Silylation
Silylation is the introduction of a (usually) substituted silyl group (R3Si–) to a molecule.
Nearly all functional groups which present a problem in gas chromatogaphic separation (hydroxyl, carboxylic acid, amine, thiol, phosphate) can be derivatized by silylation reagents. It involves the replacement of an acidic hydrogen on the compound with an alkylsilyl group, for example, trimethylsilyl (-SiMe3). The derivatives are generally less polar, more volatile and more thermally stable. The introduction of a silyl group(s) can also serve to enhance mass spectrometric properties of derivatives, by producing either more favorable diagnostic fragmentation patterns of use in structure investigations, or characteristic ions of use in trace analyses employing selected ion monitoring and related techniques.[1]
Similarly, silylation is used to introduce silyl groups as protecting groups. A general technique is to deprotonate the substrate with a suitable strong base (e.g. butyl lithium), and allow it to react with a silyl chloride (e.g. trimethylsilyl chloride). An auxiliary base is usually used to remove the hydrogen chloride formed:
- RH → R− + H+
- R− + R'3SiCl → R-SiR'3 + Cl−
Desilylation is the reverse of silylation: the silyl group is exchanged for a proton. Various fluorides (e.g. sodium, potassium, tetra-n-butylammonium fluorides) are popular for this purpose.