Silylating reagents

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

Me3Si0)2S02- This is a powerful silylating reagent, but has seen little application in organic chemistry. 

Trimethylsilyl trifluoromethanesulfonate (trimethylsilyl triflate) is the most synthetically useful representative of the family of trialkylsilyl perfluoroalkane-sulfonates (for a review, see reference 101) This reagent is commercially available or can be prepared easily by the reaction of chlorotrimethylsilane and triflic acid [101] It has wide application in organic synthesis as an excellent silylating reagent... 

Hindered ketones and enones fail to form the ketal because of competing decomposition of the silyl reagent. 

In another example treating anilines 40 with acetone under similar conditions gave the desired quinoline 41 as a single regioisomer. It has been reported that the addition of silylating reagents, in particular bis(trimethylsilyl)acetamide, may be beneficial for these types of substrates. ... 

R C Denney, Silylation reagents for chromatography Speciality Chemicals, 1983, 3, 6-7, 12... 

The oxophilicity of silicon is also the driving force that allows the reaction of the same silylated reagent 29 with oxiranes [134], aldehydes and ketones [135,136], and enones [136] for yielding functionalized AT-silylphosphazenes (Scheme 31). 

Polar functional groups such as alcohols or phenols 11 or trimethylsilanol 4 are transformed by monofunctional silylating reagents Me3SiX 12 into their hpophilic and often volatile trimethylsilyl ethers 13 whereas water is converted into persilyl-ated water (=Me3SiOSiMe3, hexamethyldisiloxane, HMDSO, 7, b.p. 100 °C). The persilylation of phenols and, in particular, catechol (or hydroquinone) systems (Scheme 2.1) protects them efficiently against air oxidation even at temperatures of up to 180 °C. (cf, e.g., the silylation-amination of purine nucleosides with dopamine hydrochloride in Section 4.2.4)... 

Alternative silylating reagents such as N,0-bis(trimethylsilyl)acetamide 22a (BSA) [39-43], N,0-bis(trimethylsilyl)trifluoracetamide 22b (BSTFA) [44], or N,N-bis(trimethylsilyl)formamide 22c (BSF) [41, 46], in which the N- and O-trimethyl-silyl groups are in equilibrium [45] (Scheme 2.4), are much more powerful silylating reagents [40, 45] but are more expensive than FIMDS 2, because they are usually prepared by heating formamides or acetamides with TCS 14/triethylamine... 

Whereas silylations with trimethylchlorosilane (TCS) 14 (b.p. 57 °C) demand the presence of a base to neutralize the HCl evolved, giving rise to the hydrochloride of the base, the use of hexamethyldisilazane (HMDS) 2 (b.p. 126 °C), in particular in the presence of 0.01-0.05 equivalents of acidic catalysts such as TCS 14 or ammonium sulfate, should normally be preferred as the preparative silylating reagent, because HMDS 2 ... 

Free aldehydes such as benzaldehyde are directly converted by 663, excess silylation reagent 463, and TMSOTf 20 to give high yields of C-substitution products such as 668 [57]. 

Whereas conversion of sulfoxides to the corresponding a-acyloxysulfides by acid anhydrides, for example acetic anhydride, the Pummerer reaction [1], has been known for quite a time, the conversion of sulfoxides with silylating reagents via the unstable intermediate O-silyl compounds to a-silyloxysulfides, the Sila-Pummerer reaction is a relatively new reaction, which has recently been reviewed [1—4-]. 

TMCS A poor silylating reagent unless used in the presence of base (e.g., pyridine, diethylamine). Causes extensive enol-ether formation with unprotected ketone groups. Mainly used to catalyze the reaction of other silylating reagents. 

There seem to be few applications for which the use of weaX "silyl donors" is either necessary or desirable. Other imwrtant considerations for the selection of the correct reagents for a particular application are summarized in. Table 8.15. The strongest silylating reagent of all is a mixture of TMSIM-BSTFA-TNCS (1 1 1). 

To improve the detectability of silyl ethers, silylation reagents containing an electron-capturing group [443,449-451,468] or cyano group for thermionic detection [469] have been prepared, the 2-cyanoethyldimethylsilyl derivatives are only marginally aore sensitive (ca. 5 fold) to the thermionic detector than to the flame ionization det ftpr which Units their usefulness. The... 

---