Alcohols trimethylsilyl

Notes, fa) Rate of metallation with t-BuLi varies from case to case. Lithiation of ally] alcohol trimethylsilyl ether proceeds to completion in 2 h at -78 °C, whereas the corresponding methallyl derivative requires 3.5 h at -33°C. 

Allenyl Silyl enol ethers, 86 Allyl alcohol trimethylsilyl ether, 84 Allyl carbonates, 114-15 9 Allyl-ay 2 octalone, 34-5 2-Allyl-2 methylcyclohexanone, 106 (Allyldimethylsilyl)methyl chloride, 58, 59 (AUyldimethylsilyl)methylmagnesium chloride, 59... 

Figure 11.7 Pyrogram of a beeswax sample obtained with a microfurnace pyrolyser at 600°C, in the presence of HMDS. TIC total ion current m/z 117 profile of carboxylic acid trimethylsilyl esters, showing a maximum with palmitic acid m/z 57 profile of hydrocarbons, showing a maximum with heneicosane m/z 103 profile of alcohol trimethylsilyl ethers, showing a max imum with docosanol. For the identification of all peaks, see Bonaduce and Colombini [70]...

Protection of Alcohols. Trimethylsilyl ethers, readily prepared from alcohols by treatment with a variety of silylating agents have found considerable use for the protection of alcohols. They are thermally stable and reasonably stable to many organometallic reagents and they are easily cleaved by hydrolysis in acid or base or by treatment with fluoride ion. t, Butyl dimethylsilyl ethers have considerably greater hydrolytic stability and are easier to work with than trimethylsilyl ethers. They are prepared from alcohols by treatment with t. butyl dimethylsilyl chloride. 

Oxidation of alcohols. Trimethylsilyl ethers of secondary alcohols are oxidized in CH2CI2 to ketones in 95-1007o yield by trityl tetrafluoroborate. Oxidation of ethers of primary alcohols is too slow to be useful. Consequently selective oxidation of ethers of secondary alcohols is possible. Oxidation of bissilyl ethers is not clean, but use of bistrityl ethers or bis-r-butyl ethers is a useful method. The latter ethers are useful for selective oxidation of some 1,3-diols, because only... 

New reagents for the primary and secondary alcohol to alkyl iodide conversion, with inversion at secondary centres, are diphosphorus tetraiodide (P2I4), a well characterized and stable solid, and mixtures of triphenylphosphine with iodine and imidazole or with 2,4,5-tri-iodoimidazole. The P2I4 system also iodinates tertiary alcohols. Trimethylsilyl iodide is known to convert alcohols into iodides (2,128), and some more systems that are believed to generate trimethylsilyl iodide in situ have been found to effect the alcohol to iodide conversion (c/. 3,151). Trimethylsilyl chloride-sodium iodide in acetonitrile produces iodides from alcohols direct or from their trimethylsilyl ethers. Hexamethyldisilane-... 

Benzyl trichloracetimidate (48) is a new reagent for acid-catalysed benzylation of alcohols in the presence of trifluoromethanesulphonic acid, and benzyl p-toluenesulphonate-potassium carbonate has been recommended as abenzylat-ing system for phenols, especially in cases where benzyl chloride-potassium carbonate gives C-alkylated impurities.Facile removal of benzyl ether protecting groups has been achieved by catalytic transfer hydrogenation with Pd(OH)2 on carbon and cyclohexene as hydrogen-donor. A new procedure for O-tritylation by treatment of an alcohol trimethylsilyl ether with trityl trimethylsilyl ether is shown in equation (6). The synthesis and characterization has been completed of 4-dimethylamino-N-triphenylmethylpyridinium chloride (49)," a postulated intermediate in the formation of trityl ethers from alcohols... 

Silylation of Alcohols. Trimethylsilyl ethers can be prepared in good yield by reacting alcohols with trimethylsilyldiethyl-... 

The most stable protected alcohol derivatives are the methyl ethers. These are often employed in carbohydrate chemistry and can be made with dimethyl sulfate in the presence of aqueous sodium or barium hydroxides in DMF or DMSO. Simple ethers may be cleaved by treatment with BCI3 or BBr, but generally methyl ethers are too stable to be used for routine protection of alcohols. They are more useful as volatile derivatives in gas-chromatographic and mass-spectrometric analyses. So the most labile (trimethylsilyl ether) and the most stable (methyl ether) alcohol derivatives are useful in analysis, but in synthesis they can be used only in exceptional cases. In synthesis, easily accessible intermediates of medium stability are most helpful. 

Trichloroacetonitrile reacts with glycosidic hydroxy groups of protected sugars to form glycosyl trichloroacetimidates (R. R. Schmidt, 1980, 1984,1985,1986 B. Wegmann, 1988). The imidate is substituted by alcohols in the presence of trimethylsilyl trifluoromethanesulfonate... 

Trimethylsilyl iodide [16029-98-4] (TMSI) is an effective reagent for cleaving esters and ethers. The reaction of hexamethyldisilane [1450-14-2] with iodine gives quantitative conversion to TMSI. A simple mixture of trimethylchlorosilane and sodium iodide can be used in a similar way to cleave esters and ethers (8), giving silylated acids or alcohols that can be Hberated by reaction with water. 

Etherification. The reaction of alkyl haUdes with sugar polyols in the presence of aqueous alkaline reagents generally results in partial etherification. Thus, a tetraaHyl ether is formed on reaction of D-mannitol with aHyl bromide in the presence of 20% sodium hydroxide at 75°C (124). Treatment of this partial ether with metallic sodium to form an alcoholate, followed by reaction with additional aHyl bromide, leads to hexaaHyl D-mannitol (125). Complete methylation of D-mannitol occurs, however, by the action of dimethyl sulfate and sodium hydroxide (126). A mixture of tetra- and pentabutyloxymethyl ethers of D-mannitol results from the action of butyl chloromethyl ether (127). Completely substituted trimethylsilyl derivatives of polyols, distillable in vacuo, are prepared by interaction with trim ethyl chi oro s il an e in the presence of pyridine (128). Hexavinylmannitol is obtained from D-mannitol and acetylene at 25.31 MPa (250 atm) and 160°C (129). 

Polarimetric analysis of sorbitol and mannitol in the presence of each other and of sugars is possible because of their enhanced optical rotation when molybdate complexes are formed and the higher rotation of the mannitol molybdate complex under conditions of low acidity (194). The concentration of a pure solution of sorbitol may be determined by means of the refractometer (195). Mass spectra of trimethylsilyl ethers of sugar alcohols provide unambiguous identification of tetritols, pentitols, and hexitols and permit determination of molecular weight (196). 

Opening by trimethylsilyl trifluoromethanesulfonate yields an adduct (54) from which trifluoromethanesulfonic acid can be eliminated to give an allylic alcohol (Scheme 47) (79JA2738) [cf. base-promoted isomerization to allylic alcohols (Section 5.05.3.2.2)]. 

A large number of silylating agents exist for the introduction of the trimethylsilyl group onto a variety of alcohols. In general, the sterically least hindered alcohols are the most readily silylated, but these are also the most labile to hydrolysis with either acid or base. Trimethylsilylation is used extensively for derivatization of... 

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. 

A,<9-Bis(trimethyIsilyl)trifluoroacetamide. The reagent is suitable for the silylation of carfjoxylic acids, alcohols, phenols, amides, and ureas. It has the advantage over bis(trimethylsilyl)acetamide in that the byproducts are more volatile. 

Trimethylsilyl cyanide. This reagent readily silylates alcohols, phenols, and carboxylic acids, and more slowly, thiols and amines. Amides and related compounds do not react with this reagent. The reagent has the advantage that a volatile gas (HCN is highly toxic) is the only byproduct. 

Me3SiI, CH2CI2, 25°, 15 min, 85-95% yield.Under these cleavage conditions i,3-dithiolanes, alkyl and trimethylsilyl enol ethers, and enol acetates are stable. 1,3-Dioxolanes give complex mixtures. Alcohols, epoxides, trityl, r-butyl, and benzyl ethers and esters are reactive. Most other ethers and esters, amines, amides, ketones, olefins, acetylenes, and halides are expected to be stable. 

This ester is formed by standard procedures and is readily cleaved with Pd(Ph3P)4 in CH2CI2 to form trimethylsilyl esters that readily hydrolyze on treatment with water or alcohol or on chromatography on silica gel (73-98% yield). Amines can be protected using.the related carbamate. ... 

Silyl esters are stable to nonaqueous reaction conditions. A trimethylsilyl ester is cleaved by refluxing in alcohol the more substituted and therefore more stable silyl esters are cleaved by mildly acidic or basic hydrolysis. 

Some of the more common reagents for the conversion of carboxylic acids to trimethylsilyl esters are listed below. For additional methods that can be used to silylate acids, the section on alcohol protection should be consulted since many of the methods presented there are also applicable to carboxylic acids. Trimethylsilyl esters are cleaved in aqueous solutions. 

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