Protection of alcohols as silyl ethers
Description: Alcohols can be converted into silyl ethers with trimethylsilyl chloride (TMSCl) or similar silly groups such as t-butyldimethylsilyl chloride (TBDMSCl). This is a useful procedure for the protection of alcohols.
Notes: Silyl ethers are inert to many reagents that react with alcohols, so they serve as excellent protecting groups.
Sometimes a base such as pyridine is added to the reaction to quench excess HCl formed during the reaction.
Silyl ethers are cleaved using strong acids or fluoride ion (such as tetrabutylammonium fluoride, TBAF). Silicon forms very strong bonds to fluorine.
TMSCl is sometimes also written as (CH3)3SiCl.
Notes: The last example shows pyridine as a base. Another common base is “imidazole”.
Mechanism: Attack of the oxygen on the silicon leads to displacement of Cl (Step 1, arrows A and B) forming a protonated silyl ether, which is then deprotonated (Step 2, arrows C and D). This reaction actually proceeds much better if a base such as pyridine is used to perform this last step.
Notes: Sometimes a base such as pyridine is added that will react with the H-Cl that is formed; this helps the reaction proceed forward.