Halogenated silyl derivatives

The pentafluorophenyldimethylsilyl derivatives, termed flophemesyl, were developed by Morgan and Poole in 

Flophemesylamine (43) was found to be a seli ctive reagent which reacts only with primary and secondary hydroxyl groups in steroids in the presence of unprotected ketone groups. 

Preparation of flophemesyl derivatives of steroids [448] To form sterol flophemesyl ethers add flophemesylamine (20 p ) to the sterol (1-5 mg) in pyridine (20 1, dried over barium oxide) at room temperatun in a Reacti-Vial (Pierce and Warriner) and allow to stand for 15 min at room temperature. Analyse directly by GC using flame ionization detection. For electron capture detection remove the solvent and re-dissolve in hexane. 

Much less thoroughly investigated, but showing considerable promise, is the use of flophemesyl derivatives to improve detection limits in GC analyses with photoionization detection (PID) [452, 453]. Simple aliphatic alcohols dissoved in acetonitrile, pyridine or triethyl-amine were found to react very rapidly with flophemesyl 

Halomethyldimethylsilyl ether derivatives were introduced in the 1960 s because of the possibility for their selective determination at trace levels in GC analyses using the electron capture detector [459, 460], Under favourable circumstances, ECD limits for halocarbon-dimethylsilyl ethers can be 3-4 orders of magnitude better than those attainable with FID. The order of response of the ECD to halogens is 1 Br Cl F. 

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Previous derivatization of the extract is necessary to improve the stability of the compounds and the sensitivity and precision of subsequent GC-MS analysis. Silyl derivatives formed for example with MSTFA [43], halogenated alkene derivatives produced with heptafluorobutyric anhydride (HFBA) [36] or pentafluoropropionic acid [58] or anhydride (PFPA), as well as acetate derivatives formed using acetic anhydride [48] have been widely employed. 

Another approach consists of an in-situ acetylation and extraction of NPEOs and further analysis of the acetyl derivatives. The method has been applied to analyse effluent water and sewage sludges [102,103], sediments [104] and river waters [105]. Silylated derivatives [106] using BSA or BSTFA have also been used to determine NPEO (n < 6) in seawater [107] and wastewater [107,108], sediments [109] and sludges from wool scour effluents [110]. Halogenated derivatives of alkylphenols (AP) can also be formed as a result of chlorination practices in water treatment or wastewater if bromide is present. Brominated OPs and NPs (BrAPEOs) have been identified by GC-MS in sewage [111] and tap water [89], respectively. 

The alkylation reaction is limited to nitro-substituted arenes and heteroarenes and is highly chemoselec-tive nucleophilic displacement of activated halogens, including fluorine, was not observed. The regio-selectivity is determined by the bulkiness of the silicon reagent. With unhindered silyl derivatives a strong preference for ortho addition was observed, as in the example of equation (6). With bulkier reagents attack took place exclusively at the para position (Scheme 1). The success of this reaction, which could not be reproduced with alkali enolates, was attributed at least in part to the essentially nonbasic reaction conditions under which side processes due to base-induced reactions of nitroarenes can be effectively eliminated.12... 

Silyl derivatives containing halogens have been used, as for other substances, also for carboxylic acids with sensitive electron-capture detection. Chloromethyldimethylsilyl derivatives have been applied to phenolic acids, as follows [176], An amount of 1 mg of phenolic acids was dissolved in pyridine (600 pi), bis(chloromethyl)tetramethyldisilazane (200 pi) and chloromethyldimethylchlorosilane (100 pi). The mixture was diluted with w-hexane after 30 min and 0.1 -0.2 pi was analysed (UCW-98,200°C). The limit of detection was reported to be 4 ng. 

Some halogenated silylating reagents are used in the synthesis of derivatives for GC analysis with... 

Catalytic Halogenation of Alcohols and their Silylated Derivatives... 

The Reforrnatsku reaction of a-halogenated carboxylic esters with silylated cyanohydrins combined with an intramolecular acylation reaction gives fluorinated derivatives of tetronic acid [28] (equation 17) It is noteworthy to mention that this particular reaction sequence only proceeds with ultrasonic irradiation A very... 

Apart from silyl shifts, other reactions that are also characteristic of this class of compounds or their derivatives are due to the easy formation of halogen-silicon bonds. Phosphonium salt 34, resulting from the addition of bromine to 33, undergoes spontaneous desilylation by the action of the bromide anion to give the P-bromophosphazene 35 [138,139] (Scheme 33). 

Instead of direct halogenation of ketones, reactions with more reactive derivatives such as silyl enol ethers and enamines have advantages in certain cases. 

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