Silylene Derivatives

Trost first introduced the di-/err-butylsilylene derivative as a means for protecting 12- and 1,3-diols during a synthesis of Pillaromycinone derivatives. Di-rerr-butylsilylene derivatives are not as robust as isopropyiidene or benzylidene acetals and their use is best reserved for systems requiring deprotection under very mild conditions. Di-isopropylsilylene derivatives are occasionally used but they usually only survive in highly crowded environments. Di-ferf-butylsily-lene derivatives survive hydroboration with 9-BBN, mild oxidation (e.g., the Dess-Martin, ozone). Lewis acids such as trifluoroborane etherate and titanium tetrachloride, mild acids (pyridinium p-toluenesulfonate). camphorsulfonic acid, strong bases such as rer/-butyllithium (THF, -50 °C), DDQ, and sodium meth-oxide in methanol at 0 — conditions used to cleave acetate esters. 

2- Diols are more easily released from their di-rer/-butylsilylene protectors than 

3- or L4-diols. The l,2-diol derivatives hydrolyse rapidly under basic conditions (5 1 THF-pH 10 buffer, room temperature, 5 min) whereas the 1.3- and 

Further examples of the application of di-rerr-butylsilylene derivatives to the protection of l,3- )iols in complex natural product synthesis include Salinomy-cin 219 Swinholide,22o Scytophycin C,22i and Muamvatin.222 

The simultaneous protection of the 3 and 5 hydroxy groups of nucleosides and the 4,6- or 3,4-hydroxyls of hexopyranoses is a common problem in organic synthesis In the case of hexopyranoses, we have already seen that benzylidene acetals and, in certain circumstances, isopropylidene acetals can be used to good effect. An alternative silicon-based group would offer a wider repertoire of conditions for mild deprotection and such a group was devised by Markiewicz the 1,1,3,3-tetraisopropyldisiloxanylidene group (abbreviated TIPDS). TIPDS groups are stable to water, 0.3 M p-toluenesulfonic acid in dioxane, 10% tri-fluoroacetic acid in chloroform, 5 M ammonium hydroxide in dioxane-H20 (4 1), and tertiary amines in pyridine. 

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Three different silylene derivatives were used to achieve selective protection of a rnore hindered diol during a taxol synthesis. Treatment of the silylene with MeLi opens the ring to afford the more hindered silyl ether. 

When the salt 152 was treated with [(COE)2IrCl]2, the product of C-Fl activation of the cyclooctene (COE) ligand 162 was obtained in 65% yield (Scheme 62) if -binding mode of the cyclooctene ligand in 162 was confirmed by X-ray crystallography and NMR spectroscopy. Complex 162 under the action of Fl2SiMes2 was converted into silylene derivative 163 observed by NMR spectroscopy <1999JA9871>. 

Two germanium-containing compounds were prepared using the route analogous to the standard route for silylene derivatives [Eq. 

Bifunctional protection of 1,2-, 1,3-, and, A-diols, dithiols, and dicarboxylic acids. The reagent (1) converts these substrates into silylene derivatives in high yield. Examples ... 

BF3-Et20, allyltrimethylsilane, toluene, 85°C, 95% yield." This is a general method for the selective ring opening of the DTBS derivative to give silyl ethers of the more hindered alcohol. The silylene derivatives of tertiary or henzylic alcohols result in elimination. 

A similar method for simultaneous derivatization of two functional groups is the formation of cyclic silylene derivatives for the same types of compounds (Fig. 

Benzenediols Di-ferr-butyl dichlorosilane Di-fert-butyl silylene derivatives... 

Source From Cyclic di-tert-butyl silylene derivatives of substituted salicylic acids and related compounds. A study by gas chromatography-mass spectrometry, in J. Chromatogr. ... 

Silylene derivatives have become very useful in synthetic organic chemistry. For example, the cyclic dimethylsilylene derivative (2) of o-hydroxyacetophenone is useful for [4 + 2] cycloadditions (eq 1).3 A glycosyl donor and acceptor have been linked as the dimethylsilylene derivative (3) and used for stereoselective glycosylation (eq 2). Also, an efficient and stereocontrolled aldol reaction of silylene enolate (4) with aldehydes has been reported (eq 3).5... 

This protecting group has seen limited use but has been applied in ribonucleoside chemistry. " The reagent reacts slowly with nucleosides in the presence of imidazole/DMF but formation of the more reactive di-t-butylsilyl dinitrate in situ, followed by addition of cytidine, results in a protected 3, 5 -silylene derivative in excellent yield (90%) (eq 3). The protecting group can be removed conveniently with tributylamine hydrofluoride. ... 

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