Silanes electrophilic substitution

Application of silanes and metalation chemistry offers an access to substituted aromatic compounds, which are difficult to prepare by classical electrophilic substitution because of harsh reaction conditions and formation of regioisomers. 

More recently, trifluoromethanesulfonic acid (triflic acid, TfOH) has been used to functionalize silanes by electrophilic substitution of aryl substituents62,63 (equation 35). The silyl triflates formed in this reaction are useful building blocks for a wide variety of products. Chlorosilanes can be obtained by treatment with lithium chloride (equation 36). 

Molecular orbitals demonstrate the smooth transition from the allyl silane, which has a k bond and a C-Si O bond, to the allylic product with a new K bond and a new o bond to the electrophile. The intermediate cation is mainly stabilized by O donation from the C-Si bond into the vacant p orbital but it has other a-donating groups (C—H, C-C, and C-E) that also help. The overall process is electrophilic substitution with allylic rearrangement. Both the site of attachment of the electrophile and the position of the new double bond are dictated by the silicon. 

There thus exists a preference for anti (or antara) hydroxylation in these cyclohexenylstannanes, where electrophilic substitutions are known to proceed faitiifully with allylic rearrangement. A more likely padiway is shown in Scheme 4, which is supported by results with optically active allylsilanes, whidi require anti attack by MCPBA on the silane conformation maximizing C—Si tr nr interaction. 

The well-established stabilization of a positive charge on carbon p to silicon has been utilized in very versatile methods for the control of aliphatic Friedel-Crafts reactions. The specificity of this stabilization lies behind the utility of both alkenyl- and allyl-silanes as substrates for electrophilic substitutions, and acylations in particular. These classes offer complementary regiospecificities in controlling both the site of acylation and the location of the double bond. This promotion of simple substitution is one of the most significant advances in aliphatic Friedel-Crafts acylations of recent times, and has recently been the subject of an exhaustive review. ... 

TABLE 13. Stereochemical outcome of electrophilic substitution at vinyl silanes... 

Electrophilic substitution on polystyrene through a chlorometallation reaction yields chlorine functionality. This has opened up the possibilities of making many derivatives of polystyrene. Starting with chlorometallated polystyrene, derivatives such as quaternary, ammonium, or phosphonium salts have been made. Similarly, ethers, esters, sulfonamides, silanes, and ketone derivatives have been made by replacing the chlorine atom on chlorometallated polystyrene. In the case of polystyrene, however, it was discovered that chain end functionalization can be realized if the chain ends were terminated by group I metals such as lithium and potassium. 

The hydrosilylation of alkynes provides facile access to a diverse range of vinyl silane products which are versatile building blocks in organic synthesis [68]. The electrophilic substitution of vinylsilanes is one of the most useful methods for the stereoselective synthesis of substituted alkenes [77, 78]. The effectiveness of... 

As an extension to the allylic silane groups formed in the chain end in polyeth-ylene-co-allyl-Si(CH3)3, a clear indication of internal vinylene unsaturation was also found (Fig. 20a, triplet at 5.19 ppm [153], / = 7.5 Hz). Mechanistically, this unsaturation can be explained by the allylic activation taking place after the chain termination of primary inserted allyl-Si(CH3)3 [23]. Most interestingly, both of these unsaturations were in the allylic position of silicon and were therefore sensitive to electrophilic substitution [149]. This was seen when the normal acidic work-up procedure (after the polymerization step) was extended overnight, whereby all of the chain end and most of the internal allylic silane groups were cleaved off (Fig. 20b) [23]. 

The electrophilic substitution of aromatics by allyl-silanes, -germanes, and -stannanes has been achieved by an in situ transformation to the allyl cationic species (107) with thallium(iii) trifiuoroacetate. This represents a reactivity umpolung of these conventionally nucleophilic reagents, and gives allyl-substituted aromatics in good yield." ... 

Moreover, unsaturated keto silanes and diketones obtained according to eq 1 can be easily reduced to saturated keto silanes or 1,6-dicarbonyl confounds unsaturated keto silanes can be transformed into enantiomerically enriched hydroxy derivatives by reduction and enzymatic kinetic resolution. MonosUylated unsaturated sulfides, precursors of a variety of dienylsilanes, can be also obtained by chemoselective electrophilic substitutions. ... 

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

The six-position may be functionalized by electrophilic aromatic substitution. Either bromination (Br2/CH2Cl2/-5°) acetylation (acetyl chloride, aluminum chloride, nitrobenzene) " or chloromethylation (chloromethyl methyl ether, stannic chloride, -60°) " affords the 6,6 -disubstituted product. It should also be noted that treatment of the acetyl derivative with KOBr in THF affords the carboxylic acid in 84% yield. The brominated crown may then be metallated (n-BuLi) and treated with an electrophile to form a chain-extender. To this end, Cram has utilized both ethylene oxide " and dichlorodimethyl-silane in the conversion of bis-binaphthyl crowns into polymer-bound resolving agents. The acetylation/oxidation sequence is illustrated in Eq. (3.54). 

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