conditions are very mild, the method is widely applicable, even for sensitive substrates. In contrast with most other methods, fluoride-induced perfluoroalkylation via silicon compounds also works for enolizable carbonyl compounds. With c(, -un-saturated substrates 1,2-addition directly to the carbonyl group is strongly preferred [64b]. If the oxygen is coordinated to a bulky Lewis acid, for example aluminum tri(2,6-bis(tert-butyl)phenoxide (ATPH), the 1,4-addition products are obtained selectively [73f] (Scheme 2.128). 

Recently Yoshida et al. have employed silyl-stabilized a-alkoxy organolithium reagents for the synthesis of a variety of carbonyl compounds. Methoxy(trimethylsilyl)methane and methoxybis(trimethyl-silyl)methane, when deprotonated with Bu Li and Bu"Li respectively, give anions which can be alkylated with a variety of electrophiles. Electrolysis of a solution of the alkylated product in methanol yields, by virtue of the reduced oxidation potential of ethers a-substituted with silicon, either the dimethyl acetal or in the latter case the orthoester. The mildness of the electrolytic process recommends the method for the preparation of a variety of carbonyl compounds. 

Carbon monoxide insertion into zirconium(IV)-silicon bond has been reported . The trimethylsilyl derivative Zr(/7 -C5H5)2(SiMe3)Cl was carbonylated under CO pressure (about 7 atm) in diethyl ether, giving the following silyl-acyl compound ... 

Despite this background reaction, the reaction rate is substantially accelerated by Lewis bases, which provides an opportunity for developing an asymmetric variant. The required enoxytrichlorosilanes 21.97 can be generated in various ways, e.g., from the corresponding trimethylsilyl enol ethers on reaction with silicon tetrachloride, catalysed by mercury(ii) acetate from carbonyl compounds or trimethylsilyl enol ethers on treatment with trichlorosilyl triflate from cx-chloroketones on reaction with trichlorosilane and triethylamine or from the corresponding tributyl-stannyl enol ethers, etc. ... 

Preparative Methods several methods for the synthesis of this azide have been reported. The procedure involving aluminum chloride is not recommended, since an explosive product is formed. Azidotrimethylsilane is now commercially available, and a representative synthetic procedure is as follows. A mixture of sodium azide and chlorotrimethylsilane is refluxed in di-n-butyl ether for 2 days and the azide is safely distilled directly from the reaction vessel. Purer compound (99% content) is obtained by redistillation of the product. Several improved conditions have been reported for the preparation of this azide. In these procedures, trimethylsUyl chloride is reacted with sodium azide either neat or in a high boiling point solvent, such as a mixture of silicone oil and polyethylene glycol. Distillation of the crude product usually provides trimethylsilyl azide (TMSA) in high purity (97.9%) and yield (97%). ... 

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