The phenyldimethylsilyl group as a masked hydroxy group
A phenyldimethylsilyl group attached to carbon can be converted into a hydroxy group 1→5, with retention of configuration at the migrating carbon, by any of three main methods. The first involves protodesilylation, to remove the phenyl ring from the silicon atom, followed by oxidation of the resulting functionalised silicon atom using peracid or hydrogen peroxide. The second uses mercuric acetate for the same purpose, and can be combined in one pot with the oxidative step using peracetic acid. This method has a variant in which the mercuric ion is combined with palladium(II) acetate, both in less than stoichiometric amounts. The third uses bromine, which can also be used in one pot in conjunction with peracetic acid. In this method, but not in the method based on mercuric acetate, the peracetic acid may be buffered with sodium acetate. The method using bromine as the electrophile for removing the benzene ring has a more agreeable variant in which it is administered in the form of potassium bromide, which is oxidised to bromine by the peracetic acid. The scope and limitations of each of these methods are reported with a range of examples possessing between them many of the common functional groups. Simple benzene rings, alcohols, ethers, esters, amides and nitriles are compatible with all three methods, and ketones do not undergo Baeyer–Villiger reaction under any of the conditions. Amines, however, are oxidised to amine oxides. Ketones may be brominated in the third of the three main recipes. The absence of acid in the third method makes it especially valuable when the phenyldimethylsilyl group has a neighbouring nucleofugal group such as hydroxy or acetoxy. Carbon–carbon double bonds are incompatible with the methods, except for terminal monosubstituted double bonds, which can survive the conditions used in the first of the three methods.