Silicon silyl chlorides

Another convenient procedure for hydrochlorination involves adding trimethylsilyl chloride to a mixture of an alkene and water. Good yields of HC1 addition products (Markovnikov orientation) are formed.3 4 These conditions presumably involve generation of HC1 by hydrolysis of the silyl chloride, but it is uncertain if the silicon plays any further role in the reaction. 

We should compare the S reaction at silicon with the S 2 reaction at carbon. There are some iportant differences. Alkyl halides are soft electrophiles but silyl halides are hard electrophiles. Alkyl halides react only very slowly with fluoride ion but silyl halides react more rapidly with fluoride [than with any other nucleophile. The best nucleophiles for saturated carbon are neutral and/or based on elements down the periodic table (S, Se, I). The best nucleophiles for silicon are charged and based on highly electronegative atoms (chiefly F, Cl, and O). A familiar example is the reaction of enolates at carbon with alkyl halides but at oxygen with silyl chlorides (Chapter 21). 

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. 

Sharpless oxidation, 8 Sila-Pummerer rearrangement, 314 Silica, 18, 185, 342, 346-347 Silicon(IV) chloride, 347 Silver carbonate, 350 Silver chloride, 347-348 Silvcr(II) dipicolinate, 348 Silver fluoride-Pyridine, 348 Silver imidazolate, 349-350 Silver(I) nitrate, 350 Silver(l) oxide, 350-351 Silver(II) oxide, 80, 352-354 Silver perchlorate, 354-355 Silver(I) trifluoracetate, 365 Silybin, 351,352 2-Silyl-l,3-dithianes, 380 Silyl nitronates, 381 SMEAH, 357... 

Quantitative relationships have been developed to examine the steric factors associated with nucleophilic attack on silicon and the solvolysis of silyl chlorides. Silyl ethers are also considered to be poor donor ligands for chelation-con-trolled reactions, and thus their use in reactions where stereoinduction is anticipated must be carefully considered. One of the properties that has made silyl groups so popular is the fact that they are easily cleaved by fluoride ion, which is attributed to the high affinity that fluoride ion has for silicon. The Si-F bond strength is 30 kcaV mol greater than the Si-0 bond strength. 

Stabilization of a lipid membrane onto a solid support by covalent attachment also provides the physical stability necessary for the development of practical sensors. An oriented membrane can be prepared by allowing self-assembly of individual amphiphilic molecules onto a solid surface through either the reaction of terminal silane moieties with a hydroxylated surface to form a silyl ether [33,34], or by the reaction of sulfur-terminated compounds (alkylthiols or disulfides) with gold surfaces [35,36]. A variety of species, both with and without polar head groups, have been deposited onto surfaces such as glass, quartz, silicon, and gold [37-39]. These include phospholipids, fatty acids, and fatty amines which were synthetically altered so as to contain either a silyl chloride or a thiol moiety at the terminus of the acyl chain [40]. Both monolayers... 

The traceless anchors based on silicon just described are limited by the need to synthesize a preformed handle consisting of the first building block and the silicon linker. Moreover, traceless release from conventional aryldialkylsilanes is complicated by their low hydrolytic stability and loss of silicon from the resin in the cleavage process. The silyl chloride resin 8h can be used to anchor aryl rings by reaction with aryllithium reagents. The anchor is stable to concentrated TFA and bases, but treatment with TBAF cleaves the final products [353]. 

Summary Aryl ligands carrying oxygen, phosphorus or sulfur donor atoms in the side chain are introduced at silicon. Here we report on the synthesis of silyl chlorides and silyl triflates. The interaction between the silicon centre and the donor atom are discussed on the basis of NMR and X-ray crystallographic data. In the reaction of bis[2-(methoxymethyl)phenyl]methylsilyl triflate 8 with nucleophiles, reactivity of oxonium as well as of siliconium ions is observed. 

Sol 3. (i) Danishefsky s diene is obtained from a, P-unsaturated ketones via treatment with trimethyl silyl chloride and zinc chloride. The zinc chloride is a Lewis acid that activates the oxygen for reaction with the silicon. The mechanism of this transformation is as follows ... 

Silanes. Dichlorodimethylsilane (1) is a precursor to a wide variety of silicon reagents. For example, reaction with an organo-metallic reagent, such as phenyl- or f-butyllithium, results in the corresponding silyl chloride (eq 4). Many of these silyl chlorides have found application for the protection of functional groups. 

---