Trialkylsilyl group, selective removal

Kinetic studies of the substitution reaction of 2-chloro-l-methylpyridinium iodide with phenoxides are consistent with the SnAt mechanism, with rate-determining nucleophilic attack.38 The effects of a variety of ring substituents on the reactivities of 2-fluoro- and 2-chloro-pyridines in reactions with sodium ethoxide in ethanol have been examined. The results were discussed in terms of the combination of steric, inductive, and repulsive interactions.39 Substitution in 2,4,6-trihalopyridines normally occurs preferentially at the 4-position. However, the presence of a trialkylsilyl group at the 3-position has been shown to suppress reaction at adjacent positions, allowing substitution at the 6-position.40 Methods have been reported for the introduction and removal of fluorine atoms for polyfluoropyridines. Additional fluorine atoms were introduced by metallation, chlorination, and then fluorodechlorination, while selective removal of fluorine was achieved by reduction with either metals or complex hydrides or alternatively by substitution by hydrazine followed by dehydrogena-tion-dediazotization.41... 

Dealkylation and de(trialkylsilyl)atlon have been covered In a review on selective removal of protecting groups. ... 

A new and efficient combination for silylation is trimethylsilyl chloride-lithium sulphide. Even hindered hydroxy-groups are silylated at room temperature in neutral conditions, although the mechanism of the process is not yet clear. 4-Dimethylaminopyridine (DMAP) has been shown to be an efficient catalyst for the silylation of alcohols by t-butyldimethylsilyl chloride and to be more selective than the imidazole traditionally used. Two new methods for removal of the t-butyldimethylsilyl group are treatment with boron trifluoride etherate and reaction with aqueous HF in acetonitrile. The O-silyl derivatives of normal carboxylic esters, i.e. keten methyl trialkylsilyl acetals (59), have been investigated in silyl transfer to alcohols (Scheme 31) they have the advantage of... 

---