Trialkylsilyl group

A trialkylsilyl group can be introduced into aryl or alkenyl groups using hexaalkyidisilanes. The Si—Si bond is cleaved with a Pd catalyst, and trans-metallation and reductive elimination afford the silylated products. In this way, 1,2-bis-silylethylene 761 is prepared from 1,2-dichloroethylene (760)[625,626], The facile reaction of (Me3Si)2 to give 762 proceeds at room temperature in the presence of fluoride anion[627]. Alkenyl- and arylsilanes are prepared by the reaction of (Me3Si)3Al (763)[628],... 

Replacement of an aromatic/heteroaromatic proton with a trialkylsilyl group can confer a variety of synthetic advantages. The silyl moiety can mask a potentially acidic proton, and it can be readily removed by electrophiles, normally resulting in a process of ipso desilylation ... 

It is well known that trialkylsilyl groups are prone to migrate via different types of molecular rearrangements [131]. This behavior serves to explain some... 

Trialkylsilyl groups have a modest stabilizing effect on adjacent carbanions (see Part A, Section 3.4.2). Reaction of the carbanions with carbonyl compounds gives (3-hydroxyalkylsilanes. (3-Hydroxyalkylsilanes are converted to alkenes by either acid or base.270 These eliminations provide the basis for a synthesis of alkenes. The reaction is sometimes called the Peterson reaction.211 For example, the Grignard reagent derived from chloromethyltrimethylsilane adds to an aldehyde or ketone and the intermediate can be converted to a terminal alkene by acid or base.272... 

Compared to the cyclic ketones, the coupling of aliphatic aldehydes to prepare 3-substituted indoles was less successful, except for phenyl acetaldehyde, which afforded 3-phenyl indole 83 in 76% yield (Scheme 4.22). The lack of imine formation or the instability of the aliphatic aldehyde towards the reaction conditions may be responsible for the inefficiency of these reactions. Therefore, a suitable aldehyde equivalent was considered. With the facile removal of a 2-trialkylsilyl group from an indole, an acyl silane was tested as a means of preparing 3-substituted indoles. Indeed, coupling of acetyl trimethylsilane with the iodoaniline 24 gave a 2 1 mixture of 2-TMS-indole 84 and indole (85) in a combined 64% yield. Evidently, the reaction conditions did lead to some desilylation. Regardless, the silyl group of 84 was quantitatively removed upon treatment with HC1 to afford indole (85). 

The chemistry of SENA originated 35 years ago. Since that time, extensive experience in handling these hydrolytically unstable but very useful derivatives of AN has been accumulated. Although no systematic investigation of the mechanism of silylation of AN has been made, the transfer of the trialkylsilyl group from the silylating agent to the AN anion is presumably the key step of this process (Scheme 3.50). 

By the action of protons and some other electrophilic agents, these isoxazolidines eliminate the corresponding silanol to give isoxazolines (224). At the same time, nucleophiles can cause elimination of the trialkylsilyl group to form oximino alcohols (225). 

Unfortunately, these studies have not been developed in more recent years, apparently due to problems associated with the instability of Li intermediates. It should be emphasized that, unlike Li derivatives, trialkylsilyl derivatives of many organic compounds are quite stable. In organic synthesis, the trialkylsilyl group is considered as a standard protecting group (448). Here AN are no exception (for more details, see Section 3.3.1.2.)... 

Steric hindrance in the silyl groups of cation (349) and nucleophile (352) has virtually no effect on the rate constant of the C,C-coupling reaction. Hence, it can be concluded that, at least for silyl-containing nucleophiles (352), elimination of the trialkylsilyl group from cationic intermediate A is not the rate-determining step of the reaction sequence (Scheme 3.207). 

Here the bis-oxyiminium cations A are also formed as intermediates. The chemistry of these cations was discussed in sufficient detail in Section 3.5.2.3. The trialkylsilyl group can be eliminated from these cations with a good leaving group X-, but the acetate ion is the reagent of choice for optimization of similar... 

The polarographic study of (idomethyl)trialkylsilanes has revealed that the introduction of a trialkylsilyl group into iodoalkanes decreases the value of the half wave potential (E1/2), i.e. favours the electron-transfer (Table 11) [91]. 

There is - as with any other ionization method - no strict upper limit for molecular mass, nevertheless a range of up to 800-1000 u is a realistic estimate. Exceptions up to 1300 u are observed, if the analyte is extremely unpolar, e.g., from numerous fluoroalkyl or trialkylsilyl groups which also significantly contribute to molecular mass. 

Trialkylsilyl substituents have proved very useful for directing the course of cy-clizations involving vinyl ethers. The trialkylsilyl group stabilises a radical centre but destabilises a carbonium ion next to the silicon atom. An example of the di-... 

Similar to their reaction with phosphaalkenes, l-diazo-2-(oxoalkyl)silanes 29 react with various heterophospholes by [3 + 2] cycloaddition of the diazocumulene system 30 (which is in equilibrium with 29) across the P=C bond. With 2-acyl-1,2,3-diazaphospholes 119 (R = Ac, Bz no reaction with R = Me, Ph up to 60 °C), the expected cycloaddition products 120 (Scheme 8.27) could be isolated (186). Elimination of N2 from these bicyclic A -pyrazolines occurred upon heating at 100 °C and furnished the tricyclic systems 122 when SiRs was a trialkylsilyl group. Apparently, the thermolysis of 120 generates the 5-aIkenylidene-l,2,5-diazaphosphole 121 (by N2 extrusion) as well as diazaphosphole 119 (by a [3 + 2] cycloreversion process), which recombine in an intermolecular cycloaddition to furnish 122. When SiRa = SiPhaf-Bu, a formal intramolecular [3 + 2] cycloaddition of the C=P=C unit with an aromatic C=C bond occurs and the polycyclic compound 123 is obtained (187). 

It should be emphasized that the formyl group remained intact in major product 3 despite the reaction under forcing conditions. This is the first example of silylformylation in which a trialkylsilyl group and a formyl group are simultaneously connected to acetylenic carbons to form 1 in a one-pot reaction. Based on this breakthrough, the Matsuda group " published refined results and coined the word silylformylation . The reaction has since been applied to a variety of substrates as described in the following sections. 

---