Silyl ethers relative stability

The following tables give a comparison of the stability of various silyl ethers to acid, base, and TBAF. The reported half-lives vary as a function of environment and acid or base concentration, but they help define the relative stabilities of these silyl groups. 

The protection of alcohols as silyl ethers has been reviewed62, as have the relative stabilities of the different trialkylsilyl groups63. Their stability under alcohol oxidation conditions and their oxidative deprotection have been discussed64. Methods for selective deprotection of the various silyl ethers have been the subject of an excellent review65. 

The synthetic potential of silyl ethers as protecting groups for hydroxyls is based on the fact that they can be easily introduced and cleaved under mild conditions and their relative stability can be tuned by varying the substituents on silicon. In carbohydrate chemistry, the tert-butyl-dimethylsilyl (TBDMS), terl-butyldiphenylsilyl (TBDPS) and triethyl-silyl (TES) ethers are the most often applied silicon-based protecting groups (Scheme 2.9).23... 

The Silyl Group as an Electron Acceptor. III. The Relative Stabilities of Some Complexes of Silyl Iodide with Ethers and Thioethers. J. inorg. nucl. Chem. 15, 87—94 (1960). 

The TES group is 10-100 times more stable to hydrolysis or nucleophilic attack than the TMS group but much more labile than the /m-butyldimethylsilyl (TBS) group. A quantitative measure of the relative stability of TES compared with the TMS and TBS groups can be gleaned from a synthesis of Rapamycin by Smith and co-workers.1 A salient problem was the choice of protecting group for the CIO hydroxyl [Scheme 4.15]. Three silyl ethers were evaluated. The TBS derivative of 15.1 was too stable and attempts to remove it under a... 

Best results with tetrafluorophcnyl-/.5-phosphane in fluorinations of silyl ethers are achieved with substrates stabilized by electron-withdrawing substituents in the a- or /3-position relative to thesiloxy group,11 15 e.g. formation of 3,13 and with steroids, e.g. formation of 4.16... 

Although relatively short-lived, radical cations are the primary products of the electron transfer (ET) step. Evidence for the formation of radical cations has been presented for silyl enol ether systems that form stabilized radical cations. Kochi and coworkers investigated the photoinduced electron transfer between different silyl enol ethers and chloranil in detail.In a system suitable for the spectroscopic study of the radical cation intermediate, chloranil (CA) is electronically excited to its triplet state CA (10). Subsequent electron transfer generates the corresponding radical ion pair (11/12). All these transient species have been observed by time-resolved spectroscopy. 

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