Catecholamines silylation

A two-step silylation has been described for catecholamines [92]. All of the hydroxyl groups are converted into TMS ethers within 2—3 h by reaction with TMS-imidazole in acetonitrile at 60°C, BSA—TMCS (2 1) reagent is added and within the next 2 h all of the primary amino groups are converted into the N,N-di-TMS derivative. Secondary (N-methyl) amino groups do not react under these conditions. A distinct speed-up of the silylation reaction on the addition of a small amount (1%, v/v) of water to the reaction mixture (see Fig. 5.6) and different effects of the solvents are interesting. These derivatives possess excellent chromatographic properties and are well separated on OV-1. 

Catecholamines are particularly polar and difficult to extract from aqueous media. An acylating agent convenient to use directly in aqueous solution is methyl chloroformate [236], Cl-CO-OMe. Amino and phenolic hydroxyl groups are converted to carbamate (MeO-CO-NHR) and carbonate (MeO-CO-OAr) groups simultaneously, with aliphatic hydroxyl groups subsequently silylated. 

Attention has been centered on the haiogenated derivatives of primary amines. This development started with the introduction of electron capture detection systems. With pentafluoro-derivatives for example, detection by electron capture is over 1000 times more sensitive than by flame ionization. If hydroxyl groups are present, they are usually trimethylsilylated, which augments the electron affinity of the product. Picogram amounts of catecholamines have been separated on methylsilicone phases and detected by electron capture after derivatization with pentafluorobenzaldehyde and silylation [37,43]. Such methods can be a great aid in the ultra-trace analysis of primary amines in biological specimens. 

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