Trimethylsilyl reagents

Figure 2.6 Reagents used for the deactivation of silanol groups on glass surfaces. A - disilazanes, B > cyclic siloxanes, and C -silicon hydride polysiloxanes in which R is usually methyl, phenyl, 3,3,3-trifluoropropyl, 3-cyanopropyl, or some combination of these groups. The lover portion of the figure provides a view of the surface of fused silica with adsorbed water (D), fused silica surface after deactivation with a trimethylsilylating reagent (E), and fused silica surface after treatment with a silicon hydride polysiloxane (F).

In a typical procedure, 10 mg of a sugar is dissolved in 1 ml of pyridine, treated successively with 0.2 ml of hexamethyldisilazane and 0.1 ml of chlorotrimethylsilane, and the mixture shaken for a few seconds. Reaction is normally complete within 5 minutes. This treatment corresponds to the original procedure used by Sweeley and colleagues,5 and is the method used in the great majority of cases. Apart from the deliberate use of other trimethylsilylating reagents, most of the variations on this fundamental reaction have been designed to meet certain special conditions. 

N,0-Bis(trimethylsilyl)acetamide (3) is an alternative trimethylsilylating reagent that has been found to trimethylsilylate L-ascorbic acid completely,139 and to react with tertiary hydroxyl groups in steroids.140,141 The latter observation is of interest, in view of the isolation from antibiotic substances of branched-chain carbohydrates possessing tertiary hydroxyl groups.142 Some authors have recommended that the highly reactive bis(trimethylsilyl)acetamide (3) be used in the absence of pyridine, in preference to a mixture of hexamethyldisilazane and chlorotrimethylsilane in this solvent.143 In the tri-... 

A powerful trimethylsilylating reagent, selective for hydroxyl groups, is N-(trimethylsilyl)imidazole (4) it is relatively tolerant to... 

Various fluoroacetamide derivatives have been used as trimethylsilylating reagents and, of these, N,0-bis(trimethylsilyl)trifluoro-acetamide (5) is the best known. It has approximately the same donor... 

Among the more important trimethylsilylation reagents are trimethylchloro-silane (TMCS) hexamethyldisilazane [(Me3Si)2NH, (HMDS)] iV-trimethyl-silylimidazole (TMSI) (1) AT,0-bis(trimethylsilyl)acetamide (BSA) (2) AT,0-bis(trimethylsilyl)trifluoroacetamide (BSTFA) (3) and A-methyl-A-(t-butyldimethylsilyl)trifluoroacetamide (MTBSTFA) (4). 

The method of Lentz (35,36) was adopted for trimethylsilylation of the aqueous silicate solutions. The mixture of cone, hydrochloric acid, water, 2-propanol and hexamethyldisiloxane was used as the trimethylsilylating reagent. Trimethylsilylated derivatives obtained were adaptable to gas-phase analysis. The distribution of silicate species in solutions, which was analyzed quantitatively by the trimethylsilylation technique combined with gas-liquid chromatography, was expressed as the Si(>2 recovery, that is, the percentage of silica as a silicate species in total silica component in the solutions. 

For trimethylsilylation of the silicate solids, the method of Gotz and Masson (37) was used. The mixture of chlorotrimethylsilane, hexamethyldisiloxane and 2-propanol was used as the trimethylsilylating reagent. Conditions for gas-liquid and gel permeation chromatographies were described elsewhere (12). 

It was subsequently found that the trimethylsilyl reagents of structure 79 provided a general and mild synthesis of 3-hydroxythioethers from a variety of epoxides, " but in the reaction of amino-acid type thiols with LTA4, the best... 

Tetrachloro(cyclopentadienyl) complexes of niobium and tantalum, and their ring-substituted derivatives can be prepared by two general, methods. One method uses tin reagents of cyclopentadienes and the other uses trimethylsilyl reagents, which are reacted with niobium or tantulum pentachloride. Here the former method (Method 1) is described for the synthesis of Ta(ii -C5Me5)Cl4, and the latter (Method 2) for the synthesis of Nb(T -C5Me5)Cl4. 

FIGURE 2.36 Structures of the most commonly used trimethylsilylating reagents. 

Alkyldimethylsilanol RMe SiOH (R = ethyl or propyl) is not eliminated from the corresponding silyl ethers as readily as is trimethylsilanol, permitting more mass-spectral information to be gained from the high-mass end of the spectrum [193]. Hence, with the use of ethyl- or propyldimethylsilylimidazole or -trifluoroacetamide, for example, some of the objections to the trimethylsilylating reagents are overcome with only modest sacrifices in volatility and reactivity. 

Limited experiments have used GC-MS to identify the amino acids of the ergot peptides following hydrolysis. GC is in practice restricted to the determination of clavine alkaloids and lysergic acid amides, as in the determination of LSD in forensic samples. LSD is derivatized at the N1 position with trifluoroacetic anhydride or trimethylsilylating reagents to decrease its polarity and increase volatility for GC-MS. 

FIGURE 3.23 Selected reagents used for deactivation of silanol groups (a) disUazanes, (b) cyclic siloxanes, (c) silicon hydride polysiloxanes. Lower portion is a view of fused-silica surface with (d) adsorbed water (e) after deactivation with a trimethylsilylating reagent and (f) after treatment with a silicon hydride polysiloxane. (Reproduced from Reference 83 and reprinted with permission from Elsevier Science Publishers.)... 

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