Silyl group

Although trimethylsilyl lactic acid derivatives are known, they are not sufficiently stable to many reaction conditions to be synthetically useful. 

A more stable, and consequently more synthetically useful protecting group, is the tert-butyldimethylsilyl (TBS) functionality. Ethyl or methyl lactate is easily protected with this group by treatment with r r -butyldimethylsilyl chloride in the presence of a suitable base. The use of imidazole in DMF furnishes 401 or 402 in 90-99% yield [126,127,128,130], whereas triethylamine/DMAP in tetrahydrofuran affords 401 in 90% yield [129]. 

Vinylation of 401 is accomplished by initial saponification with lithium hydroxide to yield the lithium salt 403, followed by addition of one equivalent of vinyllithium to give 404. After 

Selective reduction of the 4-keto group of either 410 or 411 gives an intermediate alcohol which, upon silyl deprotection, cyclizes to the bicyclic system 412 by an Sn2 mechanism. The product is obtained as a mixture of epimers at the C-2 carbon, but these are readily separated by column chromatography. 

The necessary 2-(ethylthio)allyl silyl ether 414 is prepared in good yield from 402 by conversion to thioester 413 followed by treatment with Tebbe reagent. Reaction of 414 with a variety of aldehydes in the presence of Me2AlCl produces adducts 415 in high yield. To 

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Monosubstitution of acetylene itself is not easy. Therefore, trimethylsilyl-acetylene (297)[ 202-206] is used as a protected acetylene. The coupling reaction of trimethylsilylacetylene (297) proceeds most efficiently in piperidine as a solvent[207]. After the coupling, the silyl group is removed by treatment with fluoride anion. Hexabromobenzene undergoes complete hexasubstitution with trimethylsilylacetylene to form hexaethynylbenzene (298) after desilylation in total yield of 28% for the six reactions[208,209]. The product was converted into tris(benzocyclobutadieno)benzene (299). Similarly, hexabutadiynylben-zene was prepared[210j. 

The ability to promote /S elimination and the electron-donor capacity of the /3-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. An example is the replacement of a trimethylsilyl substituent by an acetyl group by reaction with acetyl chloride. 

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

The transformation of an ester carbonyl group to a difluoromethylene group, which IS usually difficult to perform, can be accomplished by conversion to the thiaesier followed by treatment with diethylaminosulfur trifluoride (DAST). A vanety of ester types react efficiently, although the reaction fails with lactones. Remarkably, trimethylsilylmethyl esters carry through the proeedure with the silyl group intact [13] (equation 17). 

Silyl groups have found broad appeal as protective groups because their reactivity and stability can be tailored by varying the nature of the substituents on the silicon. Their ability to migrate from one hydroxyl to another is a property that can be used to advantage, but more often, it is a nuisance. The migratory apti-... 

In the well-known Brook rearrangment, silyl groups migrate from oxygen to carbon, but the following example is less obvious and not necessarily predictable ... 

The following tables give a comparison of the stability of various silyl ethers to acid, base, and TBAF. The reported half-lives vary as a function of environment and acid or base concentration, but they help define the relative stabilities of these silyl groups. 

The sisyl ether is stable to Grignard and Wittig reagents, oxidation with Jones reagent, KF/18-crown-6, CsF, and strongly acidic conditions (TsOH, HCl) that cleave most other silyl groups. It is not stable to alkyllithiums or LiAlH4. 

The bulk of the TIPS group, introduced with TIPSCl (DMF, Im, 92% yield), directs metallation away from the silyl group as illustrated. ... 

In this series of amides, hydrolysis or aminolysis of a simple ester, cleavage of a silyl groups a cis/trans isomerization, or reduction of a quinone to a hydro-quinone exposes an alcohol that then induces deprotection by intramolecular addition to the amide carbonyl. 

The role of silyl groups in condensation polymerization is different from that in GTP. The use of silylated monomers in condensation polymerization was studied first by Klebe [90-92] in 1964. N-trimethylsilyl-substi-... 

Apart from the mentioned advantages, the polymeric reagents covalently adsorbed by silica also diminish its inherent non-specific adsorptivity. One of the ways to synthesize a polymeric modifier of this type is a copolymerization of a vinylsilane with a compound of the desired functionality. The segments carrying silyl groups will condense with the surface silanols forming anchors or trains . 

This /J-silyl phosphorane is a synthon for a vinylmetal reagent, Vinylation occurs with concomitant migration of the silyl group. 

Allylsilanes in which the silyl group is at the more substituted end of the allyl system have been prepared by a reaction sequence involving the conjugate addition of silylcuprates to a, jS-unsat-urated esters followed by reduction and dehydration via selenoxide elimination38. 

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