Phenolic acids, trimethylsilyl derivative

This difference in chemical reactivity can be converted into a differential etch rate by treating the exposed film with an appropriate silylating reagent. For example, chlorotrimethylsilane, hexamethyldisilazane and bis(trimethylsilyl)acetamide are well know to react with phenolic hydroxyls, carboxylic acids and other nucleophilic species to form the corresponding trimethylsilyl derivatives. 

If the unknown is sufficiently volatile or can be made volatile by derivatization, then the LC/MS approach can be complemented by the use of GC/MS. A useful starting point in this regard is to make a trimethylsilyl derivative. Silylation is applicable to a wide range of nonsterically hindered functional groups, including alcohols, phenols, thiols, amines, oximes, and carboxylic acids (Table 6) 22,127 The dried sample (1 pg... 

Takahashi [12, 13] used gas liquid chromatography to determine BHA and BHT in breakfast cereals. He obtained quantitative recoveries of 20 to 30 ppm of the antioxidants added to cereals. To determine antioxidants and preservatives in soya sauce and other foods, Takemura [14] converted these compounds to trimethylsilyl derivatives and analysed the product by GC. The sample was shaken with the silation reagent (1 ml) for 30 seconds, then set aside for 5 minutes and 2 pi of the solution was analysed by GC on, for example 20% of SE-31 silicone on Celite 545 at 200 °C with helium as carrier gas (22 ml/min) and hexane as internal standard. The method was applied to 15 preservatives and 7 antioxidants well-separated peaks were obtained for 4-hydroxy benzoate estars, phenol, xylenol derivatives and salicylic acid. The calibration graphs for methyl, ethyl, propyl and butyl-4-hydroxybenzoates were rectilinear for the range 0.5 to 4 mg. The method was used to determine 4-hydroxybenzoates in soya sauce. 

Attempts were made to overcome the problems of partial ether formation with hydroxy and phenolic acids by the use of the trimethylsilyl ether methyl ester derivatives. Dalgliesh et al. (1966) esterified the carboxylic acid groups in organic acids using diazomethane and a short reaction time (<1 min) with immediate evaporation of the reagent and solvent, followed by... 

Fig. 4.2 Chromatogram of standard phenolic acids separated as their methyl ester-trimethylsilyl ether derivatives on a 6 ft column (0.004 m i.d.) packed with 10 per cent F-60 on silanized Gas Chrom P (80-100 mesh) using temperature programming from 100°C to 240°C at 2°C min" Peak identifications are 1, 2-hydroxyphenylacetate 2, 3-hydroxyphenylacetate 3, 4-hydroxyphenylacetate 4, indoleacetate 5, homovanil-late 6, homogentisate 7, vanillylmandelate 8, 5-hydroxyindoleacetate 9, nonadecanoate (standard). (Redrawn with modifications from Horning etal, 1966)...

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. 

Trimethylsilyl ethers are readily cleaved by fluoride ion, mild acids, and bases. If the TMS derivative is somewhat hindered, it also becomes less susceptible to cleavage. A phenolic TMS ether can be cleaved in the presence of an alkyl TMS ether [Dowex lX8(IfO ), EtOH, rt, 6 h, 78% yield]. ... 

However, when it comes to the more important 2,2-diaryl derivatives (1.25), the routes illustrated in Figure 1.7 are not very useful. For these derivatives the almost universally adopted synthetic method involves the reaction of a l,l-diarylprop-2-yn-l-ol (1.24) with a substituted phenol or naphthol in the presence of an acid catalyst. The acid catalyst can be alumina, an acidic clay or Nafion for heterogeneous reactions, or trifluoroacetic acid, p-toluenesulfonic acid and dodecylbenzenesulfonic acid for reactions carried out in solution. The alkynols are prepared by reaction of a benzophenone (1.22) with a Na or Li derivative of an alkynide, such as the trimethylsilyl acetylide (1.23), (Figure 1.8). ... 

O-Tritylation.2 A trimeihylsilylated alcohol, phenol, or carboxylic acid can be converted into the corresponding trityl derivative by reaction with I catalyzed by trimethylsilyl inflate (equation I). r-Butoxytrimethylsilane can be used, but it is less effective. 

There has been a summary of the use of insertion reactions of arynes into a-bonds to prepare nrt/io-disubs tituted arenes. A key to the success of these processes is the ability to generate benzyne under mild conditions by the reaction of readily available o-(trimethylsilyl)phenyl triflate with fluoride ions.61 Reaction of amines and their derivatives with benzynes generated in this way has been shown to be an efficient method for the production of N-arylated derivatives, as illustrated in Scheme 8. The method also works well in the O-arylation reactions of phenols and carboxylic acids 62... 

TMS production involves one specific functional group (-OH, -COOH, =NH, -NH2, or -SH), which loses an activated hydrogen and is replaced by a trimethylsilyl group (Proestos et ah, 2006). To achieve silylation, some authors have used BSTFA (N,0-hA(trimethyl-silyl)trifluoroacetamide) and TMCS (trimethylchlorosilane) successfully in several matrices (e.g. aromatic plants, cranberry fixiit) (Zuo et ah, 2002 Proestos et ah, 2006). Using silylated derivatives is advantageous for several reasons phenols and carboxylic acids are prone to silylation, these compounds can be derivatized in the same part of the process, and the minor products do not impede analysis and are well documented (Little, 1999 Stalikas, 2008). A two-step methylation procedure was used to analyze catechins and tannins in plant extracts. The first step used trimethylsilyl diazomethane (TMS-diazomethane) to pre-methylate the sample, and the second step used thermally assisted hydrolysis and methylation (THM). The pre-methylation step with TMS-diazomethane stabilized the dimer molecule m/z 540) by minimizing isomerization and reducing reactivity. (Shadkami et ah, 2009). 

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