the relative resistance is: TMS (1) < TES (64) < TBS (20 000) < TIPS (700,000) < TBDPS (5,000,000) In basic media, the relative resistance is: TMS (1) < TES (10-100) < TBS~TBDPS (20 000) < TIPS (100,000) Monoprotection of symmetrical diols It is possible to monosilylate a symmetrical diol, although this is known to be problematic occasionally. For example, the following monosilylation was reported: However, it turns out that this reaction is hard to repeat. If the reaction were controlled solely by thermodynamics then, statistically, if the dianion is of similar reactivity to the monoanion, then a corresponding statistical mixture of 1:2:1 disilylated:monosilylated:unsilylated diol will result. However, the reaction in THF is made selective by two factors, kinetic deprotonation of the first anion AND the insolubility of the monoanion. At the initial addition of TBSCl, there is only a minor amount of monoanion in solution with the rest being in suspension. This small portion reacts and shifts the equilibrium of the monoanion to draw more into solution, thereby allowing for high yields of the mono-TBS compound to be obtained. Superior results in some cases may be obtained with butyllithium: A third method uses a mixture of DMF and DIPEA. Alternatively, an excess (4 eq) of the diol can be used, forcing the reaction toward monoprotection. Selective deprotection Selective deprotection of silyl groups is possible in many instances. For example, in the synthesis of taxol: Silyl ethers are mainly differentiated on the basis of sterics or electronics. In general, acidic deprotections deprotect less hindered silyl groups faster, with the steric bulk on silicon being more significant than the steric bulk on oxygen. Fluoride-based deprotections deprotect electron-poor silyl groups faster than electron-rich silyl groups. There is some evidence that some silyl deprotections proceed via hypervalent silicon species. The selective deprotection of silyl ethers has been extensively reviewed. Although selective deprotections have been achieved under many different conditions, some procedures, outlined below, are more reliable. A selective deprotection will likely be successful if there is a substantial difference in sterics (e.g., primary TBS vs. secondary TBS or primary TES vs primary TBS) or electronics (e.g. primary TBDPS vs. primary TBS). Unfortunately, some optimization is inevitably required and it is often necessary to run deprotections partway and recycle material. Some common acidic conditions 100 mol% 10-CSA (camphorsulfonic acid) in MeOH, room temperature; a "blast" of acid, deprotects primary TBS groups within ten minutes.