Of dimethylsilyl

Although several advantages of dimethylsilyl (Me2HSi) ethers have been claimed,191 few examples of their use have been reported, except for the separation of cycloamyloses.133 Thus, dimethylsilyl derivatives (which are more volatile than trimethylsilyl ethers) require lower column-temperatures and have shorter retention times for example, for glucose, the per(trimethylsilyl) derivative has a retention time of 11.4 min, and the per(dimethylsilyl) derivative, 5.10 min. Both types of derivative may be analyzed on the same column, and, in certain cases, compounds inseparable as their O-trimethylsilyl derivatives may be separated as their dimethylsilyl ethers. The potential of di-methylsilylation for converting oligosaccharides into volatile derivatives should be examined. 

Thus far, there are only a few examples of the use of dimethylsilyl derivatives, but cyclo-hexa-, -hepta-, and -octa-amylose have been separated from each other by using these ethers.133... 

An analogous mechanistic scheme (equation 87) has been proposed for the flash vacuum pyrolysis of dimethylsilyl(trimethylsilyl)thioketene148 (256). The pyrolysis of bis(trimethylsilyl)thioketene (257) leads to a more complicated product mixture (equation 88). With 47% conversion, a mixture of trimethylsilylacetylene, 1-trimethylsilyl-1-propyne, bis(trimethylsilyl)acetylene, (trimethylsilyl)thioketene, 2,2,4,4-tetramethyl-2,4-disila-l-thietane and 2,2,4,4-tetramethyl-2,4-disila-l,3-dithietane was obtained. All products can be rationalized, however, by the assumption that carbene 258 undergoes not only a silylcarbene-to-silene rearrangement (as in the preceding two cases) but also isomerization to 2-thiirene and insertion into a methyl-C, H bond. 

Linear permethylpolysilanes Me(Me2Si) Me with n = 4-8 also undergo photolysis when their cyclohexane solutions are irradiated with a low-pressure mercury lamp (32,33). Octamethyltrisilane is always produced as the major volatile product, in addition to 1-hydroheptamethyltrisilane and polymeric substances arising from polymerization of dimethylsilyl-ene. Octamethyltrisilane (Xmax 215.0 nm) is photochemically rather stable under the conditions used, but prolonged irradiation of its dilute cyclohexane solution causes skeletal contraction, giving hexamethyldisilane with extrusion of dimethylsilylene. 

The photolysis of phenylethynylpentamethyldisilane (56) takes place simultaneously by at least two different processes. The main route proceeds through a silacyclopropene and the other involves transient formation of a silapropadiene. Irradiation of a benzene solution of 56 in the presence of acetone gives four products, 2,2,5,5-tetrameth-yl-3-trimethylsilyl-4-phenyl-l-oxa-2-silacyclo-3-pentene (57), 2,2,5,5-tetra-methyl-3-phenyl-4-trimethylsilyl- l-oxa-2-silacyclo-3-pentene (58), phen-yltrimethylsilylacetylene, and 1-pheny 1-1 -trimethylsily 1-3-methyl-1,2-butadiene (59), in 51,2,10, and 5% yield, respectively, with 81% conversion, of the starting disilane. The formation of 57 and 58 can be explained by insertion of acetone into the silacyclopropene. Liberation of dimethylsilyl-ene species from either direct photolysis of 56 or decomposition of the silacyclopropene results in the formation of PhC=CSiMe3. Product 59... 

VI. Vanden Heuvel, W. J. A., The gas-liquid chromatography of dimethylsilyl trimethylsilyl and chloromethyldimethylsilyl ethers of steroids. Mechanism of silyl ether formation and effect of trimethylsilylation upon detector response. 

In the cases of dimethylsilyl IMDA precursors, the exolendo selectivity was poor. However, this ratio could be readily and quite dramatically influenced by varying the alkyl substituents on the silicon template [12]. Thus with dienol 24, tether formation with dimethylvinylsilyl chloride and subsequent IMDA reaction afforded a 4 1 mixture of exolendo products (Scheme 10-7). The ratio could be further improved to 10 1 by using a diphenylsilyl tether, and when bulky Bu groups were used, a single stereoisomer, resulting from exo addition, was observed. This example once more illustrates the potential for tuning the stereoselectivity of the reaction by varying the steric interactions with the tether. 

Kokubo, M., and Kobayashi, S. (2008). Scandium(III) fluoride as a novel catalyst for hydroxymethylation of dimethylsilyl enolates in aqueous media. Synlett, 1562-1564. 

Since 1973, many Lewis acid-catalyzed aldol reactions have been studied, as described above. There have, however, been few examples of Lewis base-catalyzed aldol reaction using special silyl enolates, e.g. phosphoramide-catalyzed aldol reactions of trichlorosilyl enolates 69 (Eq. (36)) [47] and CaCh-catalyzed reactions of dimethylsilyl enolates 72 in aqueous DMF (Eq. (37)) [56]. 

Scheme 3-128. Reaction of dimethylsilyl enol ethers with aldehydes.

N-(Thexyl dimethylsilyl)dimethylamine (N-[2,3-dimethyl-2-butyl]dimethylsilyl dimethyl-amine) [81484-86-8] M 187.4, b 156-160°/720mm. Dissolve in hexane, filter, evaporate and distil. Colourless oil extremely sensitive to humidity. It is best to store small quatities in sealed ampoules after distillation. For estimation of purity crush an ampoule in excess O.IN HCl and titrate the excess acid with O.IM NaOH using methyl red as indicator. [Helv Chim Acta 67 2128 1984.]... 

Bradshaw and Hui, in their review of sulfur-containing macrocycles, have included the analog of compound 7 (this chapter) in which each carbon bridge is replaced by a dimethylsilyl bridge. Nevertheless, silicon-containing crown relatives remain rare. 

A/ HE, 0.1 M NaF, pH 5. THE, 25°, 2 days, 77% yield. In this substrate, a mixture of products resulted from the attempted cleavage of the t-butyl-dimethylsilyl ether with tetra-n-butylammonium fluoride, the reagent generally used. ... 

The introduction of a (dialkylamino)dimethylsilyl or dimethylphenylsilyl group makes it possible to introduce a hydroxy group stereospecifically by Tamao oxidation78 80. This was successfully applied in the synthesis of /rrw.y-4-hydroxy-5-alkylfuranones81. 

Difunctional reagents, for example the very cheap dimethyldichlorosilane 48, which is produced on a large technical scale, and the much more reactive and expensive dimethylsilyl bis(O-triflate) 49 [65-67] (Scheme 2.8) convert alcohols or phenols 11 in the presence of bases, for example triethylamine or DBU, into the silylated compounds 50. Thus 48 and 49 and other bifunctional reagents such as di-tert-butyldichlorosilane [68] or di(tert-butylsilyl)-bis(0-triflate) [69] and the subsequently described 51 and 52 combine two alcohols to silicon-tethered molecules 50, which can undergo interesting intramolecular reactions [70-74]. 

Because the trimethylsilyl enol ether of cyclohexanone 107 a is considerably more bulky than the corresponding dimethylsilyl enolate 107b, only the latter reacts with the N-tosyhmine 108 in the presence of catalytic amounts of diisopropylamine in DMF/H2O at 78°C or at room temperature to give the Mannich type compounds 109 in high yields [39] (Scheme 3.4). 

Whereas 2-methylpyridine-N-oxide 881a reacts rather slowly with TCS 14/ NaCN/NEt3 in DMF at 100-110°C, sterically hindered 2-methoxycarbonyl- 881b, 2-isopropyl- 881c, or 2-tert-butylpyridine-N-oxide 881d have not yet been reacted in the presence of NEts or DBU in DMF with the much less bulky but apparently as yet unknown dimethylsilyl cyanide Me2HSiCN 883 (which can probably be gener-... 

Silica sol-gel inunobihzed La(OTf)3 (Scheme 48.2B) previously used in the acylation of a series of alcohols and activated aromatic compounds using acetic anhydride as acylating agent, showed a poor activity compared with other various sihca sol-gel inunobihzed triflate derivatives (tert-butyl-dimethylsilyl-trifluoromethane-sulfonate (BDMST), or trifhc acid (HOTf)). Acylation at the aromatic ring occurred over the BDMST and HOTf catalysts, while the La(OTl)3 catalysts only led to O-acetylated products [22]. Such behavior is characteristic... 

It has also been shown that dimethylsilyl enolates can be activated by diisopropylamine and water and exhibit a high reactivity toward iV-tosyl imines to give Mannich-type reaction products in the absence of a Fewis acid or a Bronsted acid.51 For example, the reaction of [(1-cyclohexen-l-yl)oxy]dimethylsilane with 4-methyl-A -(phenylmethylene)benzene sulfonamide gave re/-4-methyl-N- (f )-[(15)-(2-oxocyclohexyl)phenyl-methyl] benzenesulfonamide (anti-isomer) in 91% yield stereoselectively (99 1 anti syn) (Eq. 11.30). On the other hand, Fi and co-workers reported a ruthenium-catalyzed tandem olefin migration/aldol and Mannich-type reactions by reacting allyl alcohol and imine in protic solvents.52... 

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