Silyl cleavage

Early attempts to generate ot-aryl-(3-silyl substituted carbocations by ionization of 1,1-diphenyl-2-(trimethylsilyl)ethanol 10 usipg FS03H in S02C1F even at very low temperature of -140 °C were unsuccessful (77). Only 1,1-diphenylethyl cation 11 and trimethylsilyl fluorosulfate, fhe products of P-silyl cleavage were observed. 

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). 

During the reaction silver(0) deposits on the sides of the reaction vessel. The sequence of events leading to the azomethine ylide is unclear. However, evidently single-electron transfer (SET) from the amine to silver takes place either prior or subsequent to fluoride-enabled silyl cleavage. This process is repeated with a second equivalent of silver fluoride resulting in the formation of 38 either in free form or more likely as its silver complex. The scope of the method was expanded to the synthesis of bicylic systems exemplified here by tropinone 42 (Scheme 2.11).19 Pandey has also extended the protocol to the synthesis of tricycloalkanes20 and applied it to a total synthesis of the poisonous frog alkaloid epibatidine.21... 

Likewise the l-/ -tolyl-2-(triisopropylsilyl)ethyl cation 15 is formed by protonation of ( -l-/ -tolyl-2-(triisopropylsilyl)ethene 1. Cation 15 can be generated using less acidic conditions FSO3H without addition of SbFs is sufficient, while successful generation of 12 from 13 requires FSOsH/SbFs, the much stroger superacid, to prevent p-silyl cleavage (13). 

T-Methoxysalicylaldehyde-BFs, CH2CI2, 25°C. This method generates HF in situ The following table gives the relative rates of silyl cleavage for three different reagents (TIBS — triisobutylsilyl). 

Synthesis of isocyanates,12 Thermal cleavage of urethanes gives low yields of isocyanates because of the high temperatures necessary (200-300°). However, if the urethane is N-silylated, cleavage occurs at much lower temperatures. Thus the urethane (1) is treated with trimethylchlorosilane and triethylamine in refluxing... 

Thomas investigated the chemistry of 8-azabicyclo[1.2.1]octanes, the core structure of the tropane alkaloids, and unexpectedly observed a Grob fragmentation instead of the predicted elimination reaction. Upon treatment of mesylate 27 with potassium /-butoxide in dimethyl sulfoxide, silyl cleavage affords alkoxide 28 which fragments to yield formaldehyde, a mesylate anion, and bicycle 29. Exposure of 27 to DBU in acetonitrile leads to the desired E2 reaction with complete suppression of the Grob fragmentation. ... 

Although the current results from preparations of ternary materials utilizing the silyl cleavage method are preliminary, the aforementioned data has encouraged further investigation into synAesizing precursors to III-V ternary and quaternary materials using this versatile reaction pathway. 

Numerous in-depth mechanistic studies have been performed on the Mukaiyama aldol reaction. " Although various mechanisms exist in the literature that take into account the various roles of the numerous catalysts used for the enantio- and diastereoselective Mukaiyama aldol reaction, the commonly accepted mechanism accounting for bond formation is shown below.The reaction begins with the coordination of a Lewis acid with aldehyde 4 to form complex 5. Due to its enhanced electrophilicity, complex 5 is attacked by the 7t-bond of the enol silane 6, giving rise to resonance stabilized cation 7. At this point, either intermolecular silyl cleavage upon hydrolysis or intramolecular silyl transfer to the product hydroxyl group occurs to give products such as 8 or 9. 

Alternatively, a Friedel-Crafts mechanism has been proposed to account for bond formation via the Mukaiyama aldol reaction. As stated, attack of the enol silane 11 on the activated aldehyde 12 provides carbocation 13. Prior to silyl group transfer or outright silyl cleavage seen in the mechanism above, removal of the a-hydrogen regenerates the enol silane 14. While highly dependent on specific reaction conditions, the isolation of 15 leads to the suggestion of 14 as a potential intermediate in the Mukaiyama aldol reaction. 

Epimerization at C(5) has not been observed under the conditions discussed for the preparation of C(6) epimers (see Section 5.11.3.8.4). It is possible to prepare 5 -epipenicillins, however, as shown in Scheme 28. Note particularly the successful removal of the phthaloyl group (step iii) in this sequence, a procedure which leads to /3-lactam cleavage when C(5) is the R, or natural, configuration. Silylation of (35) followed by DBN treatment afforded (36), which corresponds to epimerization at C(3), and (37), which corresponds to epimerization at C(3) and C(6). No product corresponding to only C(6) epimerization was observed (76JOC2561). 

Reaction of triethylsilyl hydrotrfoxide with electron-rich olefins to gh/e dioxetanes that react IntrarTMlecularly with a keto group in the presence of t-txrtyidimethyl silyl triflateto afford 1,2,4 Inoxanes also oxydatnre cleavage ol alkenes Also used in cleavage ol olefins... 

The cleavage proceeds by initial reduction of the nitro groups followed by acid-catalyzed cleavage. The DNB group can be cleaved in the presence of allyl, benzyl, tetrahydropyranyl, methoxy ethoxy methyl, methoxymethyl, silyl, trityl, and ketal protective groups. 

Silyl-derived protective groups are also used to mask the thiol function. A complete compilation is not given here since silyl derivatives are described in the section on alcohol protection. The formation and cleavage of silyl thioethers proceed analogously to simple alcohols. The Si—S bond is weaker than the Si—O bond, and therefore sulfur derivatives are more susceptible to hydrolysis. For the most part silyl ethers are rarely used to protect the thiol function because of their instability. Silyl ethers have been used for in situ protection of the — SH group during amide formation. ... 

Alkyldithio carbamates are prepared from the acid chloride (Et N, EtOAc, 0°) and amino acid, either free or as the O-silyl derivatives (70-88% yield). The N- i-propyldithio) carbamate has been used in the protection of proline during peptide synthesis. Alkyldithio carbamates can be cleaved with thiols, NaOH, Ph P/TsOH. They are stable to acid. Cleavage rates are a function of the size of the alkyl group as illustrated in the table below. 

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

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